DNA Replication & Repair.


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It is a short and concise slide about DNA replication and Repair. It is prepared keeping in mind for Undergraduates level but PG also might find it handy.

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DNA Replication & Repair.

  1. 1. DNA Replication & Repair Abhishek Dahal
  2. 2.  DNA is a molecule that carries Genetic information from generation to next.  Also called as Reserve Bank Of Genetic information.  Central Dogma of life: Flow of information from DNA to RNA to Protein synthesis.
  3. 3. 3 Nitrogenous Bases  Double ring PURINES Adenine (A) Guanine (G)  Single ring PYRIMIDINES Thymine (T) Cytosine (C) T or C A or G
  4. 4. 4 Chargaff’s Rule  Adenine must pair with Thymine  Guanine must pair with Cytosine  The bases form weak hydrogen bonds G C T A
  5. 5. 5 Antiparallel Strands  One strand of DNA goes from 5’ to 3’ (sugars)  The other strand is opposite in direction going 3’ to 5’ (sugars)
  6. 6. 6 DNA Replication copyright cmassengale
  7. 7. 7 Semi conservative Model of Replication  Idea presented by Watson & Crick  The two strands of the parental molecule separate, and each acts as a template for a new complementary strand  New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA
  8. 8. 8 DNA Replication  Begins at site called as "Origins of Replication"  Specific Protein Called as dna A binds to this site causing double strands to separate.  As the 2 DNA strands open at the origin, Replication Bubbles form  Prokaryotes (bacteria) have a single bubble  Eukaryotic chromosomes have MANY bubbles Bubbles Bubbles
  9. 9. 9 DNA Replication  Two strands open forming Replication Forks (Y- shaped region)  New strands grow at the forks Replication Fork Parental DNA Molecule 3’ 5’ 3’ 5’
  10. 10. 10 DNA Replication  Enzyme DNA Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds.  Single-Strand Binding Proteins (SSBP) attach and keep the 2 DNA strands separated and untwisted
  11. 11. 11 Replication Requirement  Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides  Primase is the enzyme that synthesizes the RNA Primer  DNA polymerase 3 can then add the new nucleotides and forms new strand.
  12. 12. 12 DNA Replication  DNA polymerase can only add nucleotides to the 3’ end of the DNA  This causes the NEW strand to be built in a 5’ to 3’ direction.  DNA polymerase also checks for incoming nucleotides and act as proof reading.
  13. 13. 13 Synthesis of the New DNA Strands  Leading strand synthesized 5’ to 3’ in the direction of the replication fork movement.  It is continuous  Requires a single RNA primer  Lagging strand synthesized 5’ to 3’ in the opposite direction.  Discontinuous (i.e., not continuous)  Requires many RNA primers , DNA is synthesized in short fragments.
  14. 14. 3 Polymerase III Leading strand base pairs 5’ 5’ 3’ 3’ Supercoiled DNA relaxed by gyrase & unwound by helicase + proteins: Helicase + Initiator Proteins ATP SSB Proteins RNA Primer primase 2Polymerase III Lagging strand Okazaki Fragments 1 RNA primer replaced by polymerase I & gap is sealed by ligase
  15. 15.  DNA polymerase 1 removes the RNA primer  The gap left behind is Sealed by Ligase and new Daughter DNA is formed.  Thus process of replication is ended.
  16. 16. DNA repair  The process of replication is extremely accurate but errors occurs sometime and cells posses capacity to repair these errors.  Damaged DNA must be repaired  If the damage is passed on to subsequent generations, then we use the evolutionary term - mutation.
  17. 17. Damage from where?  Consequences of DNA replication errors  Chemical agents acting on the DNA  UV light imparting energy into DNA molecule  Spontaneous changes to the DNA
  18. 18. a) Base-excision repair  Presence of the Uracil ,hypoxanthine and xanthine in DNA is a great example base- excision.  N-glycosylase enzyme replace just the defective base.  snip out the defective base  2cut the DNA strand  Add fresh nucleotide via DNA Polymerase.  Gap is sealed by LIGASE
  19. 19. b)Nucleotide-excision repair  UV light and Ionization radiation causes modification of bases, strand breaks, cross-linkage, etc.  It recognizes more varieties of damage in DNA .  Cutting of the defective piece by Exinuclease and its removal (Degraded).  Resynthesis of the cut part by DNA polymerase and ligase.  Defect in this mechanism leads to Xeroderma pigmentosa
  20. 20. c) Mismatch repair  These are normally caused by mismatched bases i.e. AG and CT.  Special enzymes scan the DNA for bulky alterations in the DNA double helix.  GATC endonuclease cuts the strand and the strand is digested by Exonuclease.  These gaps are excised and the DNA repaired by ligase and polymerase enzyme respectively.  Defect in this mechanism causes Lynch syndrome i.e. patient are of high risk of developing Colon cancer.
  21. 21. d) Double-strand break repair  High energy radiation and free radicals causes DNA breakage and leads to cell death.  Repair mechanism is of 2 type  1) Non-homologous end joining(Yeast)  2) Homologous end joining (mamals)  Defect: Cancer and Immunodeficiency syndrome.
  22. 22. THE END  Thank you……..