Viruses

Viruses

"Obligate intracellular parasite" - the virus cannot reproduce outside of their host cell.
"Virion" - the inert state the virus is in outside of the host. Virions can survive for a long time in this inert state but cannot reproduce. A transport form only.
Viral Classification
1. By genome. -SS (single stranded) or DS (double stranded). -Linear or circular. -RNA or DNA.
2. By host. Bacteria, plant, animal, other eukaryotes.
DNA vs RNA
1. DNA is more stable and does not mutate as quickly. It can be SS-DNA or DS-DNA. Eg. smallpox.
2. RNA is less stable and mutates more rapidly. It can be SS-RNA or DS-RNA. e.g. HIV.
3. A benefit of less mutations is that most mutations are harmful to itself. A benefit of more mutations means the virus may be able to 'outrun' the hosts immune system.
Sub-viral entities
1. Viroids are RNA viruses minus the protein coat. They only infect plants.
2. Prions are misfolded proteins. The prion enters the cell and inserts itself into healthy proteins, causing them to also misform. They are the cause to Creutzfeidt-Jacob disease.
How viruses can cause cancer
1. Oncogene: a gene which has the potential to cause cancer. Protoncogene: a normal gene which can become an oncogene after a mutation.
2. Normally a repressor molecule expressed by a repressor gene represses the protooncogene.
3. First hit: A virus inserts its DNA into the protooncogene. This acts a mutation of the gene which turns it into a oncogene. However the gene is still being repressed by a repressor molecule so does not cause cancer.
4. Second hit: A virus inserts its DNA into the region of DNA coding for the repressor molecule. The repressor molecule is no longer produced, and expression of the oncogene is no longer inhibited. The protein expressed leads to uncontrolled cell division which leads to cancer.
Methods of inserting viral nucleic acid into a cell.
1. 1. Direct penetration. The virus binds to specific proteins on the cell surface and injects it nucleic acid into the cell.
2. 2. Membrane fusion. The virus binds to specific proteins on the cell surface and the virus's envelope and cell's membrane fuse. This leaves the nucleic acid on the inside of the membrane so it can enter the cell.
3. 3. Endocytosis. The virus binds to specific proteins of the cell's membrane. The cell membrane then encapsulates the whole virus and brings it into the cell by endocytosis. Once inside the cell the capsid is removed by 'uncoating'.
Size: 30nm-400nm.
Viral Synthesis.
1. DS-DNA. The capsomere proteins are made in the cytoplasm then translocated to the nucleus for viral assembly.
2. DNA viruses enter the nucleus whilst RNA viruses are replicated in the cytoplasm.
3. SS-DNA. The DNA enters the nucleus of the host cell. Here a new complementary copy of the strand is synthesised. The 2 strands join and are transcribed to mRNA to form viral proteins. A new ssDNA strand is also synthesised.
4. Positive sense ssRNA. Acts directly as mRNA.
5. Negative sense ssRNA. RNA transcriptase transcribes tRNA from the negative RNA, which is then translates by the ribosomes.
Lytic and lysogenic cycles
1. The lytic cycle. Short cycle, approx 25 minutes.
2. Lysogenic cycle. The viral DNA is encorporated into the cells DNA. An integration enzyme cuts both the cell's and the bacterium's DNA, then rejoins ends. The DNA stays in the genome, not being synthesised, until an activation event (e.g. suppressed immune system) when it starts being synthesised.
Retroviruses use their own reverse transcriptase protein to change its RNA into DNA.

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	Created by Siobhan Martin about 11 years ago