1. DNA repair NIKAM C.D. ASSISTANT PROFESSOR, DEPARTMENT OF MICROBIOLOGY,BY
NIKAM C.D.
ASSISTANT PROFESSOR,
DEPARTMENT OF MICROBIOLOGY,
S.M. JOSHI COLLEGE, HADAPSAR, PUNE

2. DNA repair Damaged DNA must be repaired
If the damage is passed on to subsequent generations, then we use the evolutionary term - mutation. It must take place in the germ cells - the gametes - eggs and sperm
If damage is to somatic cells (all other cells of the body bar germ cells) then just that one individual is affected.

3. 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

4. Why repair DNA? DNA pol does a great job, but not good enough
Introduces errors in about 1 in 10E7 nucleotides added, which it does not correct
Other mechanisms exist (as we will see) to correct many of the errors left by the replication system
Most mistakes and damage corrected (99% -leaving just a few - only 1 in 10E9 errors are left)
Mutations are permanent changes left in the DNA

5. Why repair DNA?
Repair of non-replication related damage to the DNA must also be a priority for the cell.
These defects also will prevent translation and duplication of the DNA
Cell will die.
Again, any errors or changes to the DNA become Mutations - which are permanent changes left in the DNA

6. Sickle Cell Disease
This is a very good illustration of the devastating effects of even tiny changes to the DNA
Red Blood Cells
Hemoglobin -
Has a large protein component
2 beta globin chains
A single base change -substitution causes the disease

7. 06_19_sickle_cell.jpg
06_19_sickle_cell.jpg

8. Spontaneous Mutations
Involves thermal energy
Due to random molecular collisions between molecules and DNA in the cell
Cannot be prevented
Parts of the DNA molecule are stripped off and alterations introduced
Many outcomes…

9. Direct DNA Damage Some agents damage DNA directly Chemicals and light
Chemicals - alkylating agents
Methy and ethyl groups added to DNA bases
This type of damage can be repaired by direct reversal involving special enzymes
They remove the offending atoms and restore the base

10. DNA Damage
Just a few types of damage is repaired via simple reversal of the chemical change -
UV light induced dimers
Methylation of bases
Ethylation of bases
Large chemical groups added to the DNA
Most other damage require other systems…

11. Random photons of ultraviolet (UV) light induce aberrant bonding between neighbouring pyrimidines (thymine & cytosine) bases on the same strand of DNA. The will prevent the replication machine from duplicating the DNA. The cell will die!
06_24_radiation.jpg
06_24_radiation.jpg
This type of defect can be readily reversed by a process called photoreactivation. Visible light energy is used to reverse the defect (in bacteria, yeasts, protists, some plants, and some animals but NOT in humans)

12. Other forms of DNA damage
Deamination - An amino group of Cytosine is removed and the base becomes Uracil
Deamination - An amino group of Adenine is removed and the base becomes Hypoxanthine
Deamination - An amino group of Guanine is removed and the base becomes Hypoxanthine

13. And…
Depurination - the base is simply ripped out of the DNA molecule leaving a gap (like a missing tooth)…

14. 06_23_Depurination.jpg Molecular level view-
Remember these are random events
06_23_Depurination.jpg
06_23_Depurination.jpg

15. DNA level view of the same two events as last slide
06_25_mutations.jpg
06_25_mutations.jpg

16. Which is which? The cell has a big problem to overcome…
How does it tell which strand carried the correct information?
We think we know…

17. 06_21_Errors corrected.jpg
The cell has to pick the right strand to fix or else…
06_21_Errors corrected.jpg
06_21_Errors corrected.jpg

18. The cell has a mechanism of identifying new strand synthesis by
leaving nicks that DNA. There are enzymes which scan these new regions
looking for errors
06_22_DNA mismatch.jpg
06_22_DNA mismatch.jpg

19. Correction mechanisms
Direct reversal of damage - Photoreactivation (bacteria, yeast, some vertebrates - not humans) Two thymines connected together by UV light.
Excision Repair - removal of defective DNA. There are three distinct types
1) base-excision
2) nucleotide-excision
3) mismatch repair

20. base-excision
Presence of the Uracil in DNA is a great example of this type
Special enzymes replace just the defective base
1 snip out the defective base
2 cut the DNA strand
3 Add fresh nucleotide
4 Ligate gap

21. nucleotide-excision Same as previous except that
It recognizes more varieties of damage
Remove larger segments of DNA ( s of bases)

22. mismatch repair
Special enzymes scan the DNA for bulky alterations in the DNA double helix
These are normally caused by mismatched bases AG AC CT
These are excised and the DNA repaired

23. 06_26_three steps.jpg Basic mechanism is the same for all three types
Remove damaged region
Resynthesis DNA
Ligate
06_26_three steps.jpg

24. Consider… Sunlight - sunbathing or daily exposure
Impact of ozone depletion
Impact on different skin tones
Environmental degradation

25. Evolution acts on mutations
If we did not have mutation then we would all be the same!
Any changes in the environment would be deleterious to all members of the population equally
= There would be no evolution!!!!
But mutation does exist and it is supported by comparison of related organisms…

26. 06_27_humans_whales.jpg
06_27_humans_whales.jpg

27. Thank You