Which is unique to viruses?
a) SNRNPs
b) mRNA
c) receptors
d) proteases
e) polyproteins
The correct answer and explanation is:
The correct answer is e) polyproteins.
Polyproteins are unique to viruses and are a hallmark of many viral replication processes. In some viruses, especially RNA viruses, the genome encodes a large, single protein called a polyprotein. This polyprotein is subsequently cleaved into smaller functional proteins by viral proteases. These proteins play crucial roles in the viral life cycle, including forming structural components, assisting with replication, and interacting with the host cell machinery.
The polyprotein strategy allows viruses to maximize the efficiency of their limited genetic material. By encoding a single, long protein that can be cleaved into many smaller, functional proteins, viruses reduce the number of genetic sequences they need to include. This is particularly advantageous because the viral genome is often very compact.
The use of polyproteins is a common feature among certain groups of viruses, including picornaviruses, flaviviruses, and retroviruses. For example, the human immunodeficiency virus (HIV), a retrovirus, produces a large polyprotein called Gag-Pol, which is cleaved by the viral protease to produce multiple proteins needed for the assembly and function of new virions.
This strategy contrasts with cellular organisms, where individual proteins are typically transcribed and translated separately from distinct genes. In viruses, polyproteins streamline the production of necessary proteins, making it a unique feature of their biology.
In comparison to other options:
- SNRNPs (small nuclear ribonucleoproteins) are involved in splicing and are present in eukaryotic cells.
- mRNA is a universal molecule found in both viruses and cells.
- Receptors and proteases are present in both viral and host systems but are not unique to viruses.
Thus, polyproteins stand out as a viral-specific strategy to optimize genetic material use.