Biology · MU Pleven

22.DNA repair in prokaryotes and eukaryotes. POPULATION GENETICS. CELL AND REPRODUCTIVE BIOLOGY.

Covers DNA damage types and the repair pathways in prokaryotes and eukaryotes, including direct repair, base and nucleotide excision, mismatch repair and double-strand break repair, with prokaryotic and eukaryotic enzymes compared. Moves on to population genetics: gene pool, Hardy-Weinberg equilibrium with its conditions and equations, forces changing allele frequencies, types of selection and genetic drift. Ends with the cell cycle, mitosis and meiosis, gametogenesis, fertilisation, early embryonic development, assisted reproduction and chromosomal abnormalities.

Inside: 13 sections · 6 labelled pictures · 10 flashcards · 4 min

Topic 22 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 13 sections, about 500 words (4 min of reading), with 10 flashcards.

In this note

  1. DNA repair in prokaryotes and eukaryotes. POPULATION GENETICS. CELL AND REPRODUCTIVE BIOLOGY.
  2. DNA REPAIR IN PROKARYOTES & EUKARYOTES
  3. DNA Repair Mechanisms
  4. POPULATION GENETICS
  5. Hardy-Weinberg Equilibrium (HWE)
  6. Forces That Change Allele Frequencies
  7. Types of Selection
  8. CELL AND REPRODUCTIVE BIOLOGY
  9. Cell Cycle Overview
  10. Gametogenesis
  11. Early Embryonic Development
  12. Assisted Reproductive Technologies
  13. Chromosomal Abnormalities in Reproduction

DNA REPAIR IN PROKARYOTES & EUKARYOTES

  • DNA is continuously exposed to damage from replication errors, chemical agents, and physical factors like UV radiation.
  • Efficient repair mechanisms are essential for maintaining genetic stability.
  • Failure in repair can result in mutations, aging, cancer, and cell death.

Major Types of DNA Damage

  • Base modifications (oxidation, alkylation, deamination)
  • Pyrimidine dimers (UV radiation)
  • Single-strand breaks (SSBs)
  • Double-strand breaks (DSBs)
  • Bulky adducts (chemical agents)
  • Mismatched bases during replication

DNA Repair Mechanisms

Direct Repair

  • Photoreactivation repairs pyrimidine dimers using photolyase (present in prokaryotes and yeast, absent in placental mammals).
  • O6-methylguanine methyltransferase (MGMT) removes alkyl groups from guanine.

Base Excision Repair (BER)

  • Removes damaged bases caused by oxidation, deamination, or alkylation.
  • Steps:
  • DNA glycosylase removes the damaged base.
  • AP endonuclease cuts the DNA backbone.
  • DNA polymerase fills the gap.
  • DNA ligase seals the nick.
  • Present in both prokaryotes and eukaryotes.

Nucleotide Excision Repair (NER)

  • Removes bulky DNA lesions such as thymine dimers and chemical adducts.
  • Prokaryotes:
  • UvrABC endonuclease complex cuts the damaged strand.
  • UvrD helicase removes the fragment.
  • Eukaryotes:
  • Involves large multiprotein complexes including XP proteins.
  • More complex but functionally similar to prokaryotic NER.

Mismatch Repair (MMR)

  • Corrects replication errors such as mismatched bases and insertion-deletion loops.
  • Prokaryotes:
  • MutS, MutL, and MutH recognize and repair mismatches.
  • Eukaryotes:
  • MSH and MLH homologs perform similar roles.

Double-Strand Break Repair

  • Non-homologous End Joining (NHEJ):
  • Error-prone; directly ligates DNA ends.
  • Present in both prokaryotes and eukaryotes.
  • Homologous Recombination Repair (HRR):
  • Error-free; uses sister chromatid as a template.
  • Occurs mostly in S and G2 phases.

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