Every day your cells produce proteins that contain amino acids in a certain sequence.
Imagine, for example, that one day an epidermis cell in your skin produces a different protein. Suppose also that this protein is an enzyme that acts in a chemical reaction that leads to the production of a yellow pigment instead of the pigment normally found in the skin, melanin. This cell multiplies and suddenly a yellow spot appears on your skin.
Probably this protein may have changed in its amino acid sequence, having replaced one amino acid with another, which led to a change in its mechanism of action and, as a result led to the production of a different color pigment. Now, how the amino acid sequence in a protein is determined by the action of a certain gene that leads to pigment synthesis.
This change in the base sequence in the gene's DNA molecule is called a gene mutation.
Albinism is caused by a mutation in the enzyme tyrosine that transforms the amino acid thyrosine into the skin's pigment, melanin. This disease occurs in animals and plants and is hereditary.
Mutation and its consequences
If the change in amino acid sequence in the protein does not affect the functioning of the molecule and does not harm the body, it usually goes unnoticed, is indifferent.
Other times the change leads to a favor. Imagine, for example, that a certain cell in your gut starts producing an enzyme called cellulase that can digest the cellulose of the vegetables you eat. The mutation that led to this error is likely to be beneficial to you, which may even feed on shredded paper.
Often, however, the mutation can be harmful. At sickle cell anemia, the substitution of the amino acid glutamic acid for the amino acid valine in one of the hemoglobin chains leads to a change in the shape of the whole protein. This change changes the shape of the red blood cell, which is unable to carry oxygen. Another serious consequence is that sickle-shaped red blood cells stick to each other in the blood capillaries, which can cause obstructions in the pathway to the tissues.
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Mutations are hereditary
Depending on the cell in which the mutation occurs, it may be transmitted to the progeny. In the assumptions we have made concerning skin pigment and cellulase enzyme, it is clear that mutant genes will not be transmitted to their offspring.
These are somatic mutations, that is, they occurred in cells not involved in the production of gametes.
The mutation that led to sickle cell anemia must have occurred in the past in some ancestral germ line cells. The anomalous gene, then suggested, must have been carried by a gamete and then spread to the human species.
The causes of mutations
In general, mutations occur as a consequence of error in the process of DNA duplication. They happen at a very low frequency. Many of them are even corrected by special mechanisms, such as the action of the p53 gene that prevents the formation of tumors.
There are, however, certain environmental agents that may increase the rate of occurrence of genetic errors. These mutagenic agents include: substances in smoke, x-rays, ultraviolet light, mustard gas, nitrous acid and some food coloring. No wonder that in many countries there is growing concern about the thinning of the ozone gas layer (O3), which surrounds the earth's atmosphere. This gas acts as a filter for ultraviolet light from the sun. As its thickness decreases, the incidence of this radiation increases, which can affect people's skin. Lesions occur in the genetic material, which can lead to certain types of skin cancer.