Which of the following RNAs is not required for the synthesis of protein ?

The correct answer is siRNA.

Step-by-Step Explanation:

To understand which RNA is not required for protein synthesis (translation), let's look at the function of each option in the cellular process:

1. mRNA (messenger RNA): mRNA carries the genetic code transcribed from DNA in the nucleus to the ribosomes in the cytoplasm. It serves as the direct template that determines the specific amino acid sequence of the protein being synthesized. Therefore, mRNA is absolutely required for protein synthesis.

2. tRNA (transfer RNA): tRNA acts as an adapter molecule during translation. Each tRNA molecule has an anticodon sequence that recognizes a specific codon on the mRNA template and carries the corresponding amino acid to the growing polypeptide chain. Without tRNA, the genetic code on mRNA cannot be translated into amino acids, making it essential for protein synthesis.

3. rRNA (ribosomal RNA): rRNA is a major structural and catalytic component of ribosomes, the molecular machines where translation takes place. It plays a critical role in binding mRNA and tRNAs, and catalyzes the formation of peptide bonds between amino acids (peptidyl transferase activity). Thus, rRNA is vital for protein synthesis.

4. siRNA (small interfering RNA): siRNA is a double-stranded RNA molecule involved in the RNA interference (RNAi) pathway. Its primary function is gene silencing, where it targets specific mRNA molecules for degradation or prevents their translation, thereby downregulating gene expression. Instead of being required to synthesize proteins, siRNA acts to inhibit or regulate the production of specific proteins. Therefore, it is not required for the basic machinery of protein synthesis.

Thus, among the given options, siRNA is the RNA that is not required for the synthesis of proteins.