Question at position 12 What binds cut DNA and retains it in protein-DNA complex?XRCC4 with DNA ligase IVKu70/80TdTArtemisRAG1/2

Question

BlackTom AI · Accepted Answer

Ku70/80 heterodimer binds double‑stranded DNA ends and forms a ring-shaped protein that threads onto and holds the broken DNA termini in a protein–DNA complex.

1) Relevant concepts
- Nonhomologous end joining (NHEJ) is the principal pathway for repairing DNA double-strand breaks (DSBs) in mammalian cells.  
- Key NHEJ factors act in defined roles: DNA-end recognition/binding, end processing, and ligation.  
- Ku70 and Ku80 form a heterodimer (Ku) that has a central channel sized for duplex DNA and a very high affinity for DSB ends. Ku recruits DNA‑PKcs to form DNA‑PK, protects ends from exonucleases, and helps tether ends for repair.

2) Step‑by‑step reasoning
- A DSB occurs. The immediate requirement is a protein that recognizes and secures the free DNA ends.  
- Ku70/80 binds rapidly and directly to the exposed double‑stranded ends, forming a stable protein–DNA complex.  
- Ku then serves as a platform to recruit DNA‑PKcs and other NHEJ factors (e.g., Artemis for processing, XRCC4–Ligase IV for ligation). Thus Ku’s action is to bind and retain cut DNA ends while repair is organized.

3) Intermediate molecular actions (no numeric calculations)
- Ku heterodimer encircles DNA end → protection and tethering.  
- Recruitment of DNA‑PKcs → activation of end‑processing nucleases (Artemis) and polymerases (TdT in V(D)J contexts), then XRCC4–Ligase IV performs ligation.

4) Why the correct answer is correct
- Ku70/80 is the canonical DNA end‑binding and end‑retaining factor in NHEJ. Its structural ring and rapid binding explain its role in keeping cut DNA in a protein–DNA complex while processing and ligation occur.

5) Why other options are incorrect
- XRCC4 with DNA ligase IV: catalyzes final ligation, but does not initiate binding/retention of free ends.  
- TdT: a terminal transferase that adds non‑templated nucleotides (V(D)J), not an end‑retaining scaffold.  
- Artemis: nuclease that processes hairpins/overhangs after recruitment by DNA‑PK, not the primary end binder.  
- RAG1/2: introduces the DSBs during V(D)J recombination and can be associated with signal ends transiently, but is not the general DSB end‑binding/retaining factor in NHEJ.

Conclusion: Ku70/80 is the protein that binds and retains cut DNA in a protein–DNA complex. This matches the provided answer: Ku70/80.

Question at position 12 What binds cut DNA and retains it in protein-DNA complex?XRCC4 with DNA ligase IVKu70/80TdTArtemisRAG1/2单项选择题

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