A2.3 Viruses [HL] Content

5 structural features common to viruses

1. small, fixed size

2. DNA/RNA as genetic material

3. protein casing (Capsid). enclosing genetic material

4. x cytoplasm

5. few or x enzymes

virus size?

about 20-500 nm

why are viruses small?

can easily infect fells, few or no enzymes, x cytoplasm

example of single-stranded RNA

HIV, influenza, coronaviruses

example of double-stranded RNA

rotaviruses

example of single-stranded DNA

parvovirus

example of double-stranded DNA

bacteriophage lambda, variola virus, herpes viruses

diversity in viral structure?

shape, type of genetic material, enveloping, genome size

why enveloping? (3)

additional layer of protection to viral genome, help disguise virus from immune system of host cell, helps virus attach & enter new host cells

why some x enveloping?

may be more stable & resistant to environmental factors as —> lipid envelopes easily damaged by heat, light & chemicals

examples of enveloped viruses

HIV, influenza, coronavirus

examples of x enveloped viruses

norovirus, adenovirus, bacteriophage lambda

how do viruses (thought) demonstrate convergent evolution?

some viruses that infect different cell types/host organimss —> evolved similar mechanisms for enter/replicating within those cells / similar shape & sizes

why rapid rates of evolution in viruses?

1. many rna viruses retroviruses —> high mutation rates (copy genetic material using reverse transcriptase enzymes/RNA polymerase —> less accurate than DNA polymerase)

2. virus can exchange genetic material (e.g. horizontal gene transfer)

3. viruses have short generation times & high reproductive rate —> more opportunities for evolution to occur

summary: high mutation rates, exchange of gene. material, and short generation/high reproductive rate

rapid evolution in HIV

HIV is a retrovirus, using reverse transcriptase during reproduction (much more error-prone than DNA polymerase) —> lead to mutations, acquiring genes which can further increase its adaptability

rapid evolution in influenza

Influenza (flu) is a RNA virus.

1. = less accurate means of replicating genetic material (than DNA viruses or living org.) —> high mutation rates in surface proteins —> difficult for host immune system to identify/protect agains the virus & also why vaccines have relatively low effectiveness

2. do ‘reassortment’ (infects host cell already infect w/ different strain of influenza —> two exchange gene material —> mutation)

Why do RNA viruses (retrovrisues) have high rates of evolution?

They copy their genetic material using error-prone mechanisms such as reverse transcriptase enzymes and RNA polymerases.

viruses are…

obligate intracellular parasites, meaning they depend on the host cell for many vital function including replication, and cannot survive outside of the host cell

several possible origins of viruses (4)

ancient genetic material encapsulated, viroids, transposons, ancient cells that lost their ability to live independently

viriods

transpoons