📖 General Microbiology

GENETIC VARIATION

Two methods are known for genetic variation in bacteria: mutation and gene transfer.

Mutation : Any change in the sequence of bases of DNA, irrespective of detectable changes in the cell phenotype. Mutations may be spontaneous or induced by various agents which are known as mutagens. 

Spontaneous Mutations: Arise from enzymatic imperfections during DNA replications or with transient insertions of transposable elements.

Induced Mutations: Mutation by physical and chemical mutagens.

Physical mutagens  ultraviolet rays and high-energy ionizing radiations. The primary effect of UV rays on DNA is the production of pyrmidine dimers whereas ionizing radiations cause single_stranded breaks the DNA molecules.

Chemical mutagens :Affecting nucleotide sequence

(i) Agents which cause error in base pairing (e.g. nitrous acid and alkylating agents).
(ii) Agents which cause errors in DNA replication (e.g. acridine dyes such as acridine orange and profiavine).
(iii) Base analogs which are incorporated into DNA and cause replication errors (e.g. 5-bromouracil)

Gene Transfer

Transformation: Uptake of naked DNA

Transduction    : Infection by a nonlethal bacteriophage

Conjugation    : Mating between cells in contact

Protoplast fusion

Transformation: Gene transfer by soluble DNA is called as transformation. it requires that DNA be absorbed by the cell, gain entrance to the cytoplasm and undergo recombination with the host genome. 

Artificial Transformation(transfection) :Some of the bacteria (such as Escherichia coli) resist transformation until they are subjected to some special treatment such as CaCl2 to make the bacterium more permeable to DNA. Such modified cells can also take up intact double stranded DNA extracted from viruses or in the shape of plasmids. Though the process is same as transformation, it is 9 as transfection because it results in infection by an abnormal route

Transduction :The type of gene transfer in which the DNA of one bacterial cell is introduced into another bacterial cell by viral infection is known as transduction. This introduces only a small fragment of DNA. Because the DNA is protected from damage by the surrounding phage coat, transduction is an easier to perform and more reproducible process than transduction. ,

Two types of transduction are known.

- Generalized transduction When a bacteriophage picks up fragments of host DNA at random and can transfer any genes

-  Specialised transduction: phage DNA that has been integrated into the host chromosome is excised along with a few adjacent genes, which the phage can then transfer.

After entry into the host cell, the phage DNA gets incorporated into the host chromosome in such a way that the two genomes are linearly contiguous (lysogeny). The phage genome in this stage is known as prophage, The host cell acquires a significant new property as a consequence of lysogeny because it becomes immune to infection by homologous phage. This is hence called as lysogenic conversion and endow toxigenicity to Corynebacterium diphtheriae

Abortive Transduction :phage DNA fails to integrated into the host chromosome, the process is called as abortive transduction The phage DNA does not replicate and along with binary fission Of the host it goes into one of the daughter cells.

Conjugation :This is defined as the transfer of DNA directly from on bacterial. .cell to another by a mechanism that requires cell-to-cell contact. 

The capacity to donate DNA depends upon the possession of the fertility (F) factor. The F pili  also retard male-male union. Concomitant with effective male-female pair formation, the circular DNA bearing the F factor is converted to a linear form that is transferred to the female cell in a sequential manner. DNA replication occurs in the male cell and the newly synthesized, semiconserved DNA molecule remains in the male. This ensures postmating characters of the male.

Conjugation in Different Bacteria: Unusual form of plasmid transfer, called phase mediated conjugation has  been reported to occur with some strains of Staphylococcus aureus.

Protoplast Fusion: Also called as genetic transfusion. Under osmotically buffered Conditions protoplast fusion takes place by joining of cell membrane and generation of cytoplasmic bridges through which genetic material can be exchanged.

Transposons: Transposons  Tn  are  DNA sequences which are incapable of autonomous existence and which transpose blocks of genetic material back and forth between cell Chromosome and smaller replicons such as plasmids. insertion sequences (IS ) are another similar group of nucleotides which can move from one chromosome to another

Genetic material. IS and  Tn are collectively also known as transposable elements or Jumping genes. These are now recognised to play an important role in bringing about vanous types of mutations.