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Microbial Genetics (Micr340)
Lecture 1Chromosome Structure,
Replication and Segregation (I)
DNA Structure
DNA structure
DNA structure model proposed in 1953 If DNA strands are chains,
deoxyribonucleotides form the links Each “link” is made of a base, a sugar
and a phosphate Phosphodiester bonds join each “link” Two DNA strands (chains) are hold
together by hydrogen bonds between bases
DNA structure
Fig 1.1
DNA structure
Bases Purine
Adenine, guanine Pyrimidine
cytosine, uracil, thymine
Sugars 2-deoxyribose (for DNA) ribose (for RNA)
DNA structure
Fig 1.2
DNA structure
Fig 1.2
DNA structure
DNA strands are antiparallel If one strand is 5’ to 3’ direction, the
other is 3’ to 5’ direction Base pairing
A and T form two hydrogen bonds G and C form three hydrogen bonds
DNA structure
Fig 1.3
DNA structure
Fig 1.3
Mechanism of DNA Replication
Mechanism of DNA replication
Deoxynucleotide polymerization Enzymes involved
DNA polymerases Nucleases DNA ligases Primases
Mechanism of DNA replication
Fig 1.6
Mechanism of DNA replication
Fig. 1.7
Mechanism of DNA replication
Semiconservative replication
Mechanism of DNA replication
Fig 1.8
Mechanism of DNA replication
Fig 1.8
Mechanism of DNA replication
Semiconservative replication DNA strands separated by
helicases DNA polymerase III catalyzes
polymerazation Non-continuous synthesis of one of
the two strands: okazaki fragments
Mechanism of DNA replication
Fig 1.9
Mechanism of DNA replication
Fig 1.9
Mechanism of DNA replication
Semiconservative replication DNA strands separated by helicases DNA polymerase III catalyzes
polymerazation Non-continuous synthesis of one of
the two strands: okazaki fragments Linkage of okazaki fragments
Mechanism of DNA replication
Fig 1.11
Replication Errors
Editing; correcting mistake as it replicates DNA. In E. coli, DNA polymerase III has a 3’
(to 5’) exonuclease activity to perform editing
Replication Errors
Replication Errors
Methyl-directed mismatch repair Takes advantage of semiconservative
replication; old strand is heavily methylated by DAM (deoxyadenosine methylase)
The system recognizes the mismatch and removes it and its surrounding DNA on the same new strand.
Replication Errors
Fig 1.15
