1. DNA replication

2. Major points DNA is the carrier of genetic information Genetic information is passed between generation by complementary base-pairing Each DNA strand acts as a template for copying a new strand

3. Reading (4) Chapter 16, pp. 284-291

4. DNA replication I. DNA structure and function A. Genetic information B. Template copying by base-pairing II. DNA replication mechanism III. Replication machinery

5. Genetic link between generations

6. “It has not escaped our notice that the specific basepairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” DNA replication: template copying Each strand acts as a template for copying a new strand based on complementary base-pairing Watson & Crick, 1953

7. Template One DNA strand acts as template for synthesis of a new DNA strand (replication) or a new RNA strand (transcription) Template is complementary, not identical, to new strand Depends on base-pairing: A=T (or A=U), C  G, G  C, T=A (or U=A) Double helix must be locally denatured to access template for base-pairing

8. DNA replication: basic model

13. DNA replication I. DNA structure and function II. DNA replication mechanism A. General features B. Models III. Replication machinery

14. DNA replication mechanism Synthesis is 5 to 3 Specific origin Semiconservative Bidirectional (or unidirectional) Semidiscontinuous Primed by short RNAs

15. Origin of replication Unique and specific site(s) on DNA where replication (DNA synthesis) begins Why? Include an AT- rich sequence

16. Meselson-Stahl experiment

17. Semiconservative Each daughter molecule consists of one old strand (template) and one newly synthesized strand

18. Bidirectional Replication (of most DNAs) proceeds in both directions away from the origin via two growing forks moving in opposite directions One origin Two replication forks

19. Antiparallel strands paradox DNA synthesis is 5 to 3 However double helix is antiparallel

20. Semidiscontinuous Replication is continuous on one strand (leading) and discontinuous on other strand (lagging)

21. Okazaki fragments Small replication products from discontinuous replication on lagging strand Replication requires a primer (made of RNA)

22. Primers DNA polymerases cannot initiate a new strand by themselves A short RNA (or DNA) that has a free 3´ end and is complementary to template is required

23. Primers in DNA replication RNA primer is synthesized on template strand DNA is synthesized RNA is removed Gaps are filled in with DNA and joined together (ligated)

25. DNA replication I. DNA structure and function II. DNA replication mechanism III. Replication machinery A. DNA polymerase B. Proofreading

26. Replication machinery Several enzymes (biological catalysts) and DNA-binding proteins are required in DNA replication Origin-binding proteins Helicases and SSBs Primases DNA polymerases DNA ligase Telomerase

27. Polymerase overview DNA synthesis (replication) is catalyzed by DNA polymerase RNA synthesis (transcription) is catalyzed by RNA polymerase

28. DNA polymerases Synthesize DNA in a template-dependent fashion Require: –DNA template –The four nucleoside triphosphates (dNTPs) –Primer Catalyze addition of nucleotides to 3´ end of growing chain

29. Proofreading Most DNA polymerases are associated with a 3´to 5´ exonuclease activity that removes nucleotides (reverse of polymerase activity) Reduces point mutations that would have resulted from insertion of wrong nucleotide Results in high-fidelity replication (only 1 error in 10 10 incorporated nucleotides) Checks for proper base-pairing, which requires intact helix (primer requisite)

31. Bio 103 home