Genes, chromosomes also, are usually constant, however it was found that it occurs from time to time, at the various bodies, sudden variations, more or less obvious. These variations, called mutations, are due either to a single gene change or entire chromosome change or segments of it. Some of these abrupt changes, immediately became hereditary to direct descendants, have been observed since long time.
There are not known so far causes that have caused and continue to cause the appearance of mutations in nature, but many researchers have struggled to find their means of artificial generating .
Thus, in 1927 Muller managed to rise artificially in Drosophila, the occurrence of mutations, namely by the action of Roentgen rays. The radiation amounted to Drosophila the mutation rate of 0.2% (spontaneous) to over 13%. He thus introduced a new method in genetic work, start a new branch of genetics, called radiogenetics.
Provocative of mutations of Roentgen rays were confirmed shortly afterwards by Gager and Blakeslee in Datura and Stadler (1928) in maize.
After a while, after the execution of these experiences,he managed to cause mutations by ultraviolet light and ionizing radiation, other than Roentgen rays, ie rays, neutrons, etc. and by temperature.
Among physical mutagenic factors,ionizing radiations have a most effective action (corpuscular radiation - electrons, protons, neutrons, deutroni, alpha particles - as well as high-energy electromagnetic radiation - gamma rays and X-ray). Gamma rays are met in nature: they are similar to X rays, but far more penetrating, with smaller wavelength, having a strong physiological action.
Irradiation has been proven that exercises two kinds of actions, namely:
- A primary physical action, direct to the gene, changing or destroying a part of the genetic material.
- A secondary action, radiochemical, indirect, by converting water into hydrogen peroxide, which in turn, acts on the genetic material.
Exposure to X-rays and other forms of high energy particles can cause both the appearance of mutations complete similar to gene mutations, which are obtained in nature, and the structural-chromosomal mutations, the latter are due to either inhibition of cell division or chromosome breakage, which causes chromosome rearrangements and abnormalities in mitosis and meiosis .
Factors that influence the most number of mutations occurring by the action of radiation are the following:
- The dose of irradiation. Number of mutations increases generally proportional to the radiation dose applied.
- The type of radiation, neutrons compared with gamma radiation having an efficiency five times higher, and fast neutrons, to 10-20 times higher, in some plants they can overcome even 100 times.
- Species, respectively radiated variety. Plants that possess a small number of chromosomes, are more radiosensitive while plants with a large number of chromosomes are more radio resistant.
- Water content of tissue irradiated, mutation rate is much higher, eg damp seeds than the dried seeds, which demonstrates the role of water in an indirect mutagenic action.
- Temperature, high temperatures, reducing the action of X-rays.
- The content of oxygen, a high concentration of oxygen significantly increasing the number of mutations and chromosomal aberrations.
No comments:
Post a Comment