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Chapters 16 and 17. Objectives Describe the data that led Watson and Crick to suggest their model of DNA structure

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Chapters 16 and 17

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Chapters 16 and 17

Chapters 16 and 17

Chapters 16 and 17

  • Objectives

    • Describe the data that led Watson and Crick to suggest their model of DNA structure

    • Show how the double helix model of DNA conformed to prior observations of the chemistry of the molecule and, at the same time, suggested mechanisms for the roles of DNA in gene expression and chromosome replication

    • Define and explain the terms replication, transcription and translation

Chapters 16 and 17

  • Define what is meant by semi-conservative DNA replication and describe the role of Okazaki fragments

  • Use the base-pairing rules to determine the nucleic acid sequences of new DNA strands, mRNA and tRNA from DNA sequences

  • Trace the steps of protein synthesis, including the locations of the processes, the involvement of complex molecular machinery and the requirement for inputs of energy

  • Explain why the DNA code must involve a reading frame of three bases



  • Many basics of molecular biology determined using viruses that infect bacteria

    • viruses composed of protein coat and internal genetic material (DNA or RNA)

      • not living-no cell structure, no metabolism, cannot self-replicate

    • Bacterial viruses called bacteriophages

      • logical choice for experiments

Chapters 16 and 17

  • Early experiments showed DNA genetic material

  • 1928-Griffith

    • Streptococcus pneumonia is a bacteria that causes pneumonia in mammals

      • 2 strains: 1 pathogenic and 1 nonpathogenic

    • Mix heat killed pathogenic strain with living nonpath strain and some of these cells became pathogenic



  • Change in genotype and phenotype due to an assimilation of external DNA by the cell

1952 hershey and chase

1952-Hershey and Chase

  • Showed that viruses can infect bacteria

Dna is the genetic material

DNA is the Genetic Material

  • Prior to the 1950’s, it was already known that DNA is a polymer of nucleotides, each consisting of three components

  • Each monomer (nucleotide) composed of phosphate group, 5C sugar, and nitrogenous base

    • DNA-deoxyribose

    • RNA-ribose

  • Each polymer contains four nucleotides named for the nitrogenous bases

    • DNA

      • thymine-T, cytosine-C, adenine-A, and guanine-G

    • RNA

      • uracil-U replaces T

  • Chapters 16 and 17


    • DNA is hereditary information is enclosed in the chemical language of DNA

    • DNA is the blue print and program that directs the development of many traits

    Watson and crick

    Watson and Crick

    • DNA is double-stranded helix

      • Watson-Crick model based on observations from many sources

        • chemical structure of bases

        • X-ray crystallographs from Rosalind Franklin

        • chemical analysis of DNA

          • A=T and G=C

        • ratios of A+T and G+C

    Chapters 16 and 17

    final model that fit observations is double helix with sugar backbones on outside and hydrogen-bonded bases on inside

    • twisted rope ladder

    • A always bonds with T and G always bonds with C

    • led to proposed mechanisms of DNA function

    Chapters 16 and 17

    Meselson and Stahl demonstrated that DNA replication is semi-conservative, confirming the Watson-Crick model

    Dna replication

    DNA Replication

    • Copying of the DNA is extremely fast and accurate

      • Prokaryotes= 5000 nuc per second

      • Errors occur in every 1 billion nuc

    • DNA replication depends on base pairing rules

      • requires accurate and complete copies of DNA

    • known as semi-conservative replication

      • each strand of DNA molecule acts as template for synthesis of new strand

      • bases added to growing strand using base-pairing rules

    Chapters 16 and 17

    • replication initiated at special sites called origins of replication

    Chapters 16 and 17



    Chapters 16 and 17


    Chapters 16 and 17

    • Enzymes proofread DNA during replication and repair damage in existing DNA

      • mismatch repair - enzymes repair errors in base pairing during replication

      • excision repair - enzymes repair damage caused by physical and chemical agents

    Chapters 16 and 17

    3’ end of leading strand has potential to get shorter with repeated replication

    • solved by two mechanisms

      • telomeres - non-coding repeated DNA sequences at ends of linear molecules

      • telomerases - catalyzes lengthening of telomeres

    • shortening of telomeres may represent cell life - limiting factor

    Chapter 16 check list

    Chapter 16 Check list

    • What did Griffith do?

    • What did Hershey and Chase do?

    • Watson and Crick?

    • Meselson and Stahl?

    • What bases pair with each other?

    • DNA replication is _______?

    • Can stranded DNA have many replication sites?

    • What about circular DNA?

    • What are the enzymes that are used in replication, what do they do and what order do they occur in?

    • Define leading vs. lagging strands and the differences between them.

    • DNA synthesis starts with what kind of primer?

    Gene expression transcription and translation ch 17

    Gene Expression/Transcription and Translation(CH 17)

    • Genotype expressed as proteins-basis of phenotypic traits

      • one gene-one polypeptide hypothesis

      • flow of information is from DNA to RNA (transcription) to polypeptide (translation)

    Genes specify proteins via transcription and translation

    Genes specify proteins via transcription and translation

    • Evidence for this idea came from studies of metabolic defects

    • 1909, British physician Archibald Garrod was the first to dictate phenotypes through enzymes

    Nutritional mutants

    Nutritional Mutants

    • Beadle and Tatum caused bread mold to mutate by using x-rays

      • Could not survive on minimal medium

    • Using genetic crosses they determined that their mutants fell into three classes, each mutated in a different gene

    Chapters 16 and 17

    Genetic information written as codons and translated into amino acids

    occurs in two stages:

    • transcription of DNA in nucleus into mRNA

    • translation of mRNA into amino acid sequence in cytoplasm

    Chapters 16 and 17

    two languages used by cell

    • nucleotides-five letters in alphabet (T or U, A, C, G)

    • amino acids-20 letters in alphabet

    • DNA= Code

    • MRNA=Codon

    • TRNA= Anticodon

    Chapters 16 and 17

    • smallest nucleotide word that can accommodate amino acid alphabet is a three-letter word

    • triplets of bases in nucleotides specify specific amino acids-known as codons

      • first codon deciphered was UUU-phenylalanine

    • forms the basis of the genetic code

      • universal

      • redundant but not ambiguous

    Evolution of genetic code

    Evolution of Genetic Code

    • The genetic code is nearly universal and it is shared by organisms from the simplest bacteria to the most complex animals



    • Makes copies of information in DNA molecule in form of messenger RNA

      • occurs in nucleus in eukaryotes and cytoplasm in prokaryotes

      • involves RNA polymerase

      • initiated at promoter region and elongation continues until terminated at terminator region

    • Two other types of RNA-rRNA and tRNA-also transcribed

    Chapters 16 and 17

    • Promoters, specific nucleotide sequences at the start of a gene, are the site of RNA polymerase binding

    • Transcription factors help the RNA polymerase recognize the promoter sequence and bind to the DNA.

    Chapters 16 and 17

    • RNA polymerase moves along the DNA and creates the mRNA

      • Does the work of ALL the enzymes use in DNA replication

    Chapters 16 and 17


    Chapters 16 and 17

    • Eukaryotic mRNA’s are processed before leaving the nucleus

      • modified nucleotide G subunits added to 5’ end (cap) and a poly-A tail added to 3’ end

      • introns (non-coding sequences) excised from mRNA by spliceosomes and exons (coding sequences) linked together.

        • shuffling of exons contributes to protein diversity

    Function and evolution of introns

    Function and Evolution of Introns

    • The presence of introns allows for alternative RNA splicing

    • Proteins often have different structural and functional regions called domains

    • In many cases different exons code for the different domains



    • Occurs in cytoplasm

      • involves ribosomes, tRNA, mRNA enzymes and protein factors, and energy sources

    Chapters 16 and 17


    Chapters 16 and 17

    • Transfer RNA (tRNA) acts as interpreter

      • amino acids recognized by codons to anticodon match

      • tRNA-one for each amino acid-matches amino acid to codon

        • contains special region (anti-codon) that base-pairs with codon for its amino acid on mRNA

          • correct amino acid is bound to tRNA by enzyme complex (one for each tRNA-amino acid combination) using ATP

    Chapters 16 and 17


    • build polypeptides

    • composed of rRNA and protein in two subunits

    • provides stable platform for assembly of peptides

      • contains binding sites for mRNA, two tRNA-amino acid complexes and the growing polypeptide

    Chapters 16 and 17

    • mRNA is longer than genetic message it contains

      • start of message marked by initiation sequence

      • end of message marked by termination sequence

    Translation divided into three phases

    Translation divided into three phases


    • initiation sequence helps bind mRNA to small subunit of ribosome

    • methionine-tRNA binds to start codon (AUG-also codes for methionine) via anticodon of tRNA

    • large subunit binds to small subunit so that methionine tRNA fits in P site of large subunit

    Chapters 16 and 17


    • involves three steps

      • anticodon of incoming tRNA-amino acid complex binds to codon in A site

      • peptide bond formed between polypeptide (attached to tRNA in P site) and new amino acid

      • P site tRNA leaves ribosome and A site tRNA-polypetide complex moves to P site

    Chapters 16 and 17

    • Termination

      • elongation continues until stop codon (UAA, UAG or UGA) reached

      • finished polypeptide released

      • ribosome splits into two subunits

    Chapters 16 and 17

    • multiple ribosomes can process a single mRNA at the same time

      • polyribosome

    Chapters 16 and 17

    • Polypeptide develops tertiary structure during and after translation

    • After synthesis, several polypeptides can come together to form protein with quaternary structure

    Chapters 16 and 17

    Proteins destined for membranes or export from cell are synthesized on rough ER

    • signal sequence at amino end of polypeptide triggers binding to ER

    • cytosolic polypeptides lack signal sequence and are synthesized by free ribosomes

    Chapters 16 and 17

    In prokaryotes, transcription and translation can occur concurrently leading to the formation of “lampbrusharrays”



    • Mutations change the meaning of genes

      • cause a change in nucleotide sequence of DNA

      • differences in inherited traits can be traced to mutations

    Chapters 16 and 17

    • two types of alterations to DNA sequence

      • substitution-replacement of one nucleotide with another

        • usually results in replacement of one amino acid with another in polypeptide or no change if new codon codes for same amino acid

        • example-sickle cell anemia allele

          • single base change in DNA results in amino acid change in hemoglobin

    Chapters 16 and 17

    • insertion or deletion of bases in DNA sequence

      • results in phase shift of three base reading frame

      • affects all codons after mutation

        • results in different amino acid sequence

      • almost always results in non-functional polypeptide

    • mutations can occur spontaneously or by physical (radiation) or chemical mutagens

    Check list

    Check list

    • The flow of information is from…?

    • What is the codon, code and anticodon?

    • What nucleotides are in DNA and RNA?

    • Is the genetic code universal?

    • What is transcription? Describe the process.

    • What is translation? Describe the process.

    • What enzymes are involved in transcription?

    • What are the specific regions on DNA that indicate where transcription starts.

    • What are the types of DNA alterations?

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