Radiations

Radiations

	Radiations - Among the physical mutagens radiation is the most important. The energy content of a radiation depends upon its wavelength. In general, the shorter the wavelength the greater the energy value of the radiation. High energy radiations can change the atomic structure of a substance by causing the loss of an electron and the formation of anion.
Sometimes an electron pair may be moved from an inner to an outer orbital shell. This brings about excitation of the atom. In this excited state the atom is highly reactive and is called a free radical. Radiation which brings about such a state is called ionizing radiation. Alterations in nucleic acids caused by radiation are of great genetic importance. High-energy ionizing radiation and ultraviolet (UV) light are important mutagenic agents. Ionizing radiation has greater penetration power than UV-radiation and produces free radicals which tend to labilize molecules.
This type of radiation causes single-strand breaks in DNA and produces deletions. Both DNA and RNA preferentially absorb UY-light, causing their nitrogen-containing bases to become highly reactive free radicals. The resulting un stability causes the conversion of one base to another (a purine to another purine or a pyrimidine to another pyrimidine).
If this change occurs in mRNA only a few inactive proteins will be formed, because mRNA is soon broken down. Substitutions in DNA, however, may have a lasting effect. Alt the proteins coded by the DNA may be defective. Moreover, if the mutation happens to take place in germ cells the mutated DNA strands could be passed on to succeeding generations.
	The primary mutagenic effect of UV-light appears to be due to the production of thymine dimers. The 5,6 unsaturated bonds of adjacent pyrimidines become covalently linked to form a cyclobutane ring. Irradiation of a bacterial culture and subsequent extraction of DNA yields three possible types of pyrimidine dimers in DNA: Thymine-thymine - 50% Thymine-cytosine - 40% Cytosine-cytosine - 10% Pyrimidine dimers can also be formed between adjacent strands. In RNA pyrimidine dimers are formed between adjacent uracil and cytosine rings. Pyrimidine dimers cannot fit into the DNA double helix and cause distortion of the molecule. If the damage is not repaired, replication is blocked, leading to lethal effects. Distortions in DNA caused by thymine dimers can be corrected by a repair mechanism. An exonuclease recognizes the distorted region and excises it.

A second enzyme, DNA polymerase inserts the correct bases in the gap. A third enzyme, ligase, joins the inserted bases. The DNA is thus restored to its original condition.

UV -radiation also causes addition of water molecules to pyrimidines in both DNA and RNA resulting in the formation of photohydrates .

The water molecule is adde4 across the C5-C6 double bond X-rays bring about mutations by breaking the phosphate ester linkages in DNA. The breakage may take place at one or more points.

As a result, a large number of bases are lost (deletion) or rearranged. In double stranded DNA breaks may occur in one or both strands. Only the latter type are lethal. Sometimes two double-stranded breaks may occur in the same molecule and the two broken ends may rejoin.

The part of the DNA between the two breaks is eliminated, resulting in deletion. The damage caused to nucleic acids by UV light and X rays is utilised to sterilize bacteria and viruses.

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Suppressor Genes Frameshift Suppressions Molecular Basis of Mutations Spontaneous Mutations Tautomerism Induced Mutations Chemical Mutagens Base Analogues Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases Agents Producing Distortions in DNA Radiations Types of Mutations

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