Which tiny particles in the cytoplasm contain RNA and proteins, both free and attached to rough ER, and function in protein synthesis under the direction of mRNA?

• A. Microfilaments
• B. Ribosomes
• C. Microtubules
• D. Organelles

Answer: (B)

Ribosomes are the cellular machines that carry out protein synthesis. They are tiny RNA–protein particles found freely in the cytoplasm or bound to the rough endoplasmic reticulum. Their job is to translate the genetic code carried by mRNA into a growing polypeptide chain, with tRNA delivering the appropriate amino acids. When ribosomes are free, they typically synthesize proteins for the cytosol and certain organelles; when bound to the rough ER, they make proteins destined for secretion, insertion into membranes, or lysosomal delivery. This translation happens under the direction of the mRNA template, which provides the sequence of codons that guide which amino acids are added next. In contrast, microfilaments and microtubules are part of the cell’s cytoskeleton and provide structure and transport, not protein-synthesis machinery, and organelles is a broad term that doesn’t specify the site where synthesis occurs.