Bacterial Genetics: Effect Of Environment Alteration And Mutation

Bacteria is a single cell living organism. It consist a circular DNA which is called as bacterial chromosome. A part of that some bacterial cells contain plasmid. Which is an extra chromosomal DNA of bacterial cell. Because of presence of two type of circular DNA bacteria become an interesting organism for microbiologists. Although, bacteria reproduce and elongate it’s progeny mainly by bacterial chromosome. Plasmid DNA is known to provide some extra feature to bacterial cell. Like resistant to various antibiotics. Carry genes of interest or artificially inserted genes to next generations. Because plasmid is an independent dividing unit it can be inserted by some artificial genes those can replicate and show their effects.

Phenotypic Changes Due To Environmental Alteration

Bacteria, like a cell of higher organisms contain more genetic information – there genotype – is then expressed at anyone time. The extent to which this information is expressed depends on environmental conditions. For example: a facultative anaerobic bacteria will produce different end products of metabolism. Depending on the presence or absence of oxygen during growth. In such bacteria presence of oxygen determine, which enzyme system will work and which will not.

A phenotypic change of this type is not inherited, rather, it occurs in the changing of some conditions of the environment. A return to the original phenotype occurs when these conditions return to original type.

Genotypic Changes

Genotype is refer to the genetic information present in the chromosomes of a living cell. The chromosomes are decided into gene. A gene is a functional unit of inheritance; it specify the formation of a particular polypeptide as well as various type of RNA. Each gene consists of hundreds of nucleotide 

For example, if a polypeptide chain contain 300 amino acids, the gene coding for this polypeptide must contain some 900 bp.

It has been estimated that bacterial cell is capable to code for 3,500 different proteins.

Any gene is capable to undergo changes or mutation, which result in the alteration of polypeptide sequence and convert protein in a new form, which may also lead to the cell death, if protein become non-functional for its task. For example, substitution of any one amino acid in the polypeptide chain may may cause the to be non functional.

A mutation is the change in the nucleotide sequence of a gene. A cell or an organism which shows the effects of a mutation is called a mutant.

In nature, mutation are rare events which occur at random and arise spontaneously with no regard to environmental conditions. Spontaneous bacterial mutation occur at a rate of only one mutation in 1 million bacterial cells to a rate of only one mutation in 10 billion bacterial cells. Generally the mutation in a bacterial cell is masked by the greater number of unmutated cell in the population. So it become nearly impossible to isolate mutated cells from the colonies of bacterial cell.

However, microbiologists have a developed a technique to identify and isolate mutated cells from the colony if unmuted cells. For example, an antibiotic can be incorporated in a medium to select the antibiotic resistance mutants.

Types Of Mutation

At a molecular level it has been found that there are several ways by which minute changes in purine and pyrimidine may occur, resulting in mutation. Two common type of mutations have been identified (1) point mutation, (2) frameshift mutation.

Point Mutation

Point mutation occur as a result of the substitution of one nucleotide for another in the specific nucleotide of the gene. The substitution of one purine or one pyrimidine for another pyrimidine is termed a transition type of point mutation. A transversion is the replacement of one purine by the pyrimidine, or vice versa. This base pair substitution may result in one of three kinds of mutations affecting the translation process.

1. The altered gene triplet produces a coding in the mRNA which specifies an amino acid, other than in normal polypeptide chain. This mutation is called a missense mutation. Such a protein may be functionally inactive or partially active then the normal one.
2. The altered gene triplet produces a chain terminating coding in mRNA, resulting in premature termination of protein formation during the process of translation. This is called a nonsense mutation. This result in the incomplete polypeptide formation which is non-functional.
3. The altered gene triplet produces a mRNA codon which specifies the same amino acid because the codon resulting from mutation is a synonym for the original codon. This is called a neutral mutation.

Frameshift Mutation

These mutation result from an addition or loss of one or more nucleotides in a gene and are termed inserting or deletion mutations respectively. This result in a shift of the reading frame.

Frameshift mutations, therefore, generally lead to non-functional proteins, because an entirely new sequence of amino acids synthesis from frameshift mutation.

Reference: Microbiology Pelczar