Mechanisms of defense against mutations.

Date: 2015-10-07; view: 429.

Classification of mutagens

Causes of mutation.

Mutations may arise spontaneously due to certain intracellular factors or be induced by environmental factors.

Spontaneous mutations occur at random and their frequency is rather low. They are thought to arise generally by errors in the processes of replication.

Induced mutations arise after influence of environmental factors that are called mutagens or mutagenic agents.

Physical mutagens

Chemical mutagens

Biological mutagens

For example the radiations (X-rays, gamma rais, alpha and beta rays, etc.) and temperature are physical mutagens, some viruses are biological mutagens, some chemical substances (nitrous acid, bromouracil, formaldehyde, peroxides, ect.) are Chemical mutagenes.

Mechanism of action of bromouracil following. During replication of DNA bromouracil may be complementary to adenine. Therefore the daughter chain of DNA contains bromouracil instead of thymine. During next replication bromouracil may be complementary to guanine. Thus next daughter chain of DNA contains guanine instead of adenine. The sequence of nitrogenous bases and sense of DNA genetic code are changed.

Living organisms have some mechanisms of defense against mutations. There are barriers (mechanisms, which prevent an origin and manifestation of mutations) and reparations (mechanisms, which correct mutations).

The main barriers are: triplet nature and redundancy or degeneracy of genetic code, repetitions of genes, double chain of DNA, and homologous pairs of chromosomes.

An example of reparation is the postreplication reparation. This reparation takes place after replication of DNA and it includes of four phases.

The first phase of reparation. The ferments complex finds a place of mutation. The main ferment is DNA-endonuclease.

The second phase of reparation. The ferments complex cuts off the DNA in the place of mutation and adjoining to him area of DNA. The main ferment is DNA-exonuclease.

The third phase of reparation. The ferment is named as DNA-polimerase produces syntheses of normal DNA instead of DNA that has cut off.

The fourth phase of reparation. The ferment is named as DNA-ligase sutures fragments of DNA.

Due to mechanisms of defense against mutations the living organism have possibility to save constancy its genotype and survive. It is the biological significance of these mechanisms.