MUTATIONS

• A mutation is a sudden change in the structure or the amount of genetic material.

• Most mutations are harmful to an organism, some have no effect, and others are beneficial.

• Mutations appear to be normal and natural events. However, certain factors in the environment increase the rate of mutations. These factors are called mutagens.

• Mutation rate varies from gene to gene and species to species.

MUTAGENS Physical Factors

Chemical FactorsHigh temperature 1. pH changesVarious types of radiation 2. Certain chemicalsX-rays aflatoxin

(produced by molds)UV radiation benzeneIonization radiation chloroform (Alpha, beta and gamma) pesticides Cosmic radiation colchicine ozone

some food additives (nitrites etc.) mustard gas

(nerve gas)

AFLATOXIN (IT IS PRODUCED BY MOLDS)

EFFECT OF COLCHICINE

TYPES OF MUTATIONS There are two main kinds of mutations.

I. Chromosomal Mutations –

involve changes in the structure or in the number of chromosomes

II. Gene Mutations – involve changes that affect the genes. This type of mutations involves a change in one or more of the nucleotides in a strand of DNA ( gene)

Any change in the sequence of the template DNA strand is likely to change the message transcribed into mRNA. This is likely to change the structure of the protein that the cell makes.

TYPES OF MUTATIONS

Gene MutationsA) Substitution and inversion are called point

mutations.

They usually bring about only a minor change and sometimes the organism is affected only slightly or not at all.

Substitution by A

EX: CCC codes for the amino acid proline

CCA codes for the amino acid proline

• However, if the substituted amino acid occurs at a critical position in the protein, then a major defect may arise.

• Ex: Formation of hemoglobin S and sickle cell anemia. In this case, the glutamic acid code GAG for normal hemoglobin, is changed to GUG that codes for valine in hemoglobin S

Gene Mutations

DNA Sequence normal

ATG GTG CAC CTG ACT CCT GAG GAG AAG TCT GCC GTT ACT

ATG GTG CAC CTG ACT CCT GTG GAG AAG TCT GCC GTT ACT

DNA Sequence sickle

B) Insertion (addition) and deletion (removal) of one or more nucleotides result in frameshift mutations.Each leads to a major change since it causes a large portion of the gene’s DNA to be misread. The mutation usually makes the gene useless and organism lacks the protein that the gene normally specifies

Gene Mutations

EX: What would happen if the letter C were removed from the following sentence?“THE CAT ATE THE FAT RAT” (template DNA)

deletion

“THE ATA TET HEF ATR AT” Does it make any sense?

Gene Mutations

Clearly a frameshift mutation can result in a gene that codes for a completely nonfunctional protein. * Many inherited diseases are the result of this type of gene mutation.

• For a mutation to be inherited in a sexually reproducing organism, it must be present in the DNA of a gamete. Thus, the mutation must occur in a gamete or in any cell from which gamete develops.

• Mutations that occur in the body (somatic) cells normally cannot be inherited in sexually reproducing organisms, since body cells are not transmitted to offspring. Somatic cell mutations affect that particular organism only.

• The insertion or deletion of a base pair in the genetic code will cause a frameshift mutation unless the number of base pairs inserted or deleted is three.

• EX: Deletion of the second THE from the sentence “THE CAT ATE FAT RAT”

Gene Mutations

MUTATIONS AFFECTING BIOCHEMICAL PATHWAYS

• Beadle and Tatum irradiated multi-nucleate haploid asexual spores of Neurospora to produce random mutations; cultures derived from these spores were then mated with another strain to produce uni-nucleate haploid sexual spores.

Sucrose, a few salts, and one vitamin - biotin - provide the nutrients that Neurospora needs to synthesize all the macromolecules of its cells.

ONE GENE-ONE POLYPEPTIDE HYPOTHESIS

A) In this example, Beadle and Tatum tested a number of different mutant strains that require the amino acid arginine. These were grouped ( I, II or III) by their response to various intermediates in the metabolic pathway leading to arginine, which were provided as supplements to the minimal growth medium.

Analysis of the experimental results led to this model for a portion of the arginine biosynthetic pathway(which is actually a cycle). Because Group I strains grew on minimal medium supplemented with ornithine, citrulline, or arginine, they were thought to be missing enzyme A, required for formation of all three compounds.

ONE GENE-ONE POLYPEPTIDE HYPOTHESIS

Group II strains were thought to be missing enzyme B because they were unable to grow on minimal medium supplemented with ornithine but allowed the conversion of citrulline to arginine.

ONE GENE-ONE POLYPEPTIDE HYPOTHESIS

Group III strains were thought to be missing enzyme C because they grew on minimal medium to which only arginine has been added. A one-to-one correspondence between each specific enzyme and a specific gene locus was verified by special genetic crosses(not shown).

ONE GENE-ONE POLYPEPTIDE HYPOTHESIS

NEW GENETIC COMBINATIONS

• Lederberg and Tatum isolated two mutant strains of E. coli bacteria, each of which was unable to synthesize certain substances (nutrients).

• The mutants I and II were grown on a medium that supplied the nutrients they could not synthesize.

• After sometime, a few colonies grew when samples were cultured on the minimal medium.

• The survival of these bacteria resulted from a recombinant of traits (genes) from the two original mutant strains

A and B cannot be synthesized

D and E cannot be synthesized

MUTANT X MUTANT Y

ABDE

MUTANT X + MUTANT Y

MUTANT Z

MINIMAL MEDIUM