It is encoded either in DNA or, for many types of virus, in RNA. A virus has either DNA or RNA genes and is called a DNA virus or a RNA virus. The genome includes both the genes and the non-coding sequences of the DNA/RNA.
Genomes of RNA viruses encode for a limited number of proteins. RNA viral genomes are broadly divided into double stranded RNA, positive and negative strand single stranded RNAs, monopartite and multipartite RNA viruses.
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.
The viruses in this group are so simple that they do not encode most of the proteins required for replication of their DNA but depend on cellular proteins for this purpose. For this reason, they have been particularly useful in identifying and analyzing the cellular proteins involved in DNA replication (Chapter 12).
Single-stranded RNA genomes, however, exist in two forms: either the sense or the “plus” (+) strand or the nonsense or “minus” (−) strand. ssRNA(+) genomes act as mRNA, are infectious upon entry into host cells, and are immediately translated into protein, including the enzymes required for viral reproduction.
What types of genomes do viruses have?
The vast majority of viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes. Viral genomes are circular, as in the polyomaviruses, or linear, as in the adenoviruses. The type of nucleic acid is irrelevant to the shape of the genome.
What are the four types of genomes that viruses can have?
Viral genomes exhibit extraordinary diversity with respect to nucleic acid type, size, complexity, and the information transfer pathways they follow. Thus, viral nucleic acids can be DNA or RNA, double-stranded or single-stranded, monopartite or multipartite, linear or circular, as short as 2 kb or up to 2500 kb long.
How many genes do viruses have?
The smallest viruses have only a few genes; the largest viruses have as many as 200. Genetically, however, viruses have many features in common with cells.
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.
What is the process of virus reproduction?
Viral replication involves six steps: attachment, penetration, uncoating, replication, assembly, and release. During attachment and penetration, the virus attaches itself to a host cell and injects its genetic material into it.
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.
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.
A prophage is a bacteriophage (often shortened to “phage”) genome inserted and integrated into the circular bacterial DNA chromosome or exists as an extrachromosomal plasmid. This is a latent form of a phage, in which the viral genes are present in the bacterium without causing disruption of the bacterial cell.
Do RNA viruses have genes?
RNA viruses have RNA as genetic material, that may be a single-stranded RNA or a double stranded RNA.
Which type of virus has a genome that is also an mRNA?
Class IV viruses have a positive single-stranded RNA genome, the genome itself acting as mRNA (messenger RNA. Class V viruses have a negative single-stranded RNA genome used as a template for mRNA synthesis.
Positive-sense viral RNA is similar to mRNA and thus can be immediately translated by the host cell.