The Genetics of Viruses


- In the late 1800’s researchers found an infectious agent a lot smaller than a bacterium which was later identified as a virus
- A genome could be a single or double stranded DNA or single stranded RNAvirus.gif
- A virus is a genome enclosed in a protective coat
- They are small nucleic acid genomes protected by a protein capsid and sometimes a membrane envelope
- Viruses can only reproduce within a host cell
- They use enzymes, ribosomes, and small molecules/cells to synthesize viral DNA
- Phages are bacterial viruses that reproduce either using the lytic or lysogenic cycle
- In the lytic cycle a phage genome is injected into a bacterium and programs the destruction of host DNA, production of new phages, and digestion of the host’s cell wall in order to release the phages after they’re made
- The phage sticks to receptor sites on the outer suface of the cell and inserts its DNA into the host. The cell's DNA is hydrolyzed then the phage parts come together and assemble themselves into viruses. Then an enzyme digests the cell wall so the cell bursts releasing the newly made viruses.
- In the lysogenic cycle, a temperate phage injects its genome into a bacteria chromosome making itself a prophage. The genetic material is then passed on to host daughter cells until it is directed to leave the chromosome and begin a lytic cycle.
- Animal viruses often have an envelope protecting itself that allows them to enter a host cell and exit it.
- Retroviruses are RNA viruses that synthesize DNA from an RNA template. This allows the DNA to integrate into the host genome as a provirus.
- A provirus is viral DNA that remains a permanent resident of the host cell’s genome, unlike a prophage that leaves.
- Vaccines work by stimulating the immune system to defend the host against and infection
- lytic.GIFViruses can mutate very rapidly which can make it hard for researchers to find a vaccine for certain viruses. Emerging viruses that cause new outbreaks of disease in the world exist because they learned how to expand their host territory by mutation.
- Most plant viruses contain single stranded RNA. They enter plant cells through damaged cell walls or can be inherited from a parent.
- Plant diseases can be caused by viroids which are simpler than viruses.
- Viroids are naked RNA molecules that disrupt plant growth.
- Prions can also aid in plant disease. They are infectious proteins that cause brain diseases in mammals.

The Genetics of Bacteria

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- Because bacteria have a short generation span they can adapt to changing environments more efficiently
- A bacterial chromosome is a circular DNA molecule that contains few proteins.
- Plasmids are smaller rings of DNA found in bacteria that can self-replicate
- Bacteria reproduce quickly so more mutations are possible
- Gene transfers between bacteria are transformation, transduction, and conjugation.
- Transformation is when DNA enters the cell from the surroundings.
- Transduction is when bacterial DNA is carried from one cell to another by phages.
- Conjugation is the direct transfer of genetic material between two bacterial cells that are temporarily joined. This process is the bacterial version of sex.
- In conjugation the male extends sex pili and attach to a female cell. They get closer together allowing a cytoplasmic bridge to form in between them. Throught the “bridge” the male transfers DNA to the female. In detail the male contains something called an F factor that transfers DNA to the F- cell.
- R plasmids are the genes conferring resistance that are carried by plasmids
- Transposons are DNA segments that contribute to genetic shuffling because they have the ability to insert themselves into multiple sites in a cell’s DNA.
- Bacterial transposons can affect gene function and sometimes contain extra genes that help to resist antibiotics
- Cells control metabolism by regulating enzyme activity or regulating enzyme synthesis by activating and deactivating genes.
- Regulated genes are clustered into operons. One promoter serves genes that are close by.
- The operator site can turn the operon on or off. In a repressible operon, binding of specific repressor protein to the operator can stop transcription because the RNA polymerase is blocked.
- In an inducible operon an active repressor can be inactivated by the binding of an inducer. This means that the repressor can turn on the genes of the operon at any time.
- Operons can be controlled by a stimulatory activator protein like mentioned above. This can happen when a cAMP receptor protein stimulates transcription by binding to a site next to the promoter allowing it to bind RNA polymerase.