Microbial Genetics (Micr340) Lecture 2 Chromosome Structure, Replication and Segregation (II)

Replication of Bacterial Chromosome and Cell Division

Bacterial Chromosome The DNA molecule of a bacterium that carries most of its normal genes is commonly referred to as its Chromosome Most bacterial chromosomes are circular

Chromosome Replication Replication of circular chromosome initiates at a unique site - origin of chromosomal replication = oriC Replication proceeds in both directions around the circle The two replication forks proceed around the circle until they meet and terminate replication

Chromosome Replication Concepts of cis-acting vs trans-acting oriC is a cis-acting element Many proteins are trans-acting functions, capable of acting on any DNA in the same cell DnaA required only for initiation DnaB and DnaC both required for primer synthesis after DNA replication begins

Replication Initiation Fig. 1.17

Replication Initiation Fig. 1.17

Termination of Replication Termination occurs in a region called ter ter consists of clusters of sites called ter sequences of 22 bp long These sites serve as one-way gates allowing replication forks to pass through in one direction but not in the other

Termination of Replication

Segregation of 2 daughter DNAs Easy if no recombination occurs after replication Problematic if Chromosomes dimerize Solutions: General Recombination to form 2 DNAs Site-specific recombination system involving XerC and XerD and dif site Another problem: chromosomal decatenation Solutions: Topoisomerases condensation

Chromosome partitioning Each daughter cell must get one chromosome - partitioning Par proteins: ParA, ParB proteins and parS sites

Chromosome partitioning

Cell Division and DNA replication Coordination is needed Length of time for DNA replication is constant, regardless of growth rate In fast growing cells, new round of DNA replication is initiated prior to the conclusion of the old cell cycle Faster growing cells contain higher DNA content

Timing of DNA replication during cell cycle

DNA supercoiling Bacterial nucleoid Supercoiling in the nucleoid Positively supercoiled vs negatively supercoiled Supercoiling stress Topoisomerases

Supercoiling

Topoisomerases

Antibiotics affecting replication Mitomycin crosslinks guanine bases in DNA Fluoroquinolones, GyrA inhibitor Novobiocin, GyrB inhibitor DNA gyrase is made of GyrA and GyrB subunits