Microbial Genetics (Micr340) Lecture 1 Chromosome Structure, Replication and Segregation (I)

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