How mutations form in living organisms and their types

Introduction

Characteristics of mutations

• Mutations are usually recessive in nature but dominant mutations are also occur in nature.


• Mutations are the raw materials for evolution.


• Mutations are mostly harmful to organisms but rarely they may be beneficial.


• Vital mutations were successfully used by scientists in crop improvment programs.


• Mutations may occur in a single gene, multiple genes, in a chromosome or in any part of the chromosome.


• Mutation can occur at anytime in the life of an organism.


• An organism will die if a mutation affect the vital functions of an organism.


• Some genes may mutate more frequently than others.

Types of mutations

1. Somatic mutation

   It 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.



2. Germinal mutation

   Mutations 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.



3. Gametic mutation

   It 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.



4. Zygotic mutation

   When 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.



5. Dominant mutation

   If 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.



6. Recessive mutation

   When 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.



7. Chromosomal mutation

   The 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.



8. Spontaneous mutation

   Without 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.



9. Back mutation

   This 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.



10. Lethal mutation

    Sometimes, 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.



11. Induced mutation

    If 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.



12. Biochemical mutation

    Mutations 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.



13. Macro mutations

    Some mutations are large with distinct phenotypic deviations in the organism. Such mutations are called macromutations. Ex: Homeotic mutations of fruit flies (Drosophila).



14. Micro mutations

    Mutations that bring slight effects in phenotypic features are known as micro mutations. A micro mutation involves alteration at a single gene locus.



15. Gene mutations

    The 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

1. Gene mutation refers to the changes that occur in the structural and chemical composition of a gene.


2. Usually, the DNA sequence of a gene will be altered in different ways. The changes may occur through rearrangement or addition or deletion of specific nitrogenous bases present in the DNA.


3. Gene mutation may occur in random irrespective of the needs of the organism. It may occur without the need of the organism or may not occur when the organism really needs it.


4. Most often mutations produce recessive defective genes in individuals.


5. Some types of mutations occur repeatedly and are known as recurrent mutations.


6. Some times a gene induces other genes to mutate in an organism. Such special genes are known as mutator genes.


7. A single gene in an individual to undergo mutations to form different forms of genes. These genes are known as multiple alleles. The blood groups of man, the different forms of hair like smooth, curly, and straight, etc. are caused by multiple alleles.


8. Environmental factors can influence the mutation rate but it cannot decide a particular mutation to occur.


9. A mutated gene can be transmitted through many generations without any further change but after some times it may mutate again to another variant gene or it may revert back to its original condition.

Most frequently asked questions

1. Name any four diseases caused through mutations.
   Ans. Sickel cell anemia, Haemophilia, Retinitis pigmentosa, Schizophrenia.


2. Persons suffering from sickle cell anaemia normally do not suffer from malaria. What is the reason for it?
   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.


3. State any two significances of mutation.
   Ans.
   1. Mutations aids in evolution.
   
   
   2. Scintists are using principle of mutation in crop improvement.
   
   
4. Name any three mutagens that may change your DNA?
   Ans. Exposure to certain chemicals like formaldehyde, mustard gas or X-rays or UV radiations or heat/temperature.


5. Name the four types of chromosomal mutations.
   Ans. Deletions, Duplications, Inversions and Translocations.


6. Why RNA viruses are more dangerous than DNA viruses?
   Ans. RNA viruses have high mutation rates than DNA viruses.