The Molecul Chapter ar Basis 16: The M of olecular Inheritance Basis of Inheritance Fig. 16-1

Transcription

1 he Chapter Molecular 16: he Basis Molecular of Inheritance Basis of Inheritance Fig. 16-1

2 dditional Evidence hat DN Is the Genetic Material It was known that DN is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group In 1950, Erwin Chargaff reported that DN composition varies from one species to the next his evidence of diversity made DN a more credible candidate for the genetic material nimation: DN and RN Structure Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3 Chargaff s rule states that in any species there is an equal number of and bases, and an equal number of G and C bases Knowing what we already know about DN structure, why do you think that this rule would be valid? Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

4 Building a Structural Model of DN: Scientific Inquiry Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure Franklin produced a picture of the DN molecule using this technique (a) Rosalind Franklin (b) Franklin s X-ray diffraction photograph of DN

5 Franklin s X-ray crystallographic images of DN enabled Watson to deduce that DN was helical he X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases he width suggested that the DN molecule was made up of two strands, forming a double helix nimation: DN Double Helix Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

6 Fig end Hydrogen bond end 1 nm 3.4 nm 0.34 nm end end (a) Key features of DN structure (b) Partial chemical structure (c) Space-filling model

7 Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DN Franklin had concluded that there were two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule s interior - (this means two separate strands made up of, G, C and ). Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

8 t first, Watson and Crick thought the bases paired like with like ( with, and so on), but such pairings did not result in a uniform width Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data

9 Watson and Crick reasoned that the pairing was more specific, dictated by the base structures hey determined that adenine () paired only with thymine (), and guanine (G) paired only with cytosine (C) he Watson-Crick model explains Chargaff s rules: in any organism the amount of =, and the amount of G = C Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10 Fig denine () hymine () Guanine (G) Cytosine (C)

11 Fig G C G C (a) Parent molecule G C G C (c) Daughter DN molecules, each consisting of one parental strand and one new strand (b) Separation of strands G C G C G C G C

12 Getting Started Replication begins at special sites called origins of replication (meaning where replication starts), where the two DN strands are separated, opening up a replication bubble eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin, until the entire molecule is copied nimation: Origins of Replication Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

13 Fig Origin of replication Parental (template) strand Daughter (new) strand Doublestranded DN molecule wo daughter DN molecules Replication bubble Replication fork 0.5 µm (a) Origins of replication in E. coli Origin of replication Double-stranded DN molecule Bubble Parental (template) strand Daughter (new) strand Replication fork 0.25 µm wo daughter DN molecules (b) Origins of replication in eukaryotes

14 Fig Single-strand binding proteins Primase opoisomerase RN primer Helicase

15 Synthesizing a New DN Strand Enzymes called DN polymerases catalyze the elongation of new DN at a replication fork Most DN polymerases require a primer and a DN template strand he rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

16 ntiparallel Elongation he antiparallel structure of the double helix (two strands oriented in opposite directions) affects replication DN polymerases add nucleotides only to the free end of a growing strand long one template strand of DN, the DN polymerase synthesizes a leading strand continuously, moving toward the replication fork nimation: Leading Strand Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17 Fig a Overview Origin of replication Leading strand Lagging strand Primer Lagging strand Leading strand Overall directions of replication

18 Fig b Origin of replication Parental DN RN primer Sliding clamp DN pol III

19 o elongate the other new strand, called the lagging strand, DN polymerase must work in the direction away from the replication fork he lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DN ligase nimation: Lagging Strand Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20 Fig Overview Origin of replication Leading strand Lagging strand Lagging strand 2 1 Overall directions of replication Leading strand emplate strand RN primer 1 Okazaki fragment Overall direction of replication

21 Proofreading and Repairing DN DN polymerases proofread newly made DN, replacing any incorrect nucleotides In mismatch repair of DN, repair enzymes correct errors in base pairing In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DN DN polymerase DN ligase Nuclease