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DNA repair

This lecture explores the significance of repairing damaged DNA and the consequences of DNA replication errors. It discusses various factors that cause DNA damage, the need for DNA repair, and different mechanisms of DNA repair. It also highlights the devastating effects of mutations and provides examples of DNA damage caused by chemical agents, UV light, and spontaneous changes. The lecture concludes with the impact of environmental factors on DNA damage.

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DNA repair

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  1. DNA repair Lecture 13 Pp 209-215

  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

  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. 06_24_radiation.jpg 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! 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. Molecular level view- Remember these are random events 06_23_Depurination.jpg

  15. DNA level view of the same two events as last slide 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. The cell has to pick the right strand to fix or else… 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

  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 (10 -100s 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

  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

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