Gene mutations are associated with changes in the number and structure of chromosomes

Waiting for the birth of a child is the most wonderful time for parents, but also the worst thing. Many worry that a baby can be born with any flaws, physical or mental disabilities.

Science does not stand still, there is an opportunity to check for small pregnancies of the baby for deviations in development. Almost all of these tests can show whether everything is normal with the child.

Why does it happen that the same parents can be born absolutely different children - a healthy child and a child with disabilities? This is determined by genes. In the birth of an underdeveloped kid or a child with physical disabilities, genetic mutations associated with a change in the structure of DNA affect. Let's talk about this in more detail. Consider how this happens, what gene mutations are, and their causes.

What are mutations?

Mutations are physiological and biological changes in cells in the structure of DNA. The cause may be irradiation (during pregnancy, you can not take X-ray pictures, for injuries and fractures), ultraviolet rays (long exposure to the sun during pregnancy, or being in a room with ultraviolet lights on). Also, such mutations can be transmitted and inherited from their ancestors. All of them are divided into types.

Gene mutations with changes in the structure of chromosomes or their number

Chromosomal mutations are mutations in which the structure and number of chromosomes are altered. Chromosomal patches can fall out or double, move to a non-homological zone, turn from the norm by one hundred and eighty degrees.

The reasons for the appearance of such a mutation is a violation in the cross-over.

Gene mutations are associated with changes in the structure of chromosomes or their number, are the cause of serious disorders and diseases in the baby. Such diseases are incurable.

Types of chromosome mutations

There are two types of basic chromosomal mutations: numerical and structural. Aneuploidy are species by the number of chromosomes, that is, when gene mutations are associated with a change in the number of chromosomes. This is the emergence of an additional or several of the latter, the loss of any of them.

Gene mutations are associated with a change in structure in the event that the chromosomes break up, and then reunite, violating the normal configuration.

Types of Numerical Chromosomes

According to the number of chromosomes, mutations are divided into aneuploidy, that is, species. Consider the main, we'll find out the difference.

• Trisomy

Trisomy is the appearance in the karyotype of an extra chromosome. The most common phenomenon is the appearance of the twenty-first chromosome. It becomes the cause of Down's syndrome, or, as the disease is called, the trisomy of the twenty-first chromosome.

The Patau syndrome is identified by the thirteenth, and by the eighteenth chromosome is diagnosed Edwards syndrome. These are all autosomal trisomies. Other trisomy are not viable, they die in the womb and are lost with spontaneous abortions. Those individuals who have additional sex chromosomes (X, Y) - are viable. The clinical manifestation of such mutations is very slight.

Gene mutations associated with a change in number arise for certain reasons. Trisomy most often can occur when the chromosomes divide homologous in anaphase (meiosis 1). The result of this discrepancy is that both chromosomes enter only one of the two daughter cells, the second remains empty.

Uncommon chromosomes may occur less frequently. This phenomenon is called a violation in the divergence of sister chromatids. Occurs in meiosis 2. This is exactly the case when two completely identical chromosomes settle in one gamete, triggering a trisomic zygote. Nondisjunction occurs in the early stages of the process of crushing an egg that has been fertilized. Thus, a clone of mutant cells appears, which can cover a greater or lesser part of the tissues. Sometimes it manifests itself clinically.

Many associate the twenty-first chromosome with the age of the pregnant woman, but this factor to this day has no unequivocal confirmation. The reasons why chromosomes do not diverge remain unknown.

• Monosomy

Monosomy is the absence of any of the autosomes. If this happens, in most cases the fetus can not be taken out, premature birth occurs in the early stages. The exception is monosomy due to the twenty-first chromosome. The reason for the appearance of monosomy may be the non-dissociation of chromosomes, and the loss of the chromosome during its path in anaphase to the cell.

On the sex chromosomes, monosomy leads to the formation of a fetus, in which the karyotype is XO. The clinical manifestation of such a karyotype is Turner's syndrome. In eighty percent of cases out of a hundred, the appearance of monosomy on the X chromosome is due to a violation of the child's meiosis. This is due to the non-divergence of X and Y chromosomes. Basically, the fetus with KO karyotype is killed in the womb.

On the sex chromosomes, trisomy is divided into three types: 47 XXY, 47 XXX, 47 XYY. Klinefelter's syndrome is trisomy 47 XXY. With such a karyotype, the chances of bearing a child are fifty to fifty. The cause of such a syndrome can be the non-separation of X chromosomes or the non-dilatation of X and Y of spermatogenesis. The second and third karyotypes can occur only in one in a thousand pregnant women, they practically do not manifest themselves and in most cases are discovered by specialists quite accidentally.

• Polyploidy

These are gene mutations associated with a change in the haploid set of chromosomes. These sets can be tripled and quadrupled. Triploidy is most often diagnosed only when there was a spontaneous abortion. There were several cases when the mother could endure such a baby, but all of them died before reaching the age of one month. Mechanisms of gene mutations in the case of triploidy cause a complete divergence and non-divergence of all chromosomal sets of either female or male sex cells. Also, the mechanism can be the double fertilization of one egg. In this case, the placenta degenerates. Such a rebirth is called a bubble drift. As a rule, such changes lead to the development of a child's mental and physiological disorders, termination of pregnancy.

What gene mutations are associated with changes in chromosome structure

Structural changes in chromosomes are a consequence of the rupture (destruction) of the chromosome. As a result, these chromosomes unite, breaking their former appearance. These modifications may be unbalanced and balanced. Balanced do not have a surplus or a lack of material, so do not show up. They can appear only in those cases, if there was a gene on the site of the destruction of the chromosome, which is functionally important. In a balanced set, gametes may appear unbalanced. In consequence, the fertilization of an ovum with such a gamete can lead to the appearance of a fetus with an unbalanced chromosome set. With such a set, the fetus has a number of developmental defects, severe types of pathology appear.

Types of structural modifications

Gene mutations occur at the level of gamete formation. It is impossible to prevent this process, nor can it be known to know that such mutations can occur. There are several types of structural modifications.

• Deletions

This change is associated with the loss of a part of the chromosome. After such a break, the chromosome becomes shorter, and its torn part is lost with further cell division. Interstitial deletions are the case when one chromosome is broken at once in several places. Such chromosomes usually create an unviable fetus. But there are also cases when the kids survived, but because of such a set of chromosomes there was Wolf-Hirschhorn syndrome, a "cat cry".

• Duplications

These gene mutations occur at the level of the organization of dual DNA segments. In general, duplication can not cause such pathologies that cause deletions.

• Translocations

Translocation is due to the transfer of genetic material from one chromosome to another. If a break occurs simultaneously in several chromosomes and they exchange segments, this causes a reciprocal translocation. The karyotype of such a translocation has only forty-six chromosomes. The very same translocation is revealed only with the detailed analysis and study of the chromosome.

Change in nucleotide sequence

Gene mutations are associated with a change in the sequence of nucleotides, when expressed in the modification of the structures of some sections of DNA. On the consequences of such mutations are divided into two types - without a shift of the reading frame and with a shift. In order to know exactly the causes of the changes in DNA regions, it is necessary to consider each type separately.

Mutation without a frame shift

These gene mutations are associated with the change and replacement of nucleotide pairs in the DNA structure. With such replacements, the length of DNA is not lost, but loss and replacement of amino acids is possible. There is a possibility that the protein structure will be preserved, this will be the degeneracy of the genetic code. Let's consider in detail both variants of development: with replacement and without replacement of amino acids.

Mutation with the replacement of amino acids

The replacement of the amino acid residue in polypeptides is called missense mutations. In the human hemoglobin molecule there are four chains - two "a" (it is located in the sixteenth chromosome) and two "b" (encoding in the eleventh chromosome). If "b" is a normal chain, and it contains one hundred forty-six amino acid residues, and the sixth is glutamine, then hemoglobin will be normal. In this case, the glutamic acid should be encoded by the GAA triplet. If the GAA mutation is replaced by GTA, then valine is formed instead of glutamic acid in the hemoglobin molecule. Thus, instead of normal hemoglobin HbA, another HbS hemoglobin will appear. Thus, the replacement of one amino acid and one nucleotide will cause a serious serious illness - sickle cell anemia.

This disease is manifested in the fact that the red blood cells become in shape, like a sickle. In this form they are not able to deliver oxygen normally. If at the cellular level homozygotes have the formula HbS / HbS, this leads to the death of the child in the earliest childhood. If the formula is HbA / HbS, the red blood cells have a weak form of change. Such a weak change has a useful quality - the body's resistance to malaria. In those countries where there is a danger of catching malaria the same as in the cold in Siberia, this change is of a beneficial quality.

Mutation without amino acid substitution

Replacement of nucleotides without exchange of amino acids is called seymensens mutations. If in the DNA segment encoding the "b" - chain there is a change of GAA to GAG, then due to the fact that the genetic code is in excess, glutamic acid replacement can not occur. The structure of the chain will not be changed, there will be no changes in the red blood cells.

Mutations with a frame shift

Such gene mutations are associated with changes in DNA length. The length may become smaller or larger, depending on the loss or addition of nucleotide pairs. Thus, the whole structure of the protein will be changed completely.

Intragenic suppression may occur. This phenomenon occurs when there is a place for two mutations that compensate each other. This is the moment of attachment of the nucleotide pair after one has been lost, and vice versa.

Nonsense mutations

This is a special group of mutations. It occurs rarely, in its case, the appearance of stop codons occurs. This can happen both with the loss of nucleotide pairs, and with their attachment. When stop codons appear, the synthesis of polypeptides completely stops. This way, zero alleles can be formed. None of the proteins will match this.

There is such a thing as intergenic suppression. This is a phenomenon where the mutation of one gene suppresses mutations in others.

Are there any changes in pregnancy?

Gene mutations associated with changes in the number of chromosomes can in most cases be determined. To find out whether the fetus has defects in development and pathology, screening is prescribed in the first weeks of pregnancy (ten to thirteen weeks). This is a series of simple examinations: a fence for blood tests from a finger and a vein, ultrasound. On ultrasound examination, the fetus is treated in accordance with the parameters of all limbs, nose and head. These parameters, with a strong inconsistency with the norms, indicate that the baby has defects in development. This diagnosis is confirmed or refuted on the basis of blood test results.

Also under the close supervision of physicians are future mothers, whose children may have mutations at the gene level, inherited. That is, these are women who had births of a child with mental or physical abnormalities, diagnosed with Down's syndrome, Patau and other genetic diseases.