Write an essay on the structure and replication of retroviruses.
A retrovirus is a virus that has its genes encoded in RNA instead of them being encoded in DNA (it is also known as the RNA virus). Because of this, a retrovirus has to have its genes reverse transcribed into DNA before replication. This is carried out by an enzyme called reverse transcriptase. The retro virus upon infecting the host with its DNA can use the host cell’s machinery to replicate. The host cell cannot tell the retrovirus’s DNA from its own and so copies its genes as part of the cells own replication process. The structure of retroviruses allows them to fuse with their host cell ensuring effective invasion of the host for viral replication. Structure:
Retroviruses vary in size from about 100nm to 120nm. They are enveloped particles as they are surrounded by a lipid envelope (viral envelope) derived from a host cell’s membrane. This envelope is studded (as in the case of the Human Immunodeficiency Virus or H.I.V.) or spiked (as in the case of the Mouse Mammary Tumor Virus or M.M.T.V.) with surface glycoproteins that project out from it. Transmembrane glycoproteins also project through the envelope next to the surface glycoproteins to hold them in place. The surface glycoproteins recognise and bind to receptor molecules on the host cell. Inside the viral envelope, the virus’s genetic material is surrounded by a protein coat (capsid). This genetic material contains at least 3 genes: gag – codes for core proteins, pol codes for reverse transcriptase and env codes for a viral envelope protein. These viral genes are encoded in single stranded RNA. The enzyme reverse transcriptase (derived from the pol gene) is present to reverse transcribe the RNA into DNA. Retroviruses have been split into 4 groups based on their structure: * A Type – Immature intracellular particles. They are believed o be derived from endogenous retro virus like forms for example: the immature form of M.M.T.V....
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