Question

Which of the following is false?

a. Ribosomes are sites of polypeptide synthesis

b. Ribosomes are composed of RNA and protein

c. Ribosomes contain a large and small subunit in eukaryotes, but not in prokaryotes

d. Ribosomes exhibit peptidyl transferase activity

e. none of these

Answer 1

Ribosomes are the sites in a cell where the mRNA is translated into a peptide chain hence, protein synthesis takes place here. These are macromolecular structures as rightly pointed in the option b) which are made up of RNA and protein themselves. Since I have already mentioned that they are the sites of protein synthesis option a) also turns out to be true. Polypeptides is another name which can be used for protein, it is more of a generic name for protein where they are made of amino acids, the code for which is encoded on the mRNA. In order to link these aminoacids together and form a chain of polypeptide there is an important chemical reaction that takes place on ribosomes and that reaction is called peptidyl transferase. Thus, ribosomes have that activity, and option d) is correct in that sense.
This brings us to rule out option c) which says that ribosomes are made of a small and a large subunit, but, only in the case of eukaryotes and not in the case of prokaryotes. This statement which is made in the option c) is false because ribosomes respective of being present in eukaryotes or prokaryotes always have two subunits. It is just that the size of these subunits differs amongst them (the eukaryotes and prokaryotes). For instance, in eukaryotic ribosomes the larger subunit is of 60S whereas, the smaller subunit holds 40S and in the case of prokaryotes, the larger and smaller subunit of the ribosome is of 50S and 30S respectively.

Note: The S is the Svedberg Unit, a non-SI unit for sedimentation coefficient, calculated on the basis of the seddimentation of these particles in a centrigugal field.