Describe the structure of the bacterial ribosome and its subunits.

• A. 70S ribosome composed of 30S and 50S subunits.
• B. 60S and 40S subunits.
• C. 80S ribosome composed of 40S and 60S.
• D. 70S ribosome composed of 20S and 50S.

Answer: (A)

Bacteria use a 70S ribosome made of two pieces: a small 30S subunit and a large 50S subunit. The 30S subunit contains the 16S rRNA and is mainly responsible for locating the start of the mRNA and decoding the genetic message. The 50S subunit carries the 23S and 5S rRNAs and provides the peptidyl transferase activity to form peptide bonds between amino acids. Together, these two subunits assemble on an mRNA template to synthesize proteins.

This setup is distinct from eukaryotic ribosomes, which are 80S and composed of 40S and 60S subunits. The other options either mix subunits that correspond to eukaryotes or give an incorrect bacterial subunit (for example, a 20S subunit), which does not match bacterial ribosome structure.