The genetic material contained in a cell forms structurally differentiated units called chromosomes. Chromosomes are multimolecular aggregates formed mainly by DNA and protein molecules and containing a small amount of RNA, which, strictly speaking, is not a structural part of the chromosome.

The structure of chromosomes is clearly visible at the metaphase stage of mitosis. The study of chromosomes made it possible to establish the following facts:

1) in all somatic cells of any plant or animal organism, the number of chromosomes is the same;

2) the germ cells always contain half as many chromosomes as the somatic cells of a given type of organism;

3) in all organisms belonging to the same species, the number of chromosomes in the cells is the same (for example, a person has 23 pairs of chromosomes in somatic cells, and a pigeon has 40 pairs).

The number of chromosomes in somatic cells is always even, since they contain two chromosomes of the same shape and size: one from the paternal organism, and the other from the maternal one. The chromosome set of a somatic cell, in which each chromosome has a pair, is called double or diploid. Only one of each pair of chromosomes enters the germ cells, therefore the chromosome set in this case is called single or haploid.

In determining the shape of chromosomes great importance has the position of the so-called primary constriction, or centromere, the region to which the spindle tubes are attached during mitosis. The centromere divides the chromosome into two arms. The location of the centromere determines three main types of chromosomes:

1) equal-shouldered - with shoulders equal or almost equal length;

2) uneven shoulders, having shoulders of unequal length;

3) rod-shaped - with one long and the second very short, sometimes hardly detectable shoulder.

The direct carrier of hereditary information in chromosomes is Deoxyribonucleic acid (DNA) is a biological polymer consisting of two polynucleotide chains connected to each other. The monomers that make up each strand of DNA are complex organic compounds, including one of four nitrogenous bases: adenine (A), guanine (G), thymine (T) or cytosine (C); sugar - deoxyribose, after which the DNA itself, as well as the residue phosphoric acid. These compounds are called nucleotides (Fig. 1).

Rice. 1. Diagram of the nucleotide structure

In each strand, the nucleotides are joined by the formation of covalent bonds between the deoxyribose of one and the phosphoric acid residue of the next nucleotide. Two chains are combined into one molecule using hydrogen bonds between nitrogenous bases that are part of the nucleotides that form different chains. The number of such bonds between different nitrogenous bases is not the same, and as a result, they can only be connected in pairs: the nitrogenous base A of one chain of polynucleotides is always connected by two hydrogen bonds with T of the other chain, and G - by three hydrogen bonds with the nitrogenous base C of the opposite polynucleotide chain. This ability to selectively combine nucleotides is called complementarity. Complementary interaction of nucleotides leads to the formation of pairs of nucleotides. In a polynucleotide chain, adjacent nucleotides are linked together through a sugar (deoxyribose) and a phosphoric acid residue.

In 1953, an American biophysicist J. Watson(b. 1928) together with the English biophysicist and geneticist F. Creek(b. 1916) proposed a model of the spatial structure of DNA in the form of a double helix.

Thus, in structural organization of the DNA molecule one can distinguish the primary structure - a polynucleotide chain, the secondary structure - two complementary and antiparallel polynucleotide chains connected by hydrogen bonds, and the tertiary structure - a three-dimensional helix. The helix diameter is 2 nm, the pitch length is 3.4 nm. Each turn contains 10 pairs of nucleotides. The length of the helix of the DNA molecule depends on the organism to which it belongs. The DNA of simple viruses contains several thousand nucleotide pairs, bacteria - several million, and higher organisms - billions. If you line up all the DNA molecules contained in one human cell in one line, you get a thread 2 m long, that is, its length is a billion times greater than its thickness.

Hereditary information, written in the genetic code, is stored in DNA molecules and multiplies in order to provide newly formed cells with the necessary "instructions" for their development and functioning. At the same time, DNA does not directly participate in the life support of cells. The role of an intermediary, whose function is to translate the hereditary information stored in DNA into a working form, is played by ribonucleic acids (RNA).

Unlike DNA molecules, ribonucleic acids are represented by one polynucleotide chain, which consists of four types of nucleotides containing sugar - ribose (instead of deoxyribose), a phosphoric acid residue and one of four nitrogenous bases: adenine, guanine, cytosine or uracil (instead of thymine). In an RNA chain, nucleotides are joined by the formation of covalent bonds between the ribose of one nucleotide and the phosphoric acid residue of another. RNA is synthesized on DNA molecules using RNA polymerase enzymes in compliance with the principle of complementarity, and uracil is complementary to DNA adenine in RNA.

Depending on the function and location in the cell, one can distinguish three types of RNA: information (mRNA), transport (tRNA) and ribosomal (rRNA). Each of these RNAs is synthesized in a specific region of DNA. Synthesis process messenger RNA, which is called transcription - the rewriting of information, begins with the discovery by RNA polymerase special section in the DNA molecule, indicating the place where transcription begins - the promoter. After attaching to the promoter, RNA polymerase unwinds the adjacent turn of the DNA helix. Two strands of DNA diverge at this point, and on one of them the enzyme synthesizes mRNA. The size of mRNA depends on the length of the DNA segment on which it was synthesized. mRNA molecules can consist of 300-30,000 nucleotides.

In the process of synthesis, as the RNA polymerase moves along the DNA molecule, the single-stranded sections of DNA it has passed through are again combined into a double helix. The mRNA formed during transcription contains an exact copy of the information recorded in the corresponding section of DNA. Three adjacent mRNA nucleotides that code for amino acids are called codons. The mRNA codon sequence codes for the sequence of amino acids in the polypeptide chain. Codons of mRNA correspond to specific amino acids.

Chromosomes- cell structures that store and transmit hereditary information. A chromosome is made up of DNA and protein. The complex of proteins associated with DNA forms chromatin. Proteins play an important role in the packaging of DNA molecules in the nucleus.

DNA in chromosomes is packed 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 packaging of DNA takes the form of a looped structure, similar to amphibian lampbrush chromosomes or insect polytene chromosomes. The loops are maintained by proteins that recognize specific nucleotide sequences and bring them closer together. The structure of the chromosome is best seen in the 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. The structure and replication of the chromosome

When DNA replication starts, RNA synthesis stops. Chromosomes can be in two states: condensed (inactive) and decondensed (active).

The diploid set of chromosomes in an organism is called a karyotype. Modern methods research allows you to determine each chromosome in the karyotype. For this, the distribution of light and dark bands visible under a microscope (alternation of AT and GC pairs) in chromosomes treated with special dyes is taken into account. The chromosomes of the representatives have transverse striation. different types. In related species, for example, in humans and chimpanzees, the pattern of alternation of bands in the chromosomes is very similar.

Each species of organisms has a constant number, shape and composition of chromosomes. The human karyotype has 46 chromosomes - 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 certain alleles (for example, the blood clotting allele). Chromosomes of one pair are called homologous. Homologous chromosomes at the same 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 the continuity and succession of life.

asexual reproduction carried out in the following ways:

• simple division into two or many cells at once (bacteria, protozoa);
• vegetatively (plants, coelenterates);
• division of a multicellular body in half, followed by regeneration (starfish, hydra);
• budding (bacteria, coelenterates);
• dispute formation.

Asexual reproduction usually provides an increase in the number of genetically homogeneous offspring. But when spore nuclei are formed as a result of 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 bisexual individuals (hermaphrodites) produce both eggs and sperm. And in some algae, two identical germ cells merge.

Fusion of haploid gametes results in fertilization and the formation of a diploid zygote.

The zygote develops into a new individual.

All of the above is true only for eukaryotes. Prokaryotes also have a sexual process, but it happens differently.

Thus, during sexual reproduction, the genomes of two different individuals of the same species are mixed. Offspring carry new genetic combinations that distinguish them from their parents and from each other.

One of the types of sexual reproduction is parthenogenesis, or the development of individuals from an unfertilized egg (aphids, drone bees, etc.).

The structure of germ cells

Oocytes- round, relatively large, motionless cells. Sizes - 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 a yolk. The ovum 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 shell shell; 5 - liquid protein; 6 - dense protein; 7 - germinal disk; 8 - light yolk; 9 - dark yolk.

In mosses and ferns, eggs develop in archegonia, in flowering plants - in ovules localized in the ovary of the flower.

Oocytes are classified as follows:

• isolecithal - the yolk is evenly distributed and there is not much of it (in worms, mollusks);
• alecithal - almost devoid of yolk (mammals);
• telolecital - contain a lot of yolk (fish, birds);
• polylecital - contain a significant amount of yolk.

Ovogenesis is the production of eggs in females.

In the breeding zone are ovogonia - primary germ cells that reproduce by mitosis.

From the ogonium after the first meiotic division, oocytes of the first order are formed.

After the second meiotic division, second-order oocytes are formed, from which one egg and three directional bodies are formed, which then die.

spermatozoa- small, mobile cells. They have a head, neck and tail.

In front of the head is the acrosomal apparatus - an analogue of the Golgi apparatus. It contains an enzyme (hyaluronidase) that dissolves the shell of the egg during fertilization. The neck contains centrioles and mitochondria. The flagella are made up of microtubules. During fertilization, only the nucleus and centrioles of the sperm enter the egg. Mitochondria and other organelles remain outside. Therefore, cytoplasmic heredity in humans is transmitted only through the female line.

The sex cells of sexually reproducing animals and plants are formed as a result of a process called gametogenesis.

Chromosome is a DNA-containing thread-like structure in the cell nucleus that carries genes, the units of heredity, arranged in a linear order. Humans have 22 pairs of normal chromosomes and one pair of sex chromosomes. In addition to genes, chromosomes also contain regulatory elements and nucleotide sequences. They house DNA-binding proteins that control the functions of DNA. Interestingly, the word "chromosome" comes from the Greek word "chrome", meaning "color". Chromosomes got this name due to the fact that they have the peculiarity of being painted in different tones. The structure and nature of chromosomes vary from organism to organism. Human chromosomes have always been the subject of constant interest of researchers working in the field of genetics. The wide range of factors that are determined by human chromosomes, the anomalies they are responsible for, and their complex nature have always attracted the attention of many scientists.

Interesting facts about human chromosomes

Human cells contain 23 pairs of nuclear chromosomes. Chromosomes are made up of DNA molecules that contain genes. The chromosomal DNA molecule contains three nucleotide sequences required for replication. When staining chromosomes, the banded structure of mitotic chromosomes becomes apparent. Each strip contains numerous nucleotide pairs of DNA.

Man is a biological species that reproduces sexually and has diploid somatic cells containing two sets of chromosomes. One set is inherited from the mother, while the other is inherited from the father. Reproductive cells, unlike body cells, have one set of chromosomes. Crossing over (crossover) between chromosomes leads to the creation of new chromosomes. New chromosomes are not inherited from either parent. This is the reason for the fact that not all of us exhibit traits that we receive directly from one of our parents.

Autosomal chromosomes are numbered from 1 to 22 in descending order as their size decreases. Each person has two sets of 22 chromosomes, an X chromosome from the mother and an X or Y chromosome from the father.

An abnormality in the contents of a cell's chromosomes can cause certain genetic disorders in humans. Chromosomal abnormalities in humans are often responsible for the occurrence of genetic diseases in their children. Those who have chromosomal abnormalities are often only carriers of the disease, while their children have the disease.

Chromosomal aberrations (structural changes in chromosomes) are caused by various factors, namely, a deletion or duplication of a part of a chromosome, an inversion, which is a change in the direction of a chromosome to the opposite, or a translocation, in which a part of a chromosome breaks off and joins it to another chromosome.

An extra copy of chromosome 21 is responsible for a very well known genetic disorder called Down syndrome.

Trisomy 18 leads to Edwards syndrome, which can cause death in infancy.

A deletion of part of the fifth chromosome results in a genetic disorder known as 'cried cat' syndrome. In people affected by this disease, there is often a delay in mental development, and their crying in childhood resembles a cat's cry.

Sex chromosome abnormalities include Turner syndrome, in which female sex characteristics are present but underdeveloped, and XXX syndrome in girls and XXY syndrome in boys, which cause dyslexia in affected individuals.

Chromosomes were first discovered in plant cells. Van Beneden's monograph on fertilized roundworm eggs led to further research. Later, August Weissman showed that the germline was different from the soma and found that the cell nuclei contained hereditary material. He also suggested that fertilization leads to the formation of a new combination of chromosomes.

These discoveries have become cornerstones in the field of genetics. Researchers have already accumulated a fairly significant amount of knowledge about human chromosomes and genes, but much remains to be discovered.

Video

Chromosomes are an intensely colored body, consisting of a DNA molecule associated with histone proteins. Chromosomes are formed from chromatin at the beginning of cell division (in the prophase of mitosis), but they are best studied in the metaphase of mitosis. When the chromosomes are located in the plane of the equator and are clearly visible in a light microscope, the DNA in them reaches maximum helicity.

Chromosomes consist of 2 sister chromatids (doubled DNA molecules) connected to each other in the region of the primary constriction - the centromere. The centromere divides the chromosome into 2 arms. Depending on the location of the centromere, chromosomes are divided into:

the metacentric centromere is located in the middle of the chromosome and its arms are equal;

submetacentric centromere is displaced from the middle of the chromosomes and one arm is shorter than the other;

acrocentric - the centromere is located close to the end of the chromosome and one arm is much shorter than the other.

In some chromosomes, there are secondary constrictions that separate from the shoulder of the chromosome a region called the satellite, from which the nucleolus is formed in the interphase nucleus.

Chromosome Rules

1. The constancy of the number. The somatic cells of the body of each species have a strictly defined number of chromosomes (in humans -46, in cats - 38, in fruit flies - 8, in dogs -78, in chickens -78).

2. Pairing. Each chromosome in somatic cells with a diploid set has the same homologous (same) chromosome, identical in size, shape, but unequal in origin: one from the father, the other from the mother.

3. Individuality. Each pair of chromosomes differs from the other pair in size, shape, alternation of light and dark stripes.

4. Continuity. Before cell division, the DNA is doubled and the result is 2 sister chromatids. After division, one chromatid enters the daughter cells and, thus, the chromosomes are continuous - a chromosome is formed from the chromosome.

All chromosomes are divided into autosomes and sex chromosomes. Autosomes - all chromosomes in cells, with the exception of sex chromosomes, there are 22 pairs of them. Sexual - this is the 23rd pair of chromosomes, which determines the formation of the male and female body.

In somatic cells there is a double (diploid) set of chromosomes, in sex cells - haploid (single).

A certain set of chromosomes of a cell, characterized by the constancy of their number, size and shape, is called karyotype.

In order to understand a complex set of chromosomes, they are arranged in pairs as their size decreases, taking into account the position of the centromere and the presence of secondary constrictions. Such a systematized karyotype is called an idiogram.

For the first time, such a systematization of chromosomes was proposed at the Congress of Geneticists in Denver (USA, 1960)

In 1971, in Paris, chromosomes were classified according to color and alternation of dark and light bands of hetero- and euchromatin.

To study the karyotype, geneticists use the method of cytogenetic analysis, in which a number of hereditary diseases associated with a violation of the number and shape of chromosomes can be diagnosed.

1.2. The life cycle of a cell.

The life of a cell from its inception as a result of division to its own division or death is called the cell life cycle. Throughout life, cells grow, differentiate, and perform specific functions.

The life of a cell between divisions is called interphase. Interphase consists of 3 periods: presynthetic, synthetic and postsynthetic.

The presynthetic period immediately follows the division. At this time, the cell grows intensively, increasing the number of mitochondria and ribosomes.

During the synthetic period, replication (doubling) of the amount of DNA occurs, as well as the synthesis of RNA and proteins.

During the post-synthetic period, the cell stores energy, achromatin spindle proteins are synthesized, and preparations for mitosis are in progress.

There are different types of cell division: amitosis, mitosis, meiosis.

Amitosis is a direct division of prokaryotic cells and some cells in humans.

Mitosis is an indirect cell division during which chromosomes are formed from chromatin. Somatic cells of eukaryotic organisms divide by mitosis, as a result of which the daughter cells receive exactly the same set of chromosomes as the daughter cell had.

Mitosis

Mitosis consists of 4 phases:

Prophase is the initial phase of mitosis. At this time, DNA spiralization and shortening of chromosomes begin, which from thin invisible chromatin threads become short thick ones, visible in a light microscope, and arranged in the form of a ball. The nucleolus and the nuclear envelope disappear, and the nucleus disintegrates, the centrioles of the cell center diverge along the poles of the cell, and the fission spindle threads stretch between them.

Metaphase - chromosomes move towards the center, spindle threads are attached to them. Chromosomes are located in the plane of the equator. They are clearly visible under a microscope and each chromosome consists of 2 chromatids. In this phase, the number of chromosomes in a cell can be counted.

Anaphase - sister chromatids (appeared in the synthetic period when DNA is duplicated) diverge towards the poles.

Telophase (telos Greek - end) is the opposite of prophase: chromosomes from short thick visible ones become thin long ones invisible in a light microscope, the nuclear envelope and nucleolus are formed. Telophase ends with the division of the cytoplasm with the formation of two daughter cells.

The biological significance of mitosis is as follows:

daughter cells receive exactly the same set of chromosomes that the mother cell had, so a constant number of chromosomes is maintained in all cells of the body (somatic).

all cells divide except sex cells:

the body grows in the embryonic and postembryonic periods;

all functionally obsolete cells of the body (epithelial cells of the skin, blood cells, cells of the mucous membranes, etc.) are replaced by new ones;

processes of regeneration (recovery) of lost tissues occur.

Diagram of mitosis

When exposed to unfavorable conditions on a dividing cell, the spindle of division can unevenly stretch the chromosomes to the poles, and then new cells are formed with a different set of chromosomes, a pathology of somatic cells (autosomal heteroploidy) occurs, which leads to diseases of tissues, organs, body.

There are four types of chromosome structure:

Ø telocentric (rod-shaped chromosomes with a centromere located at the proximal end);

Ø acrocentric (rod-shaped chromosomes with a very short, almost imperceptible second arm);

Ø submetacentric (with shoulders of unequal length, resembling the letter L in shape);

Ø metacentric (V-shaped chromosomes with arms of equal length).

Polymer chromosomes:

Chromosomes "lampbrushes":

Lampbrush chromosomes, first discovered by W. Flemming in 1882, are a special form of chromosomes that they acquire in the growing oocytes (female sex cells) of most animals, with the exception of mammals.

In growing oocytes of all animals except mammals, during the extended diplotene stage of prophase I of meiosis, active transcription of many DNA sequences leads to the transformation of chromosomes into chromosomes shaped like glass cleaning brushes. kerosene lamps(lampbrush chromosomes). They are highly decondensed semi-bivalents consisting of two sister chromatids. Lampbrush chromosomes can be observed using light microscopy, showing that they are organized as a series of chromomeres (contain condensed chromatin) and paired lateral loops emanating from them (contain transcriptionally active chromatin).

The organization of lampbrush chromosomes in caudate and anuran amphibians, domesticated bird species, and some insect species has been described in most detail. Lampbrush chromosomes from amphibians and birds can be isolated from the oocyte nucleus using microsurgical procedures.

Lampbrush chromosomes produce a huge amount of RNA synthesized on the lateral loops. Each lateral loop always contains the same DNA sequence and remains in an extended state throughout the entire oocyte growth, up to the onset of chromosome condensation. The lateral loop may contain one or more transcription units with a polarized RNP matrix covering the DNP axis of the loop. At the same time, most of the DNA remains in a condensed state and is organized into chromomeres in lampbrush-type chromosome axes.

Due to their gigantic size and pronounced chromomere-loop organization, lampbrush chromosomes have served for many decades as a convenient model for studying the organization of chromosomes, the operation of the genetic apparatus, and the regulation of gene expression during prophase I of meiosis. In addition, chromosomes of this type are widely used for sequence mapping DNA with a high degree of resolution, studying the phenomenon of transcription of tandem DNA repeats that do not code for proteins, analyzing the distribution of chiasmata, etc.

Karyotype and idiogram of human chromosomes. The structure and types of chromosomes. Characteristics of the haploid and diploid type of chromosomes. Methods of photokaryogram analysis. Groups of chromosomes in the human karyotype.

Karyotype and idiogram of human chromosomes:

Karyotype- a set of chromosomes of a somatic cell that characterizes an organism of a given species. Chromosomes are divided into autosomes and heterochromosomes.

An idiogram is a systematized karyotype in which chromosomes are arranged as their size decreases.

The structure and types of chromosomes:

Chromosomes - cell structures that store and transmit hereditary information. A chromosome is made up of DNA and protein. The complex of proteins associated with DNA forms chromatin. Proteins play an important role in the packaging of DNA molecules in the nucleus.

There are four types of chromosome structure:

Telocentric (rod-shaped chromosomes with a centromere located at the proximal end);

Acrocentric (rod-shaped chromosomes with a very short, almost imperceptible second arm);

Submetacentric (with shoulders of unequal length, resembling the letter L in shape);

Metacentric (V-shaped chromosomes with arms of equal length).

The chromosome type is constant for each homologous chromosome and may be constant in all representatives of the same species or genus.

Characteristics of the haploid and diploid set of chromosomes:

Diploid set of chromosomes an organism is called a karyotype. Modern research methods make it possible to determine each chromosome in the karyotype. For this, the distribution of light and dark bands visible under a microscope (alternation of AT and GC pairs) in chromosomes treated with special dyes is taken into account. The chromosomes of representatives of different species have transverse striation. In related species, for example, in humans and chimpanzees, the pattern of alternation of bands in the chromosomes is very similar.

G aploid set of chromosomes(syn.: gametic set of chromosomes, single set of chromosomes) - a set of chromosomes inherent in a mature germ cell, in which of each pair characteristic of a given species there is only one chromosome; in a person G. n. X. represented by 22 autosomes and one sex chromosome.

Methods for analyzing photocardiograms: xs

Groups of chromosomes in the human karyotype:

Group A includes 3 pairs of the largest metacentric chromosomes (1-3).

Group B (4-5) includes 2 pairs of submetacentric chromosomes.

Group C (6-12) combines 7 pairs of medium-sized autosomes with a submedianly located centromere. In addition, the sex chromosome X is indistinguishable from the autosomes of this group and, when the standardly stained chromosomes are laid out, is included in the C group (6-X-12).

Group D (13-15) has 3 pairs of medium-sized acrocentric chromosomes.

In group E (16-18) - one pair of chromosomes (16) with a median localization of the centromere, pairs 17-18 differ in a shorter overall length and sizes of short arms.

The last two groups contain the smallest chromosomes: metacentric - group F (19-20) and acrocentric - group G (21-22).

The sex chromosome Y is acrocentric, similar to chromosomes 21 and 22, but can almost always be differentiated.