1. DNA STRUCTURE AND REPLICATION

2. pentose sugar nitrogen base phosphate group Sugar Base PO 4 DNA IS A POLYMER OF NUCLEOTIDES NUCLEOSIDES

3. SUGAR IN DNA

4. NITROGENOUS BASES

5. PURINES Found in DNA and RNA

6. PYRIMIDINES C in DNA and RNA T in DNA, U in RNA

7. NUCLEOSIDE/NUCLEOTIDE LINKAGES  - N- glycosidic linkage between the ribose sugar and base

8. NOMENCLATURE

9. A DNA POLY- NUCLEOTIDE (5’ - TACG- 3’) ’

10. BASE PAIRING

11. CHARGAFF’S RULE PURINES = PYRIMIDINES

12. THE DNA DOUBLE HELIX The DNA strands are anti parallel

13. STRUCTURAL FORMS OF THE DNA DOUBLE HELIX A-DNAB-DNAZ-DNA

14. CHROMATIN STRUCTURE

15. SEPARATION OF THE DNA DOUBLE HELIX

16. DNA Replication

17. “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material” James Watson and Francis Crick Nature, 1953

18. Replication: Very Simply…. DNA Replication: Very Simply….

19. DNA Replication: The Complexities

20. [1] The site of initiation of replication is not chosen randomly How does the cell know where to start replication???

21. Origins of replication

22. [2] For replication to occur, the parental DNA double helix has to be separated into single strands What protein is required to make the DNA single stranded during replication????

23. DNA helicase

24. [3] Single stranded DNA is vulnerable to damage Normally complementary single DNA strands reform the double helix What proteins are required keep the DNA single stranded during replication and to protect it from damage????

25. SSB Proteins

26. Each eukaryotic chromosome is one linear DNA double helix Average ~10 8 base pairs long With a replication rate of 2 kb/minute, replicating one human chromosome would require ~35 days (S phase of eukaryotic cell cycle = 6-8 hrs) How are eukaryotic DNA molecules replicated in time????[4]

27. Multiple Origins of replication

28. [5] The two DNA strands twist around each other to form a helix. As the two strands are pulled apart it leads to the tightening of the twists in regions upstream of replication initiation points and the introduction of ‘supercoils’. How are supercoils removed from the DNA helix????

29. SUPERCOILING

30. TOPOISOMERASE I

31. TOPOISOMERASE II

32. [6] The primary DNA replication enzyme, DNA polymerase, can only add nucleotides to an existing strand of nucleotides i.e., it cannot start the replication process by itself. How does replication get started????

34. RNA Primer

35. [7] DNA polymerase can only add nucleotides in one direction, 5’  3’ i.e., creation of both the new strands is in the 5’  3’ direction However, the two DNA strands are antiparallel to each other --- one runs 5’  3’ and the other 3’  5’. Both strands have to be replicated. How is the problem of directionality solved????

38. [8] The solution to problems 6 requires the excision of RNA primers and their replacement by DNA. Which enzyme is involved in this step and how does it work ????

39. DNA polymerase I 5’→3’ exonuclease activity 5’→3’ polymerase activity

40. [9] The solutions to problems 7 and 8 leave the newly synthesized DNA with single strand breaks or ‘nicks’. Which enzyme is involved this sealing these breaks ????

42. [10] The mis-incorporation of a single nucleotide during replication could lead to deleterious or even lethal mutations. How is replication fidelity maintained ????

44. [11] In eukaryotes DNA polymerase cannot fill the gaps at ends of chromosomes after RNA primer is removed. If this gap is not filled, chromosomes would become shorter each round of replication! How are the telomeric regions replicated without shortening of chromosomes???

45. TELOMERES