Understanding points
A2.3.1 Structural features common to viruses (HL only)
A2.3.2 Diversity of structure in viruses (HL only)
A2.3.3 Lytic cycle of a virus (HL only)
A2.3.4 Lysogenic cycle of a virus (HL only)
A2.3.5 Evidence for several origins of viruses from other organisms (HL only)
A2.3.6 Rapid evolution in viruses (HL only) |
Virus
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Virus: non-living agents that infect host cells to reproduce inside them
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Small and fixed size
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Contains nucleic acid as genetic material
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DNA virus: transcribes their DNA directly
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RNA virus:
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Positive-sense RNA virus: uses their RNA genes directly as mRNA
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Negative-sense RNA virus: transcribe their RNA genes to make mRNA
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Retrovirus: synthesizes dsDNA from their RNA genes and transcribe the DNA to produce mRNA
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Capsid: protein coat that encloses genetic material
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Envelope: viruses that infect animal cells are covered in host plasma membranes
Lytic cycle vs Lysogenic cycle
Origin of viruses
Progressive hypothesis | Regressive hypothesis |
Viruses are built in a series of steps by taking and modifying cell components
Evidence: virus-like components found in some cells
e.g. retrotransposons: the gene codes for enzymes that allow reverse transcription of RNA into DNA and insertion of the copy at random locations, causing gene duplication → similar to retroviruses | Viruses developed in steps as cells lost their components
Evidence: existence of large viruses
e.g. Mimivirus: complex structures and enzymes of its own
e.g. Chlamydia: parasitic bacteria that replicates inside a host |
Rapid evolution in viruses
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Influenza virus: ssRNA virus
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High rate of mutation due to the lack of proofreading function of RNA replicase
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HIV: retrovirus
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High rate of mutation due to lack of proofreading function in reverse transcriptase
