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Microbial Genetics (Micr340)

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

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Microbial Genetics (Micr340)

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

  2. Replication of Bacterial Chromosome and Cell Division

  3. 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

  4. 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

  5. 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

  6. Replication Initiation Fig. 1.17

  7. Replication Initiation Fig. 1.17

  8. 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

  9. Termination of Replication

  10. 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

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

  12. Chromosomepartitioning

  13. 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

  14. Timing of DNA replication during cell cycle

  15. Timing of DNA replication during cell cycle

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

  17. Supercoiling

  18. Supercoiling

  19. Topoisomerases

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

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