Operon, Unique and Repetitive DNA in a Cell

While the “golden age of genetics’ ‘ was defined from 1900 to World War II, we may be entering a new golden (or platinum) age. We can control the DNA of live organisms and make purposeful modifications to it using recombinant DNA technology. Eukaryote genetic systems are far more difficult to analyse and comprehend than prokaryotic genetic systems.

The E. coli chromosome

The circular chromosome of the common intestinal bacterium E. coli contains approximately 4.7 million base pairs. It is about 1 mm in length but just 2nm in width. The chromosome replicates in both directions, resulting in a figure that looks like the Greek letter theta. The promoter is the portion of the DNA that the RNA polymerase binds to before opening the transcribed segment.

A structural gene is a piece of DNA that codes for a specific polypeptide. On a bacterial chromosome, these are frequently seen together. The polypeptides’ position, which could be enzymes in a metabolic process, enables for speedy and effective transcription of the mRNAs. At the beginning and end of the region, there are frequently untranslated leader and trailer segments. E. coli has the ability to produce 1700 enzymes. As a result, this little bacterium contains 1700 distinct mRNA genes.

The enzyme b-galactosidase splits lactose, a milk sugar. This enzyme is inducible because it only appears in high amounts when lactose, its operating substrate, is available. In contrast, unless tryptophan is present, enzymes for the amino acid tryptophan are generated continually in developing cells. The formation of tryptophan-synthesising enzymes is inhibited when tryptophan is present.

Model of Operon

Francois Jacob and Jacques Monod proposed the operon model of bacterial gene control. Operons are units that include groups of genes that code for related proteins. Operator, promoter, regulator, and structural genes make up an operon. The regulator gene produces a repressor protein that binds to the operator and prevents the structural genes’ promoters from being transcribed. The regulator does not have to be close to the operon’s other genes. Transcription may occur if the repressor protein is eliminated.

Plasmid

Plasmids are tiny DNA fragments found in nearly all bacterial cells. Plasmids contain between two and thirty genes. Some appear to be able to enter and exit the bacterial chromosome.

A plasmid integrated into the bacterial chromosome is known as an episome. Plasmids replicate themselves in the same way that bacterial chromosomes do. The F plasmids (“sex factors”) and the R plasmids (“drug/antibiotic resistance”) have been identified for E. coli. Some of the genes on the F plasmid control the formation of F pili (proteins that run from the surface of F+, or male, cells to the surface of F-, or female, cells).

On cells with the R plasmid, drug resistance is transmitted. On a single R plasmid, up to ten resistance genes can be found. R plasmids can be passed between bacteria of the same species, viruses, and even bacteria of different species. Pathogens that cause typhoid fever, gastroenteritis, plague, undulant fever, meningitis, and gonorrhoea have been proven to have drug (antibiotic) resistance. R plasmids can be transmitted across the cell membrane in addition to the more frequent forms of transfer. The antibiotic resistance genes appear to work by either breaking down the antibiotics or bypassing the antibiotic’s block on a crucial bacterial metabolic pathway.

Viruses

A nucleic acid (DNA or RNA) is wrapped in a protein sheath by viruses (known as a capsid). As with tobacco mosaic virus, the capsid could be a single protein replicated many times (TMV). It could potentially be a combination of proteins, as in T-even bacteriophages.

The nucleic acid takes one of two pathways once inside the cell: lytic or lysogenic.

Retro viruses

With the viral RNA, retroviruses like the Human Immunodeficiency Virus (HIV) include the enzyme reverse transcriptase. Reverse transcriptase converts single-stranded viral RNA into a single-stranded viral DNA copy. 

After that, the single-stranded viral DNA is transformed into double-stranded DNA. The lytic cycle begins when viral DNA invades the host cell (remember, viruses are obligate intracellular parasites) and begins producing new viruses. The new viruses eventually force the cell to break (or lysis), releasing new viruses to continue the infection cycle. When viral DNA is incorporated into the host DNA as a prophage, the lysogenic cycle begins. The prophage is transmitted along as if it were host DNA when the cell multiplies. Once every 10,000 cell divisions, the prophage can spontaneously emerge from the host chromosome and enter the lytic cycle. The prophage can also be triggered by ultraviolet light and x-rays.

The transfer of host DNA from one cell to another by a virus is known as transduction. Because they tend to go lysogenic rather than lytic, some bacteriophages are temperate. These viruses have the ability to transduce host DNA segments.

Conclusion

The viral RNA, retroviruses like the Human Immunodeficiency Virus (HIV) include the enzyme reverse transcriptase. Reverse transcriptase converts single-stranded viral RNA into a single-stranded viral DNA copy. 

A plasmid integrated into the bacterial chromosome is known as an episome. Plasmids replicate themselves in the same way that bacterial chromosomes do. The F plasmids (“sex factors”) and the R plasmids (“drug/antibiotic resistance”) have been identified for E. coli.