There are two types of mutations: (i) Gene mutations or point mutations, and (ii) Chromosomal mutations.
A chemical change that occurs in the DNA of a cell is called a gene mutations or point mutations. Such a mutation may alter the sequence of the nucleotides within a part of the DNA molecule. This alternation changes the information on the DNA chain and results in differences in the proteins being produced.
For example, in sickle cell anemia, the mutation of a single gene causes a slight change in the structure of the protein molecule of hemoglobin, and because of that slight change, the blood cell that carries the hemoglobin takes a sickle shape.
According to many scientists, the mutations may be caused naturally by the radiation that constantly enters the earth’s atmosphere from the cosmos. For example, gene mutations are probably caused when the sex cells (gametes) of an organism are exposed to X-rays, gamma rays, cosmic rays and ultraviolet rays. In addition to this radiation, certain chemicals called mutagens can change nucleotides within DNA molecules. The details of mutagens are as follows:
Energetic, penetrating radiations such as X-rays and ultraviolet light are powerful physical mutagens. Very high energy radiations can penetrate all organisms and produce large number of a great variety of mutations. For example, X-radiation is a powerful physical mutagen which can break chromosomes, oxidise deoxyribose, deaminate and dehydroxylate bases and form peroxides.
Although less energetic than X-rays, ultraviolet light is also a powerful mutagen, largely because the base of nucleic acids absorb energy of its wavelengths. While UV’s energy can also break chromosomes, its chemical effect on bases is particularly significant mutagenesis.
Chemical mutagens such as nitrous acid, formaldehyde, ethylethane, sulphonate, etc., produce mutations by the addition or subtraction of whole nucleotides. C. Auerbach was the first to find mutations can also be induced due to certain chemicals. She made this important discovery during World War II.
The chance for a particular human gene to mutate in one generation is between 1 in 10,000 to 1 in 1,000,000. Since humans have at least 30,000 genes, it is likely that each person carries at least one mutation.
Another way for the genetic traits of an organism to be altered is through changes involving whole chromosomes or parts of chromosomes. Structural changes in chromosomes are also caused by radiation, chemicals, and even by some virus infections.
Changes occur in the structure of chromosomes, during cell division. When homologous chromosomes pair up, linked genes on the chromosomes may break apart. The genes may join another chromosome, or they may be lost.
Thus, a deletion involves the loss of a piece of chromosome. If a chromosome breaks and the parts do not reattach, the pieces may be lost. This is the most serious kind of chromosome mutation. Here, bits of genetic information are not available to the offspring.
Duplication occurs when one extra, but identical piece of chromosome is added to the normal chromosome.
When an inversion occurs, the pieces of chromosome break apart and pieces rejoin the same chromosome in a different order. Usually inversions have no harmful effects on the offspring.
Due to the mutagens two types of mutation visible and lethal are found. Mutations are located on either sex chromosomes or autosomes.