Centromeres are chromosomal structures responsible for the direction of chromosome movement during mitosis. The functions of centromeres include the adhesion of sister chromatids, kinetochore formation, pairing of homologous chromosomes, and involvement in the control of genetic expression. In most eukaryotes, centromeres do not contain a specific DNA sequence. They usually contain repeats (such as satellite DNA) that are similar but not identical. In the nematode Caenorhabditis elegans and some plants, the chromosomes are holocentric, i.e. kinetochore formation is not localized to a specific region, but occurs diffusely along the entire length of the chromosome.
Yeast centromeres
Centromere Sp 35-110 kb long (the longer the chromosome, the smaller the centromere) and consists of two domains - the central core region and the outer repeat region (otr), represented by heterochromatin (Fig. 1). The central core region consists of a region of non-repetitive DNA (cnt) and a region of inverted DNA
repeats (imt) along the edges of cnt. In the central core region, normal histone H3 is replaced by its counterpart (CENP-A in Sc) and the kinetochore is assembled at this location. Marker genes inserted into the centromeric sequence become transcriptionally inactive. Their silencing depends on the position, for example, on the outer repeats it is stronger, and in the central region it is less pronounced. Proteins Mis6, Mis12, Mal2 and Sim4 bind to the central region of the centromere. The central region is partially digested by micrococcal nuclease, which indicates a special organization of chromatin, and this organization does not depend on DNA (DNA transferred to Sp or to other parts of the chromosome does not retain such organization). The outer repeats are packaged into nucleosomes, with histones deacetylated (by the deacetylases Clr3, Clr6 and Sir2). Methyltransferase Clr4 dimethylates H3K9, on which Swi6 (an analogue of HP1) and Chp1 sit. Thus, heterochromatin is formed at the centromere
(see review of Heterochromatin). Swi6 is responsible for attaching cohesins to the outer repeat region. otr consists of dg and dh repeats, separated by other repeats. Internal and external repeats contain clusters of tRNA genes. It has been established that dg repeats have a primary role in establishing centromeric activity.
The DNA of the central core region is AT-rich and consists of three sections cnt1, cnt3 - 99% homologous, located along the lines of cnt2, which is 48% homologous with them. The left and right imr are inverted and unique to each centromere.
Rice. 1
All 16 centromeres Sc are 90 bp long and contain three elements: CDEI, CDEII and CDEIII (Fig. 2). CDEII is an AT-rich, nonconserved spacer 78–90 bp long that separates CDEI and CDEIII. CDEI is 8 bp long. This region is not essential for centromeric activity, but its deletion increases the likelihood of incorrect chromosome segregation during mitosis. CDEII - 78-90 bp, contains ~90% AT pairs. Deletions in this region interrupt centromere formation without interfering with chromosome segregation. CDEIII - 26 bp contains imperfect palindromes. A single nucleotide substitution in this region completely interrupts centromeric activity.
Rice. 2
Rice. 3 Sequences of centromeric DNA of chromosomes Sc
Human centromeres
The human centromere is a 1-4 Mb region of AT-rich α-satellite ~171 bp long ( alfoid). Other satellites are also present. Within the repeats, the site of centromere formation is established, called the neocentromere. The primary DNA sequence in an established neocentromere is not important. Not all α-satellites become a centromere; despite the presence of two α-satellite-rich loci, only one of them becomes an active centromere. Intact DNA containing the alfoid and placed in the nucleus does not form an active centromere, so the primary mechanism for the formation of an active centromere remains unclear.
By the middle of the last century, numerous cytological studies showed the decisive role of the centromere in the morphology of chromosomes. It was later discovered that the centromere, together with the kinetochore (a structure consisting mainly of proteins), is responsible for the correct segregation of chromosomes into daughter cells during cell division. The guiding role of the centromere in this process is obvious: after all, it is to it that the division spindle is attached, which, together with the cell centers (poles), constitutes the cell division apparatus. Due to the contraction of the spindle strands, chromosomes move toward the cell poles during division.
Five stages of cell division (mitosis) are usually described. For simplicity, we will focus on three main stages in the behavior of the chromosomes of a dividing cell (Fig. 2). At the first stage, gradual linear compression and thickening of chromosomes occurs, then a cell division spindle consisting of microtubules is formed. In the second, the chromosomes gradually move toward the center of the nucleus and line up along the equator, probably to facilitate the attachment of microtubules to the centromeres. In this case, the nuclear membrane disappears. At the last stage, the halves of the chromosomes - chromatids - separate. It seems that microtubules attached to the centromeres, like a tugboat, pull the chromatids towards the poles of the cell. From the moment of divergence, the former sister chromatids are called daughter chromosomes. They reach the spindle poles and come together in a parallel pattern. The nuclear envelope is formed.
Rice. 2. The main stages of mitosis.
From left to right: chromosome compaction, spindle formation; alignment of chromosomes along the equator of the cell,
attachment of the spindle to the centromeres; movement of chromatids to the poles of the cell.
With careful observation, one can notice that during the process of cell division in each chromosome, the centromere is in a constant position. It maintains a close dynamic connection with the cell center (pole). Centromere division occurs simultaneously in all chromosomes.
Sequencing methods developed in recent years have made it possible to determine the primary DNA structure of extended sections of human and fruit fly centromeres Drosophila and plants Arabidopsis. It turned out that in the chromosomes of both humans and plants, centromeric activity is associated with a block of tandemly organized DNA repeats (monomers) that are similar in size (170-180 nucleotide pairs, bp). Such sections are called satellite DNA. In many species, including those that are evolutionarily distant from each other, the size of the monomers is almost the same: various species of monkeys - 171 np, corn - 180 np, rice - 168 np, chironomus insect - 155 np. This may reflect general requirements for centromeric function.
Despite the fact that the tertiary structure of human and Arabidopsis centromeres is organized similarly, the primary nucleotide sequences (or nucleotide order) in their monomers turned out to be completely different (Fig. 3). This is surprising for a region of the chromosome that performs such an important and universal function. However, when analyzing the molecular organization of centromeres in Drosophila, a certain structural pattern was discovered, namely the presence of sections of monomers of approximately the same size. Thus, in Drosophila, the centromere of the X chromosome consists mainly of two types of very short simple repeats (AATAT and AAGAG), interrupted by retrotransposons (mobile DNA elements) and “islands” of more complex DNA. All these elements were found in the Drosophila genome and outside the centromeres, but DNA sequences characteristic of each centromere were not found in them. This means that centromeric DNA sequences themselves are insufficient and unnecessary for the formation of a centromere.
Rice. 3. DNA structure in human and plant centromeres.
The rectangles correspond to tandemly organized monomers with identical nucleotide sequences inside (primary DNA structure). In different species, the primary structure of DNA monomers varies, and the secondary structure is a helix. The sequence of monomers reflects the higher level structural organization of DNA.
This assumption is also confirmed by the manifestation of centromeric activity outside normal centromeres. Such neocentromeres behave like normal centromeres: they form a cytologically distinguishable constriction and form kinetochores that bind proteins. However, DNA analysis of two human neocentromeres and a conventional centromere did not reveal common sequences, which indicates the possible role of other structural components of the chromosome. They can be histone and non-histone proteins that bind to DNA, forming the nucleosome structure of chromatin.
The functional role of the centromeric chromatin structure is confirmed by the presence of histone H3 variants specific for each biological species in centromeric chromatin: in humans they are called CENP-A, in plants - CENH3. Among the many proteins present in the kinetochore, only two, CENH3 and centromeric protein C (CENP-C), directly bind to DNA. Perhaps it is CENH3, interacting with other histones (H2A, H2B and H4), that forms and determines the type of nucleosomes specific to centromeres. Such nucleosomes can serve as a kind of anchors for kinetochore formation. Variants of histone H3 in centromeres of various species are similar to the canonical histone H3 molecule in areas of interaction with other histone proteins (H2A, H2B, H4). However, the region of centromeric histone H3 that interacts with the DNA molecule appears to be under the influence of driving selection. As discussed, the primary structure of centromeric DNA differs between species, and centromeric histone H3 has been proposed to coevolve with centromeric DNA, particularly in Drosophila and Arabidopsis.
The discovery of the centromeric histone H3 gave rise to the extreme view that centromeric function and its complete independence from the primary DNA structure are determined by the nucleosomal organization and this histone. But are these factors sufficient for full centromere activity? Models that ignore the role of primary DNA structure must assume a random distribution of changes in centromeric DNA structure across populations in the absence of selection. However, analysis of satellite DNA in human centromeres and Arabidopsis identified conserved regions as well as regions with higher than average variability, indicating selection pressure on centromeric DNA. In addition, artificial centromeres were obtained only with human a-satellite repeats amplified from natural centromeres, but not from a-satellites of pericentromeric chromosome regions.
Models in which the decisive factor in determining the position of the centromere (preserved from generation to generation) and its functions is the tertiary (or even higher order) structure of DNA are encountered with fewer fundamental difficulties for explanation. Its conservatism allows for large variations in the nucleotide sequence and does not exclude fine tuning of the primary structure.
Henikoff and colleagues proposed a model that describes the coordinated evolution of DNA and proteins and leads to the appearance of optimally functioning centromeres using the example of female germ cell division. As is known, in the process of meiosis, one parent cell gives rise to four daughter cells through two successive divisions. Subsequently, only one of them turns into a mature female reproductive cell (gamete), transmitting genetic information to the next generation, while the other three cells die. According to this model, in the process of evolution, due to mutations and other mechanisms in chromosomes, centromeres with longer strands of satellite DNA monomers or with a primary nucleotide structure that is more conducive to binding and coordinated work with specific forms of histones CENH3 and CENP-C can arise. Moreover, in some organisms (Arabidopsis, Drosophila) evidence for positive selection pressure was obtained for CENH3, while for other species (cereals, mammals) for CENP-C (Fig. 4a). As a result, such centromeres with improved kinetochores become “stronger” and can attach a larger number of spindle microtubules (Fig. 4b). If there are more such “strong” centromeres in the gametes, then a process of meiotic drive occurs, which increases the number of such centromeres, and a new variant is fixed in the population.
Rice. 4. Model explaining the evolution of centromeres.
Top - centromeres (gray ovals) contain a specialized set of proteins (kinetochores), including histones CENH3 (H) and CENP-C (C), which in turn interact with spindle microtubules (red lines). In different taxa, one of these proteins evolves adaptively and in concert with the divergence of the primary DNA structure of centromeres.
Bottom - Changes in the primary structure or organization of centromeric DNA (dark gray oval) can create stronger centromeres, resulting in more microtubules being attached.
Comparative genomics helps to understand the mechanisms of formation and activity of centromeric regions of chromosomes. A unique example of diverse centromere structure is chromosome 8 in the rice genome. Along with satellite DNA repeats and retrotransposons, actively transcribed genes were found in it; 48 of them had sequences with high homology to known proteins. These findings refute the opinion, based on studies of centromeres in humans, Drosophila and Arabidopsis, that there are no actively working genes in centromeres.
If the molecular structure of centromeres of various eukaryotic species contains some universal characteristics (organization of DNA in the form of tandem, relatively short monomers and chromatin proteins specific to these loci), then it is difficult to identify any patterns in the sizes of these regions. Yes, in yeast Saccharomyces cerevisiae a 125 bp DNA section is taken as the minimum functional centromere, and in yeast Schizosaccharomyces pombe it is much more complex and longer (from 40 to 120 thousand words), has several levels of organization. In humans, the main component of chromosome centromeres - a-satellite DNA - forms long strands of tandemly organized monomers (from 250 thousand to 4 million bp). Among the 12 rice chromosomes, chromosome 8 has the shortest length of the strand with the CentO satellite (~64 thousand bp); the position of the centromere and its approximate size of 2 million bp were determined. It was possible to obtain the complete DNA sequence of this centromeric region and within it to determine the region (~750 thousand bp) where the kinetochore is directly formed. The main CentO cluster is located in this area.
The remarkable plasticity of centromeres, in particular the active genes found in the centromere of rice chromosome 8, suggests that there is no strict boundary between the centromere and the rest of the chromosome and even the possibility of a dispersed structure of centromeric chromatin. However, the existence of several clusters in the region of the chromosomal constriction is contradicted by recently published data on the presence of a chromatin barrier between the centromere itself and pericentromeric heterochromatin in yeast Schizosaccharomyces pombe. The barrier is the alanine tRNA gene. Deletion or modification of the barrier sequence leads to pericentromeric heterochromatin moving beyond its normal boundaries. Moreover, the absence of a barrier causes abnormal chromosome segregation in meiosis. Of course, it should be remembered that these interesting results so far concern only one type of yeast.
Depending on the functional and physiological states, a cell can divide in different ways. Division methods somatic cells: mitosis, amitosis or endomitosis. Sex cells divide by meiosis.
Mitosis – indirect cell division, accompanied by spiralization of chromosomes. There are several phases in mitosis:
I Prophase (from the Greek “pro” - before, “phases” - appearance). Spiralization and shortening of chromosomes occurs. The nucleolus and nuclear envelope disappear, the centrioles diverge to the poles of the cell, and a spindle is formed. Chromosomes consist of two chromatids connected by a centromere. Prophase is the longest phase of mitosis. Set of genetic material – 2n 4c.
II Metaphase (from the Greek “meta” - middle). Chromosomes, consisting of two chromatids, line up in the equatorial plane of the cell. The spindle filaments are attached to the centromeres. There are two types of filaments in the division spindle: 1) chromosomal, associated with the primary constrictions of chromosomes, 2) centrosomal, connecting the division poles. The set of genetic material at this moment is 2n 4c.
III Anaphase (from the Greek “ana” - up). The shortest division phase. The centromeres of the chromosomes are separated, and the chromatids (daughter chromosomes) become independent. The spindle filaments attached to the centromeres pull the daughter chromosomes to the poles of the cell. Set of genetic material – 2n 2c.
IV Telophase. Chromosomes, consisting of one chromatid, are located at the poles of the cell. Chromosomes despiral (unwind). At each pole, a nuclear membrane and nucleoli are formed around the chromosomes. The spindle threads disintegrate. The cytoplasm of the cell is divided (cytokinesis = cytotomy). Two daughter cells are formed. The set of genetic material of daughter cells is 2n 2c.
The separation of the cytoplasm by a constriction occurs differently in different cells. In animal cells, the invagination of the cytoplasmic membrane inward during cell division occurs from the edges to the center. In plant cells, a partition is formed in the center, which then increases towards the cell walls.
Biological significance of mitosis. Mitosis results in the precise distribution of genetic material between two daughter cells. Daughter cells receive the same set of chromosomes that the mother cell had - diploid. Mitosis ensures the maintenance of a constant number of chromosomes over a number of generations and serves as a cellular mechanism for growth, development of the body, regeneration and asexual reproduction. Mitosis is the basis for asexual reproduction of organisms. The number of daughter cells formed during mitosis is 2.
Amitosis(from the Greek “a” - negation, “mitos” - thread) - direct cell division, in which the nucleus is in an interphase state. Chromosomes are not detected. Division begins with changes in the nucleoli. Large nucleoli are divided by a constriction. Following this, the nucleus divides. The nucleus can be separated by only one constriction or fragmented. The resulting daughter nuclei may be of unequal size.
That. amitosis leads to the appearance of two cells with nuclei of different sizes and numbers. Often, after amitosis, two cells are not formed, i.e. After nuclear divisions, separation of the cytoplasm (cytokinesis) does not occur. 2 and multinucleate cells are formed. Amitosis occurs in aging, degenerating somatic cells.
Endomitosis- a process in which the doubling of chromosomes in a cell is not accompanied by nuclear division. As a result, the number of chromosomes in the cell multiplies, sometimes tens of times compared to the original number. Endomitosis occurs in intensively functioning cells.
Sometimes reproduction of chromosomes occurs without increasing their number in the cell. Each chromosome doubles many times, but the daughter chromosomes remain connected to each other (the phenomenon of polyteny). As a result, giant chromosomes are formed.
Meiosis - a special form of cell division in which haploid daughter cells are formed from diploid maternal germ cells. The fusion of male and female haploid gametes during fertilization leads to the appearance of a zygote with a diploid set of chromosomes. As a result, the daughter organism developing from the zygote has the same diploid karyotype that the mother organism had.
Meiosis involves two successive divisions.
The first meiotic division is called reduction. It includes 4 stages.
Prophase I. The longest stage. It is conventionally divided into 5 stages.
1) Leptotene. The core increases. The spiralization of chromosomes begins, each of which consists of two chromatids.
2) Zygotene. Conjugation of homologous chromosomes occurs. Homologous are chromosomes that have the same shape and size. Chromosomes attract and adhere to each other along their entire length.
3) Pachytene. The convergence of chromosomes ends. Double chromosomes are called bivalents. They consist of 4 chromatids. The number of bivalents = the haploid set of chromosomes of the cell. Chromosome spiralization continues. Close contact between chromatids makes it possible to exchange identical regions in homologous chromosomes. This phenomenon is called crossing over (crossing of chromosomes).
4) Diplotene. Chromosome repulsive forces arise. The chromosomes that make up the bivalents begin to move away from each other. At the same time, they remain connected to each other at several points - chiasmata. Crossing over may occur in these locations. Further spiralization and shortening of chromosomes occurs.
5) Diakinesis. The repulsion of chromosomes continues, but they remain connected at their ends into bivalents. The nucleolus and nuclear envelope dissolve, the filaments of the spindle diverge towards the poles. Set of genetic material – 2n 4c.
Metaphase I. Chromosome bivalents are located along the equator of the cell, forming a metaphase plate. The spindle threads are attached to them. Set of genetic material – 2n 4c.
Anaphase I. Chromosomes move towards the poles of the cell. Only one of a pair of homologous chromosomes reaches the poles. Set of genetic material – 1n 2c.
Telophase I. The number of chromosomes at each pole of the cell becomes haploid. Chromosomes consist of two chromatids. At each pole, a nuclear envelope is formed around a group of chromosomes, the chromosomes despiral, and the nucleus becomes interphase. Set of genetic material – 1n 2c.
After telophase I, cytokinesis begins in an animal cell, and cell wall formation begins in a plant cell.
Interphase II only found in animal cells. There is no DNA duplication.
Meiotic division II is called equatorial division. It is similar to mitosis. The difference from mitosis is that from chromosomes having two chromatids, chromosomes consisting of one chromatid are formed. Meiotic division II differs from mitosis in that during division two groups of chromosomes and, accordingly, two spindles are formed in the cell. The set of genetic material in prophase II is 1n 2c, starting from metaphase II - 1n 1c.
Biological significance of meiosis. Leads to a halving of the number of chromosomes, which determines the constancy of species on Earth. If the number of chromosomes did not decrease, then in each subsequent generation the chromosomes would double. Provides heterogeneity of gametes in gene composition (crossing over can occur in prophase, free recombination of chromosomes can occur in metaphase). A chance meeting of germ cells (=gametes) – a sperm and an egg with a different set of genes – causes combinative variability. Parents' genes are combined during fertilization, so their children may develop traits that the parents did not have. The number of cells formed is 4.
They are double-stranded, replicated chromosomes that are formed during division. The main function of the centromere is to serve as an attachment site for spindle fibers. The spindle elongates cells and separates chromosomes to ensure that each new one receives the correct number of chromosomes when completed or.
The DNA in the centromeric region of the chromosome is composed of tightly packed DNA, known as heterochromatin, which is highly compacted and therefore not transcribed. Due to the presence of heterochromatin, the centromere region is stained with dyes darker than other parts of the chromosome.
Location
The centromere is not always located in the central region of the chromosome (see photo above). A chromosome consists of a short arm (p) and a long arm (q), which join at the centromere region. Centromeres can be located either near the middle or in several positions along the chromosome. Metacentric centromeres are located near the center of the chromosomes. Submetacentric centromeres are shifted to one side from the center, so that one arm is longer than the other. Acrocentric centromeres are located near the end of the chromosome, and telocentric centromeres are located at the end or in the telomere region of the chromosome.
The position of the centromere is easily detected in the human karyotype. Chromosome 1 is an example of a metacentric centromere, chromosome 5 is an example of a submetacentric centromere, and chromosome 13 is an example of an acrocentric centromere.
Chromosome segregation in mitosis
Before mitosis begins, the cell enters a stage known as interphase, where it replicates its DNA in preparation for cell division. Sisters are formed, which are connected at their centromeres.
During prophase of mitosis, specialized areas on the centromeres called kinetochores attach chromosomes to spindle fibers. Kinetochores are composed of a series of protein complexes that generate kinetochore fibers that attach to the spindle. These fibers help manipulate and separate chromosomes during cell division.
At the metaphase stage, chromosomes are held on the metaphase plate by equal forces of polar fibers, pressing on the centromeres.
During anaphase, paired centromeres on each individual chromosome begin to diverge from each other as they first center themselves relative to the opposite poles of the cell.
During telophase, the newly formed ones include individual daughter chromosomes. After cytokinesis, two different ones are formed.
Chromosome segregation in meiosis
In meiosis, the cell goes through two stages of the division process (meiosis I and meiosis II). During metaphase I, the centromeres of homologous chromosomes are oriented to opposite poles of the cells. This means that homologous chromosomes will attach at their centromeric regions to spindle fibers extending from only one of the two poles of the cell.
When spindle fibers contract during anaphase I, homologous chromosomes are pulled toward opposite poles of the cells, but sister chromatids remain together. In meiosis II, spindle fibers extending from both cell poles attach to sister chromatids at their centromeres. Sister chromatids separate in anaphase II, when spindle fibers pull them toward opposite poles. Meiosis results in the separation and distribution of chromosomes among four new daughter cells. Each cell contains only half the number of chromosomes of the original cell.
A centromere is a region of a chromosome characterized by a specific nucleotide sequence and structure. The centromere plays an important role in the process of cell nuclear division and in the control of gene expression (the process during which the hereditary information from a gene is converted into a functional product - RNA or protein).
The centromere is involved in the connection of sister chromatids, the formation of the kinetochore (the protein structure on the chromosome to which spindle fibers are attached during cell division), the conjugation of homologous chromosomes, and is involved in the control of gene expression.
It is in the centromere region that sister chromatids are connected in prophase and metaphase of mitosis and homologous chromosomes in prophase and metaphase of the first division of meiosis. At centromeres, kinetochores are formed: proteins that bind to the centromere form an attachment point for spindle microtubules in anaphase and telophase of mitosis and meiosis.
Deviations from the normal functioning of the centromere lead to problems in the relative position of chromosomes in the dividing nucleus, and as a result, to disruptions in the process of chromosome segregation (their distribution between daughter cells). These disorders lead to aneuploidy, which can have serious consequences (for example, Down syndrome in humans associated with aneuploidy (trisomy) on chromosome 21). In most eukaryotes, the centromere does not have a specific nucleotide sequence corresponding to it. It typically consists of a large number of DNA repeats (eg, satellite DNA) in which the sequence within the individual repeat elements is similar but not identical.
Daughter chromosomes form centromeres in the same places as the mother chromosome, regardless of the nature of the sequence located in the centromeric region.
38. B– chromosomes
A chromosome present in the chromosome set in excess of the normal diploid number of chromosomes is present in the karyotype only in individual individuals in the population.; B chromosomes are known in many plants and (somewhat less frequently) in animals; their number can vary significantly (from 1 to several dozen); B-chromosomes often consist of heterochromatin (but may contain, apparently secondarily, euchromatin) and are genetically passive, although they may have side effects - for example, in insects, the presence of B-chromosomes often causes increased sperm aberrance; in cell divisions they can be stable, but more often they are unstable (sometimes they are mitotically stable, but they are unstable in meiosis, where they often form univalents); occasionally B chromosomes are isochromosomes; the mechanisms of the appearance of B chromosomes are different - fragmentation, heterochromatinization of extra chromosomes after incorrect anaphase segregation, etc. It is assumed that B chromosomes are gradually lost in somatic cells as a result of the irregularity of their inheritance
39 – 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. Polytene chromosomes can be observed in dipterans, in plants in cells associated with the development of the embryo, and 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 as early as the 1930s. The fundamental difference from other types of chromosomes is that polytene chromosomes are interphase, while all others can only be observed during mitotic or meiotic cell division.
A classic example is giant chromosomes in the cells of the salivary glands of Drosophila melanogaster larvae. 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 conjugate with each other, but also homologous chromosomes of each pair conjugate with each other. Thus, in the cells of the salivary glands a haploid number of chromosomes can be observed
40 – Lampbrush type chromosomes
Lampbrush chromosomes, first discovered by W. Flemming in 1882, are a special form of chromosomes that they acquire in the growing oocytes (female germ cells) of most animals, with the exception of mammals. This is a giant form of chromosome that arises in meiotic female cells at the diplotene stage of prophase I in some animals, particularly some amphibians and birds.
In the 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 brushes for cleaning kerosene lamp glasses (lamp brush type 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 into a series of chromomeres (containing condensed chromatin) and paired lateral loops emanating from them (containing transcriptionally active chromatin).
Lampbrush chromosomes of amphibians and birds can be isolated from the oocyte nucleus using microsurgical manipulation.
These chromosomes produce a huge amount of RNA, synthesized on the lateral loops. 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 functioning of the genetic apparatus and the regulation of gene expression during prophase meiosis I. In addition, chromosomes of this type are widely used for mapping DNA sequences with a high degree of resolution, studying the phenomenon of transcription of tandem DNA repeats that do not encode proteins, analyzing the distribution of chiasmata, etc.
Loading...