Circular bacterial chromosome

A circular prokaryote chromosome is a chromosome in bacteria and archaea, in the form of a molecule of circular DNA. Unlike the linear DNA of most eukaryotes, typical prokaryote chromosomes are circular. A circular prokaryote chromosome is a chromosome in bacteria and archaea, in the form of a molecule of circular DNA. Unlike the linear DNA of most eukaryotes, typical prokaryote chromosomes are circular. Most prokaryote chromosomes contain a circular DNA molecule – there are no free ends to the DNA. Free ends would otherwise create significant challenges to cells with respect to DNA replication and stability. Cells that do contain chromosomes with DNA ends, or telomeres (most eukaryotes), have acquired elaborate mechanisms to overcome these challenges. However, a circular chromosome can provide other challenges for cells. After replication, the two progeny circular chromosomes can sometimes remain interlinked or tangled, and they must be resolved so that each cell inherits one complete copy of the chromosome during cell division. The circular bacteria chromosome replication is best understood in the well-studied bacteria Escherichia coli and Bacillus subtilis. Chromosome replication proceeds in three major stages: initiation, elongation and termination. The initiation stage starts with the ordered assembly of 'initiator' proteins at the origin region of the chromosome, called oriC. These assembly stages are regulated to ensure that chromosome replication occurs only once in each cell cycle. During the elongation phase of replication, the enzymes that were assembled at oriC during initiation proceed along each arm ('replichore') of the chromosome, in opposite directions away from the oriC, replicating the DNA to create two identical copies. This process is known as bidirectional replication. The entire assembly of molecules involved in DNA replication on each arm is called a 'replisome.' At the forefront of the replisome is a DNA helicase that unwinds the two strands of DNA, creating a moving 'replication fork'. The two unwound single strands of DNA serve as templates for DNA polymerase, which moves with the helicase (together with other proteins) to synthesise a complementary copy of each strand. In this way, two identical copies of the original DNA are created. Eventually, the two replication forks moving around the circular chromosome meet in a specific zone of the chromosome, approximately opposite oriC, called the terminus region. The elongation enzymes then disassemble, and the two 'daughter' chromosomes are resolved before cell division is completed. The E. coli bacterial replication origin, called oriC consists of DNA sequences that are recognised by the DnaA protein, which is highly conserved amongst different bacterial species. DnaA binding to the origin initiates the regulated recruitment of other enzymes and proteins that will eventually lead to the establishment of two complete replisomes for bidirectional replication. DNA sequence elements within oriC that are important for its function include DnaA boxes, a 9-mer repeat with a highly conserved consensus sequence 5' – TTATCCACA – 3', that are recognized by the DnaA protein. DnaA protein plays a crucial role in the initiation of chromosomal DNA replication. Bound to ATP, and with the assistance of bacterial histone-like proteins DnaA then unwinds an AT-rich region near the left boundary of oriC, which carries three 13-mer motifs, and opens up the double-stranded DNA for entrance of other replication proteins. This region also contains four “GATC” sequences that are recognized by DNA adenine methylase (Dam), an enzyme that modifies the adenine base when this sequence is unmethylated or hemimethylated. The methylation of adenines is important as it alters the conformation of DNA to promote strand separation, and it appears that this region of oriC has a natural tendency to unwind. DnaA then recruits the replicative helicase, DnaB, from the DnaB-DnaC complex to the unwound region to form the pre-priming complex. After DnaB translocates to the apex of each replication fork, the helicase both unwinds the parental DNA and interacts momentarily with primase. In order for DNA replication to continue, single stranded binding proteins are needed to prevent the single strands of DNA from forming secondary structures and to prevent them from re-annealing. In addition, DNA gyrase is needed to relieve the topological stress created by the action of DnaB helicase. When the replication fork moves around the circle, a structure shaped like the Greek letter theta Ө is formed. John Cairns demonstrated the theta structure of E. coli chromosomal replication in 1963, using an innovative method to visualize DNA replication. In his experiment, he radioactively labeled the chromosome by growing his cultures in a medium containing 3H-thymidine. The nucleoside base was incorporated uniformly into the bacterial chromosome. He then isolated the chromosomes by lysing the cells gently and placed them on an electron micrograph (EM) grid which he exposed to X-ray film for two months. This Experiment clearly demonstrates the theta replication model of circular bacterial chromosomes.