Objectives• Summarize the process of DNA

replication.• Describe the role of enzymes in

DNA replication.

Section ResourcesUnit Resource Book

Study Guide pp. 69–70Power Notes p. 71Reinforcement p. 72

Interactive Reader Chapter 8Spanish Study Guide pp. 77–78

Biology Toolkit pp. C17, C19, C24

TechnologyPower Presentation 8.3Media Gallery DVDOnline Quiz 8.3

Activate Prior Knowledge Make sure students understand the complemen-tary nature of a template and what it produces. Ask, What are some everyday uses of a template? Word-processing templates give text a particular format; carpenters use templates to reproduce shapes; stencils provide openings to create specific shapes.

VocabularyAcademic Vocabulary A copy is a duplicate of what already exists. A template forms a pattern that enables someone to make a copy; it is not the same as a copy. Ask, How is a DNA sequence used as a template? The sequence of bases in one strand is a template that reproduces the sequence of bases in the other strand. Point out that the sequence of bases makes a copy of the second strand.

Plan and PreparePlan and Prepare

TeachTeach

3:07 PM3:07 PM3:07 PM

Cell Biology In Chapter 5 you learned that the cell cycle has four main stages. DNA is replicated during the S (synthesis) stage.

Connecting CONCEPTS

8.3 DNA ReplicationKEY CONCEPT DNA replication copies the genetic information of a cell.

MAIN IDEAS• Replication copies the genetic information.

• Proteins carry out the process of replication.

• Replication is fast and accurate.

VOCABULARYreplication,replication, p. 235

DNA polymerase,DNA polymerase, p. 236

Reviewbase pairing rules, S phase

Connect Do you know that some of your cells are dying right now? You may live to the ripe old age of 100, but most of your cells will have been replaced thou-sands of times before you blow out the candles on that birthday cake. Every time that cells divide to produce new cells, DNA must first be copied in a remarkable process of unzipping and zipping by enzymes and other proteins. The next few pages will take you through that process.

MAIN IDEA

Replication copies the genetic information.One of the powerful features of the Watson and Crick model was that it suggested a way that DNA could be copied. In fact, Watson and Crick ended the journal article announcing their discovery with this sentence: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

Recall that the bases that connect the strands of DNA will pair only in one way, according to the rules of base pairing. An A must bind with a T, and a C must bind with a G. If the base sequence of one strand of the DNA double helix is known, the sequence of the other strand is also known. Watson and Crick realized that a single DNA strand can serve as a template, or pattern, for a new strand. This process by which DNA is copied during the cell cycle is called replication.replication.

Suppose all of your classmates took off their shoes, placed their left shoe in a line, and tossed their right shoe into a pile. You could easily pick out the right shoes from the pile and place them with the matching left shoes. The order of the shoes would be preserved. Similarly, a new strand of DNA can be synthesized when the other strand is a template to guide the process. Every time, the order of the bases is preserved, and DNA can be accurately replicated over and over again.

Replication assures that every cell has a complete set of identical genetic information. Recall that your DNA is divided into 46 chromosomes that are replicated during the S phase of the cell cycle. So your DNA is copied once in each round of the cell cycle. As a result, every cell has a complete set of DNA.

Chapter 8: From DNA to Proteins 235

5.a Anaylze and explain the molecular basis of heredity and the inheritance of traits to succes-sive generations by using the Cen-tral Dogma of Molecular Biology.

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Differentiated Instruction

SECTION 8.3

ENGLISH LEARNERSHave students divide into groups of three. Ask each group to summarize one of the three parts of the section. Encourage students to identify the main points and restate them in concise language. Then one or more students from each group can present the group summary to the class. You may wish to have each of the other groups write questions as they listen to the summaries and ask their questions following the presentations.

Biology Toolkit, Summarizing, p. C24Chapter 8: From DNA to Protein 235

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VocabularyAcademic Vocabulary Help students distinguish between the words complement and compliment.

complement = either of two parts that completes a whole

compliment = expression of praise

Point out that the word complement has as its root the word complete. To remember the meaning of compliment, students can use the mnemonic that both praise and compliment have the letter i. Tell students that the word complimentary can also refer to something freely given, as in a gift.

Take It FurtherDNA polymerase adds nucleotides only to the 3� end of a strand, so a new DNA strand can elongate only in a 5� to 3� direction. Along one template strand, a new strand is made in one continuous piece toward the Y-shaped junction of the strands. This new strand is called the leading strand. To make the second new strand, DNA polymerase must work along the other template strand in the direction away from the Y-shaped junction of the strands. This new strand of DNA is called the lagging strand. The process is similar to backstitching in sewing, and the new strand is made in short fragments. Another enzyme bonds these fragments together to form a continuous strand.

AnswersA Apply It doubles the amount of

DNA so that both of the daughter cells resulting from mitosis have their own complete set of DNA.

B Infer Each strand of the DNA molecule dictates what the other strand must be according to the rules of base pairing. That is, because A bonds with T, and C with G, the nucleotides arrange themselves in a complementary pattern. Each replicated molecule consists of one original (template) strand and one new strand.

Teach continuedTeach continued

DNA polymerasesDNA polymerases are enzymes that form bonds between nucleotides during replication.

VISUAL VOCAB

DNA polymer ase

The ending -ase signals that this is an enzyme.

This part of the name tells what the enzyme does—makes DNA polymers.

TAKING NOTESUse a cycle diagram to take notes about processes such as replication.

existingmolecule

unzipping

two DNA molecules formed

nucleotides added

Biochemistry You read in Chapter 2 that many proteins are enzymes that function as catalysts. Enzymes decrease the activation energy and increase the rate of chemical reactions. DNA polymerase catalyzes the reaction that bonds two nucleotides together.

Connecting CONCEPTS

The fact that cells throughout the body have complete sets of DNA is veryuseful for forensic scientists. They can identify someone from nearly any cellin the body. A few cells from a drop of blood or from saliva on a cigarette buttare all detectives need to produce a DNA “fingerprint” of a criminal suspect.

Apply How does replication ensure that cells have complete sets of DNA?

MAIN IDEA

Proteins carry out the process of replication.Although people may say that DNA copies itself, the DNA itself does nothingmore than store information. Enzymes and other proteins do the actual workof replication. For example, some enzymes start the process by unzipping thedouble helix to separate the strands of DNA. Other proteins hold the strandsapart while the strands serve astemplates. Nucleotides that arefloating free in the nucleus can thenpair up with the nucleotides of theexisting DNA strands. A group ofenzymes called DNA polymerasesDNA polymerases(PAHL-uh-muh-rays) bond the newnucleotides together. When theprocess is finished, the result is twocomplete molecules of DNA, eachexactly like the original double strand.

The Replication ProcessThe following information describes the process of DNA replication ineukaryotes, which is similar in prokaryotes. As you read, follow along witheach step illustrated in FIGURE 8.8.

1 Enzymes begin to unzip the double helix at numerous places along thechromosome, called origins of replication. That is, the hydrogen bondsconnecting base pairs are broken, the original molecule separates, andthe bases on each strand are exposed. Unlike unzipping a jacket, thisprocess proceeds in two directions at the same time.

2 Free-floating nucleotides pair, one by one, with the bases on the templatestrands as they are exposed. DNA polymerases bond the nucleotidestogether to form new strands that are complementary to each templatestrand. DNA replication occurs in a smooth, continuous way on one ofthe strands. Due to the chemical nature of DNA polymerase, replicationof the other strand is more complex. It involves the formation of manysmall DNA segments that are joined together. This more complex processis not shown or described in detail here.

3 Two identical molecules of DNA result. Each new molecule has onestrand from the original molecule and one new strand. As a result, DNAreplication is called semiconservative because one old strand is con-served, and one complementary new strand is made.

Infer How does step 3 of replication show that DNA acts as a template?

236 Unit 3: Genetics

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A

BA

Differentiated Instruction

BELOW LEVELHave students relate what they see in FIGURE 8.8 on page 237 to FIGURE 8.7 on page 233. Have them visualize the replication process. Tell students they can be creative and describe not just what they see but also what they hear and feel. Have them write continuously for five minutes.

Biology Toolkit, Connect to Content through Visuals and Quick-Write, pp. C17, C19

INCLUSIONHave students practice base pairing to reinforce the concept of complementary strands. Write base sequences on the board and have students produce the complementary strand. For students who are visually impaired, make a cardboard cutout of a single-stranded base sequence, using different-shaped notches for the bases. For example, A could have a V-shaped notch. T would then have a pointed tab that would fit into the notch to base pair. The nucleotides should be T-shaped, including a fragment of backbone as well as the base.

236 Unit 3: Genetics

FIGURE 8.8 Discuss each step in the figure separately. Ask

• In step 1, how does the DNA unzip? The hydrogen bonds between the base pairs break.

• In step 2, how do the new strands compare with the template strands? Each new strand is the complement of its template strand.

• What enzyme is important in step 2? DNA polymerase

• What is the result of DNA replica-tion? two identical DNA molecules

• Why is it important for the cell to correct any errors that occur during replication? If errors were not corrected, one of the new cells that form during cell division would have DNA with incorrect genetic information.

History of ScienceThe Watson-Crick model of DNA predicted that DNA replication is semiconservative; that is, each new mol-ecule contains one original strand and one new strand. However, there were two other possible hypotheses. In the conservative model, the template molecule remained intact and a new molecule, consisting of two new strands, formed. In the dispersive model, the two original strands broke up and dispersed throughout the two new molecules that formed.

Matthew Meselson and Franklin Stahl demonstrated that the semiconservative model is correct. They used radioactive nitrogen to label the bases in bacterial DNA and then followed the path of the label through several generations of growth in an unlabeled medium. Their data showed that at first, the radioactiv-ity was all in the parent DNA. After the first generation, all the cells had DNA that was half-labeled. After two generations, half the cells had DNA that was half-labeled, and half the cells had DNA that had no label.

AnswersA Critical Viewing It is identical to the

original DNA molecule.

TEACH FROM VISUALSTEACH FROM VISUALS

DNA polymerase

DNA polymerase

nucleotide

Two molecules of DNA

Strand of DNA unzipping(colored TEM, magnification unknown)

nucleotide

nucleotide

new strands

The DNA molecule unzips in both directions.

new strandoriginal strand

1

2

3

BIOLOGYSee DNA replica-tion in action at ClassZone.com.

Two identical double-stranded DNA molecules result from replication. DNA replication is semiconservative. That is, each DNA molecule contains an original strand and one new strand.

Each existing strand of the DNA molecule is a template for a new strand. Free-floating nucleotides pair up with the exposed bases on each template strand. DNA polymerases bond these nucleotides together to form the new strands. The arrows show the directions in which new strands form.

A DNA molecule unzips as nucleotide base pairs separate. Replication begins on both strands of the molecule at the same time.

FIGURE 8.8 Replication

When a cell’s DNA is copied, or replicated, two complete and identicalsets of genetic information are produced. Then cell division can occur.

How is each new molecule of DNA related to the original molecule?

CRITICAL VIEWING

Chapter 8: From DNA to Proteins 237

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HANDS-ON ACTIVITYGive students two strands of red yarn and six strands of blue yarn—all of similar length. Have them make a model of DNA from the two red strands. Then have students go through two generations of replication, using the red yarn as template strands and the blue yarn as new strands.

Describe for students the conservative, dispersive, and semiconservative models of replication. (See History of Science.) Ask

• If the conservative model were correct, what would the model DNA look like after one generation? One molecule would be all red, and the other would be all blue.

• If the dispersive model were correct, what would the model DNA look like after one generation? Each molecule would consist of segments of red and blue.

• After two divisions, how many molecules have a strand from the original DNA? only two of four

Chapter 8: From DNA to Protein 237

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Time 5 minutes

Lab Binder Genetics, p. 37

Purpose Model the replication of DNA.

LAB MANAGEMENT• Have students compare what they

are doing to FIGURE 8.8.• The lab can be extended to model

complementary base pairing by drawing dots of four different colors on the template strands and dots of matching colors on the pieces representing free nucleotides.

AnswersAnalyze and ConcludeThe pairing of the free nucleotides in the model is not specific. Pieces of zipper can pair anywhere on the template strand, whereas free nucleo-tides can pair only with specific bases in DNA replication.

Take It FurtherDNA replication takes about eight hours in human cells. If there were only one origin, the process would take about a hundred times longer.

AnswersA Infer Our cells have a large amount

of DNA. It must be copied quickly enough to keep up with the demand for cell division and to enable the cell to carry out its normal functions.

Assess Use the Online Quiz or Section Quiz (Assessment Book, p. 149).Reteach Have students review FIGURE 8.8 and make a flow chart that lists the steps in the process of DNA replication.

Teach continuedTeach continued

QUICK LAB QUICK LAB

Assess and ReteachAssess and ReteachU spe-05U

8.3 ASSESSMENT

Connecting CONCEPTS

ONLINE QUIZClassZone.com

Q U I C K L A B

ReplicationUse two zipping plastic bags to model how complementary strands of DNA attach to template strands during replication.

PROCEDURE 1. Cut the sliding zippers off both bags. One zipper represents

the template strands of a DNA molecule.

2. Cut the other zipper into four smaller pieces and unzip each of them. These represent free nucleotides. Don’t worry about which nucleotide is which in this activity.

3. Use the pieces to model replication as shown on page 237.

ANALYZE AND CONCLUDEEvaluate What are the limitations of this model?

MO D E LI N G

MATERIALS• 2 zipping bags• scissors

FIGURE 8.9 Eukaryotic chromo-somes have many origins of repli-cation. The DNA helix is unzipped at many points along each chro-mosome. The replication “bub-bles” grow larger as replication progresses in both directions, resulting in two complete copies.

MAIN IDEA

Replication is fast and accurate.In every living thing, DNA replication happens over and over again, and it happens remarkably fast. In human cells, about 50 nucleotides are added every second to a new strand of DNA at an origin of replication. But even at this rate, it would take many days to replicate a molecule of DNA if the molecule were like a jacket zipper, unzipping one tooth at a time. Instead, replication proceeds from hundreds of origins of replication along the chromosome, as shown in FIGURE 8.9, so the process takes just a few hours.

Another amazing feature of replication is that it has a built-in “proofread-ing” function to correct errors. Occasionally, the wrong nucleotide is added to the new strand of DNA. However, DNA polymerase can detect the error, remove the incorrect nucleotide, and replace it with the correct one. In this way, errors in replication are limited to about one error per 1 billion nucleotides.

Replication is happening in your cells right now. Your DNA is replicated every time your cells turn over, or replicate themselves. Your DNA has repli-cated trillions of times since you grew from a single cell.

Infer Why does a cell need to replicate its DNA quickly?

REVIEWING MAIN IDEAS

1. Explain the function of replication.replication.2. Explain how DNA serves as its own

template during replication.

3. How do cells help ensure that DNA replication is accurate?

CRITICAL THINKING

4. Summarize Describe two major functions of DNA DNA polymerases.polymerases.

5. Infer Why is it important that human chromosomes have many origins of replication?

6. Cell Biology DNA is replicated before both mitosis and meiosis. How does the amount of DNA produced in a cell during mitosis compare with that produced during meiosis?

238 Unit 3: Genetics

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central dogma,RNA,transcription,RNA polymerase,

messenger RNA (mRNA),

ribosomal RNA (rRNA),transfer RNA (tRNA),

central dogma

RNA,

b10h

A

8.3 ASSESSMENT 1. to make a copy of all the DNA in a cell so

that it can be passed on to a new cell 2. Both strands act as a template. Because

base pairing is specific, the sequence of one strand dictates what the sequence of the other strand has to be.

3. Certain types of DNA polymerase have a built-in proofreading function that corrects most mispaired nucleotides.

4. DNA polymerases bond nucleotides together and proofread to ensure accuracy.

5. Each chromosome in a eukaryotic cell is very long. If replication started at only one place, it would take a very long time to finish. Multiple origins of replication let the process happen more quickly.

6. Cells produced by mitosis typically have twice the amount of DNA as cells pro-duced by meiosis.

238 Unit 3: Genetics

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