Viruses have been described as “organisms at the edge of life”, since they resemble living organisms, they possess genes, evolve by natural selection, and reproduce by creating multiple copies of themselves through self-assembly.
Although they have genes, they do not have a cellular structure, do not have their own metabolism, and require a host cell to make new products.
Viruses are found wherever there is life and have probably existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, molecular techniques are used to investigate how they developed.
There are three main hypotheses to explain the origins of viruses:
For this theory viruses could be small cells that parasitized larger cells, genes not required by their parasitism were lost with time.
Cellular origin hypothesis
The cellular hypothesis suggests that some viruses may have evolved from bits of DNA or RNA that “escaped” from the genes of a larger organism. The escaped DNA could have come from plasmids or transposons.
This theory proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time that cells first appeared on Earth and would have been dependent on cellular life for billions of years.
Viroids are molecules of RNA that are not classified as viruses because they lack a protein coat. They have characteristics that are common to several viruses and are often called subviral agents. Viroid are important pathogens of plants. They do not code for proteins but interact with the host cell and use the host genetic material for their replication.
The Hepatitis delta virus of humans has an RNA genome similar to viroid but has a protein coat derived from hepatitis B virus and cannot produce one of its own. It is, therefore, a defective virus. These viruses, which are dependent on the presence of other virus species in the host cell, are called satellites and may represent evolutionary intermediates of viroid and viruses.
The evidence of an ancestral world of RNA cells and computer analysis of viral and host DNA sequences are giving a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses.
Structure and Morphology
A complete virus particle, o virion, consists of nucleic acid surrounded by a protective coat of protein called capsid. These are formed from identical protein subunits called capsomeres. Viruses can have a lipid “envelope” derived from the host cell membrane. The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction.
Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid.
Based on the shape of the capsomer there are different types:
Helical, as for the tobacco mosaic virus, Icosahedral as for the rotavirus, Prolate, as for the head of bacteriophages.
Some species of virus envelop themselves in a modified form of one of the cell membranes with an outer lipid bilayer known as a viral envelope, the lipid membrane itself originate entirely from the host. The influenza virus and HIV use this strategy. Most enveloped viruses are dependent on the envelope for their infectivity.
Complex viruses possess a capsid that is neither purely helical nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall. Some bacteriophages have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibers. This tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell.
Viruses contain more structural genomic diversity than other living organisms and microorganisms. There are millions of different types of viruses.
A virus has either a DNA or an RNA genome and is called DNA virus or RNA virus, respectively. 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. In RNA viruses and certain DNA viruses, the genome is often divided into separate parts, and is called segmented. For RNA viruses, each segment often codes for only one protein and they are usually found together in one capsid. All segments are not required to be in the same virion for the virus to be infectious.
A viral genome, irrespective of nucleic acid type, is almost always either single-stranded or double-stranded. Single-stranded genomes consist of an unpaired nucleic acid, double-stranded genomes consist of two complementary paired nucleic acids. The virus particles of some virus families contain a genome that is partially double-stranded and partially single-stranded.
For most viruses with RNA genomes and some with single-stranded DNA genomes, the single strands are said to be either positive sense or negative sense, depending on if they are complementary to the viral RNA messenger, m-RNA.
Negative-sense viral RNA is complementary to m-RNA and thus must be converted to positive-sense RNA by an RNA-dependent-RNA-polymerase before translation.
In general, RNA viruses have smaller genome sizes than DNA viruses because of a higher error-rate when replicating and have a maximum upper size limit, errors during replication make the virus useless or uncompetitive. To compensate, RNA viruses often have segmented genomes reducing this way the chance that an error in a single-component genome will incapacitate the entire genome.
In contrast, DNA viruses generally have larger genomes because of the high fidelity of their replication enzymes. Single-strand DNA viruses are an exception to this rule, as mutation rates for these genomes can approach the extreme of the ss-RNA virus case.
Antigenic shift, or reassortment, can result in novel and highly pathogenic strains of human flu. Viruses undergo genetic change by several mechanisms. These include a process called antigenic shift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are “silent”—they do not change the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs.
Antigenic Shift occurs when there is a major change in the genome of the virus. This can be a result of recombination or reassortment. When this happens with influenza viruses, pandemic might result. RNA viruses often exist as swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such species are a prime target for natural selection.
Segmented genomes confer evolutionary advantages; different strains of a virus with a segmented genome can shuffle and combine genes and produce progeny viruses or offspring that have unique characteristics. This is called reassortment or ‘viral sex’.
Genetic Recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. This can occur when viruses infect cells simultaneously. Recombination is common to both RNA and DNA viruses.
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Source: Wikipedia: Virus