Gene mutations are associated with changes in the number and structure of chromosomes
Waiting for the birth of a child is the most beautifultime for parents, but also the worst. 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 the baby for small pregnancies 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 onesparents 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 changes in the structure of DNA affect the body. 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 biologicalchange of 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 whichthe structure and number of chromosomes were changed. Chromosomal patches may fall out or double, move to a nonhomologous zone, rotate by a factor of one hundred and eighty degrees.
The reasons for the appearance of such a mutation is a violation when cross-over.
Gene mutations are associated with changes in the structure of chromosomes or their number, are the cause of serious disorders and illnesses in the baby. Such diseases are incurable.
Types of chromosome mutations
Two types of basic chromosomalmutations: numerical and structural. Aneuploidy are species by the number of chromosomes, that is when the 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 karyotype of superfluouschromosomes. 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 byThe eighteenth chromosome diagnoses 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. Trisomies most often occur when the chromosomes are 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 withthe 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 born, 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.
By sexual chromosomes, monosomy leads tothe formation of the 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 baby's meiosis. This is due to the non-divergence of X and Y chromosomes. Basically, the fetus with KO karyotype dies in the womb.
On the sex chromosomes, trisomy is divided into threetype: 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-dissociation of X chromosomes or the nondisjunction 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 changehaploid 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 managed to 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-dissociation 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 the child's mental and physiological disorders, termination of pregnancy.
What gene mutations are associated with changes in chromosome structure
Structural changes in chromosomes areconsequence of the rupture (destruction) of the chromosome. As a result, these chromosomes unite, breaking their former form. 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 manifest themselves only in those cases, if there was a gene on the site of chromosome destruction, which is functionally important. In a balanced set, gametes may appear unbalanced. In consequence, the fertilization of an ovum with such a gamete can cause the appearance of a fetus with an unbalanced chromosome set. With such a set, the fetus has a number of malformations, severe types of pathology appear.
Types of structural modifications
Gene mutations occur at the level of educationgametes. 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 due to the loss of 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 transfergenetic 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 a detailed analysis and study of the chromosome.
Change in nucleotide sequence
Gene mutations are associated with a changesequence 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 change and replacementnucleotide 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 structure of the protein 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
Replacement of the amino acid residue in the compositionpolypeptides are 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 due to the mutation GAA is replaced by GTA, then instead of glutamic acid in the hemoglobin molecule, valine is formed. 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 red blood cellsbecome 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
Substitution of nucleotides without amino acid exchangeare 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, the replacement of glutamic acid can not occur. The structure of the chain will not be changed, there will be no alterations in the red blood cells.
Mutations with a frame shift
Such gene mutations are associated with a change in lengthDNA. 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 her case, there is the appearance of stop codons. This can happen both with the loss of pairs of nucleotides, and with their attachment. When stop codons appear, the synthesis of polypeptides completely stops. Thus, 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 some genes suppresses mutations in others.
Are there any changes during pregnancy?
Gene mutations associated with a change in the numberchromosomes, in most cases can 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, the fetus is treated in accordance with the parameters of all limbs, nose and head. These parameters, with a strong disparity in standards, 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 doctorsturn out to be 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 syndrome, Patau and other genetic diseases.
