1. Microbial Genetics (Micr340) Lecture 1 Chromosome Structure, Replication and Segregation (I)

2. DNA Structure

3. DNA structure DNA structure model proposed in 1953 If DNA strands are chains, deoxyribonucleotides form the links Each “link” is made of a base, a sugar and a phosphate Phosphodiester bonds join each “link” Two DNA strands (chains) are hold together by hydrogen bonds between bases

4. DNA structure Fig 1.1

5. DNA structure Bases Purine Adenine, guanine Pyrimidine cytosine, uracil, thymine Sugars 2-deoxyribose (for DNA) ribose (for RNA)

6. DNA structure Fig 1.2

7. DNA structure Fig 1.2

8. DNA structure DNA strands are antiparallel If one strand is 5’ to 3’ direction, the other is 3’ to 5’ direction Base pairing A and T form two hydrogen bonds G and C form three hydrogen bonds

9. DNA structure Fig 1.3

10. DNA structure Fig 1.3

11. Mechanism of DNA Replication

12. Mechanism of DNA replication Deoxynucleotide polymerization Enzymes involved DNA polymerases Nucleases DNA ligases Primases

13. Mechanism of DNA replication Fig 1.6

14. Mechanism of DNA replication Fig. 1.7

15. Mechanism of DNA replication Semiconservative replication

16. Mechanism of DNA replication Fig 1.8

17. Mechanism of DNA replication Fig 1.8

18. Mechanism of DNA replication Semiconservative replication DNA strands separated by helicases DNA polymerase III catalyzes polymerazation Non-continuous synthesis of one of the two strands: okazaki fragments

19. Mechanism of DNA replication Fig 1.9

20. Mechanism of DNA replication Fig 1.9

21. Mechanism of DNA replication Semiconservative replication DNA strands separated by helicases DNA polymerase III catalyzes polymerazation Non-continuous synthesis of one of the two strands: okazaki fragments Linkage of okazaki fragments

22. Mechanism of DNA replication Fig 1.11

23. Replication Errors Editing; correcting mistake as it replicates DNA. In E. coli, DNA polymerase III has a 3’ (to 5’) exonuclease activity to perform editing

24. Replication Errors

25. Methyl-directed mismatch repair Takes advantage of semiconservative replication; old strand is heavily methylated by DAM (deoxyadenosine methylase) The system recognizes the mismatch and removes it and its surrounding DNA on the same new strand.

26. Replication Errors Fig 1.15