Question

complete the following sorting exercise to be sure you understand how the two break repair mechanisms are similar and different.
sort each item to the appropriate bin.
nhej repair only
hdr repair only
both nhej and hdr repair
occurs after a dna break (including breaks made by cas9)
often results in nonfunctional (null) mutants
more error - prone; likely to cause indels and frameshift mutations
uses an undamaged homologous chromosome as a template
better for making complex substitutions
simply involves ligation of broken dna fragments
increases in frequency after single - strand breaks

Explanation:

• NHEJ Repair only:
• Occurs after a DNA break (including breaks made by Cas9). NHEJ is the main pathway for repairing double - strand breaks in the absence of a homologous template.
• More error - prone; likely to cause indels and frameshift mutations. Since it just ligates the broken ends without precise homology, insertions or deletions (indels) often occur.
• Often results in nonfunctional (null) mutants. The indels can disrupt the reading frame of a gene, leading to a non - functional protein.
• HDR Repair only:
• Uses an undamaged homologous chromosome as a template. HDR requires a homologous sequence (like a sister chromatid in the S or G2 phase) to repair the break accurately.
• Better for making complex substitutions. Because it has a template, it can precisely replace or insert DNA sequences.
• Both NHEJ and HDR Repair:
• Simply involves ligation of broken DNA fragments. Both pathways involve joining the ends of DNA (ligation), although the accuracy and requirements differ.
• Increases in frequency after single - strand breaks. Single - strand breaks can be a precursor or can lead to double - strand breaks that are then repaired by either NHEJ or HDR.

Answer:

• NHEJ Repair only: Occurs after a DNA break (including breaks made by Cas9), More error - prone; likely to cause indels and frameshift mutations, Often results in nonfunctional (null) mutants.
• HDR Repair only: Uses an undamaged homologous chromosome as a template, Better for making complex substitutions.
• Both NHEJ and HDR Repair: Simply involves ligation of broken DNA fragments, Increases in frequency after single - strand breaks.