Gene Mutation





What is Gene Mutation?

Gene alteration or Gene mutation may be defined as a change in the structure gene which is heritable. Hereditary characters are due to the effects of genes. Sometimes, a slight slip occurs in the replication of genes and this change in gene duplication is known as gene mutation. Thus gene mutations alter the information conveyed by a gene. The mutant gene and the original gene are situated at the same fixed point on a particular chromosome. Since a gene is located at a fixed point on the chromosome, a gene mutation is called point mutation. This type of mutation occurs at the molecular level usually at the time of DNA replication when new DNA strands are synthesized. Hence, gene mutations are also said to be copy-error mutations. These mutations mostly include alteration in the sequences of nucleotides in the nucleic acids which form the genetic material.




Occurrences of Gene Mutation:

i) Gene mutation occurs in one or a few nucleotides within the DNA molecule.
ii) Gene mutation generally occurs during the replication of DNA molecule.
iii) A gene may mutate several times and thus gives rise to multiple alleles.
iv) A mutant gene may even mutate back to its original form. This type of mutating is known as reverse mutation or back mutation.
v) Gene mutations may occur in nature. This type of mutation is known spontaneous mutation.
vi) Mutant gene does not express immediately because they are generally.
vii) Mutagenic agents (X-rays, gamma rays, chemicals etc.), can cause the mutatiThis type of mutation is known as induced mutation.
viii) Mutations that occur on the gametes are heritable.

Gene Mutation

Types of Gene Mutation

The gene mutations involve a change in base sequences. On the basis of alteration of base sequences, gene mutations may be of the following types—
(i) Deletion : One or more bases are deleted from DNA which represents a gene.
(ii) Insertion : One or more bases are added in DNA.
(iii) Inversion : In This case, the sequence of bases are reversed order in DNA.
Changes of genes in the above three types are due to breakage and reunion of DNA segments.
(iv) Replacement or Substitution :In this case, mutation occurs due to replacement of a base pair during replication of DNA.





Substituions are of two kinds, such as transitions and transversions.

Transitions

Transitions are those base pair replacements where a purine is replaced by another purine and a pyrimidine is replaced by another pyrimidine. It means that AT is replaced by GC and vice versa. In this case AT (Adenine and Thymine) replaced by GC (Guanine and Cytosine) and vice versa. This type of mutation is more common than transversion.

Transversions

Transversions are those base replacements where a purine is replaced by a pyrimidine and vice versa. It means that CG can be replaced by GC and AT is replaced by TA.

Both, transitions and transversions take place due to mistakes in the incorporation of nucleic acid precursors or due to mistakes committed during replication.
Gene mutations occurring during gamete formation are transmitted to all the cells of the offspring and may be significant for the future of the species. Somatic gene mutations which arise in the organism are inherited only by those cells derived from the mutant cells by mitosis. Whilst they may affect that organism, they are lost on the death of the organism. Somatic mutations are probably very common and go unnoticed.




Effect of Gene Mutation

The effects of gene mutation are extremely variable. Most minor gene mutations pass unnoticed in the phenotype since they are recessive, but there are several cases where a change in a single base in the genetic code can have a profound effect on the phenotype. Sickle cell anemia in humans is an example of base substitution mutation affecting a base in one of the genes involved in the production of hemoglobin. The respiratory pigment, hemoglobin, is made up of four polypeptide chains (two a chains and two p chains) attached to the prosthetic group haem. The polypeptide chains influence the oxygen-carrying capacity of the hemoglobin molecule. A change in the base sequence of the triplet coding for one particular amino acid out of the 146 in the P chains gives rise to the production of sickle cell hemoglobin (HbS). The amino acid sequences for the normal and abnormal chains differ in the substitution of valine for glutamic acid at one point in the abnormal polypeptide chains of hemoglobin S.



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