Question

Which series of events is NOT part of the process that generates 3'-protruding ends for strand invasion?

a The nuclease arm of RecB acts as an endonuclease and also serves to load RecA monomers onto the 3'-terminal strand after the Chi site is recognized.
b RecBC cleaves both strands 4-6 bp on the 5'-side of the Chi site.
c Chi site recognition by RecC triggers loss of helicase activity for the RecD subunit.
d RecBCD binds 3' to the Chi site at a double-strand break and unwinds the duplex.
e The RecB nuclease arm degrades both strands as it moves towards the Chi site as a member of the RecBCD complex.
f RecC contains two tunnels and recognizes the Chi sequence.

Answer 1

The incorrect option regarding the mechanism that generates 3'-protruding ends for strand invasion is 'RecBCD binds 3' to the Chi site at a double-strand break and unwinds the duplex.' RecBCD unwinds DNA until it recognizes the Chi site and then preferentially cleaves one strand, but option d incorrectly suggests something else in the process.

Step-by-step explanation:

The series of events that do NOT generate 3'-protruding ends for strand invasion include option d, which states 'RecBCD binds 3' to the Chi site at a double-strand break and unwinds the duplex.' This statement is incorrect because RecBCD actually binds at a double-stranded break and unwinds the DNA duplex, cleaving both strands until it recognizes the Chi site. At the Chi site, RecBCD's nuclease activity is modulated, such that it cleaves preferentially the 5' strand, leaving a 3' overhang to which RecA can bind.

After the Chi site is recognized, the nuclease arm of RecB acts as an endonuclease (option a). Chi site recognition by RecC causes loss of helicase activity of the RecD subunit (option c), and the RecB nuclease arm degrades both strands (option f). On the other hand, RecBC does not cleave both strands 4-6 bp on the 5'-side of the Chi site (option b) and RecC's role in recognizing the Chi sequence (option f) does not directly generate 3'-protruding ends either.