IB Biology HL A2.3 Viruses

Why are viruses non-cellular?

Viruses rely on a host cell for energy supply, nutrition, protein synthesis, and other functions of life.

What are 5 features of viruses

1. Small-size: Viruses are smaller than hosts so they can enter, they also lack cytoplasm and organelles

2. Fixed-size: viruses don't grow and are full sized when assembled in host cell

3. Nucleic acid as genetic material: Genetic code is shared, they use DNA or RNA so the hosts synthesizes proteins in the virus' genes

4. Capsid: Protein coat that protects genetic material

5. NO cytoplasm and no/very few enzymes: viruse relies on host metabolism.

Viruses are ____ ____ in shape and structure and ___ genes occur in all viruses leading scientists to believe that viruses have ______ evolutionary origins. (showed up on their own)

highly diverse, no, multiple

What are 2 diversity structures in viruses?

1. Diversity of Genetic material: Viruses have genes made of DNA or RNA with variations of shape and how genetic material is replicated

2. Enveloped and Non-enveloped: Viruses can be enveloped or covered in host cell membrane during lysis which allows it to hide and be sneaky. Non-enveloped viruses don't get covered.

What's are the two ways viruses replicate?

LYSOGENIC and LYTIC CYCLES!

Explain the lytic cycle.

1. Virus attaches to the cell

2. Injects viral DNA into the cell

3. DNA gets replicated around 100 times

4. DNA gets transcripted to make viral proteins

5. Protein synthesis, virus makes its own copies in the cell

6. Viral proteins make holes through wall and membranes

7. LYSIS - ccell contents burst with hundreds of viruses which spread to infect other cells.

What does virulent mean?

Virulent cells are ones that destroy the host during the lytic cycle. The lytic cycle allows them to spread from cell to cell and increase the severity of the disease.

How do animal cells fight off viruses?

Humans produce many antibodies that destroy all copies of a virus entering the body. But, if a viral infection is uncontrolled it can become life threatening.

What are the disadvantages of virulence/being virulant?

Viruses can be detected and destroyed by host cells or it can lose its host by killing it (lysis). Viruses must have a host to survive. If they kill a population of bacteria they risk dying out as well.

Explain the lysogenic cycle.

1. Viruses attaches to cell

2. Virus injects DNA

3. Viral DNA becomes circular and exist as a length of DNA called prophage

4. When host replicates DNA it also replicates the prophage. Daughter cells will have the viral prophage.

What are some features of the lysogenic cycle?

Virus infection in the lysogenic cycle is "temperate" and hosts aren't killed. It causes minimal harm and the virus spreads through reproduction/can't infect uninfected cells. However, the temperate virus can change to the lytic cycle in response to stimuli/environment inside or outside the cell. Temperate viruses can also benefit the host because DNA might include genes from a previous host increasing genetic diversity.

What is obligate parasitism?

Must have a host to survive. Because of this the structural features they have in common are regarded as convergent evolution, where similar structures evolved independently of each other to adapt to similar needs.

What are 3 reasons why viruses evolve rapidly?

1. Evolutionary changes happens from generation to generation. Human generation time is average 25 years but with viruses it can be less than 1 hour.

2. Evolution depends on genetic variation from mutation. Mutation rates are high among viruses because they don't have enzymes to correct or check for errors in genetic material.

3. Evolution is the result of natural selection. Intensity is high because host is trying to destroy the viruses. STRONGER.

One example of rapid evolution virus

INFLUENZA:

a. replicates using an enzyme that doesn't proofread leading to mutations

b. instead of one RNA molecule, it has 8 separate molecules. a new strain can appear if host cell is invaded by two different strains, and RNA molecule from each strain are combined.

c. transmitted between species (birds and humans)

d. two proteins in enveloping membrane can be put together in new combinations creating novel strands.

This is why we need a new virus every year and why a person can contract influenza repeatedly. 5 best guesses

Second example of rapid evolution virus

HIV:

a. reverse transcriptase doesn't proofread leading to mutations

b. mutations are also caused by cytidine deaminase an enzyme that converts cytosine to uracil

This causes HIV to have the highest known mutation rate of any virus. Rapid generation of new strains causes the virus to invade making HIV chronic rather than curable. Two or more drugs are also necessary because HIV can evolve to be resistant.