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Agriculture LibreTexts

Section D

  1. The physical agents :-Radiation
  • Gamma, X-ray, cosmic rays (very short wavelength) - Ionizing Radiation- It ionizes water and other molecules to form ions and free radicals.   Ionizing radiation has high-energy and penetrating power than ultraviolet radiation. that can break DNA strands and alter purine and pyrimidine bases that causes seriously damage DNA molecules and whole chromosomes. They act by creating free radicals and peroxides along the paths of radiation travel through the tissues.

These ions cause the initial damage of DNA ,which can either replicated or repaired incorrectly , causing both point and chromosomal mutation. Due to chemical reactions, this radiation breaks the bonds that hold DNA together , including sugar bases or breaking the DNA backbone.  

  • Non ionizing radiation -Ultraviolet rays (UV)-UV has a small wavelength of 200 – 400 nanometers but it possesses an intensive energy. UV light is a potent mutagenic agent because the purine and pyrimidine bases in DNA absorb light strongly in the ultraviolet range 254-270 nm. In this range of wavelength, UV light induces point mutations by causing photochemical (light-induced chemical) changes in DNA.UV light may cause two adjacent pyrimidine residues (cytosine or thymine) to form a dimer.UV light is absorbed by DNA and causes adjacent thymine bases on the same DNA strand to covalently bond together, forming thymine dimers or the UV light first causes two adjacent cytosine residues to form a dimer.

 

During DNA replication, both strands are used as templates to synthesize new strands. The cytosine dimer could cause adenine (instead of the normal guanine) to be incorporated into the new strand. Subsequent DNA replication will produce CC to TT mutation. UV exposure can lead to DNA lesions called thymine dimers. Although these lesions can eventually be repaired by a variety of mechanisms, including photoreactivation, excision repair, recombination repair, and SOS repair. The mechanism used depends inpart on the particular situation. LI>

Mutation rate :-

  • The frequency with which a given mutation is seen in a population provides only a rough figure of mutation rate, because mutation occurs at random.
  • Genes do, however mutate spontaneously at a characteristic rate, making it possible to assign probabilities to certain mutation events. The probability that a gene will mutate when a cell divides is called the mutation rate.
  • Spontanoeus mutation rate for the average gene is 0.000000001.
  • This shows that a mutation event is estimated to occur once in every million genes replicated.
  • Due to presence of a mutagen the rate of mutation increases from 0.00001 to 0.001.
  • This shows that a mutation event is estimated to occur once in every hundred thousand to one hundred thousand genes in the presence of a mutagen.