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DNA Damage Repair: Protecting Human Chromosomes

Updated · Standard formula, shown below

This guide explains the molecular mechanisms by which human cells detect and repair DNA damage, thereby maintaining chromosome integrity. You will learn the key repair pathways, their step-by-step processes, and common pitfalls in understanding them. No special equipment is needed—just a basic familiarity with DNA structure.

What You’ll Need

  • A basic understanding of DNA structure (double helix, base pairs)
  • Familiarity with the cell cycle phases (G1, S, G2, M)
  • A clear diagram of a chromosome (optional but helpful)

Step-by-Step

  1. Recognize the types of DNA damage. Damage can be single-strand breaks (SSBs), double-strand breaks (DSBs), base modifications (e.g., oxidation, alkylation), or crosslinks. Each type triggers a specific repair pathway.
  2. Activate the DNA damage response (DDR). Sensor proteins (like ATM and ATR) detect the damage and initiate a signaling cascade. This halts the cell cycle at checkpoints (G1/S, intra-S, G2/M) to allow time for repair.
  3. Repair single-strand damage via Base Excision Repair (BER). Damaged bases are removed by DNA glycosylases, creating an abasic site. AP endonuclease cuts the backbone, DNA polymerase fills the gap, and ligase seals it.
  4. Repair bulky lesions via Nucleotide Excision Repair (NER). A damage-recognition complex (XPC, TFIIH) unwinds the helix, endonucleases (XPF, XPG) cut on both sides of the lesion, and the oligonucleotide is removed. DNA polymerase and ligase restore the strand.
  5. Repair double-strand breaks via Non-Homologous End Joining (NHEJ). The KU70/KU80 heterodimer binds the broken ends, recruits DNA-PKcs, and processing enzymes trim or fill the ends. Ligase IV seals the break. This is error-prone but works throughout the cell cycle.
  6. Repair double-strand breaks via Homologous Recombination (HR). In S/G2 phases, the MRN complex resects the break to create 3′ overhangs. RAD51 coats the single-stranded DNA and invades a homologous sister chromatid, using it as a template for accurate repair. The Holliday junction is resolved, and ligation completes the repair.
  7. Repair interstrand crosslinks (ICLs). The Fanconi anemia pathway coordinates unhooking of the crosslink, followed by NER and HR to restore both strands.
  8. Check for repair completion. After repair, the DDR signaling is turned off, and the cell cycle resumes. If repair fails, apoptosis or senescence is triggered to prevent propagation of mutations.

Common Mistakes

  • Confusing BER and NER: BER handles small base modifications, NER handles bulky, helix-distorting lesions.
  • Thinking NHEJ is always error-prone: It can be precise if no processing is needed, but it often introduces small insertions/deletions.
  • Assuming HR is used in all cell types: HR is mainly active in S/G2 phases when a sister chromatid is available; NHEJ is dominant in G1.
  • Overlooking the role of chromatin remodeling: DNA repair requires access to damaged sites, which involves histone modifications and chromatin relaxation.

Tips

  • Use mnemonics: “BER for Bad bases, NER for Nasty lesions” helps remember the scope.
  • Visualize the steps: Draw a simple diagram of each pathway to reinforce the order of protein actions.
  • Relate to diseases: Understanding defects in repair (e.g., BRCA mutations in HR) clarifies why these pathways are critical.
  • Remember the cell cycle: Always consider which phase the cell is in when predicting whether HR or NHEJ will be used.

Troubleshooting

  • If you cannot distinguish between BER and NER: Focus on the type of damage—if it’s a single oxidized base, it’s BER; if it’s a thymine dimer or bulky adduct, it’s NER.
  • If you mix up the proteins: Create a table listing each pathway and its key proteins (e.g., BER: PARP, XRCC1; NER: XPA, XPC; NHEJ: KU, DNA-PKcs, Ligase IV; HR: MRN, RAD51, BRCA2).
  • If you are confused about when HR occurs: Remember that HR requires a homologous template, which is only available after DNA replication (S phase) and before mitosis (G2).
  • If the concept of checkpoints is unclear: Think of checkpoints as quality control gates—they stop the cell cycle until DNA integrity is verified.

Frequently Asked Questions

  1. What is the most common type of DNA damage? Single-strand breaks and base modifications are the most frequent, occurring thousands of times per cell per day due to reactive oxygen species and spontaneous hydrolysis.
  2. Why is homologous recombination considered error-free? Because it uses an undamaged sister chromatid as a template, ensuring the original sequence is restored exactly.
  3. Can DNA damage repair ever fail? Yes, especially if the damage is extensive or the repair machinery is defective (e.g., mutations in BRCA1/BRCA2). This can lead to cancer or premature aging.
  4. What happens if DNA damage is not repaired? The cell may undergo apoptosis (programmed cell death), senescence (permanent growth arrest), or continue dividing with mutations, which can lead to cancer.
  5. Does DNA repair occur in all cells? Yes, all living cells have DNA repair mechanisms, but the efficiency and pathway choice vary by cell type and cell cycle phase.
  6. How do chemotherapy drugs exploit DNA repair? Some drugs (e.g., cisplatin) cause DNA crosslinks that overwhelm repair, while PARP inhibitors block BER in cancer cells with defective HR, leading to synthetic lethality.

Related

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.