They consist of two threads - chromatid
Located parallel and connected to each other at one point called centromere
or primary constriction
On some chromosomes you can see secondary constriction.
If the secondary constriction is located close to the end of the chromosome, then the distal area limited by it is called satellite.
The terminal sections of chromosomes have a special structure and are called telomeres
The portion of the chromosome from the telomere to the centromere is called chromosome arm
Each chromosome has two arms. Depending on the ratio of arm lengths, three types of chromosomes are distinguished: 1) metacentric (equal arms); 2) submetacentric (unequal shoulders); 3) acrocentric, in which one shoulder is very short and not always clearly distinguishable.
Along with the location of the centromere, the presence of a secondary constriction and a satellite, their length is important for determining individual chromosomes. For each chromosome of a certain set, its length remains relatively constant. Measuring chromosomes is necessary to study their variability in ontogenesis in connection with diseases, anomalies, and impaired reproductive function.
Fine structure of chromosomes.
Chemical analysis of the structure of chromosomes showed the presence of two main components: deoxyribonucleic acid (DNA) and proteins such as histones and protomite (in germ cells). Studies of the fine submolecular structure of chromosomes have led scientists to the conclusion that each chromatid contains one strand - chromonema. Each chromonema consists of one DNA molecule. The structural basis of the chromatid is a protein strand. The chromonema is arranged in the chromatid in a shape close to a spiral. Evidence for this assumption was obtained, in particular, by studying the smallest exchange particles of sister chromatids, which were located across the chromosome.
Karyotype
When analyzing sets of chromosomes in cells of different species, differences in the number of chromosomes or their structure, or both at the same time, were revealed. The set of quantitative and structural features of the diploid set of chromosomes of a species is called karyotype
By determined by S. G. Navashin, karyotype
This structure is a kind of formula of the species. The karyotype contains the genetic information of an individual, changes in which entail changes in the characteristics and functions of the body of this individual or its offspring. Therefore, it is so important to know the features of the normal structure of chromosomes in order, if possible, to be able to identify changes in the karyotype.
DNA is a material carrier of the properties of heredity and variability and contains biological information - the development program of a cell or organism, recorded using a special code.
Histones are presented in five fractions: HI, H2A, H2B, NZ, H4. Being positively charged basic proteins, they bind quite firmly to DNA molecules, which prevents the reading of the biological information contained in it. these proteins perform a structural function, ensuring the spatial organization of DNA in chromosomes
Chromosome RNA is represented partly by transcription products that have not yet left the site of synthesis. Some fractions have a regulatory function.
The regulatory role of chromosome components is to “prohibit” or “permit” the copying of information from the DNA molecule.
The first level is the nucleosomal thread. DNA + histone proteins H2A, H2B, H3, H4. The degree of shortening is 6-7 times. Second: chromatin fibril. Nucleosome filament + histone H1 protein. Shortening by 42 times. Third: interphase chromosome. The chromatin fibril is folded into loops with the help of non-histone proteins. Shortening by 1600 times. Fourth. Metaphase chromosome. Supercondensation of chromatin. Shortening by 8000 times.
Structure and functions of human metaphase chromosomes
Metaphase occupies a significant part of the period of mitosis, and is characterized by a relatively stable state.
All this time, the chromosomes are held in the equatorial plane of the spindle due to the balanced tension forces of the microtubules.
In metaphase, as well as during other phases of mitosis, active renewal of spindle microtubules continues through intensive assembly and depolymerization of tubulin molecules. By the end of metaphase, a clear separation of sister chromatids is observed, the connection between which is maintained only in the centromeric regions. The chromatid arms are located parallel to each other, and the gap separating them becomes clearly visible.
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DNA helices in the nucleus"packed" into chromosomes. A human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes that form a pair on homologous chromosomes are almost or completely identical, and one often hears that all genes in the human genome have their own pair, although this is not entirely correct.
Along with DNA Chromosomes contain a lot of protein, most of which is represented by small positively charged histone molecules. They form many small, spool-like structures that, one behind the other, are wrapped around short segments of DNA.
These structures play an important role in the regulation of DNA activity, since they ensure its tight “packing” and thus make it impossible to use it as a template for the synthesis of new DNA. There are also regulatory proteins that, on the contrary, decondense small sections of histone DNA packaging, thus allowing RNA synthesis.
Video: Mitosis. Cell mitosis. Phases of mitosis
Among the main chromosome components There are also non-histone proteins, which, on the one hand, are structural proteins of chromosomes, and on the other, activators, inhibitors or enzymes as part of regulatory genetic systems.
Complete chromosome replication begins a few minutes after DNA replication completes. During this time, the newly synthesized DNA strands combine with proteins. The two newly formed chromosomes remain attached to each other until the very end of mitosis in a region close to their center and called the centromere. Such separated but not separated chromosomes are called chromatids.
Mother cell division process two daughters are called mitosis. Following the replication of chromosomes with the formation of two chromatids within 1-2 hours, mitosis automatically begins.
One of the very first changes in cytoplasm associated with mitosis, occurs late in interphase and affects centrioles. Centrioles, like DNA and chromosomes, are duplicated during interphase—usually just before DNA replication. The centriole, about 0.4 µm long and about 0.15 µm in diameter, consists of nine parallel triplets-tubules assembled in the form of a cylinder. The centrioles of each pair lie at right angles to each other. A pair of centrioles together with the substance adjacent to it is called a centrosome.
Phases of cell mitosis
Just before the start mitosis both pairs of centrioles begin to move in the cytoplasm, moving away from each other. This movement is caused by the polymerization of the protein of microtubules, which begin to grow from one pair of centrioles to another and due to this push them to the opposite poles of the cell. At the same time, other microtubules begin to grow from each pair of centrioles, which increase in length and extend radially from them in the form of rays, forming the so-called astrosphere at each pole of the cell. Its individual rays penetrate the nuclear membrane, thus promoting the separation of each pair of chromatids during mitosis. The group of microtubules between two pairs of centrioles is called the spindle, and the entire set of microtubules along with the centrioles is called the mitotic apparatus.
Prophase. As the spindle forms in the nucleus, condensation of chromosomes begins (in interphase they consist of two loosely connected chains), which due to this become clearly distinguishable.
Prometaphase. Microtubules coming from the astrosphere destroy the nuclear membrane. At the same time, other microtubules extending from the astrosphere attach to the centromeres, which still connect all the chromatids in pairs, and begin to pull both chromatids of each pair to different poles of the cell.
Video: Phases of meiosis
Metaphase. During metaphase, the astrospheres move further away from each other.
It is believed that their movement is due to microtubules extending from them. These microtubules intertwine together to form a spindle, which pushes the centrioles away from each other. It is also believed that between the spindle microtubules there are molecules of small contractile proteins, or “motor molecules” (possibly similar to actin), which ensure the mutual sliding of the microtubules in opposite directions, as occurs during muscle contraction. Microtubules attached to the centromeres pull the chromatids to the center of the cell and arrange them in the form of a metaphase plate along the equator of the spindle.
Anaphase. During this phase, the two chromatids of each pair are separated from each other at the centromere. All 46 pairs of chromatids separate and form two independent sets of 46 daughter chromosomes. Each set of chromosomes moves to opposite astrospheres, and the poles of the dividing cell at this time diverge further and further.
Telophase. In this phase, two sets of daughter chromosomes completely diverge, the mitotic apparatus is gradually destroyed, and a new nuclear envelope is formed around each set of chromosomes due to the membrane of the endoplasmic reticulum. Soon after this, a constriction appears between the two new nuclei, dividing the cell into two daughter cells. Division is caused by the formation of a ring of actin microfilaments and, possibly, myosin (two contractile muscle proteins) in the region of the constriction between the daughter cells, which ties them from each other.
Educational video: cell mitosis and its stages
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Chemical composition of chromosomes
chromatin,
Proteins make up a significant part of the substance of chromosomes.
They account for about 65% of the mass of these structures. All chromosomal proteins are divided into two groups: histones and non-histone proteins.
Histones
Number of factions non-histone
chromosomes.
Chromosome morphology
centromeres daughter chromosomes
Rice. 3.52. Chromosome shapes:
I- telocentric, II- acrocentric, III- submetacentric, IV- metacentric;
1 - centromere, 2 - satellite, 3 - short shoulder, 4 - long shoulder, 5 - chromatids
chromosomal mutations or aberrations.About them - in the next lecture.
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Chemical composition of chromosomes
The study of the chemical organization of the chromosomes of eukaryotic cells showed that they consist mainly of DNA and proteins that form a nucleoprotein complex - chromatin, received its name for its ability to be colored with basic dyes.
Proteins make up a significant part of the substance of chromosomes. They account for about 65% of the mass of these structures. All chromosomal proteins are divided into two groups: histones and non-histone proteins.
Histones represented by five fractions: HI, H2A, H2B, NZ, H4. Being positively charged basic proteins, they bind quite firmly to DNA molecules, which prevents the reading of the biological information contained in it. This is their regulatory role. In addition, these proteins perform a structural function, ensuring the spatial organization of DNA in chromosomes.
Number of factions non-histone proteins exceeds 100. Among them are enzymes of RNA synthesis and processing, DNA replication and repair. Acidic proteins of chromosomes also perform structural and regulatory roles. In addition to DNA and proteins, chromosomes also contain RNA, lipids, polysaccharides, and metal ions.
The regulatory role of chromosome components is to “prohibit” or “permit” the copying of information from the DNA molecule. Other components are found in small quantities.
Structural organization of chromatin
Chromatin changes its organization depending on the period and phase of the cell cycle. In interphase, under light microscopy, it is detected in the form of clumps scattered in the nucleoplasm of the nucleus. During the transition of a cell to mitosis, especially in metaphase, chromatin takes on the appearance of clearly visible individual intensely colored bodies - chromosomes.
The most common point of view is that chromatin (chromosome) is a spiral thread.
Chromosome morphology
In the first half of mitosis, they consist of two chromatids connected to each other in the region of the primary constriction ( centromeres) a specially organized region of the chromosome common to both sister chromatids. In the second half of mitosis, the chromatids separate from each other. They form single-filamentous daughter chromosomes distributed between daughter cells.
Depending on the location of the centromere and the length of the arms located on both sides of it, several forms of chromosomes are distinguished: equal-armed, or metacentric (with the centromere in the middle), unequal-armed, or submetacentric (with the centromere shifted to one end), rod-shaped, or acrocentric (with a centromere located almost at the end of the chromosome), and point - very small, the shape of which is difficult to determine (Fig.).
Thus, each chromosome is individual not only in the set of genes it contains, but also in the morphology and nature of differential staining.
3.52. Chromosome shapes:
I- telocentric, II- acrocentric, III- submetacentric, IV- metacentric;
1 - centromere, 2 - satellite, 3 - short shoulder, 4 - long shoulder, 5 - chromatids
Rice. 3.53. Location of loci in human chromosomes
with their differential staining:
p - short arm, q - long arm; 1-22 - serial number of the chromosome; XY - sex chromosomes
At the chromosomal level of organization, which appears in the process of evolution in eukaryotic cells, the genetic apparatus must satisfy all the requirements for the substrate of heredity and variability: have the ability to reproduce itself, maintain the constancy of its organization and acquire changes that can be transmitted to a new generation of cells.
Despite the evolutionarily proven mechanism that makes it possible to maintain a constant physicochemical and morphological organization of chromosomes over a series of cell generations, this organization can change under the influence of various influences. Changes in the structure of a chromosome, as a rule, are based on an initial violation of its integrity - breaks, which are accompanied by various rearrangements called chromosomal mutations or aberrations.About them - in the next lecture.
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The concept of “chromosome” was introduced into science by Waldeimer in 1888. Chromosome - this is an integral part of the cell nucleus, with the help of which the regulation of protein synthesis in the cell is carried out, i.e. transmission of hereditary information. Chromosomes are composed of complexes of nucleic acids and protein. Functionally, a chromosome is a strand of DNA with a huge functional surface. The number of chromosomes is constant for each specific species.
Each chromosome is formed by two morphologically identical intertwined threads of the same diameter - chromatids. They are closely connected centromere– a special structure that controls the movement of chromosomes during cell division.
Depending on the position of the chromosome, the chromosome body is divided into 2 arms. This in turn determines the 3 main types of chromosomes.
1 type – Acrocentric chromosome.
Its centromere is located closer to the end of the chromosome and one arm is long and the other is very short.
Type 2 – Submetacentric chromosome.
Its centromere is located closer to the middle of the chromosome and divides it into unequal arms: short and long.
Type 3 – Metacentric chromosome.
Its centromere is located in the very center of the chromosome body and divides it into equal arms.
The length of chromosomes varies in different cells from 0.2 to 50 μm, diameter - from 0.2 to 2 μm. Representatives of the lily family have the largest chromosomes in plants, and some amphibians have the largest chromosomes in animals. The length of most human chromosomes is 2-6 microns.
The chemical composition of chromosomes is determined mainly by DNA, as well as proteins - 5 types of histone and 2 types of non-histone, as well as RNA. The characteristics of these chemicals determine the important functions of chromosomes:
1.reduplication and transmission of genetic material from generation to generation;
2. protein synthesis and control of all biochemical processes that form the basis for the specificity of development and differentiation of the body’s cellular systems. In addition, the following were found in the chromosomes: complex residual protein, lipids, calcium, magnesium, iron.
The structural basis of chromosomes is the DNA-histone complex. In a chromosome, the DNA strand is packaged by histones into regularly repeating structures with a diameter of about 10 nm, called nucleosomes. The surface of histone molecules is positively charged, while the DNA helix is negatively charged. Nucleosomes are packaged in thread-like structures called fibrils. The chromatid is built from them.
The main substrate in which the genetic information of an organism is recorded is the euchromatic regions of the chromosomes. In contrast, there is inert heterochromatin. Unlike euchromatin, which contains unique genes, the imbalance of which negatively affects the phenotype of the organism, changes in the amount of heterochromatin have a much smaller effect or no effect at all on the development of the organism's characteristics.
In order to make it easier to understand the complex complex of chromosomes that make up the karyotype, they can be arranged in the form of an idiogram compiled by S.G. Novashin. In the idiogram, chromosomes (except sex chromosomes) are arranged in descending order of size.
However, identification by size alone is difficult because a number of chromosomes have similar sizes. The size of chromosomes is measured by their absolute or relative length in relation to the total length of all chromosomes of the haploid set. The largest human chromosomes are 4-5 times longer than the smallest chromosomes. In 1960, a classification of human chromosomes was proposed depending on morphological characteristics: size, shape, centromere position - in order of decreasing total length. According to this classification, 22 pairs of chromosomes are combined into 7 groups:
1 group 1-3 pair of chromosomes - large, metacentric.
2 group 4-5 pair of chromosomes – large, submetacentric.
3 group 6-12 pair of chromosomes - medium size, submetacentric.
4 gr. 13-15 pair of chromosomes - medium size, acrocentric.
5 group 16-18 pair of chromosomes are short, of which 16 are metacentric, 17 are submetacentric, 18 are acrocentric.
6 gr. 19-20 pair of chromosomes - short, metacentric.
7 group 21-22 pair of chromosomes – very short, acrocentric.
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Chromosomes- cell structures that store and transmit hereditary information. A chromosome consists of DNA and protein. A complex of proteins bound to DNA forms chromatin. Proteins play an important role in packaging DNA molecules in the nucleus.
The DNA in chromosomes is packaged in such a way that it fits in the nucleus, the diameter of which usually does not exceed 5 microns (5-10 -4 cm). The DNA packaging takes on the appearance of a loop structure, similar to the lampbrush chromosomes of amphibians or the polytene chromosomes of insects. The loops are maintained by proteins that recognize specific nucleotide sequences and bring them together. The structure of the chromosome is best seen in metaphase of mitosis.
The chromosome is a rod-shaped structure and consists of two sister chromatids, which are held by the centromere in the region of the primary constriction. Each chromatid is made up of chromatin loops. Chromatin does not replicate. Only DNA is replicated.
Rice. 14. Chromosome structure and replication
When DNA replication begins, RNA synthesis stops. Chromosomes can be in two states: condensed (inactive) and decondensed (active).
The diploid set of chromosomes of an organism is called a karyotype. Modern research methods make it possible to identify each chromosome in a karyotype. To do this, take into account the distribution of light and dark bands visible under a microscope (alternating AT and GC pairs) in chromosomes treated with special dyes. The chromosomes of representatives of different species have transverse striations. Related species, such as humans and chimpanzees, have very similar patterns of alternating bands in their chromosomes.
Each type of organism has a constant number, shape and composition of chromosomes. There are 46 chromosomes in the human karyotype - 44 autosomes and 2 sex chromosomes. Males are heterogametic (XY) and females are homogametic (XX). The Y chromosome differs from the X chromosome in the absence of some alleles (for example, the blood clotting allele). Chromosomes of the same pair are called homologous. Homologous chromosomes at identical loci carry allelic genes.
1.14. Reproduction in the organic world
Reproduction- this is the reproduction of genetically similar individuals of a given species, ensuring continuity and continuity of life.
Asexual reproduction carried out in the following ways:
- simple division into two or many cells at once (bacteria, protozoa);
- vegetatively (plants, coelenterates);
- dividing a multicellular body in half with subsequent regeneration (starfish, hydra);
- budding (bacteria, coelenterates);
- formation of disputes.
Asexual reproduction usually ensures an increase in the number of genetically homogeneous offspring. But when spore nuclei are produced by meiosis, the offspring from asexual reproduction will be genetically different.
Sexual reproduction- a process in which genetic information from two individuals is combined.
Individuals of different sexes form gametes. Females produce eggs, males produce sperm, and hermaphrodites produce both eggs and sperm. And in some algae, two identical sex cells merge.
When haploid gametes fuse, fertilization occurs and a diploid zygote is formed.
The zygote develops into a new individual.
All of the above is true only for eukaryotes. Prokaryotes also have a sexual process, but it occurs differently.
Thus, during sexual reproduction, the genomes of two different individuals of the same species are mixed. Offspring carry new genetic combinations that differentiate them from their parents and from each other.
One type of sexual reproduction is parthenogenesis, or the development of individuals from an unfertilized egg (aphids, drones of bees, etc.).
The structure of germ cells
Ovules- round, relatively large, immobile cells. Dimensions - from 100 microns to several centimeters in diameter. They contain all the organelles characteristic of a eukaryotic cell, as well as the inclusion of reserve nutrients in the form of yolk. The egg cell is covered with an egg membrane, consisting mainly of glycoproteins.
Rice. 15. The structure of a bird's egg: 1 - chalaza; 2 - shell; 3 - air chamber; 4 - outer subshell membrane; 5 - liquid protein; 6 - dense protein; 7 - germinal disc; 8 - light yolk; 9 - dark yolk.
In mosses and ferns, the eggs develop in archegonia; in flowering plants, in ovules located in the ovary of the flower.
Oocytes are divided as follows:
- isolecithal - the yolk is evenly distributed and there is little of it (in worms, mollusks);
- alecithal - almost devoid of yolk (mammals);
- telolecithal - contain a lot of yolk (fish, birds);
- polylecithal - contain a significant amount of yolk.
Oogenesis is the formation of eggs in females.
In the reproduction zone there are oogonia - primary germ cells that reproduce by mitosis.
From the oogonia, after the first meiotic division, first-order oocytes are formed.
After the second meiotic division, second-order oocytes are formed, from which one egg and three guiding bodies are formed, which then die.
Sperm- small, mobile cells. They have a head, neck and tail.
In the anterior part of the head there is an acrosomal apparatus - an analogue of the Golgi apparatus. It contains an enzyme (hyaluronidase), which dissolves the egg membrane during fertilization. The neck contains centrioles and mitochondria. Flagella are formed from microtubules. During fertilization, only the nucleus and centrioles of the sperm enter the egg. Mitochondria and other organelles remain outside. Therefore, cytoplasmic inheritance in humans is transmitted only through the female line.
The sex cells of sexually reproducing animals and plants are formed through a process called gametogenesis.
Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?
We propose to look into this issue in order to dot the i’s once and for all.
Looking at family photographs, you may have probably noticed that members of the same family resemble each other: children look like parents, parents look like grandparents. This similarity is passed on from generation to generation through amazing mechanisms.
All living organisms, from single-celled organisms to African elephants, contain chromosomes in the cell nucleus - thin, long threads that can only be seen with an electron microscope.
Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, implement it and transmit it.
How many chromosomes does a person have
At the end of the 19th century, scientists discovered that the number of chromosomes in different species is not the same.
For example, peas have 14 chromosomes, y have 42, and in humans – 46 (that is, 23 pairs). Hence the temptation arises to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is absolutely not the case.
Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). The sexes have morphological and structural (gene composition) differences.
In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X-chromosome and one Y-chromosome (XY-pair).
The sex of the unborn child depends on the composition of the chromosomes of the twenty-third pair (XX or XY). This is determined by fertilization and the fusion of the female and male reproductive cells.
This fact may seem strange, but in terms of the number of chromosomes, humans are inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80.
Chromosomes consist of a protein and a DNA (deoxyribonucleic acid) molecule, similar to a double helix. Each cell contains about 2 meters of DNA, and in total there are about 100 billion km of DNA in the cells of our body.
An interesting fact is that if there is an extra chromosome or if at least one of the 46 is missing, a person experiences a mutation and serious developmental abnormalities (Down's disease, etc.).
). Chromatin is heterogeneous, and some types of such heterogeneity are visible under a microscope. The fine structure of chromatin in the interphase nucleus, determined by the nature of DNA folding and its interaction with proteins, plays an important role in the regulation of gene transcription and DNA replication and, possibly, cellular differentiation.
The sequences of DNA nucleotides that form genes and serve as a template for the synthesis of mRNA are distributed along the entire length of the chromosomes (individual genes, of course, are too small to be seen under a microscope). By the end of the 20th century, for approximately 6,000 genes, it was established on which chromosome and in which part of the chromosome they are located and what the nature of their linkage is (that is, their position relative to each other).
The heterogeneity of metaphase chromosomes, as already mentioned, can be seen even with light microscopy. Differential staining of at least 12 chromosomes revealed differences in the width of some bands between homologous chromosomes (Fig. 66.3). Such polymorphic regions consist of non-coding highly repetitive DNA sequences.
The methods of molecular genetics have made it possible to identify a huge number of smaller polymorphic DNA regions that are therefore undetectable by light microscopy. These regions are identified as restriction fragment length polymorphism, tandem repeats varying in number, and short tandem repeat polymorphism (mono-, di-, tri-, and tetranucleotide). Such variability usually does not manifest itself phenotypically.
However, polymorphism serves as a convenient tool for prenatal diagnosis due to the linkage of certain markers with mutant genes that cause diseases (for example, in Duchenne myopathy), as well as in establishing the zygosity of twins, establishing paternity, and predicting transplant rejection.
It is difficult to overestimate the importance of such markers, especially highly polymorphic short tandem repeats that are widespread in the genome, for mapping the human genome. In particular, they make it possible to establish the exact order and nature of the interaction of loci that play an important role in ensuring normal ontogenesis and cell differentiation. This also applies to those loci in which mutations lead to hereditary diseases.
Regions visible under a microscope on the short arm of acrocentric autosomes (Fig. 66.1) provide rRNA synthesis and the formation of nucleoli, which is why they are called nucleolar organizer regions. In metaphase they are not condensed and do not stain. The regions of the nucleolar organizer are adjacent to condensed sections of chromatin - satellites - located at the end of the short arm of the chromosome. Satellites do not contain genes and are polymorphic regions.
In a small proportion of cells, it is possible to identify other areas decondensed in metaphase, the so-called fragile areas, where “complete” chromosome breaks can occur. Abnormalities in the only such region located at the end of the long arm of the X chromosome are of clinical significance. Such disorders cause fragile X syndrome.
Other examples of specialized regions of chromosomes are telomeres and centromeres.
The role of heterochromatin, which accounts for a significant part of the human genome, has not yet been precisely established. Heterochromatin is condensed throughout almost the entire cell cycle, it is inactive and replicates late. Most regions are condensed and inactive in all cells (), although others, such as the X chromosome, can be either condensed and inactive or decondensed and active (facultative heterochromatin). If, due to chromosomal aberrations, genes end up close to heterochromatin, then the activity of such genes can change or even be blocked. Therefore, the manifestations of chromosomal aberrations, such as duplications or deletions, depend not only on the affected loci, but also on the type of chromatin in them. Many chromosomal abnormalities that are not lethal affect inactive or inactivated regions of the genome. This may explain that trisomy on some chromosomes or monosomy on the X chromosome are compatible with life.
Manifestations of chromosomal abnormalities also depend on the new arrangement of structural and regulatory genes in relation to each other and to heterochromatin.
Fortunately, many structural features of chromosomes can be reliably detected by cytological methods. Currently, there are a number of methods for differential chromosome staining (Fig. 66.1 and Fig. 66.3). The location and width of the bands are identical in each pair of homologous chromosomes, with the exception of polymorphic regions, so staining can be used in clinical cytogenetics to identify chromosomes and detect structural abnormalities in them.
