The below mentioned article provides a study note on bacterial genomes.
Bacteria are very small unicellular organisms that do not contain nuclear envelope, mitochondria, endoplasmic reticulum, mitotic apparatus and nucleolus etc., and divide by fission. Bacteria have a rigid cell wall which surrounds their cytoplasmic membrane. Their cytoplasm contains ribosomes. mesosomes and several granular inclusions. About 1/5 of the cell volume is occupied by DNA, the genetic material.
According to their external shape, bacteria are grouped into two main classes:
(1) Cocci and
(2) Bacilli.
Cocci (= berry; Latin and Greek). These are spherical in shape; they show different patterns when their cells are incompletely separated, e.g., (i) Diplococcus: cells in pairs, (ii) Streptococci: cells chains, (iii) Staphylococci: cells in clusters, and (iv) Sarcinae: cells forming tertrads (square) or cubic packets.
Bacilli (= stick; Latin): These bacteria are rod-shaped or cylindrical, and are of different types, such as, (i) Coccobacilli: short elongated cells, (ii) Fusiform bacilli: cells tapered at both ends, (iii) Filamentous bacilli: long threads, (v) Vibrios: curved small bacilli, and (v) Spirilla: long threads curved bacilli.
Bacterial Nuclear Body:
Bacterial cells do not contain a typical nucleus. But Feulgen reaction shows one, two or more discrete nuclear bodies per cell; these are called nucleoids. The bacterial genome is confined to this nucleoid, which is more or less compact structure without any membrane.
When bacterial cells are lysed in the presence of high salt concentration, nucleoids can be recovered intact. The isolated nucleoid may be membrane free or it may be associated with membrane (mesosome). The constituents of the membrane-free nucleoid are DNA (~ 60%) RNA (-30%) and protein (- 10%) DNA forms 2-3% of the dry weight of a bacterial cell).
Bacterial Chromosomes:
Bacterial chromosome is a double-stranded circular DNA. In general, bacterial DNA ranges from 1100 pm to 1400 µm in length. An E. coli cell contains 4. 2 x 106kbp DNA which is about 1.3 mm (1300 µm) in length.
Such a long DNA molecule must be greatly folded to be packaged in a small space of 1.7 x 0.65 µm. The bacterial chromosome is folded into loops or domains which are about 100 in number. A chromosomal domain may be defined as a discrete structural entity within which supercoiling is independent of the other domains.
Thus different domains can maintain different degrees of supercoiling. The DNA chain is coiled on itself to produce supercoiling (Fig. 5.26). The ends of the loops or domains are bound in some way which does not allow rotational events to propagate from one domain to another.
If an endonuclease puts a nick in DNA strand of one domain, this loop becomes larger due to the uncoiling, but the other domains are not affected. Each domain contains about 40 kbp (13 µm) of DNA. The loops are bound by some mechanism that may involve proteins and/or RNA but the mechanism is not clearly understood.
In E. coli, a number of proteins have been isolated which have some similarities with the eukaryotic chromosomal proteins. These proteins are HU, IHF (integration host factor). HI (H-NS) and R It is suspected that HU is involved in the nucleoid condensation.
The protein HI probably has effects on gene expression. The amino acid sequence of P has some similarity with the protamine’s (DNA of certain sperms is bound with protamine’s). However, the functions of the P protein are not known.
Replication of Bacterial Chromosome:
Bacterial chromosome is a single replicon. Auto-radiographic studies have shown that it replicates bi-directionally in a semiconservative manner. A theta (0) shaped intermediate is formed during its replication. The rate of replication is 50,000 base pairs per minute which is 25 times faster than that of eukaryotic DNA (2000 bp/minute).