October 29, 2008 continued from Oct. 12 handout

3. Steps of DNA replication
a. oriC site (origin of chromosome replication): Where DNA replication begins on the bacterial chromosome
i. uncoil DNA using enzymes (DNA gyrase)
ii. have to unzip strands of DNA where it’s going to be copied
iii. the oriC site has a lot of A & T (bonds are easier to break) and no C & G
1. DnaA (enzyme) binds to bases at ori C on bacteria chromosome
2. Helicases (enzyme) binds to DnaA-oriC complex (creates a replication bubble)
3. Helicase starts unwinding DNA at oriC and begins to unzip the DNA strands
b. Single strand binding proteins attach to the replication bubble to unzipped DNA at the replication bubble
c. DNA polymerase attaches to DNA primer (small swatch of DNA, complementary to few bases that helps start DNA replication)
1. degrades quickly
2. moves in the 5’ 3 direction
d. The helicase moves ahead of DNA polymerase, unwinding & unzipping DNA as it goes
e. One strand(leading strand) is synthesized continuously (one long strand with no breaks) the other strand (lagging strand) is synthesized discontinuously
i. Okazaki fragments-the DNA that is synthesized discontinuously
ii. The fragments are patched together by DNA ligase
f. DNA polymerase falls off chromosome at terminus site
i. Because the bacterial chromosomes are circular, after replication chromosomes are interlocked
ii. Topoisomerase 4 cuts both strands of 1 chromosome and de-interlocks circles & the patches of the broken chromosome go back together so you have 2 separate chromosomes
g. DNA methylation-Methyl groups are added to some bases in the DNA
b. Mutation: change in DNA sequence; effect of mutation usually neutral but can be good (E. coli Resistance to T-4) or bad
i. Size of bacterial genomes
1. Mycoplasma genitalum (must live in host cell)~600,000 base pairs in genome
2. T. pallidum (cause syphilis; have to live in host cell)~1 million base pairs in genome
3. S. aureus (live on skin)~3million base pairs in genome
4. E.coli (survive in all types of environments)~5million base pairs in genome
5. B. anthracis (lives in the soil)~6million base pairs in genome
6. Viruses (need host cell) have smaller genome than Mycoplasma genitalum
7. Mitochondria also have very small genomes
ii. The more reliant an organism is on a host, the smaller the genome is going to be
1. Ex: viruses use host cell enzymes to copy viral DNA so they don’t need to have own genes for replication enzymes

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