Gene Mutations and their interpretation

 Gene mutations and their interpretation.

Gene Mutations:-

gene mutation is abrupt inheritable qualitative or

quantitative change in the genetic material of an organism. Since in most organisms

genes are segments of DNA molecule, so a mutation can be regarded as a change in the

DNA sequence which is reflected in the change of sequence of corresponding RNA or

protein molecules. Such a change may involve only one base/base pair or more than one

base pair of DNA. Mutations occur in a random manner, i.e., they are not directed

according to the requirements of the organism. Most mutations occur spontaneously by

the environmental effect, however, they can be induced in the laboratory either by

radiations, physical factors or chemicals (called mutagens). A unicellular organism is

more subjected to environmental on-slaughts since it is at the same time a somatic or

germ cell.In multicellular organisms the germ cells are distinct cells, and are relatively

protected from the environment. Mutation has a significant role to play in the origin of

species or evolution.

HISTORICAL BACKGROUND

The earliest record of point mutations dates back to 1791,

when Seth Wright noticed a lamb with exceptionally short legs in his flock of sheep.

Visualising the economic significance of this short-leged sheep. i.e., short legged sheep

could not cross the low stone fence and damage the crop fields in the vicinity, he

produced a flock of sheeps, each of which having short legs by employing artificial

breeding techniques. The short legged breed of sheep was known as Ancon breed. Later

on, the trait of short legs was found to be resulted from a recessive mutation and the

short legged

individuals were found to be homozygous recessive.

Hugo de Vries was the first hybridist who used the term “mutation” to describe the

heritable phenotypic changes of the evening primrose, Oenothera lamarckiana. Many

mutations described by de Vries in O. lamarckiana, are now known to be due to variation

in chromosome number or ploidy and chromosomal aberrations (viz. gross mutations).

The first scientific study of mutation was started in 1910, when Morgan started his work

on fruitfly, Drosophila melanogaster and reported white eyed male individuals among red

eyed male individuals. The discovery of white eyed mutants in Drosophila was followed

by an extensive search of other mutants of Drosophila by Morgan and his co-workers

and other geneticists. Consequently about 500 mutants of Drosophila have been reported

by geneticists all over the world. Later on, several cases of mutations have been reported

in a variety of microorganisms (e.g., bacteriophages, bacteria (Escherichia coli),

Neurospora,etc., plants (i.e., pea,

snapdragon, maize, etc.) and animals, i.e., rodents, fowls, man, etc.).

KINDS OF MUTATIONS:-

There exists a lot of controversy about the possible kinds of mutations among

geneticists. They have been classified variously according to different criteria as follows

:-

TYPE OF GENE MUTATIONS AND THEIR INTERPRETATION :-

A. Point mutation.:-- When heritable alterations occur in a

very small segment of DNA molecule, i.e., a single nucleotide or nucleotide pair, then this

type of mutations are called “point mutations”. The point mutations may occur due to

following types of subnucleotide change in the DNA and RNA.

1. Deletion mutations. The point mutation which is caused

due to loss or deletion of some portion (single nucleotide pair) in a triplet codon of a

cistron or gene is called deletion mutation.Deletion mutations have been frequently

reported in somebacteriophages (Phage T4).

2. Insertion or addition mutation. The point mutations

which occur due to addition of one or more extra nucleotides to a gene or cistron are

called insertion mutations. The insertion mutations can be artificially induced by certain

chemical sub- stances called mutagens such as acridine dye and proflavin. A proflavin

molecule, it is believed, insert between two successive bases of a DNA strand, thereby

stretching the strand lengthwise. At replication, this situation

would allow the insertion of an extra nucleotide in the

complementary chain at the position occupied by the

proflavin molecule.

The mutations which arise from the insertion or deletion of individual nucleotides and

cause the rest of the message downstream of the mutation to be read out of phase, are

called frameshift mutations. They result in the production of an incorrect, hence, inactive

protein, due to which the death of the cell may occur.

3. Substitution mutation. A point mutation in which a nucleotide of a triplet is replaced by

another nucleotide, is called substitution mutation. The substitution mutation affect only

a particular triplet codon.Such an altered code word (triplet codon) may designate a

different amino acid and may result in the production of a protein with a single amino

acid substitution. The substitution mutations alter the phenotype of an organism

variously and are of great genetical significance.They may be of following types :-

(i) Transition. When a purine (e.g., adenine) base of a triplet codon of a cistron is substituted by another purine base (e.g., guanine) or a pyrimidine

(e.g., thymine) is substituted by another pyrimidine base,(e.g., cytosine) then such kind

of substitution is called transition. The transitional substitution mutations occur due to

tautomerization.

(ii)Transversion. The substitution mutation when involves the substitution or

replacement of a purine with a pyrimidine or vice versa then that type of substitution

mutation is called transversion mutation. The existence of transversion mutation was

first of all postulated by E.Freese in 1959. We have still poor information about the

mechanism of induction, identification and characterization of transversion mutations.

Moreover, it is extremely difficult to recognize transversion mutations

genetically.However, they can be recognized only by analysis of amino acid substitutions

in proteins.

Tautomerization :-

In a DNA molecule, normally, the purine, adenine (A) is linked to the pyrimidine, thymine

(T), by two hydrogen bonds, while the purine guanine (G) is linked to the pyrimidine,

cytosine (C) by three hydrogen bonds.

A tautomeric shift is believed to occur when the amino (NH2) form of adenine is changed

to an imino (NH) form. Similarly, a tautomeric shift may occur in thymine changing it form the keto (C= O) form to the rare enol (COH) form. When a base occurs in its rare or

tatuomeric state, it cannot be linked to its normal partner. However, a purine, such as

adenine can in its rare state forms a bond with cytosine (besides thymine),

provided the cytosine is in its normal state.

Effect of Chemical Mutagens on Nucleotide Sequence :-

(a) Alteration in Resting Nucleic Acid

1. Deamination. Some chemical substances such as

nitrous acid causes transitional mutation due to oxidative deamination of DNA bases. In

the process of oxidative deamination, the amino group (NH2) of a DNA base is replaced

by hydroxyl (OH) group by the chemical mutagen. Thus, adenine is deaminated into

hypoxanthine by nitrous acid.By tautomeric shift the hypoxanthine (HX) is converted into more common or keto-tautomer which pairs with cytosine. The A: T pair, thus, can be converted to a G : C pair.Similarly, deamination converts cysosine to uracil, which has pairing properties similar to thymine and in such a case G: C pair would be changed intoA : T pair.

2. Hydroxyl-amine (HA = NH2.OH) and hydrazine

(HZ = NH2 NH2).When DNA is treated with hydroxylamine (HA), its cytosine base is the

strongest reacting base.

Hydroxylamine probably cause hydroxylation of cytosine at amino group giving rise to

hydroxylcytosine, which then subsequently pair with adenine. Thus, hydroxylamine (HA)

induces in DNA a GC—> ATbase pair transition

The hydrazine affects DNA by breaking of rings of uracil and cytosine giving rise to

pyrazolone and 3-aminopyrasole, respectively. The treatment of RNA or DNA with

anhydrous hydrazine results in the destruction of their pyrimidines.

3. Alkylating agents. Some alkylating agents carry one, two, or more alkyl groups in a

reactive form and act as strong mutagens. Examples of some most extensively studied

alkylating agents include diethyl sulphate(DES), dimethyl sulphate (DMS), methyl

methane sulphonate (MMS), ethyl ethane sulphonate (EES) and ethyl methane

sulphonate(EMS).

(b) Alteration during Replication of Nucleic Acid

1. Base analogues. Certain chemical substances have molecular structure similar to the

usual DNA bases that, if they are available, such analogues may be incorporated into a

replicating DNA strand.For example, 5-bromouracil (5BU) or its nucleoside

5-bromodeoxyuridine (5-BUdR) in its usual (keto) form is a structural analogue of

thymine (5-methyl uracil) and it will substitute for thymine. Thus, an A-T pair becomes

and remains A-BU. There is some in vitro evidence to indicate the BU immediately

adjacent to an adenine in one of DNA strands causes the latter to pair with guanine. But,

in its rare (enol) state, 5BU behaves similar to the tautomer of thymine and pairs with

guanine.This converts A : T to G : C

2–Aminopurine (2-AP) is another base analogue which is a relatively undifferentiated

purine that apparently can pair with cytosine and thymine. It is thought that 2-AP acts by

“switching” pyrimidines: for example, it may be incorporated opposite thymine during

one round of replication and then pair with a cytosine at the next round to produce an AT

→ GC transition (see Goodenough and Levine, 1974).

Mutagenic agents. The mutagenic agents are of the following kinds :

A. Radiations. The radiations which are important in mutagenesis are of two categories :

one type is ionizing radiations such as X-rays and gamma rays; alpha and beta rays;

electrons, neutrons, protons and other fast moving particles.

The second type is non-ionizing radiations such as ultraviolet and visible light.

Dimerization. The ultraviolet radiation produces several effects on DNA, one being the

formation of chemical bonds between two adjacent pyrimidine molecules in a

polynucleotide and particularly, between adjacent thymine residues.As the two thymine

residues associate, or dimerize to form a dimer, their position in the DNA helix becomes

so displaced that they can no longer form hydrogen bonds with the opposing purines

and thus regularity of the helix becomes distorted. Thus, dimerization interferes with the proper base pairing of thymine with adenine, and may result in thymine’s pairing with guanine. This will produce a T-A To C-G transition.

Chemical mutagens. Many chemical substances have been responsible to increase the mutability of genes. The ability of chemicals to induce mutation was first of all demonstrated by Auerbach and Robson in 1947 using mustard gas and related compounds as the nitrogen and sulphur mustards, mustard oil and chloracetone in experiments with male Drosophila melanogaster