Bacterial Genome is consisted from 2 subsystems. Name and describe them. What properties of the cells are carried by plasmids.
Bacterial
Genome is consisted from 2 subsystems:1.Bacterial
chromosomes (part of the nucleoid).2. Plasmids subsystem.
Nucleoid:
A
bacterium’s chromosome is composed of base pairs of nucleotides,
the basic structural components of DNA. Only four nucleotides exist
in DNA — adenine, thymine, guanine, and cytosine. These
nucleotides always bond together in the same way — adenine bonds
with thymine and guanine bonds with cytosine. All four together form
what is known as a "base pair."
Prokaryotic cells contain DNA called the bacterial chromosome that
is haploid (composed of a single DNA copy) and circular. Most cells
have a single chromosome, but some species may have two.
Vibrio
cholerae
and Agrobacterium
tumefaciens
are examples of bacterial cells having two chromosomes.
protein
s,supercoiled
domain.
Bacterial
chromosomes a single large circular double stranded DNA no histone
proteins. The molecule is further twisted by the enzyme gyrase
(topoisomerase). The only proteins associated with the bacterial
chromosomes are the ones for DNA replication, transcription etc.
Since
the amount of DNA necessary to provide the genetic information
necessary for the life of a bacterial cell far exceeds the actual
volume of the cytoplasm, the chromosome is folded into loops about
50,000 to 100,000 bp in length. When a bacterium such as E.
coli
is "gently lysed"
the chromosomal DNA leaks out of the cell as a continuous molecule that is many
times longer than the length of the cell. Plasmids are extrachromosomal rings of DNA found in some bacterial cells. These are considerably smaller than the bacterial chromosome, ranging in size from a few thousand to several million bp. Cell Properties Carried by Plasmids: Drug resistance - Resistance (R) factors carry genes that allow a cell to be resistant to antibiotics or other antimicrobial compounds. Fertility (F) factors are plasmids that carry genes necessary for a cell to transfer DNA to another compatible cell through a process called conjugation. Production of antimicrobial agents - Bacteriocidin factors carry genes for the production of toxins that kill other bacteria that might compete with the cell for nutrients and space. Metabolic activities - Examples of these genes are those that allow bacteria to utilize unique or unusual materials for carbon or energy sources. Many of the genes for these metabolic pathways are on transmissible plasmids. Virulence (1) Toxins - Enterotoxins (Escherichia coli, Vibrio cholerae), exfoliative toxin (Staphylococcus aureus), dermotoxin of Bacillus anthracis, the neurotoxin of Clostridium tetani, and the pesticide toxin of Bacillus thuringiensis. (2) Adhesins - such as produced by the plasmids of Yersinia enterocolitica, Shigella flexneri, Escherichia coli strains that produce dysentery, and Yersinia pestis. (3) Growth factors - Other plasmid borne virulence factors act to directly aid the bacteria in competing with mammalian host cells for growth. Col V of Escherichia coli contains genes for iron sequestering compounds. The acquisition of iron is essential for the survival of Escherichia coli in mammalian infections. Some species of bacteria supplement their chromosome using additional chunks of DNA. Plasmids are small, circular, extrachromosomal DNA strands found in some bacteria. Plasmids do not carry any genes essential for normal bacterial growth or function. Instead, you can consider these as accessory genes. Removal of a species' plasmids would not result in the death of the cell, but it might change some of its abilities or characteristics. In fact, some plasmids carry genes that provide a selective advantage in the environment. Some plasmids carry genes that allow for bacterial conjugation, antibiotic resistance, or even antibiotic production.