A2.3 Viruses Flashcards
Structural features common to viruses
Size: 20-300nm
No growth: fixed size.
Nucleic acid: can be DNA or RNA.
Protein Capsid: symmetrical protein units on outside.
No cytoplasm and few enzymes (rely on host to do everything).
All viruses have two common parts: nucleic acid inside, capsid outside.
How is diversity shown in structure of Viruses
Genetic material: DNA or RNA; they use it in different ways.
Envelope: Enveloped vs non-enveloped; envelope is cell membrane that a virus has burst through.
How do viruses infect a host?
Injecting its genetic material into host and then either go through:
Lysogenic cycle
OR
Lytic cycle.
Describe the process of the Lytic cycle
Virus attaches to protein in outer membrane of host - attachment depends on many factors (E.g. protein compatibility).
Virus injects viral DNA.
DNA replicates making many copies.
Transcription occurs: DNA –> mRNA
Translation occurs: mRNA –> proteins
Now there are many viruses: genetic material with protein capsid.
Lysis: new viruses accumulated and then burst out of cell; kills cell.
New host cell required.
If a virus is too virulent: there will be not enough hosts.
If a virus is not virulent enough: immune system will kill virus.
Describe the Lysogenic cycle
Virus attaches to host - attachment depends on many factors (E.g. protein compatibility).
Virus inject genetic material.
Viral DNA becomes integrated with host DNA.
This new DNA will be replicated by cell division.
Does not kill host.
Undetectable.
Certain stimuli can cause virus to switch to lytic cycle.
Evidence for there being several origins of viruses from other organisms.
Diversity in genetic material and structure. Convergent evolution?
Had to evolve after cell because viruses need cellular host.
Viruses evolved from cells.
Two hypothesis on how:
Regressive: viruses evolved from loss of cell components.
Progressive: viruses evolved from modified cell components.
Why do viruses evolve rapidly?
Short generation time.
Lots of mutations which leads to variations, particularly for RNA viruses.
Natural selection: favors viruses with genetic traits that help it evade detection or destruction.
Explain how the Influenza virus
An RNA virus: RNA mutates very quickly.
has 8 separate RNA molecules meaning high mutation rates and several variants forming.
Transmission between species can produce new strains.
Antigens can change and form new combination that result in new strains.
