Ap Chapter 16

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AP Chap 16 DNA

Text of Ap Chapter 16

  • 1. Chapter 16 The Molecular Basis of Inheritance

2. Concept 16.1: DNA is the genetic material

  • Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 3.

  • When T. H. Morgans group showed that genes are located on chromosomes, the two components of chromosomes DNA and protein became candidates for the genetic material.
  • Was it the DNA or the protein that was the genetic material?
  • The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 4. Evidence That DNA Can Transform Bacteria

  • The discovery of the genetic role of DNA began with research byFrederick Griffithin 1928
  • Griffith worked with two strains of a bacterium, one pathogenic and one harmless

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 5. Fig. 16-2 Living S cells (control) Living R cells (control) Heat-killedS cells (control) Mixture of heat-killedS cells and living R cells Mouse dies Mouse dies Mouse healthy Mouse healthy Living S cells RESULTS EXPERIMENT 6. What happened?

  • When he mixed heat-killed remains of the pathogenic S strain with living cells of the harmless R strain, some living cells became pathogenic
  • He called this phenomenontransformation , now defined as a change in genotype and phenotype dueto assimilation of foreign DNA

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 7.

  • In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that thetransforming substance was DNA
  • Their conclusion was based on experimental evidence thatonly DNAworked in transforming harmless bacteria into pathogenic bacteria
  • Many biologists remained skeptical, mainly because little was known about DNA

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 8. Evidence That Viral DNA Can Program Cells

  • More evidence for DNA as the genetic material came from studies of viruses that infect bacteria
  • Such viruses, calledbacteriophages(or phages), are widely used in molecular genetics research

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: Phage T2 Reproductive Cycle 9. Fig. 16-3 Bacterialcell Phage head Tail sheath Tail fiber DNA 100 nm 10.

  • In 1952,Alfred Hershey and Martha Chaseperformed experiments showing that DNA is the genetic material of a phage known as T2
  • To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters anE. colicell during infection
  • They concluded thatthe injected DNA of the phage provides the genetic information

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: Hershery-Chase Experiment 11.

  • They labeled the protein with S 35and the DNA with P 32.

12. Fig. 16-4-3 EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Centrifuge Pellet Pellet (bacterialcells and contents) Radioactivity (phageprotein)in liquid Radioactivity (phage DNA)in pellet 13. Additional Evidence That DNA Is the Genetic Material

  • In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next
  • This evidence of diversity made DNA a more credible candidate for the genetic material

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: DNA and RNA Structure 14.

  • Chargaffs rulesstate that in any species there is an equal number of A and T bases, and an equal number of G and C bases

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 15. Building a Structural Model of DNA

  • After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role
  • Maurice Wilkins and Rosalind Franklinwere using a technique called X-ray crystallography to study molecular structure
  • Franklin produced a picture of the DNA molecule using this technique

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 16. Fig. 16-6 (a) Rosalind Franklin (b) Franklins X-ray diffractionphotograph of DNA 17.

  • Franklins X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
  • The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases
  • The width suggested that the DNA molecule was made up of two strands, forming adouble helix

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: DNA Double Helix 18. Fig. 16-7b (c) Space-filling model 19. Fig. 16-7a Hydrogen bond 3 end 5 end 3.4 nm 0.34 nm 3 end 5 end (b) Partial chemical structure (a) Key features of DNA structure 1 nm 20.

  • Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA
  • Franklin had concluded that there were two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecules interior
  • At first, W and C thought that like bases paired.

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 21. Fig. 16-UN1 Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data 22. 23.

  • Watson and Crick reasoned that the pairing was more specific, dictated by the base structures
  • They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
  • The Watson-Crick modelexplains Chargaffs rules : in any organism the amount of A = T, and the amount of G = C

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 24. 25. Fig. 16-8 Cytosine (C) Adenine (A) Thymine (T) Guanine (G) 26. 27.

  • The relationshipbetween structure and functionis manifest in the double helix
  • Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 28. 29. Fig. 16-9-3 A T G C T A T A G C (a) Parent molecule A T G C T A T A G C (c) Daughter DNA molecules, each consisting of one parental strand and onenew strand (b) Separation of strands A T G C T A T A G C A T G C T A T A G C 30. 31.

  • Watson and Crickssemiconservative modelof replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or conserved from the parent molecule) and one newly made strand
  • Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new)

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 32. 33. Fig. 16-10 Parent cell First replication Second replication (a) Conservative model (b) Semiconserva- tive model (c) Dispersive model 34. 35.

  • Experiments byMatthew Meselson and Franklin Stahlsupported the semiconservative model
  • They labeled the nucleotides of the old strands with a heavy isotope ofnitrogen , while any new nucleotides were labeled with a lighter isotope

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 36. 37. Fig. 16-11 EXPERIMENT RESULTS CONCLUSION 1 2 4 3 Conservative model Semiconservative model Dispersivemodel Bacteria cultured in medium containing15 N Bacteria transferred tomedium containing14 N DNA sample centrifuged after 20 min (after first application) DNA sample centrifuged after 40 min (after second replication) More dense Less dense Second replication First replication 38. DNA Replication:A Closer Look

  • The copying of DNA is remarkable in its speed and accuracy
  • More than a dozenenzymesand other proteins participate in DNA replication

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 39. Getting Started

  • Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up areplication bubble
  • A eukaryotic chromosome may have hundreds or even thousands of origins of replication
  • Replication proceeds inboth directionsfrom each origin, until the entire molecule is copied

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Animation: Origins of Replication 40. 41. Fig. 16-16 Overview Origin of replication Leading strand Leading strand Lagging strand Lagging strand Overall directions of replication Template strand RNA primer Okazaki fragment Overall direction of replication 1 2 3 2 1 1 1 1 2 2 5 1 3 3 3 3 3 3 3 3 3 5 5 5 5 5 5 5 5 5 5 5 3 3 42. 43. The DNA Replication Complex

  • The prot