Question

multiple choice question
what are the roles of the guide rna and cas9 in the crispr/cas9 system?
the guide rna activates the cas9 which acts as a nuclease.
cas9 binds a 9 nt sequence in the target dna and the guide rna edits the bound sequence.
the guide rna binds the dna target and cas9 acts as a nuclease.
the guide rna binds the target dna and cas9 acts as a dna repair enzyme.

Explanation:

In the CRISPR/Cas9 system, the guide RNA (gRNA) is designed to be complementary to a specific DNA target sequence. It binds to that target DNA. Cas9 is an enzyme that acts as a nuclease. A nuclease is an enzyme that can cleave (cut) nucleic acids (in this case, DNA).

• The first option is incorrect because the guide RNA doesn't "activate" Cas9 in the sense described. Cas9 is already an enzyme with nuclease activity, and the gRNA guides it to the target.
• The second option is wrong. Cas9 doesn't bind a 9 - nt (nucleotide) sequence in that way, and the guide RNA doesn't edit the bound sequence; it guides Cas9 to the sequence for cleavage.
• The fourth option is incorrect. Cas9 is not a DNA repair enzyme. Its role is to cleave the DNA at the site guided by the gRNA.

Answer:

The guide RNA binds the DNA target and Cas9 acts as a nuclease.