Bacterial chromosome circular

Question What is the form taken by a replicating bacterial chromosome (circular DNA molecule) ... 

The bacterial chromosome is a closed, double-stranded circular DNA molecule having a single origin of replication. Separation of the two parental strands of DNA creates two replication forks that move away from each other in opposite directions around the circle. Replicatioii is,... 

Re combinational DNA repair of a circular bacterial chromosome, while essential, sometimes generates deleterious byproducts. The resolution of a Holliday junction at a replication fork by a nuclease such as RuvC, followed by completion of replication, can give rise to one of two products the usual two monomeric chromosomes or a contiguous dimeric chromosome (Fig. 25-41). In the latter case, the covalently linked chromosomes cannot be segregated to daughter cells at cell division and the dividing cells become stuck. A specialized site-specific recombination system in E. coli, the XerCD system, converts the dimeric chromosomes to monomeric chromosomes so that cell division can proceed. The reaction is a site-specific deletion reaction (Fig. 25-39b). This is another example of the close coordination between DNA recombination processes and other aspects of DNA metabolism. 

Circular bacterial chromosomes are initiated by RNA primers. At some stage the RNA primers must be eliminated and replaced by DNA. Due to the circular nature of the chromosome an upstream DNA molecule can always serve as a primer for regions from which RNA primers are eventually removed. This guarantees that the primer requirement does not interfere with complete replication of the chromosome. 

DNA replication proceeds by the synthesis of one new strand on each of the parental strands. This mode of replication is called semiconservative, and it appears to be universal. DNA synthesis initiates from a primer at a unique point on a prokaryotic template such as the E. coli chromosome. From the initiation point, DNA synthesis proceeds bidirectionally on the circular bacterial chromosome. The bidirectional mode of synthesis is not followed by all chromosomes. For some chromosomes, usually small in size, replication is unidirectional. 

The DNA of a bacterial cell, such as Escherichia coli, is a circular double-stranded molecule often referred to as the bacterial chromosome. In E. coli this DNA molecule contains 4.6 million base pairs. The circular DNA is packaged into a region of the cell called the nucleoid (see Topic Al) where it is organized into 50 or so loops or domains that are bound to a central protein scaffold, attached to the cell membrane. Fig. la illustrates this organization, although only six loops are shown for clarity. Within this structure, the DNA is actually not a circular double-stranded DNA molecule such as that shown in Fig. lb but is negatively supercoiled, that is, it is twisted upon itself (Fig. lc) and is also complexed with several DNA-binding proteins, the most common of which are proteins HU, HLP-1 and H-NS. These are histone-like proteins (see below for a description of histones). 

The process of chromosomal replication in bacteria is complex. Bacterial chromosomes are double-stranded DNA and almost always circular. DNA replication starts at a specific sequence, the origin, on the chromosome and proceeds in two directions towards another specific region, the terminus, as shown in Figure 8-10a. 

Figure 10.28 Formation of negatively supercoiled circular DNA by gyrase. This type of DNA is present in bacterial chromosomes and plasmids. Positively supercoiled DNA has the opposite handedness.

Bacterial cells frequently contain additional DNA molecules called plasmids. These are relatively small (up to 200 kb) and are present in the form of circular duplexes. They can replicate independently of the bacterial chromosome and multiple copies can be present in a cell. Normally, they exist within a cell in a negatively supercoiled conformation, as does all DNA. Eukaryotic cells can also contain additional DNA to that present in the nucleus. Such extrachromosomal DNA is present in mitochondria and chloroplasts. 

A chromosome contains a single DNA molecule, which is generally very large e.g., some bacterial chromosomes are composed of as many as 4 x 106 base pairs. Furthermore, in many cases, the DNA is a closed or circular structure. Some bacterial chromosomes are linear. We will concentrate first on the topology of replication of the circular bacterial ( . coli) chromosome. Little is known about linear bacterial chromosomes in this regard. 

Question Do all circular, double-stranded DNA molecules replicate in the same manner as the bacterial chromosome ... 

Plasmids usually resemble chromosomes except that they are approximately 0.1-1.0% of the size of a bacterial chromosome, and there are a few that are linear rather than circular. Plasmid genes are not essential for the normal functioning of the cell but may code for a property that affords a survival advantage in certain environmental conditions bacteria possessing the plasmid in question would therefore be selected when such conditions exist. Properties which can be coded by plasmids include the ability to utilize unusual sugars or food sources, toxin production, production of pili that facilitate the attachment of a cell to a substrate (e.g. intestinal epithelium) and antibiotic resistance. A cell may contain multiple copies of any one plasmid and may contain two or more different plasmids. However, some plasmid combinations cannot co-exist inside the same cell and are said to be incompatible this phenomenon enables plasmids to be classified into incompatibility groups. 

The bacterial chromosome exists as a singular, covalently closed circular molecule of double-stranded DNA comprising approximately 4600 kilobase pairs. It is complexed with small amounts of proteins and RNA, but unlike eukaryotic DNA,... 

---