Every cell in the human body contains exactly 46 chromosomes. Chromosomes are always paired. In the cell, there is always two chromosomes of each species pairs differ from each other in length, shape, and the presence of nodules or constrictions. In most cases, the chromosomes are different enough to cytologist could distinguish pairs of chromosomes (23 pairs total). It should be noted that in all somatic cells (all cells of the body, except sex) chromosome pairs are always the same in size, shape, location of the centromere, while the sex chromosomes (23rd pair) in men is not the same (X), and women are the same (XX).
The chromosomes in the cell under the microscope, you can see only during division – mitosis, during the metaphase stage. These are called metaphase chromosomes. When a cell is not divided chromosomes have the form of thin, dark-colored threads called chromatin.
Chromatin is deoxyribonucleoproteins, detectable by light microscopy in thin threads and beads. In the process of mitosis (cell division) chromatin by spiraling forms clearly visible (especially in metaphase) intensely stained structure – chromosome.
Metaphase chromosome consists of two longitudinal strands deoxyribonucleoproteins – chromatids are attached to each other in the primary constriction – centromere. Centromere – specially organized a chromosome is common to both sister chromatids. Centromere of chromosome divides the body into two shoulder. Depending on the location of the primary constriction following types of chromosomes-equal (metacentric), when the centromere is located in the middle, and the shoulders of approximately equal length; neravnoplechie (submetacentric), when the centromere is offset from the center of the chromosome, and the shoulders of unequal length, rod-shaped (acrocentric) when centromere is shifted to one end of the chromosome and one shoulder is very short. There are more of point (telotsentricheskie) chromosome, they have one arm is missing, but in the karyotype (chromosomes) of a person they are not. In some chromosomes may be secondary constrictions separating from the body portion of the chromosome, called a satellite.
Study the chemical organization of chromosomes in eukaryotic cells revealed that they consist mainly of DNA and proteins. As has been proven by numerous studies, DNA is the material properties of the carrier of heredity and variation encompasses biological information – the program of the cell, the organism, recorded using a special code. Proteins make up a large part of the substance of chromosomes (65% of the mass of these structures).
Chromosome as a set of genes is an evolutionarily developed structure inherent in all individuals of a given species. The relative position of the genes in the chromosome plays an important role in the nature of their operation.
Change in the number of chromosomes in the human karyotype can lead to various diseases. The most common chromosomal disorder in humans is Down syndrome, caused by trisomy (to a pair of normal chromosomes added another is the same extra) on chromosome 21. This syndrome occurs with a frequency of 1-2 in 1000. Often, trisomy of chromosome 21 pair cause the death of the fetus, but sometimes people with Down syndrome live to considerable age, although in general, their life expectancy reduced. Known for trisomy 13th chromosome – Patau Syndrome, and on the 18th chromosome – Edwards syndrome, in which the viability of infants dramatically reduced. They are killed in the first few months of life because of multiple malformations.
Quite often, a person found changes in the number of sex chromosomes. Among them is known monosomy X (of a pair of chromosomes is present only one (X0)) – is a syndrome of Turner. Less common trisomy X and Klinefelter’s syndrome (XXY, HHHU, XYV etc.). People with a change in the number of sex chromosomes in the presence of the Y chromosome develop male pattern. This is due to the fact that the factors that determine the type of man, are in the Y-chromosome. In contrast to mutations in autosomes (all chromosomes except sex), defects in mental development in patients expressed less clearly, for many it is in the normal range, or even above average. However, they have always observed genital malformations and growth. Less common malformations of other systems.