Types of mutation

MICROBIOLOGY

1. Types Of Mutation Name-Samadrita Banik ST. George College of Management and Science 2 nd Semester M.Sc Microbiology

2. INTRODUCTION In biology, a mutation is an alteration in the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA.  Mutations result from errors during DNA replication, mitosis, and meiosis or other types of damage to DNA which then may undergo error-prone repair or cause an error during other forms of repair or else may cause an error during replication. Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements. Mutations may or may not produce discernible changes in the observable characteristics of an organism. Mutations play a part in both normal and abnormal biological processes including: evolution, cancer, and the development of the immune system, including junctional diversity.

3. HISTORY English farmer Seth Wright recorded case of mutation first time in 1791 in male lamb with unusual short legs. The term mutation is coined by Hugo de Vries in 1900 by his observation in Oenothera . Systematic study of mutation was started in 1910 when Morgan genetically analyzed white eye mutant of Drosophila. H.J Muller induced mutation in Drosophila by using X-rays in 1972, he was awarded with Nobel prize in 1946.

4. Characteristics of Mutation Generally mutant alleles are recessive to their wild type or normal alleles Most mutations have harmful effect but some mutations are beneficial. Spontaneous mutations occurs at very low rate. Some genes show high rate of mutation such genes are called as mutable gene. Highly mutable sites within a gene are known as hot spots. Mutations can occur in any tissue or cell of an organism .

5. Types of Mutation There are three types of DNA Mutations: Base substitutions Deletions Insertions.

6. Base Substitutions Single base  substitutions  are called point mutations, recall the point mutation Glu -----> Val which causes sickle-cell disease. Point mutations are the most common type of mutation and there are two types. Transition : this occurs when a purine is substituted with another purine or when a pyrimidine is substituted with another pyrimidine . Transversion : when a purine is substituted for a pyrimidine or a pyrimidine replaces a purine .

7. Deletions A deletion, resulting in a frameshift , results when one or more base pairs are lost from the DNA. If one or two bases are deleted the translational frame is altered resulting in a garbled message and nonfunctional product. A deletion of three or more bases leave the reading frame intact. A deletion of one or more codons results in a protein missing one or more amino acids. This may be deleterious or not.

8. Insertion The insertion of additional base pairs may lead to frameshifts depending on whether or not multiples of three base pairs are inserted. Combinations of insertions and deletions leading to a variety of outcomes are also possible .

9. Chromosomal Mutation A chromosome mutation is a mutation that changes the structure of an individual chromosome, leading to imbalance involving only a part of a chromosome, such as duplication, deletion, or translocation. Different types of chromosome mutations can originate in the germinal cells. Nonreduction of the whole chromosome set will lead to  polyploid  gametes, and  nondisjunction  of single chromosomes leads to  aneuploidy  and chromosome breakage to structural chromosome aberrations. These three types of events have probably different mechanisms of origin. Therefore, the assumption seems likely that a given agent will mainly induce only one of these types of mutations.

10. Based on tissue of origin Somatic Mutation : A mutation occuring in somatic cell is called somatic mutation. In asexually reproducing species somatic mutations transmits from one progeny to the next progeny. Germinal Mutation : When mutation occur in gametic cells or reproductive cells are known as germinal mutation.In sexually reproductive species only germinal mutation are transmitted to the next generation

11. Mutation Hotspot Mutation hotspots (or mutational hotspots) are segments of  DNA  that are especially prone to genetic alteration. The increased susceptibility of these areas of DNA to mutation is attributed to interactions between mutation-inducing factors, the structure and function of the DNA sequence, and enzymes involved in DNA repair, replication, and modification.

12. Effect of Mutation A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations with small effects. Mutational effects can be beneficial, harmful, or neutral, depending on their context or location. Most non-neutral mutations are deleterious. In general, the more base pairs that are affected by a mutation, the larger the effect of the mutation, and the larger the mutation's probability of being deleterious. To better understand the impact of mutations, researchers have started to estimate distributions of mutational effects (DMEs) that quantify how many mutations occur with what effect on a given property of a biological system. In evolutionary studies, the property of interest is fitness, but in molecular systems biology, other emerging properties might also be of interest. To make things even more difficult, many mutations also interact with each other to alter their effects; this phenomenon is referred to as  epistasis . However, despite all these uncertainties, recent work has repeatedly indicated that the overwhelming majority of mutations have very small effects.

13. Conclusion Mutation rates can vary within a genome and between genomes. Much more work is required before researchers can obtain more precise estimates of the frequencies of different mutations. The rise of high-throughput genomic sequencing methods nurtures the hope that we will be able to cultivate a more detailed and precise understanding of mutation rates. Because mutation is one of the fundamental forces of evolution, such work will continue to be of paramount importance.