Mutation - Sudden heritable change

MUTATION 
SUDDEN CHANGE IN A CHROMOSOMES /GENES

Mutation types and molecular basis of spontaneous and induced mutations
üSudden heritable change.
üThe term introduced by De Vries.
üIt is the basis of discontinuous variation in population.
üIt may occur at chromosomal level or gene level.
üDobzhansky stated that mutation is a mistake or misprint in cell division.
Genome mutation/chromosomal mutation
—The changes in the structure and number of chromosome (genome) are called Genome mutation/chromosomal mutation.
— Mutation occur in chromosomes – chromosomal abberations.

—Genome-total genetic material contained within the chromosome of an organisms

—46 chromosomes = our genome.

—Genome mutation -1/10000 to 1/1000000.

Types of Genome mutation/chromosomal mutation

—Changes in the structure of chromosomes

—Change in the number of chromosomes

—Changes in the structure of chromosomes

—Change in the number of chromosomes

Changes in the structure of chromosomes

Deletion

— A segment is repeated.

—Set of genes present in double doses.

—Position effect-phenotypic effect produced by change in position of a gene/gene groups.

Inversion

—The segment of chromosomes breaks and reunites in the reverse order.

—No loss or gain of genes in chromosome.—Rearranged in reverse order.

—2 types

—Paracentric inversion- centromere is not included.
—Pericentric inversion – centromere is included. 

.—It produces variation and speciation.

Translocation

—Non –homologous chromosomes exchange segments.

—Cross shaped structure during pairing.

—It causes position effect.

—It alters the linkage groups.

Change in the number of chromosomes

—It is called ploidy.

—Loss or gain of chromosome of a set / changes in the number of chromosome sets.—2 types
—Aneuploidy

—Euploidy

Aneuploidy

—Gain / loss of one/more chromosomes in set

—It is caused by non disjunction of chromosomes.

—3 types

—Monosomy

—Nullisomy

—Trisomy

Monosomy

—Loss of 1 chromosome from a pair.

—It is represented by 2n-1.

Nullisomy

—Both chromosomes are lost.

—It is represented by 2n-2.

Trisomy

— One chromosome is added in pair.

— It is represented by 2n+1.

 Mutagens

The agents cause changes in the genetic code which are then passed to future generations.

—The changes caused in the genetic code –mutations.—The formation of mutations –mutagenesis.

Mutagens cause mutation by several mechanisms such as 

—Replacement of base by another base.

—Addition of a new base.

—Deletion of a base.

—Insertion of a base.

—Deamination of base.

—Incorporation of base analogous.

—Tautomerization of a base.

Types of mutation

—Physical (radiations,temperature,light,etc.)and

—chemical mutagens(mustard gas,nitrous acid,Nitrogen mustard,sulphur mustard,colchicine,peroxides,formaldehyde,caffeine,phenol,etc)

—Biological (infectious agents)

Mode of action of chemical mutagens

—Alkylating mutagens donate alkyl groups to the DNA bases resulting in altered base pairing. 

—Alkylating agents are

—mustard gas,

—methyl methyl sulphonate

—ethyl methyl sulphonate

— Base analogue(similar to the DNA bases)

—Base analogous are similar to DNA bases .5-bromouracil (5-Bu) is a base similar to thymine.

—Hence 5-Bu is a base analogue.It is a mutagenic agent.

—5-Bu can substitute thymine during the replication of DNA.

—Frequently it changes into tautomeric form(enol form).

—In such a state 5-Bu pairs with guanine.

—Hence A-T pair may be converted into G-C pair which is a mutant caused by 5-Bu.

—Acridine dyes

—Acridine dyes  such as proflavin and acridine orange cause deletion/insertion.

—They induce mutations by intercalating themselves between the base pairs of DNA.

—It leads to weakening of bonds.

—It leads to addition /deletion of nucleotides.

Mode of action of radiations

—X-rays,alpha rays,beta rays, gamma rays, UV rays,infra red rays,etc.—Ionizing radiations are high energy radiations.

—They are powerful mutagenic agents.
—The non-Ionizing radiations are low energy radiations.

—They are weak mutagenic agents.

Direct hit

—The ionizing  radiations have high penetrating power.so they penetrate through the body coverings ,cell wall ,cell membrane,cytoplasm,nuclear membrane,nucleoplasms,etc. and attackthe DNA itself –direct hit.

—Ionizing radiations –biological molecules - collide with atoms –release electrons - these atoms are called ions -process of formation of ions =ionization.

Indirect hit

—Non-ionizing radiations –low penetrating power –do not reach the DNA directly (but they attack molecules)–indirect hit.