TITLEMUTATION

PREPAREDREZGAR I.S.A. SINJARI

Gene:A gene is a segment of the DNA molecule that codes for the production of a protein.

Chromosome:A chromosome is a structure that is composed of a sequence of genes.

Mutation:Mutation refer to sudden heritable change in the phenotype of an individual. (A mutation is a change in the nucleotide sequence of the DNA molecule.)Somatic mutations affect only the individual in which they arise.Germ cell mutations alter gametes, affecting the next generation.The effect of mutation depends on both the type mutation and its location .

Importance of mutation:The mutation is the main source of all genetic variations and because it

Types of Mutations according to Cause:

1 -Nature (Spontaneous) Mutation

2- Artificially (Induced) Mutation

Nature (Spontaneous) MutationMost mutations are spontaneous, rather than induced by a mutagenSpontaneous mutations result from DNA replication errorsThe cause of a spontaneous mutation is unknown.The spontaneous mutation rate in eukaryotes is between 10-4-to-10-6 per gene per generation

2- Artificially (Induced) Mutation An induced mutation is brought about by a

mutagen.

A mutagen is a physical agent or a chemical agent that causes an alteration of the base sequence of the DNA molecule.

Types of Mutagens: Physical mutagens

heat ultraviolet radiation X-ray Gama ray Alpha and Beta particles

Chemical mutagens Alkylating agents benzpyrenes aflatoxin Base analogues

Ultraviolet Radiation

Ultraviolet radiation causes the formation of thymine dimers.

Thymine dimers block DNA replication. Cell excises (cuts out) the thymine

dimers using enzymes and repairs the damage.

A mutation occurs when the repair is faulty

Types of Mutations:1- Gene mutation

2- chromosome mutation

Types of Gene Mutations

a. Transitions convert a purine-pyrimidine pair to the other purine-pyrimidine pair (e.g., AT to GC or TA to CG).

b. Transversions convert a purine-pyrimidine pair to a pyrimidine-purine pair (e.g., AT to TA, or AT to CG).

c. Missense mutations have a base-pair change resulting in a different mRNA codon, and therefore a different amino acid in the protein.

d. Nonsense mutations change a codon in the ORF to a stop (nonsense) codon, resulting in premature termination of translation, and a truncated (often nonfunctional) protein

E. Neutral mutations change a codon in the ORF, but the resulting amino acid substitution produces no detectable change in the function of the protein (e.g., AAA to AGA substitutes arginine for lysine. The amino acids have similar properties, so the protein’s function may not be altered).

F. Silent mutations occur when the mutant codon encodes the same amino acid as the wild-type gene, so that no change occurs in the protein produced (e.g., AAA and AAG both encode lysine, so this transition would be silent).

G. Frameshift mutations result from insertions or deletions when the number of affected base pairs is not divisible by three.

2 -chromosome mutation

Variations in chromosome structure or number can arise spontaneously or be induced by chemicals or radiation. Chromosomal mutation.

All chromosome structure mutations begin with a break in the DNA, leaving ends that are not protected by telomeres, but are “sticky” and may adhere to other broken ends.

Mutations involving changes in chromosome structure occur in four common types:

a. Deletions.

b. Duplications.

b. Inversions

d. Translocations

1-Deletion

In a deletion, part of a chromosome is missing. Deletions do not revert, because the DNA is missing. Deletion of the centromere results in an acentric chromosome that is lost, usually with serious or lethal consequences. (No known living human has an entire autosome deleted from the genome.)

2-Duplication

Duplications result from doubling of chromosomal segments, and occur in a range of sizes and locations.

a. Tandem duplications are adjacent to each other. b. Reverse tandem duplications result in genes arranged in the opposite order of the original.

c. Tandem duplication at the end of a chromosome is a terminal tandem duplication

3-InversionInversion results when a chromosome segment excises and reintegrates oriented 1800 from the original orientation. There are two types.

a. Pericentric inversions include the centromere. b. Paracentric inversions do not include the centromere.

Inversions generally do not result in lost DNA, but phenotypes can arise if the breakpoints are in genes or regulatory regions.

4- TranslocationA change in location of a chromosome segment is a translocation. No DNA is lost or gained. Simple translocations are of two types.a. Intrachromosomal, with a change of position within the same chromosome. b. Interchromosomal, with transfer of the segment to a nonhomologous chromosome. i. If a segment is transferred from one chromosome to another, it is nonreciprocal. ii. If segments are exchanged, it is reciprocal. Gamete formation is affected by translocations.

Mutation breeding mutation is one of the special methods of

crop movement.

Mutation breeding is commonly used in self pollination and asexually propagated species.

mutation breeding is rarely used for genetic improvement of cross pollination species.

mutation breeding will be rewarding under following situations(Chopra and Sharma, 1985)

1) When a high yielding variety has oligogenic defect such as susceptibility to disease, mutation breeding is the best source of a line for breeder.

2) When there is a tight linkage between desirable and undesirable characters mutation breeding is the best way of overcoming such problems.

3) When in fruit crops, the improvement has to be made without change in the taste and colour of the fruit it can best be achieved through mutation breeding.

4) In those species where generation cycle is very long, such as fruit trees and forest trees, mutation breeding is the only short cut method.

Comparison of hybridization breeding and Mutation breeding

Mutation breeding

1. One genotype or variety sufficient.

2. Crossing is not involved.

3. It comes under special method of plant breeding.

4. It takes lesser time for release of a new variety.

5. Required mutagenic treatment .

6. It required more skill in handing mutagens and treated material.

Hybridization breeding

1. Minimum two genetically different genotypes are required.

2. Crossing is involved.

3. It comes under general method of plant breeding.

4. It takes 14-15 years for release of a new variety.

5. non Required mutagenic treatment.

6. It required less skill in handing materials

QuestionIf you wanted to change the one character any way (type) you will be choosing1- gene mutation

2- chromosome mutation