(2) microbial genetics

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MICROBIAL GENETICS

Julia Hartati, dr

General Principle of Microbial Genetics

Microbial genetics has revealed that genes consist of DNA, an observation that laid the foundation for molecular biology.

Subsequent investigations of bacteria revealed the presence of restriction enzymes that cleave DNA at specific sites, giving rise to DNA restriction fragments.

Amplification of specific regions of DNA also can be achieved with bacterial enzymes using the polymerase chain reaction (PCR) or other enzyme-based methods of nucleic acid amplification (e.g. transcription-mediated amplification).

The Prokaryotic Genome

Genome sequence data have indicated that most prokaryotic genomes consist of a single circular DNA molecule containing from 580 kbp to more than 4600 kbp of DNA.

Bacteria may also contain extrachromosomal genetic elements such as plasmids or bacteriophages (bacterial viruses).

Many bacteria contain additional genes on plasmids that range in size from several to 100 kbp.

Plasmids are small genetic elements that replicate independently of the bacterial chromosome.


Most plasmids are circular, double-stranded DNA molecules varying from 1500 to 400,000 base pairs

DNA circles (chromosome and plasmid), which contain genetic information necessary for their own replication, are called replicons.

Bacteria usually have only one copy of their chromosomes (they are therefore haploid).

Because bacteria have only one chromosome, alteration of a gene (mutation) will have a more obvious effect on the cell.


Many plasmids carry genes that mediate their transfer from one organism to another as well as other genes associated with acquisition or rearrangement of DNA.

Therefore, genes with independent evolutionary origins may be assimilated by plasmids that are widely disseminated among bacterial populations.

Transposons are genetic elements that contain several kbp of DNA, including the information necessary for their migration from one genetic locus to another.


The involvement of relatively short transposons (750–2000 bp long), known as insertion elements, produces the majority of insertion mutations

These insertion elements (also known as insertion sequence [IS] elements) carry only the genes for enzymes needed to promote their own transposition. Almost all bacteria carry IS elements, with each species harboring its own characteristic ones.

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The Viral Genome

Viruses are capable of survival, but not growth, in the absence of a cell host.

Replication of the viral genome depends upon the metabolic energy and the macromolecular synthetic machinery of the host.


The Viral Genome

Therefore, successful propagation of the virus requires a stable form that allows the virus to survive in the

absence of its host, a mechanism for invasion of a host cell, genetic information required for replication of the

viral components within the cell, additional information that may be required for

packaging the viral components and liberating the resulting virus from the host cell.


Bacteriophage

Distinctions are frequently made between viruses associated with eukaryotes and viruses associated with prokaryotes, the latter being termed bacteriophage

Bacteriophages are bacterial viruses. These extrachromosomal genetic elements can

survive outside of a host cell because a protein coat protects the nucleic acid genome

Many phages contain double-stranded DNA, others contain single-stranded RNA, and some contain single-stranded DNA.

Bacteriophage

Unusual bases such as hydroxymethylcytosine are sometimes found in the phage nucleic acid.

Many phages contain specialized syringe-like structures that bind to receptors on the cell surface and inject the phage nucleic acid into a host cell

Bacteriophages infect bacterial cells and either replicate to large numbers and cause the cell to lyse (lytic infection), or in some cases integrate into the host genome without killing the host (the lysogenic state), such as the E. coli bacteriophage lambda.


Bacteriophage

Some lysogenic bacteriophages carry toxin genes (e.g., corynephage beta carries the gene for the diphtheria toxin).

Bacteriophage lambda remains lysogenic as long as a repressor protein is synthesized and prevents the phage from becoming unintegrated and replicating independently of the host chromosome.

This reaction can be triggered if the host cell DNA is damaged by radiation or by another means or if the cell can no longer make the repressor protein, a signal that the host cell is unhealthy and is no longer a good place for "freeloading."


Lysogenic infection of bacterium with temperate bacteriophage. A, The phage infects a sensitive bacterium, and the phage DNA is injected. B, The phage DNA becomes integrated into the bacterial chromosome. C, The bacterium multiplies, apparently unaffected by the infection. It has been lysogenized. D, Occasionally the phage DNA is excised from the bacterial chromosome, takes control of the cell, and replicates. E, An individual cell (or, by induction, all the cells) produces phage components. F, The components are later assembled into phage particles. G, Ultimately, the cell lyses and releases mature phage particles

Recombination & Plasmids

The exchange of genetic material between bacterial cells may occur by one of three mechanisms: conjugation, which is the mating or quasisexual

exchange of genetic information from one bacterium (the donor) to another bacterium (the recipient);


transformation, which results in acquisition of new genetic markers by the incorporation of exogenous or foreign DNA;

transduction, which is the transfer of genetic information from one bacterium to another by a bacteriophage. Once inside a cell, a transposon can jump between different DNA molecules (e.g., plasmid to plasmid or plasmid to chromosome


CONJUGATION

Conjugation usually occurs between members of the same or related species but has also been demonstrated to occur between prokaryotes and cells from plants, animals, and fungi.

Conjugation occurs for E. coli, bacteroides, enterococci, streptococci, streptomyces, and clostridia.

Conjugation is the process by which DNA is passed directly by cell-to-cell contact during the mating of the bacteria.


CONJUGATION

Conjugation results in one-way transfer of DNA from a donor (or male) cell to a recipient (or female) cell through the sex pilus.

Conjugative R (antibiotic resistance) for gram-positive bacteria, such as streptococci, streptomyces, and clostridia, are brought together by the presence of an adhesin molecule on the surface of the donor cell instead of pili.


TRANSDUCTION

Genetic transfer by transduction is mediated by bacterial viruses (bacteriophages), which pick up fragments of DNA and package them into bacteriophage particles.

The DNA is delivered to infected cells and becomes incorporated into the bacterial genomes.

Transduction can be classified as specialized if the phages in question transfer particular genes (usually those adjacent to their integration sites in the genome) or generalized if the selection of the sequences is random because of accidental packaging of host DNA into the phage capsid

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(2) microbial genetics