Chapter 12: Chapter 12: Molecular GeneticsMolecular Genetics12.1 DNA: The Genetic Material12.1 DNA: The Genetic Material12.2 Replication of DNA12.2 Replication of DNA12.3 DNA, RNA, and Protein12.3 DNA, RNA, and Protein12.4 Gene Regulation and 12.4 Gene Regulation and MutationMutation

12.1 DNA: The Genetic 12.1 DNA: The Genetic MaterialMaterial

►Main idea: The discovery that DNA is the Main idea: The discovery that DNA is the genetic code involved many experiments.genetic code involved many experiments.

►Objectives:Objectives: Summarize the experiments leading to the Summarize the experiments leading to the

discovery of DNA as the genetic materialdiscovery of DNA as the genetic material Diagram and label the basic structure of DNADiagram and label the basic structure of DNA Describe the basic structure of eukaryotic Describe the basic structure of eukaryotic

chromosomechromosome► Review VocabularyReview Vocabulary

Nucleic acid: complex biomolecule that stores Nucleic acid: complex biomolecule that stores cellular information in the form of a codecellular information in the form of a code

► VocabularyVocabulary Double helixDouble helix nucleosomenucleosome

Discovery of the Genetic Discovery of the Genetic MaterialMaterial

►Once Mendel’s work was rediscovered Once Mendel’s work was rediscovered in the 1900’s, scientists began to in the 1900’s, scientists began to search for the molecule involved in search for the molecule involved in inheritanceinheritance

►Scientists knew that the genetic Scientists knew that the genetic information was carried on the information was carried on the chromosomes in eukaryotic cells, and chromosomes in eukaryotic cells, and the two main components of the two main components of chromosomes are DNA and protein.chromosomes are DNA and protein.

GriffithGriffith

► In 1928, Fredrick Griffith performed the first In 1928, Fredrick Griffith performed the first major experiment that led to the discovery of major experiment that led to the discovery of DNA as the genetic materialDNA as the genetic material

►Griffith studied two strains of the bacteria Griffith studied two strains of the bacteria Streptococcus pneumoniaeStreptococcus pneumoniae

► He found that one strain could be transformed, He found that one strain could be transformed, or changed, into the other formor changed, into the other form

►Of the two strains he studied, one had a sugar Of the two strains he studied, one had a sugar coat and one did not. coat and one did not. Coated strain caused pneumonia – Smooth (S) strainCoated strain caused pneumonia – Smooth (S) strain Noncoated strain does not cause pneumonia – Noncoated strain does not cause pneumonia –

Rough (R) strain; without the coat, colonies have Rough (R) strain; without the coat, colonies have rough edgesrough edges

Griffith’s ExperimentGriffith’s Experiment

► This This experiment experiment set the set the stage for stage for the search the search to identify to identify the the transformintransforming g substance. substance.

AveryAvery► In 1931, Oswald Avery identified the molecule that In 1931, Oswald Avery identified the molecule that

transformed the R strain of bacteria into the S strain.transformed the R strain of bacteria into the S strain.► He isolated different macromolecules, such as DNA, He isolated different macromolecules, such as DNA,

proteins, and lipids from killed S cells.proteins, and lipids from killed S cells.► Then he exposed live R cells to the macromolecules Then he exposed live R cells to the macromolecules

separately.separately.► When the live R cells were exposed to the S strain When the live R cells were exposed to the S strain

DNA, they were transformed into S cells.DNA, they were transformed into S cells.► Avery concluded that when the S cells in Griffith’s Avery concluded that when the S cells in Griffith’s

experiment were killed, DNA was released.experiment were killed, DNA was released.► Some of the R bacteria incorporated this DNA into Some of the R bacteria incorporated this DNA into

their cells, and this changed the bacteria into S cells.their cells, and this changed the bacteria into S cells.► Avery’s conclusions not widely accepted; scientists Avery’s conclusions not widely accepted; scientists

continued to question whether the transforming continued to question whether the transforming material was DNA or proteins.material was DNA or proteins.

Hershey and ChaseHershey and Chase► In 1952, Alfred Hershey In 1952, Alfred Hershey

and Martha Chase provided and Martha Chase provided definitive evidence that definitive evidence that DNA is the transforming DNA is the transforming factor.factor.

► They performed They performed experiments using experiments using bacteriophages (viruses bacteriophages (viruses that attack bacteria) and that attack bacteria) and radioactive labelingradioactive labeling

►They concluded that the viral DNA was injected into the cell and provided the genetic information needed to produce new viruses.

Hershey and ChaseHershey and Chase

Summary of Hershey-Chase ResultsSummary of Hershey-Chase Results

Group 1 : DNA is radioactiveGroup 1 : DNA is radioactive

(Viruses labeled with (Viruses labeled with 32P)Group 2: Protein is radioactiveGroup 2: Protein is radioactive

(Viruses labeled with (Viruses labeled with 3535S)S)

Infected Infected BacteriaBacteria

Liquid with Liquid with VirusesViruses

Infected Infected BacteriaBacteria

Liquid with Liquid with VirusesViruses

►Labeled viral Labeled viral DNA (DNA (32P) found in the bacteria►Viral replication occurred►New viruses contained (32P)

►No labeled No labeled DNADNA►No viral No viral replicationreplication

►No labeled No labeled viral proteins viral proteins ((3535S)S)►Viral Viral replication replication occurredoccurred►New viruses New viruses did not have a did not have a labellabel

►Labeled Labeled proteins foundproteins found►No viral No viral replicationreplication

DNA StructureDNA Structure

►Hershey & Chase’s experiment insured Hershey & Chase’s experiment insured confidence in scientists that DNA was confidence in scientists that DNA was the genetic material, but they the genetic material, but they questioned how nucleotides came questioned how nucleotides came together to form DNA and how DNA together to form DNA and how DNA could communicate information.could communicate information.

►Nucleotides basic structure was Nucleotides basic structure was determined by P.A. Levine in the determined by P.A. Levine in the 1920’s.1920’s.

NucleotidesNucleotides► Consist of a five-carbon sugar, a phosphate group, and a

nitrogenous base► DNA –sugar (deoxyribose), phosphate group, and nitrogenous

base (Adenine, Guanine, Cytosine, or Thymine).► RNA –sugar (ribose), phosphate group, and a nitrogenous base

(Adenine, Guanine, Cytosine, or Uracil).

ChargaffChargaff

► Data published in Data published in 1955.1955.

► Chargaff found Chargaff found that the amounts that the amounts of guanine nearly of guanine nearly equals the amount equals the amount of cytosine, and of cytosine, and the amount of the amount of adenine nearly adenine nearly equals the amount equals the amount of thymine within of thymine within a speciesa species

► Charfaff’s rule:Charfaff’s rule:C = G and T = AC = G and T = A

The Structure QuestionThe Structure Question

► Four scientists joined the search for the DNA Four scientists joined the search for the DNA structure and the meaning and importance structure and the meaning and importance of Chargaff’s rule became quite clear.of Chargaff’s rule became quite clear.

► Rosalind Franklin and Maurice Wilkins used Rosalind Franklin and Maurice Wilkins used X-ray diffraction (aiming X-rays at a DNA X-ray diffraction (aiming X-rays at a DNA molecule) to produce photo 51.molecule) to produce photo 51.

► Photo 51 indicated that DNA was a double Photo 51 indicated that DNA was a double helix or a twisted ladder shape, formed by helix or a twisted ladder shape, formed by two strands of nucleotides twisted around two strands of nucleotides twisted around each othereach other

► James Watson and Francis Crick used James Watson and Francis Crick used Franklin and Wilkin’s data and Chargaff’s Franklin and Wilkin’s data and Chargaff’s data to create the double helix modeldata to create the double helix model

Watson and Crick’s DNA Watson and Crick’s DNA ModelModel

►Two outside strands consist of Two outside strands consist of alternating deoxyribose and alternating deoxyribose and phosphatephosphate

►Cytosine and guanine bases pair to Cytosine and guanine bases pair to each other by three hydrogen bondseach other by three hydrogen bonds

►Thymine and adenine bases pair to Thymine and adenine bases pair to each other by two hydrogen bondseach other by two hydrogen bonds

DNA StructureDNA Structure► DNA often is compared to a twisted ladder.► Rails of the ladder are represented by the

alternating deoxyribose and phosphate.► The pairs of bases (cytosine–guanine or thymine–

adenine) form the steps. ► Purine bases equal the number pyrimidine bases► Adenine and guanine are purines and cytosine and

thymine are pyramidines► C=G and A=T; therefore C + T = G + A ► Complementary base pairing is used to describe the

precise pairing of purine and pyrimidine bases between strands of nucleic acids.

► It is the characteristics of DNA replication through which the parent strand can determine the sequence of a new strand.

DNA OrientationDNA Orientation

► Carbon molecules can be numbered in organic molecules, the orientation of the numbered carbons in the sugar molecules of each strand is depicted above.

► On the top rail, the strand is said to be oriented 5′ to 3′.► The strand on the bottom runs in the opposite direction

and is oriented 3′ to 5′.► The orientation of the two strands are called antiparallel.

Chromosome StructureChromosome Structure► In prokaryotes, DNA molecules are contained in cytoplasm and In prokaryotes, DNA molecules are contained in cytoplasm and

consists mainly of a ring of DNA and associated proteins.consists mainly of a ring of DNA and associated proteins.► Eukaryotic DNA is organized in individual chromosomes.Eukaryotic DNA is organized in individual chromosomes.► DNA is tightly coiled around a group of beadlike proteins called DNA is tightly coiled around a group of beadlike proteins called

histones.histones.► The phosphate groups in DNA create a negative charge, which The phosphate groups in DNA create a negative charge, which

attracts the DNA to the positively charged histone proteins and attracts the DNA to the positively charged histone proteins and forms a nucleosome.forms a nucleosome.

► The nucleosomes then group together into chromatin fibers, The nucleosomes then group together into chromatin fibers, which supercoil to make up the DNA structure recognized as a which supercoil to make up the DNA structure recognized as a chromosome.chromosome.

12.2 Replication of DNA12.2 Replication of DNA►Main idea: DNA replicates by making a Main idea: DNA replicates by making a

strand that is complementary to each strand that is complementary to each original strand.original strand.

►Objectives:Objectives: Summarize the role of the enzymes involved in Summarize the role of the enzymes involved in

the replication of DNA.the replication of DNA. Explain how leading and lagging strands are Explain how leading and lagging strands are

synthesized differently.synthesized differently.► Review VocabularyReview Vocabulary

Template: a molecule of DNA that is a pattern for Template: a molecule of DNA that is a pattern for synthesis of a new DNA moleculesynthesis of a new DNA molecule

►New VocabularyNew Vocabulary Semiconservative replicationSemiconservative replication DNA polymeraseDNA polymerase Okazaki fragmentsOkazaki fragments

Semiconservative ReplicationSemiconservative Replication

►Parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA.

Semiconservative ReplicationSemiconservative Replication►Occurs in three main stages: Occurs in three main stages:

Unwinding, Base pairing & JoiningUnwinding, Base pairing & Joining UnwindingUnwinding

►DNA helicase, an enzyme, is responsible for unwinding and unzipping the double helix.

►RNA primase adds a short segment of RNA, called an RNA primer, on each DNA strand.

Base pairing►DNA polymerase continues adding appropriate

nucleotides to the chain by adding to the 3′ end of the new DNA strand.

►Two strands made in slightly different manner.Two strands made in slightly different manner.

Base PairingBase Pairing► One strand is called the leading strand and is elongated as the DNA One strand is called the leading strand and is elongated as the DNA

unwinds; built continuously by addition of nucleotides to the 3’ unwinds; built continuously by addition of nucleotides to the 3’ end.end.

► The other strand, the lagging strand, elongates away from the The other strand, the lagging strand, elongates away from the replication fork. replication fork.

► It is synthesized discontinuously into small segments, called It is synthesized discontinuously into small segments, called Okazaki fragments, by the DNA polymerase in the 3’ to 5’ direction.Okazaki fragments, by the DNA polymerase in the 3’ to 5’ direction.

► DNA ligase later binds these fragments together.DNA ligase later binds these fragments together.► Because one strand is synthesized continuously and the other Because one strand is synthesized continuously and the other

discontinuously, DNA replication is said to be semicontinuous as discontinuously, DNA replication is said to be semicontinuous as well as semiconservative.well as semiconservative.

JoiningJoining

►DNA polymerase removes the RNA primer and fills in the place with DNA nucleotides.

►DNA ligase links the two sections.

Comparing DNA Replication in Eukaryotes and Prokaryotes

►Eukaryotic DNA unwinds in multiple areas as DNA is replicated.

►In prokaryotes, the circular DNA strand is opened at one origin of replication.

12.3 DNA, RNA, and Protein12.3 DNA, RNA, and Protein

►Main idea: DNA codes for RNA, which guides Main idea: DNA codes for RNA, which guides protein synthesisprotein synthesis

►Objectives:Objectives: Explain how messenger RNA, ribosomal RNA, and Explain how messenger RNA, ribosomal RNA, and

transfer RNA are involved in the transcription transfer RNA are involved in the transcription and translation of genes.and translation of genes.

Summarize the role of RNA polymerase in the Summarize the role of RNA polymerase in the synthesis of messenger RNA.synthesis of messenger RNA.

Describe how the code of DNA is translated into Describe how the code of DNA is translated into messenger RNA and is utilized to synthesize a messenger RNA and is utilized to synthesize a particular protein.particular protein.

12.3 DNA, RNA, and Protein 12.3 DNA, RNA, and Protein (cont.)(cont.)

► Review VocabularyReview Vocabulary Synthesis: the composition or combination of Synthesis: the composition or combination of

parts to form a wholeparts to form a whole►New VocabularyNew Vocabulary

RNA PolymeraseRNA Polymerase Messenger RNAMessenger RNA Ribosomal RNARibosomal RNA Transfer RNATransfer RNA TranscriptionTranscription RNA polymeraseRNA polymerase CodonCodon IntronIntron ExonExon TranslationTranslation

Central DogmaCentral Dogma

►““Dogma”means- a way something happensDogma”means- a way something happens►Geneticists now accept that the basic Geneticists now accept that the basic

mechanism of reading and expressing mechanism of reading and expressing genes is from DNA to RNA to protein.genes is from DNA to RNA to protein.

►Central Dogma of Biology: DNA codes for Central Dogma of Biology: DNA codes for RNA, which guides the synthesis of protein.RNA, which guides the synthesis of protein.

►RNA contains the sugar ribose, the base RNA contains the sugar ribose, the base uracil replaces thymine, and is usually uracil replaces thymine, and is usually single strandedsingle stranded

Three Major Types of RNAThree Major Types of RNA

►Messenger RNA (mRNA) - Messenger RNA (mRNA) - Long strands of RNA nucleotides that are formed complementary to one strand of DNA. They . They travel from the nucleus to the ribosome to travel from the nucleus to the ribosome to direct the synthesis of a specific protein.direct the synthesis of a specific protein.

► Ribosomal RNA (rRNA) - Ribosomal RNA (rRNA) - Associates with proteins to form ribosomes in the cytoplasm.

► Transfer RNA (tRNA) - Transfer RNA (tRNA) - Smaller segments of RNA nucleotides that transport amino acids to the ribosome.

Three Major Types of RNA Three Major Types of RNA (cont.)(cont.)

TranscriptionTranscription►Through transcription, the DNA code is

transferred to mRNA in the nucleus.►DNA is unzipped in the nucleus and RNA

polymerase binds to a specific section where an mRNA will be synthesized.

Transcription (cont.)Transcription (cont.)► As the DNA strand unwinds, the RNA polymerase As the DNA strand unwinds, the RNA polymerase

initiates mRNA synthesis and moves along one of the initiates mRNA synthesis and moves along one of the DNA strands in the 3’ to 5’ direction.DNA strands in the 3’ to 5’ direction.

► Template strand – read by RNA polymerase, and Template strand – read by RNA polymerase, and mRNA is synthesized by a complement to the DNA mRNA is synthesized by a complement to the DNA nucleotides.nucleotides.

► Nontemplate strand – not read by RNA PolymeraseNontemplate strand – not read by RNA Polymerase► The mRNA transcript is manufactured in a 5’ to 3’ The mRNA transcript is manufactured in a 5’ to 3’

direction, adding each new RNA nucleotide to the 3’ direction, adding each new RNA nucleotide to the 3’ end.end.

► Uracil is incorporated instead of thymine as the mRNA Uracil is incorporated instead of thymine as the mRNA molecule is made.molecule is made.

► Eventually, the mRNA is released, and the RNA Eventually, the mRNA is released, and the RNA polymerase detaches from the DNA.polymerase detaches from the DNA.

► The new mRNA then moves out of the nucleus The new mRNA then moves out of the nucleus through the nuclear pore into the cytoplasm.through the nuclear pore into the cytoplasm.

RNA ProcessingRNA Processing►The code on the DNA is interrupted

periodically by sequences that are not in the final mRNA.

► Intervening sequences are called introns.►Remaining pieces of DNA that serve as the

coding sequences are called exons. ►Other processing includes adding a

protective cap on the 5’ end and adding a tail of many adenine nucleotides, called the poly-A tail, to the 3’ end of the mRNA.

►The cap aids in ribosome recognition but scientists do not understand the full function of the poly-A tail.

►The mRNA that reaches the ribosome has been processed.

The CodeThe Code

►Scientist knew that 20 amino acids were used to make proteins, so they knew that the DNA must provide at least 20 different codes.

►Experiments during the 1960s demonstrated that the DNA code was a three-base code.

►The three-base code in DNA or mRNA is called a codon.

►Each of the three bases of the codon in the DNA is transcribed into the mRNA code.

Dictionary of the Genetic Dictionary of the Genetic CodeCode►Notice that all but Notice that all but

three codons are three codons are specific for an specific for an amino acid – they amino acid – they are stop codons.are stop codons.

► Codon AUG codes Codon AUG codes for the amino for the amino acid methionine acid methionine and also and also functions as the functions as the start codon.start codon.

TranslationTranslation

►In translation, tRNA molecules act as the interpreters of the mRNA codon sequence.

►At the middle of the folded strand, there is a three-base coding sequence called the anticodon.

►Each anticodon is complementary to a codon on the mRNA.

Transcription & TranslationTranscription & Translation

The Role of the RibosomeThe Role of the Ribosome

► When the mRNA leaves the nucleus , the two parts of When the mRNA leaves the nucleus , the two parts of the ribosome come together and attach to the mRNA the ribosome come together and attach to the mRNA to complete the ribosome.to complete the ribosome.

► Once the mRNA is associated with the ribosome, tRNA Once the mRNA is associated with the ribosome, tRNA with the anticodon carrying its respective amino acid with the anticodon carrying its respective amino acid will move in and bind to the mRNA codon at the 5’ end.will move in and bind to the mRNA codon at the 5’ end.

► The rRNA in the ribosome now acts as enzyme The rRNA in the ribosome now acts as enzyme catalyzing the formation of a peptide bond between catalyzing the formation of a peptide bond between the amino acids creating the amino acid chain or the amino acids creating the amino acid chain or peptide chain.peptide chain.

► As the amino acids join the tRNA is released.As the amino acids join the tRNA is released.► This process continues until the ribosome contains a This process continues until the ribosome contains a

stop codon and signals the end of protein synthesis.stop codon and signals the end of protein synthesis.► Protein release factors cause the mRNA to be released Protein release factors cause the mRNA to be released

from the last tRNA and the ribosome disassemble.from the last tRNA and the ribosome disassemble.

One Gene – One EnzymeOne Gene – One Enzyme

► In the 1940’s the Beadle and Tatum experiment showed that one gene codes for one enzyme. We now know that one gene codes for one polypeptide.

12.4 Gene Regulation and 12.4 Gene Regulation and MutationMutation

►Main idea: Gene expression is Main idea: Gene expression is regulated by the cell, and mutations regulated by the cell, and mutations can affect this expression.can affect this expression.

►Objectives:Objectives: Describe how bacteria are able to Describe how bacteria are able to

regulate their genes by two types of regulate their genes by two types of operons.operons.

Discuss how eukaryotes regulate Discuss how eukaryotes regulate transcription of gene.transcription of gene.

Summarize the various types of mutationsSummarize the various types of mutations

12.4 Gene Regulation and 12.4 Gene Regulation and Mutation (cont.)Mutation (cont.)

►Review VocabularyReview Vocabulary Prokaryote: organism that does not have Prokaryote: organism that does not have

membrane-bound organelles and DNA membrane-bound organelles and DNA that is organized in chromosomesthat is organized in chromosomes

►New VocabularyNew Vocabulary Gene regulationGene regulation OperonOperon MutationMutation MutagenMutagen

Prokaryote Gene RegulationProkaryote Gene Regulation

►Ability of an organism to control which genes are transcribed in response to the environment

►An operon is a section of DNA that contains the genes for the proteins needed for a specific metabolic pathway. Operator Promoter Regulatory gene Genes coding for protein

The Trp OperonThe Trp Operon

The Lac OperonThe Lac Operon

Eukaryote Gene RegulationEukaryote Gene Regulation

► Controlling transcription Transcription factors ensure that a gene is

used at the right time and that proteins are made in the right amounts

The complex structure of eukaryotic DNA also regulates transcription.

Hox GenesHox Genes

►Hox genes are responsible for the general body pattern of most animals.

RNA InteferenceRNA Inteference

►RNA interference can stop the mRNA from translating its message.

MutationsMutations

►A permanent change that occurs in a cell’s DNA is called a mutation.

►Types of Mutations Point mutation Insertion Deletion

Mutations (cont.)Mutations (cont.)

Protein Folding and StabilityProtein Folding and Stability

►Substitutions also can lead to genetic disorders.

►Can change both the folding and stability of the protein

Causes of MutationsCauses of Mutations

►Can occur spontaneouslyCan occur spontaneously►Chemicals and radiation also can Chemicals and radiation also can

damage DNAdamage DNA►High-energy forms of radiation, such

as X rays and gamma rays, are highly mutagenic.

Body Cell Versus Sex Cell Body Cell Versus Sex Cell MutationsMutations

►Somatic cell mutations are not passed on to the next generation.

►Mutations that occur in sex cells are passed on to the organism’s offspring and will be present in every cell of the offspring.