The viral genome is packed inside a symmetric protein capsid, composed of either a single or multiple proteins, each of them is encoding a single viral gene. Due to this symmetric structure, viruses could encode all the necessary information for constructing a large capsid using a small set of genes.
All retroviruses contain gag, pol, and env genes that, respectively, encode structural proteins, the reverse transcriptase, and proteins embedded in the viral coat. These genes generally exist as polyproteins that are processed into several protein products.
Chemical Composition and Mode of Replication: The genome of a virus may consist of DNA or RNA, which may be single stranded (ss) or double stranded (ds), linear or circular. The entire genome may occupy either one nucleic acid molecule (monopartite genome) or several nucleic acid segments (multipartite genome).
To replicate their genomes in the host cell, the RNA viruses must encode their own enzymes that can replicate RNA into RNA or, in the retroviruses, into DNA. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases, and therefore often make mistakes during transcription.
Viral genomes are very diverse, since they can be DNA or RNA, single- or double-stranded, linear or circular, and vary in length and in the number of DNA or RNA molecules. The viral replication process begins when a virus infects its host by attaching to the host cell and penetrating the cell wall or membrane.
Viral genomes consist of DNA or RNA only, never both. DNA and RNA molecules can be double stranded or single stranded, linear or circular (Fig. 1.6), segmented (composed of multiple pieces of nucleic acid) or nonsegmented.
The double-stranded RNA viruses.
Transcription of each segment results in production of mRNA encoding a single protein. The protein coat of these viruses contains its own RNA-dependent RNA polymerase, which begins transcription of the viral genome shortly after infection within the cytoplasm of the host cell.
Viral genomes may vary in the type of genetic material (DNA or RNA) and its organization (single- or double-stranded, linear or circular, and segmented or non-segmented). In some viruses, additional proteins needed for replication are associated directly with the genome or contained within the viral capsid.
Do viruses have metabolic enzymes?
Viruses are non-living entities and as such do not inherently have their own metabolism. However, within the last decade, it has become clear that viruses dramatically modify cellular metabolism upon entry into a cell. Viruses have likely evolved to induce metabolic pathways for multiple ends.
What are the four genome groups found in viruses?
- Group I: double-stranded DNA viruses.
- Group II: single-stranded DNA viruses.
- Group III: double-stranded RNA viruses.
- Group IV: positive sense single-stranded RNA viruses.
- Group V: negative sense single-stranded RNA viruses.
- Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle.
What type of viruses must encode their own polymerases?
Viruses that spend their entire life cycle in the cytoplasm do not have access to host polymerases and thus need to encode their own polymerases for transcription and replication.
Why do some viruses have an RNA genome?
Some genes of RNA virus are important to the viral replication cycles and mutations are not tolerated. For example, the region of the hepatitis C virus genome that encodes the core protein is highly conserved, because it contains an RNA structure involved in an internal ribosome entry site.
Do viruses encode ribosomal proteins?
Viruses modulate ecosystems by directly altering host metabolisms through auxiliary metabolic genes. However, viral genomes are not known to encode the core components of translation machinery, such as ribosomal proteins (RPs).
Explanation: When viral genome can become integrated into the bacterial genome they are known as prophages. They carry DNA that can behave as a kind of episome in bacteria. 3. Bacteriophages inject their nucleic acid into the bacterium.
In the lytic cycle, the phage replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations.
How is a virus organized?
All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. Some viruses are also enclosed by an envelope of fat and protein molecules. In its infective form, outside the cell, a virus particle is called a virion.