Small interfering rnas (sirnas)a. protect germ cells from the harmful effects of mobile dna sequences. b. determine the ends of the intron to be spliced out from pre-mrna. c. are involved in regulating gene expression. d. protect cells from exogenous rnas.

C

Explanation:

Small interfering RNAs (SiRNAs) is a class of double-stranded non coding RNA molecules used to interfere with the translation of proteins by binding to and promoting the degradation of mRNA at specific sequences. This process is called RNA interference (RNAi) or SiRNA silencing.

RNA interference is a mechanism by which double stranded RNA (dsRNA) suppress specific transcripts (mRNA) in a sequence-dependent manner. During RNA interference, long double-stranded RNA (dsRNA) is cut or diced into small pieces, usually 21 nucleotides long, by an enzyme called DICER. These small pieces are referred to as Small interfering RNAs (SiRNAs).

These SiRNA bind to proteins from a specific family; Argonaute (Ago) proteins. After binding to an Argonaute protein, one strand of dsRNA is removed, leaving the remaining strand available to bind to mRNA target sequences according to the rule of base pairing; A-U, G-C. Once bound, the Argonaute protein can either cleave to the mRNA, destroying it or recruit accessory factors to regulate the target sequence in other ways.

In a nutshell, SiRNAs play a part in regulating gene expression by interfering with the expression of specific genes with complementary nucleotide sequences by degrading mRNA after transcription, preventing translation.

RNAi is now widely used by researchers as a tool for gene silencing.