What is the small ribosomal subunit of a Prokaryote?

• A. 30S
• B. 40S
• C. 60S
• D. 50S

Answer: (A)

The main idea here is that prokaryotic ribosomes are made up of a small and a large subunit that together form a 70S ribosome. The small subunit in bacteria is called the 30S. The “S” is a Svedberg unit, which reflects how fast a particle sediments during ultracentrifugation, not its exact size, and units are not simply additive. The 30S subunit contains 16S rRNA and about 20–21 proteins, and it plays a key role in decoding the mRNA and coordinating the start of translation. The large subunit, 50S, carries the enzymatic activity for forming peptide bonds and contains 23S and 5S rRNA with many proteins. For contrast, eukaryotic ribosomes are 80S, with a 40S small subunit and a 60S large subunit. So, in prokaryotes, the small ribosomal subunit is 30S.