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

The correct option is siRNA.

To understand why siRNA (small interfering RNA) is not required for the synthesis of proteins, let us look at the role of each RNA type listed in the options:
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 primary template that specifies the amino acid sequence of the protein being synthesized.
2. tRNA (transfer RNA): tRNA acts as an adapter molecule. It reads the codons on the mRNA template and delivers the corresponding amino acids to the ribosome, ensuring they are added to the growing polypeptide chain in the correct order.
3. rRNA (ribosomal RNA): rRNA, along with ribosomal proteins, makes up the structure of ribosomes. It plays both a structural and catalytic role (peptidyl transferase activity) in facilitating the translation process.
4. siRNA (small interfering RNA): siRNA is involved in the RNA interference (RNAi) pathway. Its main function is to bind to specific mRNA molecules and target them for degradation, thereby preventing protein synthesis (gene silencing) rather than assisting in it.

Therefore, mRNA, tRNA, and rRNA are actively required for translation (protein synthesis), whereas siRNA is regulatory and not required for synthesizing proteins.