Mutations are changes in the cell’s genetic material. Gene mutations are changes that occur in the sequence of bases of the DNA molecule that makes up the genes, which undergo a change in their structure.
Mutations can occur in somatic cells or in germ cells; in the latter case, they can be passed down through the generations, from father to children. Somatic mutations are restricted to the individual in which they occur and are not transmitted to descendants.
Gene mutations are more punctual, that is, they affect only the nucleotide and lead to small changes in the sequence or number of nucleotides. Chromosomal mutations, on the other hand, are more serious and may alter the amount of chromosomes , their shape and even their structure.
In populations, mutations provide the appearance of new types of genes, being responsible for gene variability . Gene mutations can occur by adding, losing or replacing one or a few nucleotides in the DNA strand during duplication.
Gene mutations by addition, loss or substitution
- Addition or loss: when the gene mutation occurs by addition or loss of bases it modifies the genetic code and defines a new sequence of bases. As a consequence, this new sequence may modify the type of amino acid present in the protein chain, changing the function of the protein or inactivating the phenotypic expression. Example: when there is a deletion (loss) of segments of the gene, some types of cancer may manifest. As a result, the cells begin to grow and divide in an uncontrolled manner, giving rise to the tumor.
- Substitution: gene mutation by substitution occurs by exchanging a purine nitrogenous base (adenine and guanine) for another purine or a pyrimidine (cytosine and thymine) for another pyrimidine. Example: defective hemoglobin molecule, causing sickle cell anemia.
There are mutations that do not alter the amino acid in the polypeptide chain. This happens when the substitution of a nitrogenous base does not result in a codon for another amino acid. For example: suppose that one of the exon cracks is AAA and has changed to AAG. The AAA crack, when transcribed in mRNA, will correspond to the UUU codon, and the AAG crack, to the UUC codon.
Looking at the genetic code table, we can see that both code for the same amino acid: phenylalanine. Therefore, the mutation did not lead to a change in the polypeptide. Such mutations are called “silent” and are responsible for genetic variability that is always greater than the diversity of characteristics. Example: synthesis of the amino acid phenylalanine. In this case, the mutation of the AAA crack to AAG, produced the same amino acid, without changing the polypeptide.
In general, mutations happen due to some error in the DNA duplication process , however, there are certain factors in the environment that can increase the incidence rate of these genetic errors. Excessive exposure to x-rays, substances present in smoke, ultraviolet light, nitrous acid and some dyes present in food, for example, can favor the appearance of mutations.
There is an entire system of repairing these changes in our cells, which drastically reduces the number of mutations that persist. Although most cases of gene mutations are harmful, that is, they cause damage to the organism, they are very important in evolutionary terms , and are the primary source of genetic variability in a population.
The greater the genetic variability in a population, the greater the chance of survival of that population to changes in environmental conditions. Larger mutations, which affect the number or shape of chromosomes, are called chromosomal aberrations and, like gene mutations, are generally deleterious.
The consequences of gene mutation
The effects of a mutation on the phenotype can vary widely. When the change in the amino acid sequence in the protein does not affect the functioning of the molecule, it usually goes unnoticed .
However, the mutation is often harmful , as in the case of sickle cell anemia, in which the amino acid glutamic acid is replaced by the amino acid valine, changing the shape of the protein, resulting in the change in the shape of the red blood cell, which becomes unable to carry oxygen.
Sickle cell anemia
Sickle cell anemia is a disease in which red blood cells have a sickle aspect, hence the name sickle cell. This is due to the presence of defective hemoglobin molecules . As a result, red blood cells do not efficiently transport oxygen. These red blood cells are more fragile and can rupture, causing problems for the person, such as severe pain. In certain cases, the disruption is so intense and rapid that it can lead to death.
The defective hemoglobin molecule results from a change in the gene in the CTT crack to CAT. The mRNA codon changes from GAA to GUA, which refers to the amino acid valine, which causes the disease.
The case of insulin
The substitution of one or more amino acids, however, does not always result in the loss or alteration of the protein’s function . Certain regions of a molecule may not be essential to its functioning. Insulin, for example, is a hormone present in all vertebrates, but the molecule is not identical in all species.
When we compare the amino acid sequence of insulin from two or more different species, we observe changes in the sequence that, however, do not impair the shape and function of that protein. We then say that functionally neutral mutations occur , conserved in the DNA of individuals over the generations.
On the other hand, there are regions responsible for the three-dimensional shape of the protein, thus guaranteeing its function. If these essential regions have amino acid substitutions, the molecule may cease to be functional.
Some examples of gene mutation
- Progeria: a lethal disease that manifests itself in children aged between 5 and 6 years, making them, at 8 or 9 years old, already look like an elderly person. That is, the exact causes of progeria are not well known, but involve gene mutations.
- Alzheimer’s disease: this disease can have several causes. One of them is related to the mutation in a certain gene on chromosome 21, which leads to degeneration of the central nervous system.
- Adrenoleukodystrophy: disease caused by a mutation in an X chromosome gene. This mutation incapacitates the body to metabolize certain types of lipids (oils), causing a degenerative neurological disease that can lead to death.