Chapter 18

• Virus genetics– Scientists learned about viruses by studying

the Tobacco Mosaic virus in plants.» Viruses are small, some smaller than a

ribosome. (20 nm)

• Viral genome– double-stranded DNA– Single-stranded DNA– double-stranded RNA– single-stranded RNA– The smallest virus has 4 genes

• Capsids and envelopes– The protein shell of a virus is the capsid

Built from capsomeres• Membranes around the capsid are called

envelopes.– These are made from the host’s phospholipids

and proteins

• Bacteria viruses are called bacteriophages

– Many have been found that infect E.coli– Some viruses carry enzymes in their capsids

• Virus reproduction- Obligate intracellular parasites

– Each virus has a specific host range– They recognize their host by receptor sites on

the host cell» Swine flu has a broad host range while the

cold virus has a specific range of only the respiratory tract cells.

– Viruses hijack a cell and use it’s DNA and enzymes to reproduce more viruses.

• There are 3 patterns of viral replication– DNA -> DNA= uses _DNA polymerase_ from

the host cell– RNA -> RNA = Virus will have a gene for RNA

Polymerase – RNA -> DNA-> RNA = Virus will have a gene

for Reverse Transcriptase(HIV)– The virus will use the host’s Enzymes, tRNA,

ribosomes etc.

• Lytic cycle– Virulent- viruses that infect the host cells and

results in death of the cell– Phage attaches to surface of cell– Phage inserts DNA– Hydrolytic enzymes destroy the host DNA and the

host begins replicating and replicates viral genome.– Genome directs the synthesis of viral components.

It uses the nucleotides from the chopped up host DNA to make more viral DNA.

– Cell lyses and releases new viruses– If the cell has mutant receptor proteins or

restriction endonucleases it might save itself. (natural selection)

• Lysogenic cycle- Coexist with their host. The genome is incorporated into the cell’s DNA

– Temperate viruses- lie in wait- wait for right opportunity-can use lytic or lysogenic cycle.

– Phage attaches to the surface of cell– Phage inserts DNA into cell– DNA inserts by genetic recombination.

• Important animal viruses– Viruses like Polio, rubella, and measles are

prevented by vaccines, which are variants or derivatives of the virus that stimulate the immune system.

• Emerging viruses seem to jump onto the scene, Ebola and HIV are examples

– 1. SARS is another example made of single stranded RNA

– 2. The bird flu is the next potential outbreak.

• Some viruses have been associated with cancer, like Hepatitis B and liver cancer and Papilloma virus cervical cancer.

• Oncogenes –viral genes in human genome- associated with cancer

• D. Some viruses like Herpes stay hidden as minichromosomes in nerve cells and can reappear and cause sores.

• E.Prions are infectious proteins that spread disease.

– 1. They appear to cause mad cow disease and Creutzfeldt- Jacob in humans.

– 2. They are probably transmitted by food and can incubate for 10 years.

• They are virtually indistructable

• This is the herpes virus on the surface of a cell

• F.HIV is a retrovirus which contains reverse transcriptase which can make DNA from RNA

• The genetics of bacteria- – Short life span allows for adaptations– Made of one circular DNA– Plasmid- Small circular ring of DNA that carries

extra genetic material.– Reproduce by binary fission after the DNA is

replicated• Genetic recombination in Bacteria

– Transformation= gene recombination from foreign DNA

» Progeny will have foreign gene» E. coli can be induced to pick up new genes

• Transduction = Phages transfer genes from one host to another

• Plasmids- There is a lot of info here, you just need to know what they are because we will be using them in our lab

– R- plasmid carry genes for antibiotic resistance.

• Operons – How bacterial genes are turned on and off.

– Operon- a group of genes that are related» Contain a promoter where RNA polymerase will

bind.» Produce messenger RNA that is RNA with

several genes coded for on it.» This is a good thing because it saves the cell

energy by only making what it needs.» Operator = Located within the promoter region

it allows polymerase to bind or not.» The operator is always on unless a protein

repressor switches it off.