Final answer:
Dam methylation is not a part of excision repair; it is involved in mismatch repair for identifying parental strands in bacteria. Excision repair consists of damage recognition, incision by endonuclease, and excision by exonuclease, followed by DNA synthesis and ligation.
Step-by-step explanation:
The step that is NOT part of excision repair is A) Dam methylation. Excision repair is a DNA repair mechanism involved in fixing DNA damage such as thymine dimers typically caused by UV light. The process involves several key steps:
- Damage recognition - The repair system identifies the damaged DNA segment.
- Incision by endonuclease - An enzyme makes cuts around the damaged DNA.
- Excision by exonuclease - The damaged portion of DNA is removed.
- DNA synthesis - New, correct DNA is synthesized to replace the excised section.
- DNA ligation - The new DNA segment is sealed into the rest of the DNA strand.
Dam methylation is a process involved in mismatch repair, specifically in bacterial systems such as E. coli. It is the methylation of adenine residues in the DNA sequence GATC, which helps in identifying the parental strand during repair. This step is not part of nucleotide or base excision repair mechanisms.