1. Viruses & Bacteria
Ch. 18

2. Ch. 18 Structure Genome – double or single stranded DNA or RNA
Capsid – protein shell
Viral envelopes – in some animal viruses, help it to infect derived partly from animal cell membrane

3. Viral reproduction All viruses are parasitic
Can reproduce only in host cell – receptors on host cell surface
Viral genome provides the blueprint, host cell provides nucleotides, animo acids, tRNA, enzymes, ATP, and ribosomes to make more viruses

4. Viral cycles – Phage reproduction
Lytic cycle – virulent phage

5. Lysogenic cycle – temperate phage

6. Bacteria will sometimes defend themselves from viruses with restriction endonucleases\
restriction endonuclease, (any of the enzymes that cut nucleic acid at specific restriction sites and produce restriction fragments; obtained from bacteria (where they cripple viral invaders); used in recombinant DNA technology)
Animal viruses
Often coated with membrane which is incorporated into host membranes as they enter
Some, like herpes, may enter nucleus & stay there, emerging only at times of stress (fever blisters)

7. Retroviruses have the enzyme reverse transcriptase – the DNA that is made becomes part of the host DNA – ex. HIV
Vaccination – innoculation with harmless variant of virus that stimulates immune system to produce antibodies
RNA viruses mutate quickly since there is no replication step in which mistakes might get caught
Prions- proteins that cause disease – mis-folded proteins especially in brain cells – ex. Mad cow

8. Bacteria
Structure
DNA is circular & naked – 500 times longer than the cell
Nucleoid - the area it occupies
Plasmid is an additional, smaller circle of DNA
Replication from single origin followed by binary fission
E. coli replicates every 20 min., asexual, but mutations occur about 1 in 10 million cells – 9 million per day per human
These mutations are the raw material on which natural selection works and are significant in rapidly dividing bacteria

9. Genetic Recombination
Transformation – uptake of naked foreign DNA from the environment
Some bacteria are more likely to do this than others
Ca++ and heat shock make bacteria more likely to transform
Transduction – phages carry DNA from one host to another

10. Conjugation – direct transfer of DNA from bacteria that are temporarily joined
Donor cell produces pilus
Pilus attaches to recipient cell and brings the two cells together.
The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cells.
Both cells synthesize a complementary strand to produce a double stranded circular plasmid and also reproduce pili; both cells are now viable donors.

11. Transposons – piece of DNA that can move from one place to another
“jumping genes” – they may simply move or copy and move
This may alter other genes or may add genes to plasmids

12. Control of Genes - Bacterial
Operons – stretch of DNA necessary to code for the enzyme production for a pathway involving several genes – only in prokaryotes
Trp operon – Tryptophan is manufactured from a precursor in 5 steps involving 5 enzymes. The enzymes are all needed together.
Promotor Structural genes
Operator 1 2 3 4 5

13. Parts of trp Operon
Promotor – DNA segment where RNA polymerase attaches
Operator – switch that can be blocked or open to allow transcription
Repressor – protein that attaches to operator & blocks transcription – specific to each operator
Co-repressor – tryptophan attaches to the repressor to activate it.
The presence of tryptophan activates repressor & turns off the operator – no transcription, no enzymes
Lack of tryptophan will inactivate repressor, operator is free – transcription leads to enzymes being made to make tryptophan
Negative feedback – trp can turn off transcription or inactivate an enzyme

14. Lac operon
Genes code for enzymes to breakdown lactose. These enzymes are needed when lactose is present and glucose is absent.
Lactose is an inducer which will inactivate the repressor and allow transcription. Presence of lactose induces transcription of genes for enzymes to break down glucose
When lactose is broken down, the lactose is not on the repressor, so it becomes active and turns off transcription.
When glucose levels are low, the cell will increase its cyclic AMP (cAMP). cAMP binds to cAMP receptor protein and together they bind to the DNA right in front of the promotor to bend the DNA so RNA polymerase more readily attaches.
This receptor protein is an activator of transcription.
Positive Feedback

15. Chapter 19 Eukaryotic chromosome structure
Each human chromosome – 2 X 108 base pairs
6 cm long! – must be folded in the cell
Histones are globular protein s that DNA wraps around in its first level of packing.
A nucleosome is a “bead” formed from DNA wrapped around 8 histones
The histones leave DNA briefly during replication and stay with DNA during transcription
The DNA coils much more during mitosis.

16. Of all the DNA on chromosomes, only euchromatin is transcribed
Of all the DNA on chromosomes, only euchromatin is transcribed. The heterochromatin never fully uncoils and does not transcribe.
Much of telomere & centromere regions don’t transcribe – transposons create many copies of genes, many nonfunctional
Control of genes is similar to bacteria, but much more complex

17. Cancer
Proto-oncogene – gene that promotes cell growth – mutates in 30% of cancers
Oncogene – occurs when the proto-oncogene is mutated or amplified so it becomes hyperactive. The change is dominant genetically.
Tumor suppressor genes are supposed to inhibit cell division if they are mutated, they lose the ability to control cell division. They are recessive genetically – mutates in 50% of cancers
Usually at least half a dozen defective genes are necessary to cause a cell to become cancerous.