Polytene chromosomes. Chromosomes in meiosis There is no conjugation of homologous chromosomes

Crossing-over: exchange of genetic material between chromosomes as a result of “breaking” and joining of chromosomes; the process of exchanging sections of chromosomes during the crossing of chromosomes (Fig. 118, B4).

During pachytene (the stage of thick filaments), homologous chromosomes are in a state of conjugation for a long period: in Drosophila - four days, in humans - more than two weeks. All this time, individual sections of chromosomes are in very close contact. If in such a region a break in the DNA chains occurs simultaneously in two chromatids belonging to different homologs, then when the break is restored, it may turn out that the DNA of one homologue will be connected to the DNA of another, homologous chromosome. This process is called crossing-over.

Since crossing over is the mutual exchange of homologous sections of chromosomes between homologous (paired) chromosomes of the original haploid sets, individuals have new genotypes that differ from each other. In this case, a recombination of the hereditary properties of the parents is achieved, which increases variability and provides richer material for natural selection.

Genes are mixed due to the fusion of gametes of two different individuals, but genetic changes are not carried out only in this way. No two offspring of the same parents (unless they are identical twins) will be exactly alike. During meiosis, two different types of gene reassortment occur.

One type of reassortment is the result of the random distribution of different maternal and paternal homologues between daughter cells during the first meiotic division, each gamete receiving its own different selection of maternal and paternal chromosomes. It follows from this that the cells of any individual can, in principle, form 2 to the power of n genetically different gametes, where n is the haploid number of chromosomes. However, in fact, the number of possible gametes is immeasurably greater due to crossing over (crossover) - a process that occurs during the long prophase of the first division of meiosis, when homologous chromosomes exchange sections. In humans, in each pair of homologous chromosomes, crossing over occurs on average at 2 - 3 points.

During crossing over, the DNA double helix is broken in one maternal and one paternal chromatid, and then the resulting segments are reunited “crosswise” (the process of genetic recombination). Recombination occurs in prophase of the first meiotic division, when the two sister chromatids are so closely packed together that they cannot be seen separately. Much later in this extended prophase, the two separate chromatids of each chromosome become clearly distinguishable. At this time, it is clear that they are connected by their centromeres and are closely aligned along their entire length. The two homologues remain linked at the points where crossing over occurred between the paternal and maternal chromatids. At each such point, which is called a chiasm, two of the four chromatids intersect. Thus, this is the morphological result of the crossing over that has occurred, which in itself is not observable.

Twice. Occurs in two stages (reduction and equational stages of meiosis). Meiosis should not be confused with gametogenesis - the formation of specialized germ cells, or gametes, from undifferentiated stem cells.

With a decrease in the number of chromosomes as a result of meiosis, a transition from the diploid phase to the haploid phase occurs in the life cycle. Restoration of ploidy (transition from the haploid phase to the diploid phase) occurs as a result of the sexual process.

Due to the fact that in the prophase of the first, reduction stage, pairwise fusion (conjugation) of homologous chromosomes occurs, the correct course of meiosis is possible only in diploid cells or in even polyploids (tetra-, hexaploid, etc. cells). Meiosis can also occur in odd polyploids (tri-, pentaploid, etc. cells), but in them, due to the inability to ensure pairwise fusion of chromosomes in prophase I, chromosome divergence occurs with disturbances that jeopardize the viability of the cell or developing from it a multicellular haploid organism.

The same mechanism underlies the sterility of interspecific hybrids. Since interspecific hybrids combine chromosomes of parents belonging to different species in the cell nucleus, the chromosomes usually cannot enter into conjugation. This leads to disturbances in the segregation of chromosomes during meiosis and, ultimately, to the non-viability of germ cells, or gametes. Certain restrictions on the conjugation of chromosomes are also imposed by chromosomal mutations (large-scale deletions, duplications, inversions or translocations).

Phases of meiosis

Meiosis consists of two successive divisions with a short interphase between them.

Prophase I- prophase of the first division is very complex and consists of 5 stages:

Phase leptotene or leptonemes- condensation of DNA to form chromosomes in the form of thin threads.
Zygotene or zygonema- conjugation (connection) of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents.
Pachytena or pachynema- crossing over (crossover) exchange of sections between homologous chromosomes; homologous chromosomes remain connected to each other.
Diplotena or diplonema- partial decondensation of chromosomes occurs, while part of the genome can work, the processes of transcription (RNA formation), translation (protein synthesis) occur; homologous chromosomes remain connected to each other.
Diakinesis- DNA condenses to the maximum again, synthetic processes stop, the nuclear membrane dissolves; homologous chromosomes remain connected to each other.

Metaphase I- bivalent chromosomes line up along the equator of the cell.
Anaphase I- microtubules contract, bivalents divide and chromosomes move towards the poles. It is important to note that, due to the conjugation of chromosomes in zygotene, entire chromosomes, consisting of two chromatids each, diverge to the poles, and not individual chromatids, as in mitosis.
Telophase I

The second division of meiosis follows immediately after the first, without a pronounced interphase: there is no S period, since DNA replication does not occur before the second division.

Prophase II- condensation of chromosomes occurs, the cell center divides and the products of its division disperse to the poles of the nucleus, the nuclear membrane is destroyed, and a fission spindle is formed.
Metaphase II- univalent chromosomes (consisting of two chromatids each) are located at the “equator” (at an equal distance from the “poles” of the nucleus) in the same plane, forming the so-called metaphase plate.
Anaphase II- univalents divide and chromatids move to the poles.
Telophase II- chromosomes despiral and a nuclear envelope appears.

Secondly, lateral conjugation of chromatids. Cells that have polytene chromosomes lose the ability to divide, they are differentiated and actively secreting, that is, polytenization of chromosomes is a way to increase the number of copies of genes for the synthesis of any product. Polytene chromosomes can be observed in dipterans, in plants in cells associated with the development of the embryo, in ciliates during the formation of the macronucleus. Polytene chromosomes increase significantly in size, which makes them easier to observe and which made it possible to study gene activity back in the 1930s. The fundamental difference from other types of chromosomes is that polytene chromosomes are interphase, whereas all others can only be observed during mitotic or meiotic cell division.

A classic example is the giant chromosomes in the salivary gland cells of larvae. Drosophila melanogaster. DNA replication in these cells is not accompanied by cell division, which leads to the accumulation of newly constructed DNA strands. These threads are tightly connected along their length. In addition, somatic synapsis of homologous chromosomes occurs in the salivary glands, that is, not only sister chromatids are conjugated with each other, but also the homologous chromosomes of each pair are conjugated with each other. Thus, a haploid number of chromosomes can be observed in the cells of the salivary glands.

Story

The term “polytene chromosome” was proposed by P. Koller ( P. Koller) in 1935, and was finally introduced into science by S. Darlington in 1937.

Dimensions

Polytene chromosomes are many times larger than the chromosomes of ordinary somatic cells. They are usually 100-200 times longer and 1000 times thicker (contain up to 1000 chromosomes) than the chromosomes of many interphase cells (both germ and somatic). Yes, in the larvae D.melangaster the total length of four pairs of chromosomes in the salivary glands is 2000 microns, and in ordinary somatic cells this value is 7.5 microns.

Structure

Striations

The characteristic shape and size of polytene chromosomes are achieved due to their maximum despiralization and repeated reproduction of chromosomes without their subsequent divergence, that is, they are formed as a result of endomitosis.

Polytene chromosomes have a characteristic transverse striation due to the presence of areas of denser spiralization of chromonemas - chromomeres. Dark areas (i.e., chromomeres) contain coiled, inactive chromatin, while light bands indicate an area of increased transcriptional activity. The clear distinction between dark discs and light interdiscal areas is explained by the nondivergence of daughter chromonemas. For this reason, all the features of an individual chromonema, including the chromomeric pattern, become more pronounced.

Essentially, polytene chromosomes are a pair of giant homologous chromosomes in a state of perfectly precise somatic conjugation. In this case, the disks and interdisk regions of homologs are located strictly parallel and closely approximated. Such conjugation is not typical for the vast majority of somatic cells.

The polytene chromosome map was first compiled in 1935 by Calvin Bridges, and it is still widely used today.

The uniqueness of the structure of polytene chromosomes, namely the ability to clearly distinguish the details of their structure, was used by T. Paytner to study their rearrangements and the nature of conjugation. In general, the striation of polytene chromosomes is extremely useful for research; in particular, using the example of polytene chromosomes, visualization of areas of active and inactive chromatin was obtained. They can also be used to study the general structure of chromatin.

In addition, polytene chromosomes help identify bell mosquito larvae ( Chironomid), which are difficult to distinguish in any other way.

Poufs

In polytene chromosomes, the transcription process is accompanied by the formation of the so-called. poufs- characteristic swellings of certain disks, formed as a result of local decompactization of DNA in them. Active transcription in these regions is indicated by active incorporation of 3 H-uridine in the puff region. Large poufs are called Balbiani rings(in some sources the terms “pouf” and “Balbiani rings” are used synonymously).

Thus, the formation of poufs is a prime example differential transcription. Another famous example of this process is the formation of lampbrush chromosomes.

Functions

Polytene chromosomes contain a large number of gene copies, which greatly enhances gene expression. This, in turn, increases the production of essential proteins. For example, in the cells of the salivary glands of larvae D. melanogaster polytenization of chromosomes is necessary to produce large amounts of sticky substance prior to pupation.

Notes

, With. 66-70.
, With. 69.
Balbiani E. G. Sur la structure du noyau des cellules salivaires chez les larves de Chironomus(French) // Zoologischer Anzeiger (English) Russian: magazine. - 1881. - Vol. 4 . - P. 637-641.

DNA within chromosomes can be arranged at different densities, depending on their functional activity and stage of the cell cycle. In this regard, two states of chromosomes are distinguished - interphase and mitotic. Mitotic chromosomes are formed in a cell during mitosis. These are non-functioning chromosomes, and the DNA molecules in them are packed extremely tightly. Suffice it to say that the total length of metaphase chromosomes is approximately 104 times less than the length of all DNA contained in the nucleus. Due to this compactness of mitotic chromosomes, an even distribution of genetic material between daughter cells during mitosis is ensured.

Ticket 33 special or giant chromosomes

Polytene chromosomes - giant interphase chromosomes that arise in some types of specialized cells as a result of two processes: first, multiple DNA replication not accompanied by cell division, and second, lateral conjugation of chromatids. Cells that have polytene chromosomes lose the ability to divide, they are differentiated and actively secreting, that is, polytenization of chromosomes is a way to increase the number of copies of genes for the synthesis of any product. The characteristic shape and size of polytene chromosomes are achieved due to their maximum despiralization and repeated reproduction of chromosomes without their subsequent divergence, that is, they are formed as a result of endomitosis. Polytene chromosomes have a characteristic transverse striation due to the presence of areas of denser spiralization of chromonemas - chromomeres. Dark regions (i.e., chromomeres) contain coiled, inactive chromatin, while light bands indicate an area of increased transcriptional activity. The clear distinction between dark discs and light interdiscal areas is explained by the nondivergence of daughter chromonemas. For this reason, all features of an individual chromonema, including the chromomeric pattern, become more pronounced. In essence, polytene chromosomes are a pair of giant homologous chromosomes in a state of perfectly precise somatic conjugation. In this case, the disks and interdisk regions of homologs are located strictly parallel and closely approximated. Such conjugation is not typical for the vast majority of somatic cells

In polytene chromosomes, the transcription process is accompanied by the formation of the so-called. puffs - characteristic swellings of certain disks formed as a result of local decompactization of DNA in them. Large poufs are called Balbiani rings.

Poofing is characteristic of the larval stage. The formation and disappearance of puffs is regulated by the internal environment of the body in accordance with the stage of development.

Polytene chromosomes contain a large number of gene copies, which greatly enhances gene expression. This, in turn, increases the production of essential proteins. For example, in the salivary gland cells of D. melanogaster larvae, polytenization of chromosomes is necessary for the formation of a large amount of adhesive substance before pupation

Ticket 35 ultrastructure of mitochondria, their function, origin.

Mitochondria, regardless of their size or shape, have a universal structure, their ultrastructure is uniform. Mitochondria are bounded by two membranes .

Outer membrane The outer membrane of the mitochondrion has a thickness of about 7 nm, does not form invaginations or folds, and is closed on itself. The main function is to separate the mitochondrion from the cytoplasm. The outer membrane of the mitochondrion consists of lipids interspersed with proteins. Plays a special role porin- channel-forming protein: it forms holes in the outer membrane with a diameter of 2-3 nm, through which small molecules and ions weighing up to 5 kDa can penetrate. Large molecules can only cross the outer membrane through active transport through mitochondrial membrane transport proteins. The outer membrane of the mitochondrion can interact with the membrane of the endoplasmic reticulum; it plays an important role in the transport of lipids and calcium ions.

Intermembrane space

The intermembrane space is the space between the outer and inner membranes of the mitochondrion. Its thickness is 10-20 nm. Since the outer membrane of the mitochondrion is permeable to small molecules and ions, their concentration in the periplasmic space differs little from that in the cytoplasm. On the contrary, large proteins require specific signal peptides for transport from the cytoplasm to the periplasmic space; therefore, the protein components of the periplasmic space and the cytoplasm are different. One of the proteins contained not only in the inner membrane, but also in the periplasmic space is cytochrome c

Inner membrane

The inner membrane consists mainly of protein complexes and forms numerous comb-like folds - crista, A characteristic feature of the composition of the inner membrane of mitochondria is the presence in it cardiolipin- a special phospholipid containing four fatty acids at once and making the membrane absolutely impermeable to protons. The outer and inner membranes touch in some places; there is a special receptor protein that promotes the transport of mitochondrial proteins encoded in the nucleus into the mitochondrial matrix.

One of the main functions of mitochondria is the synthesis of ATP, a universal form of chemical energy in any living cell.

According to the theory symbiogenesis, mitochondria appeared as a result of the capture of bacteria by primitive cells (prokaryotes). Cells that could not use oxygen themselves to generate energy had serious limitations in their development capabilities; bacteria (progenotes) could do this. In the process of developing such relationships, the progenotes transferred many of their genes to the now formed eukaryotic nucleus, thanks to increased energy efficiency.

CONJUGATION - Haploid gametes formed during the division of a diploid cell by meiosis contain one chromosome of each homologous pair (paternal or maternal origin), i.e. only half the original number of chromosomes. In this regard, an additional requirement is imposed on the cell division apparatus: homologues must “recognize” each other and pair up before they line up at the spindle equator. This pairing, or conjugation, of homologous chromosomes of maternal and paternal origin occurs only in meiosis. During the first division of meiosis, DNA replication occurs, and each chromosome then consists of two chromatids, homologous chromosomes are conjugated along their entire length, and crossing over occurs between the chromatids of paired chromosomes

CROSSINGOVER (crossing-over): exchange of genetic material between chromosomes, as a result of “breaking” and joining of chromosomes; the process of exchanging sections of chromosomes during the crossing of chromosomes (Fig. 118, B4).

One type of reassortment is the result of the random distribution of different maternal and paternal homologues between daughter cells during the first meiotic division, each gamete receiving its own, different selection of maternal and paternal chromosomes. It follows from this that the cells of any individual can, in principle, form 2 to the power of n genetically different gametes, where n is the haploid number of chromosomes. However, in fact, the number of possible gametes is immeasurably greater due to crossing over, a process that occurs during the long prophase of the first division of meiosis, when homologous chromosomes exchange sections. In humans, in each pair of homologous chromosomes, crossing over occurs on average at 2 - 3 points.

During crossing over, the DNA double helix is broken in one maternal and one paternal chromatid, and then the resulting segments are reunited “crosswise” (the process of genetic recombination). Recombination occurs in prophase of the first division of meiosis, when the two sister chromatids are so close together that they cannot be seen separately. Much later in this extended prophase, the two separate chromatids of each chromosome become clearly distinguishable. At this time, it is clear that they are connected by their centromeres and are closely aligned along their entire length. The two homologs remain linked at the points where crossing over occurred between the paternal and maternal chromatids. At each such point, which is called a chiasm, two of the four chromatids intersect. Thus, this is the morphological result of the crossing over that has occurred, which in itself is not observable.

At this stage of meiosis, the homologs in each pair (or bivalent) remain connected to each other by at least one chiasm. In many bivalents there is a greater number of chiasmata, since multiple crossings between homologues are possible