Introduction Mutations are permanent changes in a gene or chromosome or DNA molecule of an organism that brings a sudden heritable change in the character of an organism. These changes may occur spontaneously or can be induced. Scientist Dobzhansky explained mutations as a mistake or misprints in cell division. The term 'mutations' was first introduced by De Vries.
The Dutch Botanist Hugo de Vries in 1901 proposed the concept of mutations as the sudden appearance of new characteristics of an organism. This scientist worked on a plant evening primrose (Oenothera lamarckiana) to establish mutation traits of inheritance. Later T.H. Morgan worked on insect Drosophilia and established 500 mutations in this insect. H.J.Muller in the 1920s established that the rate of mutations could be increased artificially by the effects of X-rays. Now it was fully established mutation rates can be significantly increased by the effects of high energy electromagnetic radiations such as UV light, X-rays, and gamma rays.
A Tulip flower showing partial yellow petal due to gene mutation (Courtesy:-wikipedia.org)
Genes are the basic heritable units present on the chromosomes or DNA material of cells. These are reproduced or copied exactly during mitosis or meiosis or gamete formation. Sometimes mistakes occur occasionally during the copying or replication of genes during cell divisions. These mistakes result in mutations or changes in the traits of an individual. Once a mutation occurs in an individual, the changed genetic material gets reproduced and copied exactly and will be passed on to the future generations. Mutations are the basis for discontinuous variation in the population. Mutations occur in gametes through meiosis cell division and are inherited. On the other hand, mutations that occur through mitosis cell division are inherited only by daughter cells. Such mutations that occur through mitotic cell divisions are known as somatic mutations.So mutations may occur before, during, and after mitosis and meiosis.
Characteristics of mutations
Types of mutationsMutations can be classified into various types according to the nature, stage of development, rate of efficiency, etc. The various types of mutations were discussed below.
Somatic mutationIt is the type of mutation that occurs in the somatic cells of the organisms. Somatic mutations are not inherited. They disappear with the death of the organisms. Ex:- Most of the human cancer growths are somatic mutations.
Germinal mutationMutations occur in the germ cells (reproductive cells) is called germinal mutations. These mutations are heritable and can be passed on from generation to generation. Ex: -A childhood malignancy of the kidney, the eye tumour retinoblastoma, and Wilms tumour.
Gametic mutationIt is the type of mutation that occurs in gametes (male and female reproductive cells). If an affected gamete is involved in fertilization, the resulting zygote will forward mutation to all the cells formed from it.
Zygotic mutationWhen mutation occurs in the zygote, it is called zygotic mutation. Sometimes, if a change occurs in the genome of the zygotic cell results in post-zygotic mutation.
Dominant mutationIf a mutation produces dominant gene and it is called dominant mutation. The dominant mutation expresses the character immediately. Dominant mutations will result in a mutant phenotype in the presence of a normal copy of the gene. A loss or gain of function is the result of the dominant mutation. Ex: Huntington's disease is an example of a dominant autosomal genetic disorder.
Recessive mutationWhen the mutation of a gene produces a recessive gene, the mutation caused is known as recessive mutation. Most of the natural mutations are mainly recessive in nature. Recessive mutations are usually not expressed immediately in females because the recessive gene needs to be in double doses in the individual. But in males, one dose of the recessive gene is enough to express the defect in the individual as they include X and y different chromosomes. An X-linked mutant recessive gene in males get easily express the defect as it has only one X-chromosome in it. But in females, the defect can't get easily expressed as the defective recessive gene has to be present on both the X-chromosomes of the individual. Recessive defective mutant gene shows its effect only in homozygous condition. Ex: Colorblindness, Haemophilia, etc.
Chromosomal mutationThe sudden changes that occur in the number of genes, the arrangement of genes, and in the number of chromosomes is known as chromosomal mutations or chromosomal aberrations.
Spontaneous mutationWithout any obvious cause if a mutation occurs naturally is said to be a spontaneous mutation. Most of the mutations in nature are this type only. Ex:-Sickle cell anaemia, the origin of Cabbage, Cauliflower, Broccoli, Kohlrabi, Brussels sprouts, Kale are originated from a mustard plant through spontaneous mutations only.
Back mutationThis is a reversal of mutation. Rarely sometimes a mutant gene changes into its original gene. In this case, the original character of an individual get restored.
Lethal mutationSometimes, the mutation affects the vital functions of the organism and that results in a mutant to die. This type of mutation is called a lethal mutation.
Induced mutationIf mutations are caused by external factors , the mutations are said to be induced mutations. The external factors that cause mutations are known as mutagenic agents or mutagens. The mutagenic agents may be physical, chemical, or biological agents The mutagens include ionizing radiations like X-rays, alpha rays, beta rays, gamma rays, infrared rays, UV rays, chemicals like mustard gas, peroxides, colchicine, dimethyl sulfate, formaldehyde, nitrogen, nitrous acid, Acridine dyes, and various environmental factors like light, heat, and temperature. Arsenic, chromium, Cadmium, Nickel, and their compounds may act as mutagenic substances to cause mutations. Induced mutations were first studied by T.H. Muller (1937) on Drosophilia insect with the help of X-rays.
Biochemical mutationMutations that cause changes in the metabolites or their end-products is called biochemical mutation. Most of the enzymes produced in our body are proteins. These enzymes are produced by genes. Each enzyme produced in our body by a specific gene. If a particular gene that synthesizes a specific enzyme undergoes mutation, the enzyme in question can't be synthesized. Every enzyme will act on a specific substrate to convert into another substance. If such a specific enzyme is not synthesized, the substrate gets accumulated in the body. Suppose if this substrate is a toxic one and its accumulation in the body leads to serious complications. Phenyl-ketonuria and Alcaptonuria are the conditions that are caused by metabolic errors due to biochemical mutation.
Macro mutationsSome mutations are large with distinct phenotypic deviations in the organism. Such mutations are called macromutations. Ex: Homeotic mutations of fruit flies (Drosophila).
Micro mutationsMutations that bring slight effects in phenotypic features are known as micro mutations. A micro mutation involves alteration at a single gene locus.
Gene mutationsThe sudden change of a gene from one form into another is known as gene mutation. Permanent alteration in DNA sequence results in gene mutations. This type of gene mutations is known as point mutations as it affects only a particular locus of the chromosome. Gene mutations are so common in individuals but most of these gene mutations are harmless.
Salient features of gene mutations
Most frequently asked questions
Ans. Sickel cell anemia, Haemophilia, Retinitis pigmentosa, Schizophrenia.
Ans. When a person infected with a malarial parasite, the parasite invades the RBC cells and reproduce inside it. The RBC cells of Sickel cell affected patients, the RBC cells life span is only 30 days. The short life span of RBC cells of the patients do not allow the parasite to reproduce properly and thus they are not affected by malaria.
Ans. Exposure to certain chemicals like formaldehyde, mustard gas or X-rays or UV radiations or heat/temperature.
Ans. Deletions, Duplications, Inversions and Translocations.
Ans. RNA viruses have high mutation rates than DNA viruses.
Excellent article by the author on understanding the basics of mutation. If we see the evolution of life on Earth then there must be mutations of various degrees and effectiveness which changed the primitive organisms from one form to other and those which could stand the vagaries of nature survived and others perished. This gave birth to some new species having characters of earlier species plus some new ones that were created due to the mysterious mutation mechanism. So today if we see so many varieties of the fauna life on Earth that is definitely the ultimate outcome of those earlier mutations that happened millions of years before to create newer and newer life forms that originated from the same parental mass. This helps in explaining the variety of life available today across the globe. In the lower level lifeforms the mutations are still going on creating new forms of the same species and for the scientist, it is an important area to study as it would help in understanding how a virus can mutate and become more aggressive or cease to become fatal whatever the case may be. Some of the present medical researches are targeted in these areas which would definitely help the mankind in a great way.