Bacterial Chromosome Structure: What, How And Detailed Facts

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A nucleoid, which has a well-defined cytoplasmic structure, contains bacterial chromosomes. Here, the double helix DNA is covered with proteins that are like histone.

The bacterial chromosome differs from the bacterial genome. The bacterial chromosome structure has a strong bond of protein-DNA-RNA that can differ in arrangement, DNA content, dimensions, and conditions related to growth. On the other hand, a genome is the carrier of genetic information for an organism.

Chromosomes of bacteria have clear cytoplasmic shapes that are present in nucleoid. Proteins have structures similar to histone that coat the double helix DNA in the nucleoid. Many bacteria may have chromosomes that are circular in shape, large in size, and single, but it is not the same for all bacteria. Other bacteria may also have multiple chromosomes just like Rhodobacter sphaeroides.

Rhodobacter sphaeroides has more than one chromosome, out of which one is of size 3.0 mb and another is of size 0.9 mb, whereas Burkholderia cepacia has three chromosomes and their sizes include 3.6, 3.2, and 1.1 mb, respectively. Some other species also have linear chromosomes, like the spirochete Borrelia burgdorferi and the gram-positive Streptomyces coelicolor.

Moreover, several bacteria have some extra-chromosomal components in them, like plasmids. The genome size of B. burgdorferi is higher than 0.56 mb as it is composed of approximately 0.9 mb of linear chromosomes and more or less 19 mb of linear and circular plasmids. Chromosome structure has a significant impact on chromosome replication. In contrast to eukaryotes, the initiation of replication takes place in a single location on bacterial chromosomes.

E. coli has a single chromosome with a circular shape, and its replication begins at the oriC site (origin of replication). In contrast to that of eukaryotes, the process of replication progresses in both directions in a manner similar to that of a semiconservative manner.

DNA replication is seen all over the chromosome, which is circular in shape until both of the replication forks link in the terminal ends, which makes up a hindrance to the progress of the replication fork.

bacterial chromosome structure
Image credit: Circular chromosome- Wikipedia

Are bacterial chromosomes circular or linear?

All bacteria do not have circular chromosomes. Out of several genera of bacteria, linear chromosomes are the most common, such as Borrelia, Streptomyces, and Agrobacteria. 

Few bacteria have more than one chromosome, whereas several bacteria have linear plasmids and chromosomes. When compared to the linear chromosomes present in eukaryotic cells, researchers found that the bacteria have circular chromosomes that are solitary and covalently closed.

The linear chromosomes are believed to have emerged from the circular ancestral chromosomes. To prove that the bacterial chromosome was circular, electron microscopy was used. This process was carried out in both gram-negative bacteria (such as Escherichia coli) and gram-positive bacteria (such as Bacillus subtilis). Bacterial plasmids have been discovered to be circular as well.

Chromosome replication is suggested to be a general mechanism of early fixation. The region of origin of replication has a gene organization. The genesis of replication region gene arrangement is evolutionarily consistent across several bacteria lineages (e.g., E. coli of the phylum Proteobacteria and gram-positive B. subtilis), implying that a generic mechanism for chromosome replication was fixed early.

Telomers are the last parts of the DNA molecules that are linear. They have two difficulties that do not suit the DNA molecules that are circular. Firstly, as we know that the loose ends of the double-stranded DNA are too delicate for degradation by the intracellular nucleases, there should be a procedure that can preserve the ends.

Secondly, the end portions of the DNA molecules that are linear have a particular process for the replication of DNA. These problems can be solved by the characteristics of telomers. There are two varieties of telomers that have been discovered in bacteria, namely, invertron telomers and hairpin telomers.

What does a chromosome look like in bacteria?

While bacterial chromosomes are circular in shape, human chromosomes have open ends. It means that the bacterial chromosomes are attached to each other.

Bacterial chromosomes have distinct cytoplasmic morphologies that are visible in the nucleoid. Proteins coat the double helix DNA in the nucleoid with structures similar to histone. Although many bacteria have circular chromosomes that are huge in size and single in number, this is not the case for all bacteria.

Bacteria are naturally distinct from human beings. Almost all bacteria have only a single chromosome. The reason why chromosomes can get fit in the bacterial cell is that they have folds in them. A nucleoid is where a chromosome can be found. This is more or less similar to the nucleus present in human cells, but it is not the same thing.

While the human nucleus has a membrane of its own, the nucleoid in the bacterial chromosome does not. Thus, the DNA does not break away from the cell. The DNA is enfolded around the DNA binding proteins. This is useful as it helps the chromosome fit into the cell due to the folds.

Current studies in the fields namely cell biology and microscopic methods disclosed that DNA of bacterial chromosome has folds like structure that help them occupying small space in the cell. Bacterial chromosome which is present in nucleoid is independently assembled in supercoiled loops known as domains.  

The shape of nucleoid is highly active as the arrangement of the domain permits the DNA chromosome to go through the changes of structure during various cellular processes such as segregation, replication and transcription that occurs in the cells of bacteria concurrently.

How are bacterial chromosomes arranged?

Bacterial chromosomes are structured in stereotyped configurations in daughter cells, which are consistently and vigorously recreated.

Bacterial chromosomes have spatial organization patterns that categorize under two broad classifications: where chromosome is oriented longitudinally in ori-ter pattern ad where chromosome occupy space in transverse configuration with two arms- left and right, called replichores which are present separately in cell halves in left-ori-right pattern.

The most common type of organization pattern in chromosome of bacteria includes the longitudinal positioning, also known as ori-ter organization. Here, the origin is found at or close to the pole of old cell and the terminus is located near the new cell. Between them are present the left arm and right arm that are situated next to each other. Before the evolution of bacterial cell biology, the pattern was first proposed in sporulating B. subtilis cells.

The longitudinal pattern of ori-ter has both the features of being uncomplicated and instinctive. Still, a methodical observation of E. coli that was growing slowly and experimented with long ago showed a spectacularly different orientation. At the initial stage of the replication of E. coli, the origin occupies its position in the middle of the cell, whereas the left and right arms are found to be in the cell halves separately. The terminus region has a size of nearly 300 kb and it helps in connecting the left and right arms to form a complete circle.

Therefore, the ori-ter axis is kept perpendicular to the long axis of cell that generates a transverse organization (left-ori-right (transverse) organization). As the origins get replicated, they are separated to the quarter regions of the cell. The left arm and the right arm that are replicated newly are separated to the either side so that it can regenerate the transverse pattern in the next origination or generation.

300px Origins of DNA replication Figure 1
Image credit: Origin of replication- Wikipedia

Is the bacterial chromosome structure double-stranded?

The bacterial chromosome is generally defined as a circular, solitary and double helix DNA components that is consist of nearly all genetic information of a cell.

Several bacteria contain a haploid genome. A single bacterial chromosome consists of a round, double helix DNA strands. The chromosome of bacteria is a genetic element which is a round DNA molecule that has the capability to self replicate.

When the bacterial genome gets replicated, each strand present in double helix DNA plays a role in the synthesis of the newly formed complementary strand. Every daughter DNA molecule, which is also a double helical molecule, contains an old strand of polynucleotide and a new strand that is synthesized. This particular kind of DNA replication is known as semiconservative.

The bacterial chromosome is a single molecule of DNA. This is a super coiled DNA molecule which is helical, double stranded. In several bacteria, the end parts of the double helix DNA molecules get bonded to each other covalently to give rise to a genetic and physical circle. The DNA molecules that are linear in bacteria are protected by two different kinds of telomeres such as invertron telomeres and palindromic hairpin loops.

The telomere named palindromic hairpin loops are secured by the absence of the loose ends that are double stranded, whereas, invertron telomeres are preserved by proteins that gets bind to 5 prime ends (5’). These two procedures are also useful for some eukaryotic viruses, phage and some eukaryotic plasmids.

Invertron telomeres are made up of a protein that is covalently bonded to the 5′ ends of DNA molecules. The 5′-terminal protein or TP for short, is the name given to this 5′ end. At the telomere, the DNA polymerase connects with the terminal protein, which promotes the formation of a covalent bond between a dNTP and a TP. The dNTP coupled to TP has a loose group of 3′-OH that acts as a precursor for chain elongation.

Bacteria are often thought of as inert cells that copy themselves without altering them. However, this is not always the situation. Bacteria are extremely adaptable microorganisms. Even if they’re from distinct species, many bacteria can exchange genetic information.

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