Genetics

Genetics is a discipline of biology, is the science of genes, heredity, and variation in living organisms. The word is derived from Ancient Greek word “genetikos”, (genitive) and that from genesis meaning origin.

Genetics deals with the

• molecular structure and function of genes;
• Gene behavior in context of a cell or organism;
• Patterns of inheritance from parent to offspring and
• Gene distribution, variation and change in populations.

Gene is the genetic unit of function. A gene may encode a polypeptide or a molecule of non-translated RNA (e.g. ribosomal RNA, transfer RNA, or a regulatory RNA). As per the Mendel’s observation, gene is an unit of inheritance.

Given that genes are universal to living organisms, genetics can be applied to the study of all living systems, from viruses and bacteria, through plants and domestic animals, to humans.

Basics of Genetics

DNA and Chromosomes

The DNA in a chromosome exists as one long double helix associated with various proteins that regulate genetic activity.

The eukaryotes have linear DNA fragmented into large-sized physical units called chromosomes. Whereas, the bacterial DNA is circular; the chromosome of E. coli, for example, contains about 4 million base pairs and is approximately 1000 times longer than the cell.

Genomics is the molecular characterization of genomes. Information contained in the DNA is transcribed into RNA and translated into proteins.

Enzymes that associated with DNA:

• DNA polymerase – An enzyme that synthesize a new strand of DNA in 5’ to 3’ direction using anti parallel strand as template
• Restriction Endonuclease – An enzyme which recognizes and makes double strand DNA breaks at specific sequence of DNA
• RNA polymerase – An enzyme that synthesize RNA in 5’ to 3’ direction using anti parallel DNA strand as template
• DNA ligase – An enzyme that lygate or joint double strand DNA fragments
• DNA gyrase – This enzyme introduce super coiling of DNA in prokaryotes (also referred as Topoisomerase II)
• Topoisomerase I – An enzyme which removes the super coiling of DNA (Super coiling refers the highly twisted form of DNA)

Plasmids

Plasmids are replicons that are maintained as discrete, extra chromosomal genetic elements in bacteria. They are usually much smaller than the bacterial chromosome, varying from less than 5 to more than several hundred kbp, though plasmids as large as 2 Mbp occur in some bacteria. Plasmids usually encode traits that are not essential for bacterial viability, and replicate independently of the chromosome. Most plasmids are supercoiled, circular, double-stranded DNA molecules, but linear plasmids have also been demonstrated in Borrelia and Streptomyces.

Many plasmids control medically important properties of pathogenic bacteria, including resistance to one or several antibiotics, production of toxins, and synthesis of cell surface structures required for adherence or colonization. Plasmids that determine resistance to antibiotics are often called R plasmids (or R factors) and the plasmids responsible for conjugation are F plasmids.

Plasmids are classified based on their function in the host cell. Some of the important plasmids found in bacteria are:

1. F plasmid: A plasmid called the fertility or F factor plays a major role in conjugation in E. coli and was the first to be described. The F factor is about 100 kilobases long and bears genes responsible for cell attachment and plasmid transfer between specific bacterial strains during conjugation. (Conjugation refers the genetic recombination between two organisms through physical contact).
2. R Plasmid: Plasmids often confer antibiotic resistance on the bacteria that contain them. R factors or plasmids typically have genes that code for enzymes capable of destroying or modifying antibiotics. They are not usually integrated into the host chromosome. Genes coding for resistance to antibiotics such as amphicillin, chloramphenicol and kanamycin have been found in plasmids. Some R plasmids have only a single resistance gene, whereas others can have as many as eight.
3. Col plasmid: Bacteria also harbor plasmids with genes that may give them a competitive advantage in the microbial world. Bacteriocins are bacterial proteins that destroy other bacteria. They usually act only against closely related strains. Bacteriocins often kill cells by forming channels in the plasma membrane, thus increasing its permeability. They also may degrade DNA and RNA or attack peptidoglycan and weaken the cell wall. Col plasmids contain genes for the synthesis of bacteriocins known as colicins, which are directed against E. coli. Similar plasmids carry genes for bacteriocins against other species. For example, Col plasmids produce cloacins that kill Enterobacter species. Clearly the host is unaffected by the bacteriocin it produces.
4. Metabolic plasmids or degradative plasmids: They carry genes for enzymes that degrade substances such as aromaticcompounds (toluene), pesticides (2,4-dichlorophenoxyaceticacid), and sugars (lactose).
5. Virulence plasmids: The plasmids that make their hosts more pathogenic because the bacterium is better able to resist host defense or to produce toxins. For example, enterotoxigenic strains of E. coli cause traveler’s diarrhea because of a plasmid that codes for an enterotoxin.