Question

Which of the following is not true of RNA processing?

1. Exons are cut out before mRNA leaves the nucleus.

2. Nucleotides may be added at both ends of the RNA.

3. Ribozymes may function in RNA splicing.

4. RNA splicing can be catalyzed by spliceosomes.

Short answer

1. The statement “Exons are cut out before mRNA leaves the nucleus” istrue.

2. The statement “Nucleotides may be added at both ends of the RNA” isfalse.

3. The statement “Ribozymes may function in RNA splicing” is false.

4. The statement “RNA splicing can be catalyzed by spliceosomes” is false.

Step-by-step solution

RNA processing

All eukaryotic mRNA molecules undergo RNA processing before they are released into the cytoplasm for translation. RNA processing of mRNA involves 5’ capping, polyadenylation, and RNA splicing.

Explanation of option (A)    

The synthesized mRNA molecule contains both introns (non-coding sequences of genes) and exons (coding sequences). The introns of the mRNA molecule are removed, and the remaining exons are joined. This process is called RNA splicing.

Thus, introns, and not exons, are removed from mRNA molecules before they are released into the cytoplasm.

Therefore, the given option is not true and thus is the correct option.

Explanation of option (B)

RNA processing includes 5’capping and polyadenylation. In 5’capping, the 5’end of the mRNA molecule is modified. In this process, a 7-methylguanosine cap is added to the 5’end of the mRNA molecule.

In polyadenylation, a string of adenine (A) residues is added to the 3’ end of the mRNA molecule. The adenine residues are called the poly-A tail. Both 5’ cap and poly-A tail protect the mature mRNA from degradation.

Thus, nucleotides are added at both ends of the mRNA molecule.

Therefore, the given option is true and thus is the incorrect option.

Explanation of option (C)

Ribozymes are RNA molecules that act as enzymes. They catalyze biochemical reactions and play an essential role in the splicing of introns from mRNA molecules.

Thus, ribozymes play an essential role in RNA splicing.

Therefore, the given option is true and thus is the incorrect option.

Explanation of option (D)

RNA splicing is catalyzed by a large RNA –protein complex called spliceosomes. This complex comprises ribonucleoproteins that interact with introns and mediate its removal.

Thus, spliceosomes catalyze RNA splicing.

Therefore, the given option is true and thus is an incorrect option.