Virus: Shapes and structure, life cycle
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Introduction
Borrowed life Viruses lack metabolic enzymes and equipment for making proteins, such as ribosomes. They can reproduce only within a host cell.
Structure:
The size of an ordinary virus usually varies from 10 to 300 nm and it is composed of a viral genome and a capsid. Sometimes there is also an enveloppe. More complex viruses also occur, with two protein shells for example.
The enveloppe is a lipid membrane.
The viral genome
The viral genome of a virus is a nucleic acid, DNA or RNA. It can be double-stranded (ds) or single-stranded(ss), and it might have a sense. The sense is a property of some nucleic acids, the ssRNA. It traduces the complementarity of the ssRNA to the mRNA. A positive sense (plus strand) means that the RNA is identical to the mRNA, and it can therefore be translated at once. A negative sense (minus strand) means that the RNA is complementary to mRNA and therefore needs to be converted to mRNA before it can be translated.
The capsid
The capsid of the virus is a protein shell. It consists of several identical subunits called capsomers, which are made of proteins encoded by the viral genome.
The enveloppe
More complex structures
Shape:
The arguments that rule for the packing of the virus are genetic economy, lowest possible energy (=strength) and simplicity. Genetic economy: The proteins that constitute the subunits are encoded by the viral genome. Therefore many different subunits means more genetic information to carry for the virus. Therefore the subunits of a capsid are identical. The capsid will seek the package with lowest possible energy. As the proteins of a capsid aren’t symmetric, the lowest possible energy arrangement for non-symmetric particles is symmetric. The capsids are therefore helical, isocahedral or both.
The helical capsid is made of only one type of capsomers ordered helically around a sentral axis. The genome (ssDNA or ssRNA) is bound to the proteinhelix by the interactions between the negative charge on the nucleic acid and the positive charge on the protein.
The second package is icosahedral. A regular icosahedron has 20 equilateral triangles as faces.
Head-tail: An icosahedral head containing the viral genome and a helical tail.
Life cycle:
General features of viral reproduction cycles: The infection of a virus is composed of six basic stages, even thought there are a lot of variations between species. • Attachement. The first step in the life cycle of a virus. This is a binding between proteins on the viral capsid and the specific receptors of the host cell. Which bindings take place defines the host range of the virus. • Penetration. The entire virus, or only the viral gemone (depends on the structure of the virus) enters the host cell. • Uncoating: The viral capsid is degraded by enzymes in the host cell. • Replication: Synthesis of viral mRNA (exept for positive sense RNA), viral proteins and viral genome replication. • Assembly • Releasing by lysis or budding.
One of the variations from this simplified description is the reproductive cycle of phages, bacterial viruses. The viral genome can reproduce by two alternative mechanisms: the lytic cycle and the lysogenic cycle.
DNA-viruses: The replication of the genome takes place in the host cell’s nucleus. RNA viruses: Their genome replication takes place in the cytoplasm. Retroviruses: Retroviruses are RNA viruses that replicate by using a viral reverse transcriptase enzyme to transcribe its RNA into DNA in the host cell. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus then replicates as part of the host cell's DNA.
Virus classification:
Virus classification means naming and placing viruses into a taxonomic system. There is still an ongoing debate on virus classification, but the most well-known and recognized one is the Baltimore classification, named after David Baltimore, a Nobel Prize-winning biologist.This classification divides viruses into seven groups, depending on their combination of nucleic acid, strandedness, sense and method of replication.
| Group | Combination | Example |
|---|---|---|
| I | dsDNA | Herpesvirus |
| II | ssDNA | Parvovirus |
| III | dsRNA | Reovirus |
| IV | (+)ssRNA | Picornavirus |
| V | (-)ssRNA | Rhabdovirus |
| VI | ssRNA-RT | |
| VII | dsDNA-RT | Hepadnavirus |
Sources:
Wikipedia Introduction to modern Virology, by N.J.Dimmock, A.J.Easton, and K.N.Leppard Fundamentals of molecular virology, by Nicholas H. Acheson Biology, by Campbell and Reece
