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DNA and GenesDNA and Genes
280
What You’ll Learn■ You will relate the structure
of DNA to its function. ■ You will explain the role of
DNA in protein production.■ You will distinguish among
different types of mutations.
Why It’s ImportantAn understanding of geneticdisorders, viral diseases, cancer,aging, genetic engineering, andeven criminal investigationsdepends upon knowing aboutDNA, how it holds information,and how it plays a role in pro-tein production.
Robert Harding World Imagery/Alamy Images
Visit to• study the entire chapter
online• access Web Links for more
information and activities onDNA and genes
• review content with theInteractive Tutor and self-check quizzes
Shetland ponies originated inShetland—a group of islands off the coast of Scotland. Theislands are mostly barren andhave extremely cold winters. Dueto the isolation of these islandsin the past, the characteristics ofthe pony—small stature, thickhair (coat, mane, and tail),strength, and hardiness—arefirmly imprinted in its DNA.
Understandingthe Photo
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11.1SECTION PREVIEWObjectivesAnalyze the structure ofDNA.Determine how the struc-ture of DNA enables it toreproduce itself accurately.
Review Vocabularynucleotide: subunit of a
nucleic acid formed froma simple sugar, a phos-phate group, and anitrogenous base (p. 163)
New Vocabularynitrogenous basedouble helixDNA replication
11.1 DNA: THE MOLECULE OF HEREDITY 281
What is DNA? Although the environment influences how an organism develops, the
genetic information that is held in the molecules of DNA ultimatelydetermines an organism’s traits. DNA achieves its control by determiningthe structure of proteins. Living things contain proteins. Your skin con-tains protein, your muscles contain protein, and your bones contain pro-tein mixed with minerals. All actions, such as eating, running, and eventhinking, depend on proteins called enzymes. Enzymes are critical for anorganism’s function because they control the chemical reactions neededfor life. Within the structure of DNA is the information for life—thecomplete instructions for manufacturing all the proteins for an organism.
DNA as the genetic materialIn the early 1950s, many scientists believed that protein was the genetic
material, mainly because the structure of these large molecules was sovaried. In 1952, however, Alfred Hershey and Martha Chase performedexperiments using radioactively labeled viruses that infect bacteria.
Life’s InstructionsUsing an Analogy Can youimagine all of the informationthat could be contained in 1000textbooks? Remarkably, thatmuch information—andmore—is carried by the genesof a single organism. Scientistshave found that the DNA con-tained in genes holds this infor-mation. Because of the uniquestructure of DNA, new copiesof the information can beeasily reproduced.List Make a list of currentevents issues concerningDNA that you may have readabout in a newspaper. As youread this section, refer to yourlist and add explanations from the text.
(t)Aaron Haupt, (inset)CNRI/Phototake, NYC
Model of a DNA molecule
DNA: The Molecule of Heredity
Standard 5b Students know how to apply base-pairing rules to explainprecise copying of DNA during semiconservative replication and transcription of informationfrom DNA into mRNA.
California Standards
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These viruses were made of only pro-tein and DNA. Hershey and Chasecreated two different types of viruses.One type had radioactive DNA andthe other type had radioactive pro-tein. Each type of virus infected aseparate bacteria culture. Only theDNA entered the bacteria and pro-duced new viruses. These results wereconvincing evidence that DNA is thegenetic material.
The structure of nucleotidesDNA is capable of holding all its
information because it is a very longmolecule. Recall that DNA is a polymermade of repeating subunits callednucleotides. Nucleotides have threeparts: a simple sugar, a phosphategroup, and a nitrogenous base. Thesimple sugar in DNA, called deoxyri-bose (dee ahk sih RI bos), gives DNA its name—deoxyribonucleic acid. Thephosphate group is composed of one
atom of phosphorus surrounded byfour oxygen atoms. A nitrogenousbase is a carbon ring structure that con-tains one or more atoms of nitrogen. InDNA, there are four possible nitroge-nous bases: adenine (A), guanine (G),cytosine (C), and thymine (T). Thus, inDNA there are four possible nucleo-tides, each containing one of these fourbases, as shown in Figure 11.1.
Nucleotides join together to formlong chains, with the phosphate groupof one nucleotide bonding to thedeoxyribose sugar of an adjacentnucleotide. The phosphate groups anddeoxyribose molecules form the back-bone of the chain, and the nitrogenousbases stick out like the teeth of a zipper.In DNA, the amount of adenine isalways equal to the amount of thymine,and the amount of guanine is alwaysequal to the amount of cytosine. Youcan see this in the Problem-Solving Labon the next page.
282 DNA AND GENES
Purines Pyrimidines
Nucleotide
O
N
N
N
NN
Adenine (A)
O
O–
O = P – O
OH
H
H
H H
H
H
HSugar
(deoxyribose)
Nitrogenous basePhosphate group
CH2
H
N
N
N
N
N
Guanine (G)
O
N
N
N
N
N
N
Thymine (T)
H
CH3
Cytosine (C)
O
OH
H
N
N
N
H
H
O
AA BB
CCFigure 11.1Double-ringed nitrogenousbases—adenine and guanine—arecalled purines (A). Single-ringednitrogenous bases—thymine andcytosine—are called pyrimidines(B). Each of the four nucleotidesthat make up DNA contains aphosphate group, the sugardeoxyribose, and one of four different nitrogenous bases (C).
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283
The structure of DNAIn 1953, James Watson and Francis
Crick published a letter in a journalthat was only one page in length, yetmonumental in importance. Watsonand Crick proposed that DNA ismade of two chains of nucleotidesheld together by nitrogenous bases.Just as the teeth of a zipper hold thetwo sides of the zipper together, thenitrogenous bases of the nucleotideshold the two strands of DNA togetherwith weak hydrogen bonds. Hydrogenbonds can form only between certainbases, so the bases on one stranddetermine the bases on the otherstrand. Specifically, adenine on onestrand pairs only with thymine on theother strand, and guanine on onestrand pairs only with cytosine on theother strand. These paired bases,called complementary base pairs,explain why adenine and thymine arealways present in equal amounts.Likewise, the guanine-cytosine basepairs result in equal amounts of thesenucleotides in DNA. Watson andCrick also proposed that DNA isshaped like a long zipper that is twistedinto a coil like a spring. When some-thing is twisted like a spring, the shapeis called a helix. Because DNA is com-posed of two strands twisted together,its shape is called a double helix. Thisshape is shown in Figure 11.2.
The importance of nucleotide sequences
A cattail, a cat, and a catfish are alldifferent organisms composed of differ-ent proteins. If you compare the chro-mosomes of these organisms, you willfind that they all contain DNA made upof the same four nucleotides with ade-nine, thymine, guanine, and cytosine as their nitrogenous bases. How canorganisms be so different from eachother if their genetic material is made of the same four nucleotides?
Interpret the DataWhat does chemical analysis reveal about DNA? Much ofthe early research on the structure and composition of DNAwas done by carrying out chemical analyses. The data fromexperiments by Erwin Chargaff provided evidence of a rela-tionship among the nitrogenous bases of DNA.
Solve the ProblemExamine the table below. Compare the amounts of adenine,guanine, cytosine, and thymine found in the DNA of each ofthe cells studied.
Thinking Critically1. Compare and Contrast Compare the amounts of A, T, G,
and C in each kind of DNA. Why do you think the relativeamounts are so similar in human liver and thymus cells?
2. Compare and Contrast How do the relative amounts ofeach base in herring sperm compare with the relativeamounts of each base in yeast?
3. Summarize What fact can you state about the overallcomposition of DNA, regardless of its source?
Dan Richardson/The Stock Shop
Figure 11.2DNA normally exists inthe shape of a doublehelix. This shape is simi-lar to that of a twistedzipper.
Percent of Each Base in DNA Samples
Source of Sample A G C T
Human liver 30.3 19.5 19.9 30.3
Human thymus 30.9 19.9 19.8 29.4
Herring sperm 27.8 22.2 22.6 27.5
Yeast 31.7 18.2 17.4 32.6
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Their differences result from thesequence of the four different nucleo-tides along the DNA strands, as shownin Figure 11.3.
The sequence of nucleotides formsthe unique genetic information of anorganism. For example, a nucleotidesequence of A-T-T-G-A-C carriesdifferent information from a sequenceof T-C-C-A-A-A. In a similar way,two six-letter words made of the sameletters but arranged in different orderhave different meanings. The closerthe relationship is between twoorganisms, the more similar theirDNA nucleotide sequences will be.
The DNA sequences of a chimpanzeeare similar to those of a gorilla, butdifferent from those of a rosebush.Scientists use nucleotide sequences to determine evolutionary relation-ships among organisms, to determinewhether two people are related, andto identify bodies of crime victims.
Replication of DNAA sperm cell and an egg cell of two
fruit flies, both produced throughmeiosis, unite to form a fertilized egg.From one fertilized egg, a fruit flywith millions of cells is produced bythe process of mitosis. Each cell has acopy of the DNA that was in the orig-inal fertilized egg. As you havelearned, before a cell can divide bymitosis or meiosis, it must first makea copy of its chromosomes. The DNAin the chromosomes is copied in aprocess called DNA replication.Without DNA replication, new cellswould have only half the DNA of theirparents. Species could not survive, and
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Sugar-phosphate backbone
Hydrogen bonds betweennitrogenous bases
Chromosome
A
A
A
P
P
P
P
D
D
D
D
D
D
D
D
P
T
T
T
G
G
C
C
284 DNA AND GENES
Figure 11.3The structure ofDNA is shownhere.
In each chain of nucleotides,the sugar of one nucleotideis joined to the phosphategroup of the next nucleotideby a covalent bond.
A
Complementary base pairing produces a long, two-stranded molecule that is often compared to a zipper. As you can see, the sides of the zipper areformed by the sugar and phosphate units, while the teeth of the zipper are the pairs of bases.
C
The two chains of nucleotidesin a DNA molecule are heldtogether by hydrogen bondsbetween the bases. In DNA,cytosine forms three hydrogen
bonds with guanine, andthymine forms two
hydrogen bondswith adenine.
B
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individuals could not grow or repro-duce successfully. All organisms un-dergo DNA replication. Figure 11.4Bshows bacterial DNA replicating.
How DNA replicatesYou have learned that a DNA mole-
cule is composed of two strands, eachcontaining a sequence of nucleotides.As you know, an adenine on one strandpairs with a thymine on the otherstrand. Similarly, guanine pairs withcytosine. Therefore, if you know theorder of bases on one strand, you canpredict the sequence of bases on theother, complementary strand. In fact,part of the process of DNA replicationis done in just the same way. Duringreplication, each strand serves as a pat-tern, or template, to make a new DNAmolecule. How can a molecule serve asa template? Examine Figure 11.5 onthe next page to find out.
Replication begins as an enzymebreaks the hydrogen bonds betweenbases that hold the two strands
together, thus unzipping the DNA.As the DNA continues to unzip,nucleotides that are floating free inthe surrounding medium are attachedto their base pair by hydrogen bond-ing. Another enzyme bonds thesenucleotides into a chain.
This process continues until theentire molecule has been unzippedand replicated. Each new strandformed is a complement of one of theoriginal, or parent, strands. Theresult is the formation of two DNAmolecules, each of which is identicalto the original DNA molecule.
When all the DNA in all the chro-mosomes of the cell has been copiedby replication, there are two copies ofthe organism’s genetic information.In this way, the genetic makeup of anorganism can be passed on to newcells during mitosis or to new genera-tions through meiosis followed bysexual reproduction.
Explain why DNAmust unzip before it can be copied.
11.1 DNA: THE MOLECULE OF HEREDITY 285Dr. Gopal Murti/Science Photo Library/Photo Researchers
Replication
DNA
Replication
Figure 11.4DNA replication produces two mol-ecules from one. Compare Howdo the new molecules comparewith the original?
When a DNAmolecule replicates,two molecules areformed. Eachmolecule has oneoriginal strand andone new strand.Newly synthesizedstrands are shownin red.
A
This DNA is replicating. Replication is takingplace at the points where the DNA hasunzipped to create a “bubble.”
B
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286 DNA AND GENES
Separation of strands When acell begins to copy its DNA, the twonucleotide strands of a DNA moleculeseparate when the hydrogen bondsconnecting the base pairs are broken.As the DNA molecule unzips, thebases are exposed.
AA
Base pairing The bases infree nucleotides pair withexposed bases in the DNAstrand. If one nucleotide on astrand has thymine as a base,the free nucleotide that pairswith it would be adenine. Ifthe strand contains cytosine,a free guanine nucleotide willpair with it. Thus, each strandbuilds its complement by basepairing—forming hydrogenbonds—with free nucleotides.
BB
Bonding of bases The sugar andphosphate parts of adjacent nucleotidesbond together with covalent bonds toform the backbone of the new strand.Each original strand is now hydrogen-bonded to a new strand.
CC
Copying DNAFigure 11.5DNA is copied during interphase prior to mitosis andmeiosis. It is important that the new copies are exactlylike the original molecules. The structure of DNA providesa mechanism for accurate copying of the molecule. Theprocess of making copies of DNA is called DNA replica-tion. Critical Thinking What might be the outcome ifmitosis occurred before replication took place?
D
P
D
P
D
P
P
D
D
P
D
P
D
P
Original DNA strand
New DNA strand
Original DNA
A
A
A
AA
T
T
T
T
G
G
G
G
C
C
C
C
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11.1 DNA: THE MOLECULE OF HEREDITY 287Dr. Gopal Murti/Science Photo Library/Photo Researchers
Understanding Main Ideas1. Describe the structure of a nucleotide.
2. How do the nucleotides in DNA bond with eachother within a strand? How do they bond witheach other across strands?
3. Explain why the structure of a DNA molecule isoften described as a zipper.
4. How does DNA hold information?
Thinking Critically5. The sequence of nitrogenous bases on one strand
of a DNA molecule is GGCAGTTCATGC. Whatwould be the sequence of bases on the comple-mentary strand?
6. Get the Big Picture Sequence the steps thatoccur during DNA replication. For more help, referto Get the Big Picture in the Skill Handbook.
SKILL REVIEWSKILL REVIEW
Results of replication The process ofreplication produces two molecules of DNA. Eachnew molecule has one strand from the originalmolecule and one strand that has been newlysynthesized from free nucleotides in the cell.
DD
Free nucleotides
New DNA molecule
New DNA molecule
TOriginal DNA strand
Two molecules of DNA from one
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From DNA to Protein11.2
Genes and ProteinsThe sequence of nucleotides in DNA contain information. This infor-
mation is put to work through the production of proteins. Proteins foldinto complex, three-dimensional shapes to become key structures andregulators of cell functions. Some proteins become important structures,such as the filaments in muscle tissue. Other proteins, such as enzymes,control chemical reactions that perform key life functions—breakingdown glucose molecules in cellular respiration, digesting food, or makingspindle fibers during mitosis. In fact, enzymes control all the chemicalreactions of an organism. Thus, by encoding the instructions for makingproteins, DNA controls cells.
You learned earlier that proteins are polymers of amino acids. Thesequence of nucleotides in each gene contains information for assemblingthe string of amino acids that make up a single protein.
Explain how DNA controls the activities of cells.
RNARNA, like DNA, is a nucleic acid. However, RNA structure differs from
DNA structure in three ways, shown in Figure 11.6. First, RNA is singlestranded—it looks like one-half of a zipper—whereas DNA is doublestranded. The sugar in RNA is ribose; DNA’s sugar is deoxyribose.
Sequence After you read Section 11.2, begin at the top tab and sequence the five steps of translation.
Protein Synthesis Make the following Foldable to help you understand the process of protein formation.
Collect 3 sheets of paper and layer them about 1.5 cm apart vertically. Keep the edges level.
Fold up the bottom edges of the paper to form 6 equal tabs.
Fold the papers and crease well to hold the tabs in place. Staple along the fold. Label each tab.
STEP 1
STEP 3
STEP 2
288 DNA AND GENES
SECTION PREVIEWObjectivesRelate the concept of thegene to the sequence ofnucleotides in DNA.Sequence the stepsinvolved in protein synthesis.
Review Vocabularypolymer: a large molecule
formed from smaller sub-units that are bondedtogether (p. 158)
New Vocabularymessenger RNAribosomal RNAtransfer RNAtranscriptioncodontranslation
Standard 4b Students know how to apply the genetic coding rules topredict the sequence of amino acids from a sequence of codons in RNA.
California Standards
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Finally, both DNA and RNA containfour nitrogenous bases, but ratherthan thymine, RNA contains a simi-lar base called uracil (U). Uracilforms a base pair with adenine inRNA, just as thymine does in DNA.
What is the role of RNA in a cell?Perhaps you have seen a car beingbuilt on an automobile assembly line.Complex automobiles are built inmany simple steps. Engineers tellworkers how to make the cars, andworkers follow directions to buildthe cars on the assembly line.Suppliers bring parts to the assemblyline so they can be installed in the car. Protein production is similarto car production. DNA providesworkers with the instructions formaking the proteins, and workers
build the proteins. Other workersbring parts, the amino acids, over tothe assembly line. The workers forprotein synthesis are RNA mole-cules. They take from DNA theinstructions on how the proteinshould be assembled, then—aminoacid by amino acid—they assemblethe protein.
There are three types of RNA thathelp build proteins. Extending thecar-making analogy, you can considerall three of these RNA molecules to be workers in the protein assemblyline. One type of RNA, messengerRNA (mRNA), brings instructionsfrom DNA in the nucleus to the cell’s factory floor, the cytoplasm. On the factory floor, mRNA moves to the assembly line, a ribosome.
11.2 FROM DNA TO PROTEIN 289
CH
C
C CH
C
C
N
N
O
C
Hydrogenbonds Adenine
Uracil
Ribose
A
OH OH
H
O
H
O
O
O�
O P O CH2
Phosphate
NH
N
N
N
N
H
H
C
C C
C
C H
P
R
C
P
RU
RG
C
H
Figure 11.6The three chemical differ-ences between DNA and RNAare shown here. InterpretScientific Illustrations Whatis the three-dimensionalshape of RNA?
The nitrogenous base uracil (U) replaces thymine (T)in RNA. In RNA, uracil forms base pairs with adeninejust as thymine does in DNA.
C
Ribose is the sugarin RNA, rather thanthe deoxyribosesugar in DNA.
B
An RNA molecule usually consists of asingle strand of nucleotides, not a doublestrand. This single-stranded structure isclosely related to its function.
A
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The ribosome, made of ribosomalRNA (rRNA), binds to the mRNAand uses the instructions to assemblethe amino acids in the correct order.The third type of RNA, transferRNA (tRNA) is the supplier. TransferRNA delivers amino acids to the ribo-some to be assembled into a protein.
TranscriptionHow does the information in DNA,
which is found in the nucleus, move tothe ribosomes in the cytoplasm?Messenger RNA carries this informa-tion through the nuclear envelope tothe ribosomes for manufacturing pro-teins, just as a worker carries informa-tion from the engineers to the assemblyline for manufacturing a car. In thenucleus, enzymes make an RNA copyof a portion of a DNA strand in a process called transcription(trans KRIHP shun). Follow the steps in Figure 11.7 as you read about transcription.
The main difference between tran-scription and DNA replication is thattranscription results in the formationof one single-stranded RNA moleculerather than a double-stranded DNAmolecule.
DNAstrand
DNAstrand
RNAstrand
RNAstrand
A
A
A
T
T
U U
G
G G
G
G
C
C
C
C
Figure 11.7Messenger RNA ismade during theprocess of transcription.
290 DNA AND GENES
The process oftranscriptionbegins as enzymesunzip the moleculeof DNA in theregion of the geneto be transcribed.
A
Free RNAnucleotides formbase pairs with theircomplementarynucleotides on theDNA strand. ThemRNA strand iscomplete when theRNA nucleotides bondtogether.
B
The mRNA strandbreaks away, andthe DNA strandsrejoin. The mRNAstrand leaves thenucleus and entersthe cytoplasm.
C
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11.2 FROM DNA TO PROTEIN 291
Modeling transcription in theBioLab on pages 302–303 will help youto understand this process. You canfind out how scientists use new micro-scopes to “watch” transcription takeplace by reading the Biotechnology atthe end of the chapter.
RNA ProcessingNot all the nucleotides in the DNA
of eukaryotic cells carry instructions—or code—for making proteins. Genesusually contain many long noncodingnucleotide sequences, called introns(for intervening regions), that are scat-tered among the coding sequences.Regions that contain information arecalled exons because they areexpressed. When mRNA is transcribedfrom DNA, both introns and exons arecopied. The introns must be removedfrom the mRNA before it can functionto make a protein. Enzymes in thenucleus cut out the intron segmentsand paste the mRNA back together.The mRNA then leaves the nucleusand travels to the ribosome.
The Genetic CodeThe nucleotide sequence tran-
scribed from DNA to a strand of mes-senger RNA acts as a genetic message,the complete information for thebuilding of a protein. Think of thismessage as being written in a languagethat uses nitrogenous bases as itsalphabet. As you know, proteins con-tain chains of amino acids. You couldsay that the language of proteins usesan alphabet of amino acids. A code isneeded to convert the language ofmRNA into the language of proteins.There are 20 common amino acids,but mRNA contains only four types ofbases. How can four bases form a codefor all possible proteins? The Problem-Solving Lab shows you how.
Formulate ModelsHow many nitrogenous bases determine an amino acid?After the structure of DNA had been discovered, scientists triedto predict the number of nucleotides that code for a singleamino acid. It was already known that there were 20 aminoacids, so at least 20 groups were needed. If one nucleotide codedfor an amino acid, then only four amino acids could be repre-sented. How many nucleotides are needed?
Solve the ProblemExamine the three safes.Letters representingnitrogenous bases havereplaced numbers on thedials. Copy the data table.Calculate the possiblenumber of combinationsthat will open the safe ineach diagram using the formula provided in the table. The 4corresponds to the number of letters on each dial; the super-script refers to the number of available dials.
Thinking Critically 1. Use Models Using Safe 1, write down several examples of
dial settings that might open the safe. Do the total possi-ble combinations seen in Safe 1 equal or surpass the totalnumber of common amino acids?
2. Analyze Could a nitrogenous base (A, T, C, or G) takenone at a time code for 20 different amino acids? Explain.
3. Use Numbers Using Safe 2, write down several examplesof dial combinations that might open the safe. Do thetotal possible combinations seen in Safe 2 equal or surpassthe total number of common amino acids?
4. Analyze Could nitrogenous bases taken two at a timecode for 20 different amino acids? Explain.
5. Analyze Could nitrogenous bases taken three at a timecode for 20 different amino acids? Explain.
6. Draw Conclusions Does the analogy prove that threebases code for an amino acid? Explain.
T
G
C
A
T
G
C
A
T
G
C
A T
G
C
A
T
G
C
A
T
G
C
A
Safe 1 Safe 2 Safe 3
Data Table
Number TotalNumber of Letters Possible Formulaof Dials per Dial Combinations
Safe 1 41
Safe 2 42
Safe 3 43
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Biochemists began to crack thegenetic code when they discoveredthat a group of three nitrogenousbases in mRNA code for one aminoacid. Each group is known as a codon.For example, the codon UUU resultsin the amino acid phenylalanine beingplaced in a protein.
Sixty-four combinations are possi-ble when a sequence of three bases isused; thus, 64 different mRNAcodons are in the genetic code,shown in Table 11.1. Some codonsdo not code for amino acids; theyprovide instructions for making theprotein. For example, UAA is a stopcodon indicating that the proteinchain ends at that point. AUG is astart codon as well as the codon forthe amino acid methionine. As youcan see, more than one codon cancode for the same amino acid.However, for any one codon, therecan be only one amino acid.
All organisms use the same geneticcode. For this reason, it is said to beuniversal, and this provides evidencethat all life on Earth evolved from acommon origin. From the chloro-phyll of a birch tree to the digestiveenzymes of a bison, a large number ofproteins are produced from DNA. It may be hard to imagine that onlyfour nucleotides can produce so many diverse proteins; yet, thinkabout computer programming. Youmay have seen computer code, such as 00010101110000110. Through a binary language with only twooptions—zeros and ones—manytypes of software are created. Fromcomputer games to World WideWeb browsers, complex software isbuilt by stringing together the zerosand ones of computer code into longchains. Likewise, complex proteinsare built from the long chains ofDNA carrying the genetic code.
292 DNA AND GENES
Table 11.1 The Messenger RNA Genetic Code
First ThirdLetter Second Letter Letter
U C A G
U Phenylalanine (UUU) Serine (UCU) Tyrosine (UAU) Cysteine (UGU) U
Phenylalanine (UUC) Serine (UCC) Tyrosine (UAC) Cysteine (UGC) C
Leucine (UUA) Serine (UCA) Stop (UAA) Stop (UGA) A
Leucine (UUG) Serine (UCG Stop (UAG) Tryptophan (UGG) G
C Leucine (CUU) Proline (CCU) Histidine (CAU) Arginine (CGU) U
Leucine (CUC) Proline (CCC) Histidine (CAC) Arginine (CGC) C
Leucine (CUA) Proline (CCA) Glutamine (CAA) Arginine (CGA) A
Leucine (CUG) Proline (CCG) Glutamine (CAG) Arginine (CGG) G
A Isoleucine (AUU) Threonine (ACU) Asparagine (AAU) Serine (AGU) U
Isoleucine (AUC) Threonine (ACC) Asparagine (AAC) Serine (AGC) C
Isoleucine (AUA) Threonine (ACA) Lysine (AAA) Arginine (AGA) AMethionine; Threonine (ACG) Lysine (AAG) Arginine (AGG) GStart (AUG)
G Valine (GUU) Alanine (GCU) Aspartate (GAU) Glycine (GGU) U
Valine (GUC) Alanine (GCC) Aspartate (GAC) Glycine (GGC) C
Valine (GUA) Alanine (GCA) Glutamate (GAA) Glycine (GGA) A
Valine (GUG) Alanine (GCG) Glutamate (GAG) Glycine (GGG) G
0288-0295 C11S2 BDOL-829900 8/5/04 9:51 AM Page 292
Translation: FrommRNA to Protein
How is the language of the nucleicacid mRNA translated into the lan-guage of proteins? The process of con-verting the information in a sequenceof nitrogenous bases in mRNA into asequence of amino acids in protein isknown as translation. You can sum-marize transcription and translation bycompleting the MiniLab.
Translation takes place at the ribo-somes in the cytoplasm. In prokary-otic cells, which have no nucleus, themRNA is made in the cytoplasm. In eukaryotic cells, mRNA is madein the nucleus and travels to thecytoplasm. In the cytoplasm, a ribo-some attaches to the strand ofmRNA like a clothespin clampedonto a clothesline.
The role of transfer RNAFor proteins to be built, the 20 dif-
ferent amino acids dissolved in thecytoplasm must be brought to the ribo-somes. This is the role of transfer RNA(tRNA), modeled in Figure 11.8. EachtRNA molecule attaches to only onetype of amino acid.
PredictTranscribe and Translate Molecules of DNA carry thegenetic instructions for protein formation. Converting theseDNA instructions into proteins requires a series of coordinatedsteps in transcription and translation.
Procedure! Copy the data table.@ Complete column B by writing the correct mRNA codon
for each sequence of DNA bases listed in the columnmarked DNA Base Sequence. Use the letters A, U, C, or G.
# Identify the process responsible by writing its name on thearrow in column A.
$ Complete column D by writing the correct anticodon thatbinds to each codon from column B.
% Identify the process responsible by writing its name on thearrow in column C.
^ Complete column E by writing the name of the correctamino acid that is coded by each base sequence. UseTable 11.1 on page 292 to translate the mRNA basesequences to amino acids.
Analysis1. Recognize Spatial Relationships Where within the cell:
a. are the DNA instructions located?b. does transcription occur?c. does translation occur?
2. Formulate Models Describe the structure of a tRNA molecule.
3. Use Scientific Explanations Explain why specific basepairing is essential to the processes of transcription andtranslation.
293
T
Anticodon
Aminoacid
Chain of RNAnucleotides
ransfer RNAmolecule
AA
U A C
Figure 11.8A tRNA molecule is composed of about 80 nucleotides. Each tRNA rec-ognizes only one amino acid. The amino acid becomes bonded to oneside of the tRNA molecule. Located on the other side of the tRNA mol-ecule are three nitrogenous bases, called an anticodon, that pair upwith an mRNA codon during translation.
Data Table
A B C D EDNA Base mRNA tRNA AminoSequence Process Codon Process Anticodon Acid
AAT → →GGG → →ATA → →AAA → →GTT → →
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Figure 11.9A protein is formed by the process of translation.
AUGA U G AAG U GCUC C A A
mRNA codon
Ribosome
Methionine
AUGA
A
U
U
G AAG U GCUC
C
C A A
tRNA anticodon
AUGA U G AAG
G
U
U
GCUC
C
C A A
Alanine
U AC
Peptide bond
AUGA U G AAG U GCUC C A A
U AC G UC
Alanine
Peptidebond
Methionine
AAA U G AAG UC
A A C
C A
Stopcodon
A U GU U
294 DNA AND GENES
As translation begins, a ribosome attaches to themRNA strand. Molecules of tRNA, each carrying aspecific amino acid, approach the ribosome.
AA
The codon AUG, which codes for the amino acidmethionine, signals the start of protein synthesis. The tRNA molecule carrying methionine attaches to the ribosome and mRNA strand.
BB A new tRNA molecule carrying an amino acidattaches to the ribosome and mRNA strand next to the previous tRNA molecule. The amino acids on the tRNA molecules join by peptide bonds.
CC
After the peptide bond is formed, the ribosomeslides along the mRNA to the next codon. The tRNAmolecule no longer carrying an amino acid isreleased. A new tRNA molecule carrying an aminoacid can attach to the ribosome and mRNA strand.
DD A chain of amino acids is formed until a stop codon is reached on the mRNA strand.
EE
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Correct translation of the mRNAmessage depends upon the joining ofeach mRNA codon with the correcttRNA molecule. How does a tRNAmolecule carrying its amino acid rec-ognize which codon to attach to? Theanswer involves base pairing. There isa sequence of three nucleotides on theopposite side of the transfer-RNAmolecule from the amino-acid attach-ment site, that is the complement ofthe nucleotides in the codon. Thesethree nucleotides are called an anti-codon because they bind to the codonof the mRNA. The tRNA carriesonly the amino acid that the anti-codon specifies. For example, onetRNA molecule for the amino acidcysteine has an anticodon of ACA.This anticodon binds to the mRNAcodon UGU.
Translating the mRNA codeFollow the steps in Figure 11.9 as
you read how translation occurs. Astranslation begins, a tRNA moleculebrings the first amino acid to themRNA strand that is attached to theribosome, Figure 11.9A. The anti-codon forms a base pair with the codonof the mRNA strand, Figure 11.9B.This places the amino acid in the cor-rect position for forming a peptidebond with the next amino acid. Theribosome slides down the mRNA chain
to the next codon, and a new tRNAmolecule brings another amino acid,Figure 11.9C. The amino acids bond; the first tRNA releases its aminoacid and detaches from the mRNA,Figure 11.9D. The tRNA molecule isnow free to pick up and deliver anothermolecule of its specific amino acid to aribosome. Again, the ribosome slidesdown to the next codon; a new tRNAmolecule arrives, and its amino acidbonds to the previous one. A chain ofamino acids begins to form. When astop codon is reached, translation ends,and the amino acid strand is releasedfrom the ribosome, Figure 11.9E.
Amino acid chains become proteinswhen they are freed from the ribo-some and twist and curl into complexthree-dimensional shapes. Each pro-tein chain forms the same shape everytime it is produced. These proteinsbecome enzymes and cell structures.
Now that you have followed theprocess of protein synthesis, you haveseen that the pathway of informationflows from DNA to mRNA to pro-tein. This scheme is called the centraldogma of biology and is found in allorganisms from the simplest bac-terium to the most complex plant andanimal. The formation of protein,originating from the DNA code, produces the diverse and magnificent living world.
Understanding Main Ideas1. How does the DNA nucleotide sequence deter-
mine the amino acid sequence in a protein?2. What is a codon, and what does it represent?3. What is the role of tRNA in protein synthesis?4. Compare DNA replication and transcription.
Thinking Critically5. You have learned that there are stop codons that
signal the end of an amino acid chain. Why is it
important that a signal to stop translation be partof protein synthesis?
6. Get the Big Picture Sequence the steps involvedin protein synthesis from the production of mRNAto the final translation of the DNA code. For morehelp, refer to Get the Big Picture in the SkillHandbook.
SKILL REVIEWSKILL REVIEW
11.2 FROM DNA TO PROTEIN 295
codon from theLatin word codex,meaning “a tabletfor writing”; Acodon is the three-nucleotide sequencethat codes for anamino acid.
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11.3SECTION PREVIEWObjectivesCategorize the differentkinds of mutations thatcan occur in DNA.Compare the effects of different kinds of mutations on cells andorganisms.
Review Vocabularycancer: diseases believed to
be caused by changes inthe genes that controlthe cell cycle (p. 211)
New Vocabularymutationpoint mutationframeshift mutationchromosomal mutationmutagen
296 DNA AND GENES
When Things Go WrongUsing Prior Knowledge Youknow that DNA controls thestructures and functions of a cell.What happens when the sequenceof DNA nucleotides in a gene ischanged? Sometimes it may havelittle or no harmful effect—likethe thick, tightly crimped fur onthis American Wirehair cat—andthe DNA changes are passed onto offspring of the organism. Atother times, however, the change can cause the cell to behave differ-ently. For example, in skin cancer, UV rays from the sun damage theDNA and cause the cells to grow and divide rapidly. Recognize Cause and Effect Why might a mutation have little or no harmfuleffect on an organism?
MutationsIf the DNA in all the cells of an adult human body were lined up end
to end, it would stretch nearly 100 billion kilometers—60 times the dis-tance from Earth to Jupiter. This DNA is the end result of thousands ofreplications, beginning with the DNA in a fertilized egg. Organisms haveevolved many ways to protect their DNA from changes. In spite of thesemechanisms, however, changes in the DNA occasionally do occur. Anychange in the DNA sequence is called a mutation. Mutations can becaused by errors in replication, transcription, cell division, or by externalagents. One such cause is discussed in Figure 11.10.
Mutations in reproductive cellsMutations can affect the reproductive cells of an organism by changing
the sequence of nucleotides within a gene in a sperm or an egg cell. If thiscell takes part in fertilization, the altered gene would become part of thegenetic makeup of the offspring. The mutation may produce a new traitor it may result in a protein that does not work correctly, resulting instructural or functional problems in cells and in the organism.Sometimes, the mutation results in a protein that is nonfunctional, andthe embryo may not survive.
In some rare cases, a gene mutation may have positive effects. An organ-ism may receive a mutation that makes it faster or stronger; such a mutationmay help an organism—and its offspring—better survive in its environment.
Genetic Changes
Richard Katris
American Wirehair cat
Gamma radiationas a waveGamma radiation,or gamma rays, areelectromagneticwaves. Gammaradiation is emittedby processes thatoccur in the nucleiof atoms. Theyhave the shortestwavelengths andhighest frequenciesof any waves in theelectromagneticspectrum.
Standard 4c Students know how mutations in the DNA sequence of agene may or may not affect the expression of the gene or the sequence of amino acids in anencoded protein.
California Standards
PhysicalScience
Connection
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You will learn later that mutations thatbenefit a species play an important rolein the evolution of that species.
Mutations in body cellsWhat happens if powerful radiation,
such as gamma radiation, hits the DNAof a nonreproductive cell, a cell of thebody such as in skin, muscle, or bone? Ifthe cell’s DNA is changed, this muta-tion would not be passed on to off-spring. However, the mutation maycause problems for the individual.Damage to a gene may impair the func-tion of the cell; for example, it maycause a cell in the stomach to lose itsability to make acid needed for diges-tion, or a skin cell may lose its elasticity.When that cell divides, the new cellsalso will have the same mutation. Manyscientists suggest that the buildup ofcells with less than optimal functioningis an important cause of aging.
Some mutations of DNA in bodycells affect genes that control cell divi-sion. This can result in the cells grow-ing and dividing rapidly, producingcancer. As you learned earlier, canceris the uncontrolled dividing of cells.
(tl)SIU/Visuals Unlimited, (tr)Visuals Unlimited, (b)Grace Moore/The Stock Shop/Medichrome
Genetic Counselor
A re you fascinated by how youinherit traits from your par-
ents? If so, you could become agenetic counselor and help peopleassess their risk of inheriting orpassing on genetic disorders.
Skills for the JobGenetic counselors are medical pro-
fessionals who work on a health careteam. They analyze families’ medical histories to determinethe parents’ risk of having children with genetic disorders,such as hemophilia or cystic fibrosis. Counselors also edu-cate the public and help families with genetic disorders findsupport and treatment. These counselors may work in amedical center, a private practice, research, or a commerciallaboratory. To become a counselor, you must earn a two-year master’s degree in medical genetics and pass a test tobecome certified. The most important requirement is theability to listen and to help families make difficult decisions.
For more careers in related fields, visit
11.3 GENETIC CHANGES 297
AA BB
Figure 11.10Nuclear power plant workers wear radiationbadges (A) and pocket dosimeters (B) to mon-itor their exposure to radiation. Radiation cancause changes to DNA.
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Cancer results from gene mutations.For example, ultraviolet radiation insunlight can change the DNA in skincells. The cells reproduce rapidly,causing skin cancer.
Explain how muta-tions in body cells cause damage.
The effects of point mutationsConsider what might happen if an
incorrect amino acid were insertedinto a growing protein chain duringthe process of translation. The mis-take might affect the structure of theentire molecule. Such a problem canoccur if a point mutation arises. Apoint mutation is a change in a sin-gle base pair in DNA.
A simple analogy can illustratepoint mutations. Read the followingtwo sentences. Notice what happens
when a single letter in the first sentence is changed.
THE DOG BIT THE CAT.THE DOG BIT THE CAR.
As you can see, changing a singleletter changes the meaning of theabove sentence. Similarly, a change ina single nitrogenous base can changethe entire structure of a proteinbecause a change in a single aminoacid can affect the shape of the pro-tein. Figure 11.11A shows what canhappen with a point mutation.
Frameshift mutationsWhen the ribosome moves along
the mRNA strand, a new amino acid is added to the protein for every codon on the mRNA strand. What would happen if a single base were lost from a DNA strand?
298 DNA AND GENES
Figure 11.11The results of a pointmutation and aframeshift mutationare different. The dia-grams show the mRNAand protein that wouldbe formed from eachcorresponding DNA.Infer Which type ofmutation is likely tocause greater geneticdamage?
Proteins that are produced as a result of frameshift mutations seldom functionproperly because such mutations usually change many amino acids. Adding ordeleting one base of a DNA molecule will change nearly every amino acid in theprotein after the addition or deletion.
BB
In this point mutation, the mRNA produced by the mutated DNA had the baseguanine changed to adenine. This change in the codon caused the insertion ofserine rather than glycine into the growing amino acid chain. The error may ormay not interfere with protein function.
AA
Normal
Pointmutation
Frameshiftmutation
. . .
. . .
AC AUGUGA U G A A G U U U G G C U AmRNA
StopMet Lys Phe Gly Ala LeuProtein
mRNA
StopMet Lys Phe Ser Ala LeuProtein
A AC UGUGA U G A A G U U U U AG C
Replace G with A
A
mRNA
Met Lys Leu Ala His CysProtein
G AC AAUGUGA U G A G U U G C U
UDeletion of U
A
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11.3 GENETIC CHANGES 299
This new sequence with the deletedbase would be transcribed into mRNA.But then, the mRNA would be out ofposition by one base. As a result,every codon after the deleted basewould be different, as shown inFigure 11.11B. This mutation wouldcause nearly every amino acid in theprotein after the deletion to bechanged. In the sentence THE DOGBIT THE CAT, deleting a G wouldproduce the sentence THE DOBITT HEC AT. The same effect wouldresult from the addition of a singlebase. A mutation in which a singlebase is added to or deleted from DNAis called a frameshift mutationbecause it shifts the reading of codonsby one base. In general, point muta-tions are less harmful to an organismbecause they disrupt only a single co-don. The MiniLab on the next pagewill help you distinguish point muta-tions from frameshift mutations, andthe Problem-Solving Lab on this pagewill show you an example of a com-mon mutation in humans.
Compare and contrast the cause and effect of a point mutation and a frameshiftmutation.
ChromosomalAlterations
Changes may occur in chromo-somes as well as in genes. Alterationsto chromosomes may occur in a vari-ety of ways. For example, sometimesparts of chromosomes are broken offand lost during mitosis or meiosis.Often, chromosomes break and thenrejoin incorrectly. Sometimes, theparts join backwards or even join tothe wrong chromosome. These struc-tural changes in chromosomes arecalled chromosomal mutations.
Chromosomal mutations occur inall living organisms, but they areespecially common in plants. Suchmutations affect the distribution ofgenes to the gametes during meiosis.Homologous chromosomes do notpair correctly when one chromosomehas extra or missing parts, so separa-tion of the homologous chromosomesdoes not occur normally. Gametesthat should have a complete set ofgenes may end up with extra copies ora complete lack of some genes.
Make and Use TablesWhat type of mutation results in sickle-cell anemia?A disorder called sickle-cell anemia results from a genetic changein the base sequence of DNA. Red blood cells in patients withsickle-cell anemia have molecules of hemoglobin that are mis-shapen. As a result of this change in protein shape, sickled bloodcells clog capillaries and prevent normal flow of blood to bodytissues, causing severe pain.
Solve the ProblemThe table below shows the sequence of bases in a short seg-ment of the DNA that controls the order of amino acids in theprotein hemoglobin.
Thinking Critically1. Interpret Data Use Table 11.1 on page 292 to transcribe
and translate the DNA base sequence for normal hemo-globin and for sickled hemoglobin into amino acids.Remember that the table lists mRNA codons, not DNAbase sequences.
2. Define Operationally Does this genetic change illustratea point mutation or frameshift mutation? Explain youranswer.
3. Explain Why is the correct sequence of DNA bases impor-tant to the production of proteins?
4. Analyze Assume that the base sequence reads GGG CTTCTT AAA instead of the normal sequence for hemoglobin.Would this result in sickled hemoglobin? Explain.
DNA Base Sequences
Normal hemoglobin GGG CTT CTT TTT
Sickled hemoglobin GGG CAT CTT TTT
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300 DNA AND GENES
A B C D E F G H A B C E F G H
A B C D E F G H A B C B C D E F G H
A B C D E F G H A D C B E F G H
A B C D E F G H W X A B C D E F G H
TranslocationW X Y Z Y Z
Deletion
Insertion
Inversion
Figure 11.12Study the four kinds of chromosomalmutations.
When a part of a chromosomeis left out, a deletion occurs.
A
When part of a chromatid breaksoff and attaches to its sisterchromatid, an insertion occurs.The result is a duplication ofgenes on the same chromosome.
B
When part of a chromosomebreaks off and reattachesbackwards, an inversion occurs.
C
When part of one chromosomebreaks off and is added to adifferent chromosome, atranslocation occurs.
D
Few chromosomal mutations arepassed on to the next generationbecause the zygote usually dies. In caseswhere the zygote lives and develops,the mature organism is often sterile andthus incapable of producing offspring.The most important of these muta-tions—deletions, insertions, inversions,and translocations—are illustrated inFigure 11.12.
Causes of MutationsSome mutations seem to just hap-
pen, perhaps as a mistake in basepairing during DNA replication.These mutations are said to be spon-taneous. However, many mutationsare caused by factors in the environ-ment. Any agent that can cause achange in DNA is called a mutagen(MYEW tuh jun). Mutagens includeradiation, chemicals, and even hightemperatures.
Forms of radiation, such as X rays,cosmic rays, ultraviolet light, and
Make and Use TablesGene Mutations and ProteinsGene mutations often have seriouseffects on proteins. In this activity,you will demonstrate how suchmutations affect protein synthesis.
Procedure! Copy the following base sequence of one strand of an
imaginary DNA molecule: AATGCCAGTGGTTCGCAC.@ Then, write the base sequence for an mRNA strand that
would be transcribed from the given DNA sequence.# Use Table 11.1 to determine the sequence of amino acids
in the resulting protein fragment.$ If the fourth base in the original DNA strand were
changed from G to C, how would this affect the resultingprotein fragment?
% If a G were added to the original DNA strand after thethird base, what would the resulting mRNA look like? Howwould this addition affect the protein?
Analysis1. Define Operationally Which change in DNA was a point
mutation? Which was a frameshift mutation?2. Infer How did the point mutation affect the protein?3. Analyze How did the frameshift mutation affect the
protein?
DNA segment
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nuclear radiation, are dangerous muta-gens because the energy they containcan damage or break apart DNA. Thebreaking and reforming of a double-stranded DNA molecule can result indeletions. Some kinds of radiation canconvert a base into a different, incor-rect base or fuse two bases together.
Chemical mutagens include diox-ins, asbestos, benzene, and formalde-hyde, substances that are commonlyfound in buildings and in the environ-ment, Figure 11.13. These mutagensare highly reactive substances thatinteract with the DNA molecule andcause changes. Chemical mutagensusually cause substitution mutations.
Repairing DNAThe cell processes that copy
genetic material and pass it from onegeneration to the next are usuallyaccurate. This accuracy is importantto ensure the genetic continuity ofboth new cells and offspring. Yet,mistakes sometimes do occur. Thereare many sources of mutagens in anorganism’s environment. Althoughmany of these are due to humanactivities, others—such as cosmicrays from outer space—have affectedliving things since the beginning oflife. Repair mechanisms that fixmutations in cells have evolved.
Much like a book editor, enzymesproofread the DNA and replaceincorrect nucleotides with correctnucleotides. These repair mecha-nisms work extremely well, but theyare not perfect. The greater theexposure to a mutagen such as UVlight, the more likely is the chancethat a mistake will not be corrected.Thus, it is wise for people to limittheir exposure to mutagens.
Understanding Main Ideas1. What is a mutation?2. Describe how point mutations and frameshift
mutations affect the synthesis of proteins.3. Explain why a mutation in a sperm or egg cell has
different consequences than one in a heart cell.4. How are mutations and cancer related?
Thinking Critically5. The chemicals in cigarette smoke are known to
cause cancer. Propose a series of steps that couldlead to development of lung cancer in a smoker.
6. Recognize Cause and Effect In an experimentwith rats, the treatment group is exposed to radi-ation while the control group is not. Months later,the treatment group has a greater percentage of rats with cancer and more newborn rats withbirth defects than the control group. Explain howthe exposure to radiation may have affected thetreatment group’s DNA. Infer how these effectscould have caused cancer and birth defects. Formore help, refer to Recognize Cause and Effect inthe Skill Handbook.
SKILL REVIEWSKILL REVIEW
11.3 GENETIC CHANGES 301Tom Tracy/The Stock Shop/Medichrome
Figure 11.13Asbestos was formerlyused to insulate build-ings. It is now knownto cause lung cancerand other lung diseasesand must be removedfrom buildings, as theseworkers are doing.
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302 DNA AND GENES
Before You BeginAlthough DNA remains inthe nucleus of a cell, itpasses its information intothe cytoplasm by way ofanother nucleic acid, mes-senger RNA. The basesequence of this mRNA iscomplementary to thesequence in the strand ofDNA. It is produced bybase pairing during tran-scription. In this activity,you will demonstrate theprocess of transcriptionthrough the use of paperDNA and mRNA models.
Adenine
Deoxyribose
Parts for DNA Nucleotides Extra Parts for RNANucleotides
Phosphate
Ribose
Phosphate
Guanine
Uracil
Thymine
Cytosine
RNA Transcription
ProblemHow does the order of bases in DNA determine the order ofbases in mRNA?
ObjectivesIn this BioLab, you will:■ Formulate a model to show how the order of bases in
DNA determines the order of bases in mRNA.■ Infer why the structure of DNA enables it to be easily
transcribed.
Materialsconstruction paper, 5 colorsscissorsclear tape
Safety PrecautionsCAUTION: Be careful when using scissors. Always use gogglesin the lab.
Skill HandbookIf you need help with this lab, refer to the Skill Handbook.
PREPARATIONPREPARATION
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11.3 GENETIC CHANGES 303
1. Copy the parts for the four different DNA nucleotidesonto your construction paper, making sure that each dif-ferent nucleotide is on a different color paper. Make tencopies of each nucleotide.
2. Using scissors, carefully cut out the shapes of eachnucleotide.
3. Using any order of nucleotides that you wish, construct adouble-stranded DNA molecule. If you need morenucleotides, copy them as in step 1.
4. Fasten your molecule together using clear tape. Do nottape across base pairs.
5. As in step 1, copy the parts for A, G, and C RNA nucleo-tides. Use the same colors of construction paper as instep 1. Use the fifth color of construction paper to makecopies of uracil nucleotides.
6. With scissors, carefully cut out the RNA nucleotideshapes.
7. With your DNA molecule in front of you, demonstratethe process of transcription by first pulling the DNAmolecule apart between the base pairs.
8. Using only one of the strands of DNA, begin matchingcomplementary RNA nucleotides with the exposed baseson the DNA model to make mRNA.
9. When you are finished, tape your new mRNA moleculetogether.
PROCEDUREPROCEDURE
ANALYZE AND CONCLUDEANALYZE AND CONCLUDE
1. Observe and Infer Does the mRNA model more closely resemble theDNA strand from which it was transcribed or the complementary strandthat wasn’t used? Explain your answer.
2. Recognize Cause and Effect Explain howthe structure of DNA enables the moleculeto be easily transcribed. Why is this impor-tant for genetic information?
3. Relate Concepts Why is RNA important tothe cell? How does an mRNA molecule carryinformation from DNA? Where does themRNA molecule take the information?
Research Do research to find out moreabout how the bases in DNA were identifiedand how the base pairing pattern was determined.
Web Links To find out more aboutDNA, visit
Matt Meadows
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Scanning ProbeMicroscopes
Have you ever heard of someone dissecting achromosome to get a closer look at DNA?
Imagine an instrument small enough to allowyou to grab hold of one of the nucleotides in astrand of DNA, yet powerful enough to providea detailed image.
Scanning probe microscopes can show thearrangement of atoms on the surface of a mole-cule. They make it possible for scientists to pickup molecules, and even atoms, and move themaround. They can also be used to observe howbiological molecules interact. There are manytypes of scanning probe microscopes. All ofthem use a very sharp probe that may be only asingle atom wide at its tip. The probe sits veryclose to the specimen but does not actuallytouch it. As the probe moves across, or scans,the specimen, it measures some property of thespecimen.
The scanning tunneling microscope (STM)The STM uses a probe through which a tinyamount of electric current flows. As the probescans a molecule, it encounters ridges and val-leys formed by the different kinds of atoms onthe molecule’s surface. The probe moves upand down as needed to keep the current con-stant. The movements of the probe arerecorded by a computer, which produces animage of the molecule.
The atomic force microscope (AFM) TheAFM can measure many different properties,including electricity, magnetism, and heat. Asthe probe moves across the specimen, changes inthe property being measured move the probe.These changes are used to create the image.
What can they do? One of the primary advan-tages of scanning probe microscopy, besides itsatomic-level resolution, is the ability to observe
molecules in air or liquid. This means that biol-ogists can “watch” molecules interact as theywould inside a cell.
Scanning probe microscopes are being used tostudy patterns in liquid crystals, wave convec-tion, molecular surfaces of materials and molec-ular events such as the function of bacterialtoxins and ion channels, actin and myosin inter-actions in muscle cells, and protein folding.
Imaging transcription Biologists currentlyare imaging factors that are involved in the tran-scription of DNA. These factors includeenzymes that cut DNA and those that repairDNA errors. By using the AFM, biologists arebeginning to identify the DNA binding sites forthese enzymes, which will be used in mappingstudies of DNA. This will allow further under-standing of the process of transcription.
Think Critically What advantages do scanningprobe microscopes have over other types of micro-scopes? Predict some future applications that wouldmake use of these advantages.
To find out more about microscopes, visit
Magnification: unavailable
304 DNA AND GENES
STM image of DNA
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Photo Researchers
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Section 11.1STUDY GUIDESTUDY GUIDE
CHAPTER 11 ASSESSMENT 305Dr. Gopal Murti/Science Photo Library/Photo Researchers
To help you reviewDNA, use the Organizational StudyFold on page 288.
Color-enhanced TEMMagnification: 75 000�
Key Concepts■ Alfred Hershey and Martha Chase demon-
strated that DNA is the genetic material.■ DNA, the genetic material of organisms,
is composed of four kinds of nucleotides. A DNA molecule consists of two strands of nucleotides with sugars and phosphateson the outside and bases paired by hydro-gen bonding on the inside. The pairedstrands form a twisted-zipper shape calleda double helix.
■ Because adenine can pair only withthymine, and guanine can pair only withcytosine, DNA can replicate itself withgreat accuracy.
VocabularyDNA replication (p. 284)double helix (p. 283)nitrogenous base
(p. 282)
DNA: TheMolecule ofHeredity
Key Concepts■ Genes are small sections of DNA. Most
sequences of three bases in the DNA of a gene code for a single amino acid in aprotein.
■ Messenger RNA is made in a processcalled transcription. The order ofnucleotides in DNA determines the orderof nucleotides in messenger RNA.
■ Translation is a process through which theorder of bases in messenger RNA codes forthe order of amino acids in a protein.
Vocabularycodon (p. 292)messenger RNA (p. 289)ribosomal RNA (p. 290)transcription (p. 290)transfer RNA (p. 290)translation (p. 293)
From DNA toProtein
Section 11.2
Key Concepts■ A mutation is a change in the base
sequence of DNA. Mutations may affectonly one gene, or they may affect wholechromosomes.
■ Mutations in eggs or sperm affect futuregenerations by producing offspring withnew characteristics. Mutations in bodycells affect only the individual and mayresult in cancer.
Vocabularychromosomal mutation
(p. 299)frameshift mutation
(p. 299)mutagen (p. 300)mutation (p. 296)point mutation (p. 298)
GeneticChanges
Section 11.3
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Review the Chapter 11 vocabulary words listed inthe Study Guide on page 305. Distinguish betweenthe vocabulary words in each pair below.
1. transcription—translation2. point mutation—frameshift mutation3. transfer RNA—messenger RNA4. mutation—mutagen5. nitrogenous base—codon
6. Which of the following processes requiresprior DNA replication?A. transcription C. mitosisB. translation D. protein synthesis
7. In which of the following processes does theDNA unzip?A. transcription and translationB. transcription and replicationC. replication and translationD. mutation
8. Which DNA strand can base pair with the DNA strandshown at the right?A. T-A-C-G-A-T C. U-A-C-G-A-UB. A-T-G-C-T-A D. A-U-G-C-U-A
9. Which of the following nucleotide chainscould be part of a molecule of RNA?A. A-T-G-C-C-A C. G-C-C-T-T-GB. A-A-T-A-A-A D. A-U-G-C-C-A
10. Which of the following mRNA codonswould cause synthesis of a protein to termi-nate? Refer to Table 11.1.A. GGG C. UAGB. UAC D. AAG
11. The genetic code for an oak tree is ________.A. more similar to an ash tree than to a squirrelB. more similar to a chipmunk than to a
maple treeC. more similar to a mosquito than to an
elm treeD. exactly the same as for an octopus
12. A deer is born normal, but UV rays cause amutation in its retina. Which of the follow-ing statements is least likely to be true?A. The mutation may be passed on to the
offspring of the deer.B. The mutation may cause retinal cancer.C. The mutation may interfere with the
function of the retinal cell.D. The mutation may interfere with the
structure of the retinal cell.
13. Chemical Q causes the change in thesequence of nucleotides shown below. Thischange is an example of a(n) ________. A. point mutationB. frameshift mutationC. translocationD. inversion
14. Open Ended Explain why a mutation in a lung cell would not be passed on to offspring.
15. Open Ended Write the sequence for a seg-ment of DNA that codes for the followingchain of amino acids: valine-serine-proline-glycine-leucine. Compare your DNAsequence with another student’s sequence.Why are they likely to be different?
16. Open Ended Would the amount of cytosineand guanine be equal to each other in anRNA molecule? Explain your answer.
17. Make Inferences Explain how the univer-sality of the genetic code is evidence that allorganisms alive today may have evolvedfrom a common ancestor in the past.
TAA T G AA T A AA A
AA TT G C
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18. Summarize Using the diagram to the right, describe the way DNA replicates.Refer to the red and bluestrands in your answer.
19. Cystic fibrosisis an inherited genetic disorder that affectsabout 1 in 3900 live births of all Americans.Visit to find out whattype of changes in DNA cause this disorder.How are these changes reflected in thesymptoms of the disorder?
REAL WORLD BIOCHALLENGE
CHAPTER 11 ASSESSMENT 307
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Multiple ChoiceUse the diagram to answer questions 20–22.
20. What process is shown in diagrams A–C?A. transcription C. translocationB. translation D. replication
21. What occurred between diagrams A and B?A. DNA was copied.B. DNA was translated.C. DNA was unzipped.D. DNA mutated.
22. What structure is represented by the circlemarked “X”?A. nitrogenous base C. nucleotideB. deoxyribose D. phosphate group
The following graph records the amount of DNA inliver cells that have been grown in a culture so thatall the cells are at the same phase in the cell cycle.Study the graph and answer questions 23 and 24.
23. What process are the cells undergoing atpart A of the graph to cause the observedchange in DNA content?A. transcriptionB. translationC. DNA replicationD. meiosis
24. During what part of the graph is translationmost likely to occur?A. at part AB. at part BC. at part CD. at both part A and part C
Change in DNA Content Per Cell
DN
A (
pic
og
ram
s/ce
ll)
Time (hours)
0.1
0
0.2
0.3
0.4
0.5
0.6
2 6 10 14 18 22 26
A
C
B
X
A.
B.
C.
A T
G
G
C
C
A T
G
G
C
C
A T
G
G
C
C
A T
G
G
C
C
A T
G
G
C
C
Constructed Response/Grid InRecord your answers on your answer document.
25. Open Ended A point mutation that doesn’t change the amino acid sequence of a protein is knownas a silent mutation. Explain why a silent mutation might not affect the protein for which it codes.
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5b
5b
5b
5a
4c
The assessed California standard appears next to the question.
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