Chapter 18: Viruses and Bacteria
Viruses: Living or Not?
Living…
 They reproduce
 They have nucleic acid and proteins
 They can die?
Or not?
 They can only reproduce inside a host
 No Metabolism (they don’t eat/respire)
 They can’t make their own proteins
Viruses are made of protein shell and nucleic
acids. Usually only a couple of genes. Much
smaller than bacteria.
How Viruses Reproduce:
1. Viruses invade a host cell. Most viruses invade a particular host. Some can jump hosts (zoonosis).
Side note: where did viruses come from?
 Outer space, meteor…
 Renegade piece of DNA, that broke free from our genome. Jumping gene.
2.
Once they invade, they look for a cell, and inject their nucleic acid.
 If RNA, it can be translated directly into viral proteins.
 If it’s DNA it can incorporate into our nucleus and be translated.
 If it’s retroRNA, the RNA will be reverse transcribed into DNA (HIV)
3.
Two main life cycles:
 Lytic: More immediate, infection leads to the reproduction of new viruses right away.
 Lysogenic: Slower to affect. Infection, then incubation, DNA is copied into the nucleus, and waits before
turning into new viruses.
Vaccines: There are no cures for viruses. But there is prevention (vaccines).
 Edward Jenner: Small Pox, is related cow pox. Took a cow pox and weakened it, and injected into humans, our body
made the antibodies against it.
 Other treatments that slow down viruses. In HIV there are drugs that prevent reverse transcriptase from working.
Some viruses can cause cancer, by inserting genes that tell the cell to reproduce.
Prions – proteins that are really tough. Causes mad cow disease. They affect nerves.
Why are new viruses showing up?
 They were isolated in jungles, and now there are roads and planes.
 There are always mutations, so can jump hosts.
Avian Bird Flu…a case study.
Notes 18 Continued…
Genetics of Bacteria
Bacteria don’t have chromosomes… just a nucleoid (clump of DNA) inside. Also have plasmids, which are small rings of
DNA (1-2 genes).
Ways Bacteria can change their DNA:
Transformation: When one bacteria gets a plasmid from another. Usually during times of stress. They release plasmids into
the solution, other bacteria suck the extra plasmids. The plasmid will then be then expressed by the new bacteria… which may
help it survive the stress.
Transduction: When a bacteria will pick new DNA carried by a virus (by mistake). A way of delivering new genes using
viruses is basically transduction.
Conjugation: a.k.a. bacteria sex. A bacteria will form a bridge with another bacteria (made out of cell membrane). They can
hold to each other using, hooks called pilli. DNA will be exchanged, then they let go.
Operons are ways to control a gene in bacteria. Sometimes you want to turn a gene on or off. You don’t want to make proteins
that you don’t need, b/c that would be a waste of energy and resources.
Tryptophan Operon (trp):
If there is trp, then repressor is active = operator blocked = no transcription = no enzyme (that makes trp) made.
If there’s no trp, then repressor is inactive = operator is not blocked = transcription happens and enzyme is made (to make
tryptophan).
Lac Operon: Genes for proteins (lactase) that break down lactose (sugar).
If there is lactose, then the repressor is inactive, so the gene will be transcribed and the proteins (that break down lactose
lactase) will be made.
If there isn’t lactose, then the repressor is active, so the gene will not be transcribed and the proteins won’t be made. (do you
know why?)