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 CLASSIFICATIONPoint 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 mutationMeans mutation in an amino acid codon

that results in a codon of another amino

acid.

TTG GTG

Tryptophan 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 inducibilityMutation 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 mutationIt 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).