UV irradiation of bacterial cells causes mutations in DNA because of:
[Question ID = 532]
1. Deamination of bases [Option ID = 2127]
2. Double-stranded breaks in DNA [Option ID = 2126]
3. Tautomerization of bases [Option ID = 2125]
4. Formation of thymidine dimers. [Option ID = 2128
Answers
Answer:
Genomes of bacteria exist on a single double-stranded circular DNA molecule that contains approximately 4000 kb of DNA and are regulated by operons. A mutation is a change in the nucleotide sequence and can create new cellular functionalities or lead to the dysfunction of others. Mutations can occur spontaneously or be caused by exposure to mutation-inducing agents. [1]
Function
While the majority of bacterial genes exist on one circular chromosome, there are other genetic elements within the bacterial genome. Elements like plasmids, transposons, integrons, or gene cassettes are shorter sequences that mainly contribute to recombination events. Bacterial DNA replication and transcription co-occur and utilize the same template DNA. Replication forks proceed bi-directionally with a single origin of replication, OriC.
Bacterial genes with similar functions often share one promoter (RNA polymerase binding site) and are transcribed simultaneously; this system is called an operon. Typical operons consist of several structural genes that code for the enzymes required for the pathway. Regulation occurs through transcription factors binding to a short sequence of DNA between the promoter region and the structural genes called an operator. [2]
A mutation is a change in the nucleotide sequence of a short region of a genome, and phenotypic results may vary on the severity and location of the mutation. Mutations can be the result of errors during DNA replication or induced by exposure to mutagens (like chemicals and radiation). Spontaneous mutations occur at a rate of 1 in 10^5 to 10^8 and contribute to random population variation. [3] Since bacteria are haploid for the majority of their genes and have short generation turnover, phenotypic variation due to point mutations can occur relatively quickly.
Results of mutations can produce changes in structural or colony characteristics or loss in sensitivity to antibiotics. Some potential consequences of mutations are as follows:
Spontaneous Mutations
Spontaneous mutations occur without mutation induction and are the result of errors during DNA replication. When DNA Pol III is synthesizing a new strand of DNA, occasionally, a nucleotide will be mispaired, added, or omitted. [4] Thus, a point mutation will occur. For example, when nucleotides are mispaired, it will appear that one nucleotide substitutes for another leading to one mutated granddaughter DNA strand. Two separate malfunctions must happen in the bacteria's DNA replication machinery for this to occur: [5]
DNA pol III pairs an incorrect complementary nucleotide base onto the parent strand in the replication fork
The chemical activity of the mispairing is not enough to slow the polymerase portion of DNA polymerase so that the exonuclease can remove the mispair
Studies with Escherichia coli show that spontaneous mutations occur 20 times more often on the lagging strand than the leading strand. [6]
DNA bases can exist in many different forms, referred to as tautomers. Nucleotide bases dominantly exist in the keto (C-O) and amino (C-NH2) forms, while the imino (C[equivalent]NH) and enol (C-OH) occur rarely. Tautomerization, during DNA replication, will alter nucleotide base pair formation. For example, assume that thymine undergoes keto-enol tautomerization during replication. This enol species will preferentially bind to guanine during the first replication cycle. Due to the semiconservative nature of DNA replication, at the end of the 2nd round of replication, there will be (3) A-T base pairs and (1) G-C in the locus of mutation. [7]
Mutation Induction
Mutagens may be of physical, chemical, or biological origin. Mostly they act on the DNA directly, causing damage, which may result in errors during replication. Although severely damaged DNA can prevent replication and cause cell death. SOS is an example of cellular response to DNA damage that results in cell cycle arrest and induction of mutagenesis. Rec A induces SOS response by recognizing single-stranded DNA and activating mutagenic DNA polymerases