Bacterial Genetics

Introduction:

Genetics is the study of genes including the genetic material structure, what information is stored in the DNA, how the genes are expressed and how genetic information is transferred. Genetics is also the science of inheritance and variation.

The gene structure within species is its genotype, and the physical characteristics of an organism that is dependent on its genotype and the relationship with its environment make up its phenotype. The sequence of DNA bases determines the bacterium 's genotype.

Several genes may have alternate forms within a particular organism. Such alternative gene forms are termed alleles. The cell's genome is deposited in chromosomes, which are chains of double stranded DNA

Genes are nucleotide sequences within DNA which code for functional proteins. Bacteria and plasmids have DNA as their genetic material. Replication and expression are two primary functions of genetic material.

Parts of each DNA strand will be shared at times as two pieces of DNA come into contact with one another. This is typically done through a process called crossing over in which the DNA breaks and is attached to the other DNA strand which leads to gene transfer and probably to the formation of new genes. 

Genetic recombination is the transmission of DNA from one organism to the next. The transferred donor DNA may then be incorporated by various mechanisms into the recipient's nucleoid. In the case of homologous recombination, homologous fragments of DNA with almost the same nucleotide sequences are shared by splitting and reuniting paired segments of DNA.

Genetic information can be transferred from organism to organism by vertical transfer (from parent to offspring) or by forms of horizontal transfer, such as conjugation, transformation and transduction. Bacterial genes are usually transferred to members of the same species but can also occur occasionally in other species.

Bacterial Transformation:

Transformation involves the passage of DNA molecules across the bacterial cell wall and into the bacterial cytoplasm. Transformation is probably regularly occurring in nature and allows bacteria to potentially acquire useful DNA sequences.

However, it doesn’t have any clinical importance, and its primary importance in biomedical research is it is a key tool for experimental purposes to generate recombinant DNA. 

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Bacterial Transduction:

Transduction in bacteria involves exchanging genetic material between bacteria through "Phages" or bacteria-infecting viruses.

The capsid connects bacteria and injects the genome together with accessory proteins to replicate the bacterial metabolic machinery and then lyses the cell to release its offspring.

In some cases, called lysogenic conversion, the phage genome integrates stably into an infected bacterium and does not replicate or lysis the host organism until certain environmental conditions are met when the phage genome is removed and the replication program begins.

There are two types of Transduction, they are Generalized and Specialized transduction.

Generalized Transduction

In a small fraction of cases, pieces of the infected bacterial genome may be packaged and released inside an infectious phage capsid, rather than the phage genome.

When the mispackaged capsid injects its internal nucleic acid into a fresh recipient, genetic fragments of the previous bacteria are inserted and in some cases incorporated into the genome of the new organism.

Specialized Transduction:

In some cases, small fragments of the host bacterial genome and a lysogenically combined phage genome are clipped out.

Consequently, when infected with a new bacterium, genomic fragments of the previous bacteria that are tagging along can be integrated into the new victim. Specialized transduction is an important mechanism for the exchange of various important bacterial exotoxins.

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Bacterial Conjugation:

Conjugation involves the transfer of genetic material through a specific "Sex Pilus" between plasmids. The sex pilus and conjugation machinery proteins are based on a specialized plasmid called the "F Plasmid" (F is for Fertility).

Note that plasmids are small, circular pieces of double stranded DNA, which can replicate within the bacterial cytoplasm independently of the bacterial genome. Bacteria that produce the F plasmid (F+) will synthesize a sex pilus that binds to bacteria that do not possess the F plasmid (F-).

The F plasmid in F+ cells then produces a single - stranded copy of itself which is transferred to the F-bacteria via the "Sex Pilus" The F plasmid becomes integrated into the bacterial host genome in a small fraction of cases, and is subsequently clipped out before F plasmid transfer.

Often fragments of the bacterial host genome can be mistakenly removed together with the F plasmid and passed to other bacteria together with the F plasmid.

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Key Points:

• A bacterium takes up a piece of DNA that is floating in its environment during transformation. 

• In transduction, a virus accidentally transfers DNA from one bacterium to another. 

• DNA is transferred between bacteria in conjugation through a tube between the cells.