GENETICS TOPIC – MUTATIONS AND MOLECULAR BASIS OF MUTATION .

POINT MUTATIONS : Change in one or a few base pairs in a gene. These are very small changes when compared with the large-scale mutations. Point mutations can be divided into two general categories: Base-pair substitutions Base-pair insertions or deletions (frame shift mutations)

BASE – PAIR SUBSITUTION A base-pair substitution is the replacement of one nucleotide and its partner in the complementary DNA strand with another pair of nucleotides. THEY ARE DIVIDED INTO TWO TYPES: TRANSITION TRANSVERSION

TRANSITION AND TRANSVERSION Whenever a purine is replaced by a purine (A -G) or a pyrimidine is replaced by pyrimidine (C-T). They therefore involve bases of similar shape.    Transversions are interchanges of purine for pyrimidine bases, and vice versa. They therefore involve exchange of one-ring and two-ring structures.

FRAME SHIFT MUTATION: Whenever there is ADDITION OR DELETION of one or more than one base pair, it leads to change in the reading frame of the triplet codon. Proteins are built incorrectly.

BACK (REVERSE) MUTATION: Back Mutation is always II Mutation : Whenever a wild type phenotype changes to a mutant type this is known as FORWARD MUTATION. If a second mutation occurs at exactly at the same point where the first mutation has occurs and reverse back the effect of first mutation ( changes mutant to wild type ). This is called BACK MUTATION.

SUPPRESSOR MUTATION : SUPPRESSES THE EFFECT OF FIRST MUTATION. Whenever a mutation suppresses the effect of first mutation in the DNA, The second mutation occurring at some other point as compared to the first mutation. This is known as SUPPRESSOR MUTATION. SOMATIC MUTATIONS: Whenever a mutation occurs in somatic cell , it will be carried by all the other cell derived from this somatic cell . They will not pass to progeny. They will remain consistent to specific region or tissue.

GERMINAL MUTATION : OCCURS IN GAMETES (MAY NOT BE EXPRESSED UNTIL OFFSPRINGS). Mutation occurring in the reproductive cells or the gametes or the germ cell is known GERM TYPE MUTATION. This type of mutation is carried to the next progeny.

SPONTANEOUS MUTATION: They are mainly caused during DNA replication or by incorporation of incorrect nucleotide in the growing DNA chain. They occur naturally by changes in DNA sequence during replication. Caused by certain Mutagenic agents present in the environment

INDUCED MUTATION : Caused by changes in DNA brought about some environmental factor called MUTAGEN. E.g. – UV light, x-rays, gamma rays etc. EXAMPLE OF MUTAGENS : Ultraviolet light. Alpha, beta , gamma radiations. Radioactive dust.

MOLECULAR BASIS OF MUTATIONS:

MOLECULAR BASIS OF MUTATIONS : Each of the bases in DNA can appear in one of several forms, called tautomers, which are isomers that differ in the positions of their atoms and in the bonds between the atoms. The forms are in equilibrium. The keto form of each base is normally present in DNA whereas the imino and enol forms of the bases are rare. The ability of the wrong tautomer of one of the standard bases to mispair and cause a mutation in the course of DNA replication. was first noted by Watson and Crick when they formulated their model for the structure of DNA. Structures of bases are not static. Hydrogen atoms move from one position to another. Such changes are known as TAUTAMERIC SHIFT.

… ADENINE AND CYTOSINE are present in stable AMINO FORM. But sometimes changes into less stable IMINO FORM. THYMINE AND GUANINE are present in stable KETO FORM. But may change into less stable ENOL FORM. These shifts are reversible. Tautameric shift can lead to TRANSITION AND TRANSVERSION.

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