Genetic variations and Gene Rearrangements Mutation
Def.: It is a physical change of one or more nucleotide pairs in the DNA of a cell. The change is inherited by every descendant of the mutant cell.
Classification: Genotypic classification. Phenotypic classification (functional). Classification according to the inducibility.
A. Genotypic CLASSIFICATION Point mutation (micro lesion): Changes in a single nucleotide base pair by substitution, addition or deletion (frame shift ) Substitution of purine by purine or pyrimidine by pyrimidine is called Transition. Substitution of purine by pyrimidines is called transversion.
Multisite Mutation ( macro lesion ): Change of multiple nucleotides :- Deletion: Missing of nucleotides Duplication: a number of nucleotides are repeated. Insertion: addition of novel base pairs. Rearrangements: all the base pairs are present but the order changes. Frame shift: may be point (addition or deletion of one bp) also multisite (addition or deletion of more than one base pairs).
Original: The fat cat ate the wee rat. Frame Shift ( a added): The fat caa tet hew eer at
B. Phenotypic mutation Because not all mutations are harmful and many point mutations are silent this classification is done on the basis of the effect of mutation ( phenotypic changes ) Same sense Missense Non-sense
Same sense mutation: Means mutation in one base in a codon that results in another codon for the same amino acid for example leucine. UUA UUG Leucine Mutation Leucine
Missense mutation Means mutation in an amino acid codon that results in a codon of another amino acid. TTG Tryptophan GTG Glycine The effect of missense depends on the location of the changed amino acid in the polypeptide chain.
Nonsense mutation: Means mutation that changes a codon into one of the 3 chain termination codons ( UAG, UAA, UGA) The effect of nonsense mutation depends on where the chain is terminated i.e. the truncated protein may have no activity, some activity or full activity.
C. Classification according to inducibility Mutation can occur spontaneously with no external applied agent, also can be induced by the application of physical or chemical agents (mutagens). Spontaneous mutation Induced mutation Physical (UV- ionizing irradiation) Chemical -Base analogs -Base modifiers -Intercalators
Spontaneous mutation It can arise as a result of: DNA polymerase incorporation errors during replication or repair by mispairing and misrepairing. (errors of DNA polymerase 10-4 base pair/cell/generation. After editing dropped to 10-6 to 10-7 ; after repairing dropped to 10-9 to 10-10. Tautomeric shift in the bases (structure isomer).
involves the movement of hydrogen and shifting of bonding
Induced mutation Mutation rates can be increased by chemical or physical mutagens Physical agents 1. Ultra violet light (UV): Cause pyrimidine dimers (covalent linking of two adjacent pyrimidine on the same strand) like T-T (68%) C-T (13%), T-C (19%) and C-C (3%).. 2. Ionizing radiation Causes DNA strand break either single or double strands and inter stand covalent cross linking (hydrogen bonds changes to covalent bonds).
Chemical mutagens Chemicals that make mutation in DNA can be categorized in 3 groups 1. Chemicals that mimic normal DNA bases (Base analogs ). 2. Chemical that react with DNA bases (Base modifiers ) 3. Chemicals that bind DNA bases (Intercalators).
1. Chemicals that mimic normal DNA bases ( Base analogs ): 5- Bromouracil mimics Thymine. 2- Aminopurine mimics Adenine These analogs are structurally related to bases but differ in pairing manner.
2. Chemical that react with DNA bases ( base modifiers ) These chemical react directly with the nucleotide bases, altering the chemical structure as examples:- Nitrous oxide and Hydroxyl amine. Alkylating agents: These are the most commonly mutagenic agents adding methyl or ethyl group to the oxygen of bases. Examples : Nitrosoguanidine (NTG) nitrogen mustard gas, methyl-methane sulfonate (MMS) and ethyl methane sulfonate (EMS).
3. Chemicals that bind DNA bases (Intercalators). Some molecules such as Acridine dyes and Acridine like derivatives (proflavin, ethidium bromide) have the same dimensions as the normal bases so can slide between two adjacent base pairs ( intercalating ) causing frame shift.
Effects of Mutation 1- Have no effect on the expression of a gene means silent mutation. 2- Changes the level of gene expression (increase or decrease). 3- Produce a related but structurally different protein. 4- Mutation may proceed to carcinogenesis. 5- Deletion mutation of the virulence gene in bacteria can be used as a reference strain for vaccine (low virulent or avirulent strain).