Chromosome structure. The structure and functions of chromosomes. reproduction in the organic world. The structure of germ cells. Unusual types of chromosomes
Chromosomes are the main structural elements cell nucleus, which are carriers of genes in which hereditary information is encoded. Having the ability to reproduce themselves, chromosomes provide genetic connection generations.
The morphology of chromosomes is related to the degree of their spiralization. For example, if at the stage of interphase (see Mitosis, Meiosis) the chromosomes are maximally deployed, i.e., despiralized, then with the onset of division, the chromosomes intensively spiralize and shorten. The maximum spiralization and shortening of the chromosome is reached at the metaphase stage, when relatively short, dense, intensely stained with basic dye structures are formed. This stage is most convenient for studying the morphological characteristics of chromosomes.
The metaphase chromosome consists of two longitudinal subunits - chromatids [reveals in the structure of chromosomes elementary filaments (the so-called chromonema, or chromofibrils) 200 Å thick, each of which consists of two subunits].
The sizes of chromosomes of plants and animals fluctuate considerably: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes lie in the range of 1.5-10 microns.
The chemical basis of the structure of chromosomes are nucleoproteins - complexes (see) with the main proteins - histones and protamines.
Rice. 1. The structure of a normal chromosome.
A - appearance; B - internal structure: 1-primary constriction; 2 - secondary constriction; 3 - satellite; 4 - centromere.
Individual chromosomes (Fig. 1) are distinguished by the localization of the primary constriction, i.e., the location of the centromere (during mitosis and meiosis, spindle threads are attached to this place, pulling it towards the pole). With the loss of the centromere, fragments of chromosomes lose their ability to disperse during division. The primary constriction divides the chromosomes into 2 arms. Depending on the location of the primary constriction, chromosomes are divided into metacentric (both arms of equal or almost equal length), submetacentric (arms of unequal length) and acrocentric (the centromere is shifted to the end of the chromosome). In addition to the primary, less pronounced secondary constrictions can occur in the chromosomes. A small terminal section of chromosomes, separated by a secondary constriction, is called a satellite.
Each type of organism is characterized by its specific (in terms of the number, size and shape of chromosomes) so-called chromosome set. The set of a double, or diploid, set of chromosomes is designated as a karyotype.
Rice. 2. Normal female chromosome set (two X chromosomes in the lower right corner).
Rice. 3. Normal chromosomal set of a man (in the lower right corner - sequentially X- and Y-chromosomes).
Mature eggs contain a single, or haploid, set of chromosomes (n), which is half of the diploid set (2n) inherent in the chromosomes of all other cells of the body. In a diploid set, each chromosome is represented by a pair of homologues, one of which is maternal and the other paternal. In most cases, the chromosomes of each pair are identical in size, shape, and genetic composition. The exception is the sex chromosomes, the presence of which determines the development of the organism in the male or female direction. The normal human chromosome set consists of 22 pairs of autosomes and one pair of sex chromosomes. In humans and other mammals, the female is determined by the presence of two X chromosomes, and the male is determined by the presence of one X and one Y chromosome (Fig. 2 and 3). In female cells, one of the X chromosomes is genetically inactive and is found in the interphase nucleus in the form (see). The study of human chromosomes in normal and pathological conditions is the subject of medical cytogenetics. It has been established that deviations in the number or structure of chromosomes from the norm that occur in the sex! cells or early stages crushing of a fertilized egg, cause disturbances in the normal development of the body, causing in some cases the occurrence of a part of spontaneous abortions, stillbirths, congenital deformities and developmental anomalies after birth (chromosomal diseases). Examples of chromosomal diseases are Down's disease (an extra G chromosome), Klinefelter's syndrome (an extra X chromosome in men) and (absence of a Y or one of the X chromosomes in the karyotype). In medical practice, chromosomal analysis is carried out either by a direct method (on bone marrow cells) or after a short-term cultivation of cells outside the body (peripheral blood, skin, embryonic tissues).
Chromosomes (from the Greek chroma - color and soma - body) are thread-like, self-reproducing structural elements of the cell nucleus, containing heredity factors in a linear order - genes. Chromosomes are clearly visible in the nucleus during the division of somatic cells (mitosis) and during the division (maturation) of germ cells - meiosis (Fig. 1). In both cases, the chromosomes are intensely stained with basic dyes, and are also visible on unstained cytological preparations in phase contrast. In the interphase nucleus, the chromosomes are despiralized and are not visible under a light microscope, since their transverse dimensions are beyond the resolving power of a light microscope. At this time, individual sections of chromosomes in the form of thin threads with a diameter of 100-500 Å can be distinguished using an electron microscope. Separate non-despiralized sections of chromosomes in the interphase nucleus are visible through a light microscope as intensely stained (heteropyknotic) sections (chromocenters).
Chromosomes continuously exist in the cell nucleus, undergoing a cycle of reversible spiralization: mitosis-interphase-mitosis. The main regularities of the structure and behavior of chromosomes in mitosis, meiosis and during fertilization are the same in all organisms.
Chromosomal theory of heredity. For the first time chromosomes were described by I. D. Chistyakov in 1874 and Strasburger (E. Strasburger) in 1879. In 1901, E. V. Wilson, and in 1902 W. S. Sutton paid attention to parallelism in the behavior of chromosomes and Mendelian factors of heredity - genes - in meiosis and during fertilization and came to the conclusion that genes are located in chromosomes. In 1915-1920. Morgan (T. N. Morgan) and his collaborators proved this position, localized several hundred genes in Drosophila chromosomes and created genetic maps of chromosomes. Data on chromosomes obtained in the first quarter of the 20th century formed the basis chromosome theory heredity, according to which the continuity of the characteristics of cells and organisms in a number of their generations is ensured by the continuity of their chromosomes.
Chemical composition and autoreproduction of chromosomes. As a result of cytochemical and biochemical studies of chromosomes in the 30s and 50s of the 20th century, it was established that they consist of permanent components [DNA (see Nucleic acids), basic proteins (histones or protamines), non-histone proteins] and variable components (RNA and associated acidic protein). Chromosomes are based on deoxyribonucleoprotein filaments with a diameter of about 200 Å (Fig. 2), which can be connected into bundles with a diameter of 500 Å.
The discovery by Watson and Crick (J. D. Watson, F. H. Crick) in 1953 of the structure of the DNA molecule, the mechanism of its auto-reproduction (reduplication) and the nucleic code of DNA and the development of molecular genetics that arose after that led to the idea of genes as sections of the DNA molecule. (see Genetics). The regularities of autoreproduction of chromosomes [Taylor (J. N. Taylor) et al., 1957], which turned out to be similar to the regularities of autoreproduction of DNA molecules (semiconservative reduplication), were revealed.
Chromosomal set is the totality of all chromosomes in a cell. Each biological species has a characteristic and constant set of chromosomes, fixed in the evolution of this species. There are two main types of chromosome sets: single, or haploid (in animal germ cells), denoted n, and double, or diploid (in somatic cells, containing pairs of similar, homologous chromosomes from mother and father), denoted 2n.
The sets of chromosomes of individual biological species differ significantly in the number of chromosomes: from 2 (horse roundworm) to hundreds and thousands (some spore plants and protozoa). The diploid numbers of chromosomes of some organisms are as follows: humans - 46, gorillas - 48, cats - 60, rats - 42, Drosophila - 8.
The size of the chromosomes different types are also different. The length of chromosomes (in the metaphase of mitosis) varies from 0.2 microns in some species to 50 microns in others, and the diameter is from 0.2 to 3 microns.
Chromosome morphology is well expressed in the metaphase of mitosis. Metaphase chromosomes are used to identify chromosomes. In such chromosomes, both chromatids are clearly visible, into which each chromosome is split longitudinally and the centromere (kinetochore, primary constriction) connecting the chromatids (Fig. 3). The centromere is visible as the narrowed site which is not containing chromatin (see); threads of the achromatin spindle are attached to it, due to which the centromere determines the movement of chromosomes to the poles in mitosis and meiosis (Fig. 4).
The loss of the centromere, for example, when a chromosome is broken by ionizing radiation or other mutagens, leads to the loss of the ability of a piece of the chromosome devoid of a centromere (acentric fragment) to participate in mitosis and meiosis and to its loss from the nucleus. This can lead to severe cell damage.
The centromere divides the body of the chromosome into two arms. The location of the centromere is strictly constant for each chromosome and determines three types of chromosomes: 1) acrocentric, or rod-shaped, chromosomes with one long and the second very short arm resembling a head; 2) submetacentric chromosomes with long arms of unequal length; 3) metacentric chromosomes with arms of the same or almost the same length (Fig. 3, 4, 5 and 7).
Rice. Fig. 4. Scheme of the structure of chromosomes in the metaphase of mitosis after longitudinal splitting of the centromere: A and A1 - sister chromatids; 1 - long shoulder; 2 - short shoulder; 3 - secondary constriction; 4-centromere; 5 - spindle fibers.
Characteristic features of the morphology of certain chromosomes are secondary constrictions (which do not have the function of a centromere), as well as satellites - small sections of chromosomes connected to the rest of its body by a thin thread (Fig. 5). Satellite filaments have the ability to form nucleoli. A characteristic structure in the chromosome (chromomeres) is thickening or more densely spiralized sections of the chromosome thread (chromonema). The chromomere pattern is specific for each pair of chromosomes.
Rice. 5. Scheme of chromosome morphology in the anaphase of mitosis (chromatid moving towards the pole). A - the appearance of the chromosome; B - the internal structure of the same chromosome with two chromonemes (semichromatids) that make it up: 1 - primary constriction with chromomeres that make up the centromere; 2 - secondary constriction; 3 - satellite; 4 - satellite thread.
The number of chromosomes, their size and shape at the metaphase stage are characteristic of each type of organism. The totality of these features of a set of chromosomes is called a karyotype. A karyotype can be represented as a diagram called an idiogram (see human chromosomes below).
sex chromosomes. Sex-determining genes are localized in a special pair of chromosomes - the sex chromosomes (mammals, humans); in other cases, iol is determined by the ratio of the number of sex chromosomes and all the rest, called autosomes (drosophila). In humans, as in other mammals, the female sex is determined by two identical chromosomes, designated as X chromosomes, the male sex is determined by a pair of heteromorphic chromosomes: X and Y. As a result of reduction division (meiosis) during the maturation of oocytes (see Ovogenesis) in women All eggs contain one X chromosome. In men, as a result of the reduction division (maturation) of spermatocytes, half of the sperm contains the X chromosome, and the other half the Y chromosome. The sex of a child is determined by the random fertilization of an egg by a sperm that carries an X or Y chromosome. The result is a female (XX) or male (XY) fetus. In the interphase nucleus in females, one of the X chromosomes is visible as a lump of compact sex chromatin.
Chromosome Function and Nuclear Metabolism. Chromosomal DNA is a template for the synthesis of specific messenger RNA molecules. This synthesis occurs when a given region of the chromosome is despiralized. Examples of local activation of chromosomes are: the formation of despiralized loops of chromosomes in the oocytes of birds, amphibians, fish (the so-called X-lamp brushes) and swellings (puffs) of certain chromosome loci in multifilamentous (polytene) chromosomes of the salivary glands and other secretory organs of Diptera insects (Fig. 6). An example of the inactivation of an entire chromosome, i.e., its exclusion from the metabolism of a given cell, is the formation of one of the X chromosomes of a compact body of sex chromatin.
Rice. Fig. 6. Polytene chromosomes of the dipteran insect Acriscotopus lucidus: A and B - the area bounded by dotted lines, in a state of intensive functioning (puff); B - the same site in a non-functioning state. Numbers indicate individual loci of chromosomes (chromomeres).
Rice. 7. Chromosomal set in the culture of male peripheral blood leukocytes (2n=46).
The discovery of the mechanisms of functioning of polytene chromosomes such as lampbrushes and other types of spiralization and despiralization of chromosomes is of decisive importance for understanding the reversible differential activation of genes.
human chromosomes. In 1922, T. S. Painter established the diploid number of human chromosomes (in spermatogonia) equal to 48. In 1956, Tio and Levan (N. J. Tjio, A. Levan) used a set of new methods for studying human chromosomes : cell culture; the study of chromosomes without histological sections on total cell preparations; colchicine, which leads to the arrest of mitosis at the metaphase stage and the accumulation of such metaphases; phytohemagglutinin, which stimulates the entry of cells into mitosis; treatment of metaphase cells with hypotonic saline solution. All this made it possible to clarify the diploid number of chromosomes in humans (it turned out to be 46) and to give a description of the human karyotype. In 1960, in Denver (USA), an international commission developed a nomenclature of human chromosomes. According to the proposals of the commission, the term "karyotype" should be applied to a systematized set of chromosomes of a single cell (Fig. 7 and 8). The term "idiotram" is retained to represent a set of chromosomes in the form of a diagram built on the basis of measurements and a description of the morphology of the chromosomes of several cells.
Human chromosomes are numbered (somewhat serially) from 1 to 22 in accordance with morphological features that allow their identification. Sex chromosomes do not have numbers and are designated as X and Y (Fig. 8).
A connection has been found between a number of diseases and birth defects in human development and changes in the number and structure of its chromosomes. (see. Heredity).
See also Cytogenetic studies.
All these achievements have created a solid basis for the development of human cytogenetics.
Rice. 1. Chromosomes: A - at the stage of anaphase of mitosis in shamrock microsporocytes; B - at the metaphase stage of the first division of meiosis in pollen mother cells in Tradescantia. In both cases, the helical structure of the chromosomes is visible.
Rice. Fig. 2. Elementary chromosome filaments with a diameter of 100 Å (DNA + histone) from the interphase nuclei of the calf thymus gland (electron microscopy): A - filaments isolated from the nuclei; B - thin section through the film of the same preparation.
Rice. 3. Chromosomal set of Vicia faba (horse beans) in the metaphase stage.
Rice. 8. Chromosomes of the same as in fig. 7, sets classified according to Denver nomenclature into pairs of homologues (karyotype).
As part of the capsid.
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✪ Chromosomes, chromatids, chromatin, etc.
✪ Genes, DNA and chromosomes
✪ The most important terms of genetics. loci and genes. homologous chromosomes. Coupling and crossing over.
✪ Chromosomal diseases. Examples and reasons. Biology video lesson Grade 10
✪ Cellular technologies. DNA. Chromosome. Genome. Program "In the first approximation"
Subtitles
Before diving into the machine cell division , I think it would be useful to talk about vocabulary related to DNA. There are many words, and some of them sound similar to each other. They can be confusing. First, I would like to talk about how DNA generates more DNA, makes copies of itself, or how it makes proteins in general. We already talked about this in the video about DNA. Let me draw a small piece of DNA. I have A, G, T, let me have two Ts and then two Cs. Such a small area. It continues like this. Of course, this is a double helix. Each letter corresponds to its own. I will paint them with this color. So, A corresponds to T, G corresponds to C, (more precisely, G forms hydrogen bonds with C), T - with A, T - with A, C - with G, C - with G. This whole spiral stretches, let's say, in this direction . So there are a couple of different processes that this DNA has to carry out. One of them has to do with your body cells - you need to produce more of your skin cells. Your DNA has to copy itself. This process is called replication. You are replicating DNA. I'll show you replication. How can this DNA copy itself? This is one of the most remarkable features of the structure of DNA. Replication. I'm making a general simplification, but the idea is that two strands of DNA are separating, and it doesn't happen on its own. This is facilitated by the mass of proteins and enzymes, but in detail I will talk about microbiology in another video. So these chains are separated from each other. I'll move the chain here. They separate from each other. I'll take another chain. This one is too big. This circuit will look something like this. They separate from each other. What can happen after that? I'll remove extra pieces here and here. So here is our double helix. They were all connected. These are base pairs. Now they are separated from each other. What can each of them do after separation? They can now become a matrix for each other. Look... If this chain is on its own, now, all of a sudden, a thymine base can come along and join here, and these nucleotides will begin to line up. Thymine and cytosine, and then adenine, adenine, guanine, guanine. And so it goes. And then, in this other part, on the green chain that was previously attached to this blue one, the same thing will happen. There will be adenine, guanine, thymine, thymine, cytosine, cytosine. What just happened? By separating and bringing in complementary bases, we have created a copy of this molecule. We'll do the microbiology of this in the future, it's just for general idea about how DNA copies itself. Especially when we look at mitosis and meiosis, I can say, "This is the stage where replication occurs." Now, another process that you'll hear a lot more about. I talked about him in the DNA video. This is a transcription. In the DNA video, I didn't pay much attention to how DNA doubles itself, but one of the great things about the double strand design is that it's easy to duplicate itself. You just separate 2 strips, 2 spirals, and then they become a matrix for another chain, and then a copy appears. Now transcription. This is what must happen to DNA in order to form proteins, but transcription is an intermediate step. This is the stage where you move from DNA to mRNA. Then this mRNA leaves the cell nucleus and goes to the ribosomes. I will talk about this in a few seconds. So we can do the same. These chains are again separated during transcription. One is separating out here, and the other is separating... and the other will be separating out here. Perfectly. It may make sense to use only one half of the chain - I will remove one. That's the way. We're going to transcribe the green part. Here she is. I will delete all this. Wrong color. So, I'm deleting all of this. What happens if instead of deoxyribonucleic acid nucleotides that pair with this DNA strand, you have ribonucleic acid, or RNA, that pairs. I will depict RNA in magenta. RNA will pair with DNA. Thymine, found in DNA, will pair with adenine. Guanine, now when we talk about RNA, instead of thymine, we will have uracil, uracil, cytosine, cytosine. And it will continue. This is mRNA. Messenger RNA. Now she is separating. This mRNA separates and leaves the nucleus. It leaves the nucleus, and then translation takes place. Broadcast. Let's write this term. Broadcast. It comes from mRNA... In the DNA video, I had a small tRNA. The transfer RNA was like a truck transporting amino acids to the mRNA. All this happens in a part of the cell called the ribosome. Translation occurs from mRNA to protein. We've seen it happen. So, from mRNA to protein. You have this chain - I'll make a copy. I will copy the whole chain at once. This chain separates, leaves the core, and then you have these little trucks of tRNA, which, in fact, drive up, so to speak. So let's say I have tRNA. Let's see adenine, adenine, guanine and guanine. This is RNA. This is a codon. A codon has 3 base pairs and an amino acid attached to it. You have some other parts of tRNA. Let's say uracil, cytosine, adenine. And another amino acid attached to it. Then the amino acids combine and form a long chain of amino acids, which is a protein. Proteins form these strange complex shapes. To make sure you understand. We'll start with DNA. If we make copies of DNA, that's replication. You are replicating DNA. So if we make copies of DNA, that's replication. If you start with DNA and create mRNA from a DNA template, that's transcription. Let's write down. "Transcription". That is, you transcribe information from one form to another - transcription. Now, when the mRNA leaves the nucleus of the cell... I'll draw a cell to draw attention to it. We will deal with cell structure in the future. If it's a whole cell, the nucleus is the center. This is where all DNA is, all replication and transcription takes place here. The mRNA then leaves the nucleus, and then in the ribosomes, which we will discuss in more detail in the future, translation occurs and protein is formed. So from mRNA to protein is translation. You are translating from the genetic code into the so-called protein code. So this is the broadcast. These are exactly the words that are commonly used to describe these processes. Make sure you use them correctly by naming the various processes. Now another part of DNA terminology. When I first met her, I thought she was extremely confusing. The word is "chromosome". I'll write down the words here - you can appreciate how confusing they are: chromosome, chromatin and chromatid. Chromatid. So, the chromosome, we've already talked about it. You may have a DNA strand. This is a double helix. This chain, if I enlarge it, is actually two different chains. They have connected base pairs. I just drew base pairs connected together. I want to be clear: I drew this little green line here. This is a double helix. It wraps around proteins called histones. Histones. Let her turn around like this and something like this, and then something like this. Here you have substances called histones, which are proteins. Let's draw them like this. Like this. It is a structure, that is, DNA in combination with proteins that structure it, causing it to wrap around further and further. Ultimately, depending on the stage cell life , different structures will be formed. And when you talk about nucleic acid, which is DNA, and combine it with proteins, you are talking about chromatin. So chromatin is DNA plus the structural proteins that give DNA its shape. structural proteins. The idea of chromatin was first used because of what people saw when they looked at a cell... Remember? Each time I drew the cell nucleus in a certain way. So to speak. This is the nucleus of the cell. I drew very distinct structures. This is one, this is another. Maybe she's shorter, and she has a homologous chromosome. I drew the chromosomes, right? And each of these chromosomes, as I showed in the last video, are essentially long structures of DNA, long strands of DNA wrapped tightly around each other. I drew it like this. If we zoom in, we'll see one chain, and it's really wrapped around itself like this. This is her homologous chromosome. Remember, in the video on variability, I talked about a homologous chromosome that codes for the same genes, but a different version of them. Blue is from dad and red is from mom, but they essentially code for the same genes. So this is one strand that I got from my dad with the DNA of this structure, we call it a chromosome. So chromosome. I want to make it clear, DNA only takes this form at certain life stages when it reproduces itself, ie. is replicated. More precisely, not so ... When the cell divides. Before a cell becomes capable of dividing, the DNA assumes this well-defined shape. For most of a cell's life, when the DNA is doing its job, when it's making proteins, meaning the proteins are being transcribed and translated from the DNA, it doesn't fold in that way. If it were folded, it would be difficult for the replication and transcription system to get to the DNA, make proteins, and do anything else. Usually DNA... Let me draw the nucleus again. Most of the time, you can't even see it with a regular light microscope. It is so thin that the entire helix of DNA is completely distributed in the nucleus. I draw it here, another one might be here. And then you have a shorter chain like this one. You can't even see her. It is not in this well-defined structure. It usually looks like this. Let there be such a short chain. You can only see a similar mess, consisting of a jumble of combinations of DNA and proteins. This is what people generally call chromatin. This needs to be written down. "Chromatin" So the words can be very ambiguous and very confusing, but the common usage when you talk about a well-defined single strand of DNA, well-defined structure like this, is chromosome. The concept of "chromatin" can refer either to a structure such as a chromosome, a combination of DNA and proteins that structure it, or to a disorder of many chromosomes that contain DNA. That is, from many chromosomes and proteins mixed together. I want this to be clear. Now the next word. What is a chromatid? Just in case I haven't done it already... I don't remember if I flagged it. These proteins that provide structure to chromatin or make up chromatin and also provide structure are called "histones". There are different types that provide structure at different levels, we'll look at them in more detail later. So what is a chromatid? When the DNA replicates... Let's say it was my DNA, it's in a normal state. One version is from dad, one version is from mom. Now it is replicated. The version from dad first looks like this. It's a big strand of DNA. It creates another version of itself, identical if the system is working properly, and that identical part looks like this. They are initially attached to each other. They are attached to each other at a place called the centromere. Now, despite the fact that I have 2 chains here, fastened together. Two identical chains. One chain here, one here ... Although let me put it differently. In principle, this can be represented in many different ways. This is one chain here, and here is another chain here. So we have 2 copies. They code for exactly the same DNA. So. They are identical, which is why I still call it a chromosome. Let's write it down too. All this together is called a chromosome, but now each individual copy is called a chromatid. So this is one chromatid and this is the other. They are sometimes called sister chromatids. They can also be called twin chromatids because they share the same genetic information. So this chromosome has 2 chromatids. Now, before replication, or before DNA duplication, you can say that this chromosome right here has one chromatid. You can call it a chromatid, but it doesn't have to be. People start talking about chromatids when two of them are present on a chromosome. We learn that in mitosis and meiosis these 2 chromatids separate. When they separate, there is a strand of DNA that you once called a chromatid, now you will call a single chromosome. So this is one of them, and here's another one that could have branched off in that direction. I'll circle this one in green. So this one can go to this side, and this one that I circled in orange, for example, to this ... Now that they are separated and no longer connected by a centromere, what we originally called one chromosome with two chromatids, now you call two separate chromosomes. Or you could say that you now have two separate chromosomes, each consisting of one chromatid. I hope this clears things up a bit meaning of terms associated with DNA. I have always found them rather confusing, but they will be a useful tool when we start mitosis and meiosis and I will talk about how a chromosome becomes a chromatid. You will ask how one chromosome became two chromosomes, and how a chromatid became a chromosome. It all revolves around vocabulary. I would choose another instead of calling it a chromosome and each of these individual chromosomes, but that's what they decided to call for us. You might be wondering where the word "chromo" comes from. Maybe you know an old Kodak film called "chrome color". Basically "chromo" means "color". I think it comes from the Greek word for color. When people first looked at the nucleus of a cell, they used a dye, and what we call chromosomes was stained with the dye. And we could see it with a light microscope. The part "soma" comes from the word "soma" meaning "body", that is, we get a colored body. Thus the word "chromosome" was born. Chromatin also stains... I hope this clarifies a little the concepts of "chromatid", "chromosome", "chromatin", and now we are prepared for the study of mitosis and meiosis.
The history of the discovery of chromosomes
The first descriptions of chromosomes appeared in articles and books by various authors in the 70s. years XIX century, and priority is given to the discovery of chromosomes different people. Among them are such names as I. D. Chistyakov (1873), A. Schneider (1873), E. Strasburger (1875), O. Büchli (1876) and others. Most often, the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming, who in his fundamental book "Zellsubstanz, Kern und Zelltheilung" collected and streamlined information about them, supplementing the results of his own research. The term "chromosome" was proposed by the German histologist G. Waldeyer in 1888. "Chromosome" in literal translation means "colored body", because the basic dyes are well linked by chromosomes.
After the rediscovery of Mendel's laws in 1900, it took only one or two years for it to become clear that chromosomes during meiosis and fertilization behave exactly as expected from "heredity particles". In 1902 T. Boveri and in 1902-1903 W. Setton ( Walter Sutton) independently put forward a hypothesis about the genetic role of chromosomes.
In 1933, for the discovery of the role of chromosomes in heredity, T. Morgan received Nobel Prize in Physiology and Medicine.
Morphology of metaphase chromosomes
In the metaphase stage of mitosis, chromosomes consist of two longitudinal copies called sister chromatids, which are formed during replication. In metaphase chromosomes, sister chromatids are connected in the region primary constriction called the centromere. The centromere is responsible for separating sister chromatids into daughter cells during division. At the centromere, the kinetochore is assembled - a complex protein structure that determines the attachment of the chromosome to the microtubules of the spindle division - the movers of the chromosome in mitosis. The centromere divides chromosomes into two parts called shoulders. In most species, the short arm of the chromosome is denoted by the letter p, long shoulder - letter q. Chromosome length and centromere position are the main morphological features of metaphase chromosomes.
Three types of chromosome structure are distinguished depending on the location of the centromere:
This classification of chromosomes based on the ratio of arm lengths was proposed in 1912 by the Russian botanist and cytologist S. G. Navashin. In addition to the above three types, S. G. Navashin also singled out telocentric chromosomes, that is, chromosomes with only one arm. However, according to modern ideas There are no truly telocentric chromosomes. The second arm, even if very short and invisible in a conventional microscope, is always present.
An additional morphological feature of some chromosomes is the so-called secondary constriction, which outwardly differs from the primary one by the absence of a noticeable angle between the segments of the chromosome. Secondary constrictions are of various lengths and can be located at various points along the length of the chromosome. In the secondary constrictions, as a rule, there are nucleolar organizers containing multiple repeats of genes encoding ribosomal RNA. In humans, secondary constrictions containing ribosomal genes are located in the short arms of acrocentric chromosomes; they separate small chromosome segments from the main body of the chromosome, called satellites. Chromosomes that have a satellite are called SAT chromosomes (lat. SAT (Sine Acid Thymonucleinico)- without DNA).
Differential staining of metaphase chromosomes
With monochrome staining of chromosomes (aceto-carmine, aceto-orcein, Fölgen or Romanovsky-Giemsa staining), the number and size of chromosomes can be identified; their shape, determined primarily by the position of the centromere, the presence of secondary constrictions, satellites. In the vast majority of cases, these signs are not enough to identify individual chromosomes in the chromosome set. In addition, monochrome-stained chromosomes are often very similar across species. Differential staining of chromosomes, various methods of which were developed in the early 1970s, provided cytogenetics with a powerful tool for identifying both individual chromosomes as a whole and their parts, thereby facilitating the analysis of the genome.
Differential staining methods fall into two main groups:
Levels of compaction of chromosomal DNA
The basis of the chromosome is a linear DNA macromolecule of considerable length. In the DNA molecules of human chromosomes, there are from 50 to 245 million pairs of nitrogenous bases. The total length of DNA from one human cell is about two meters. At the same time, a typical human cell nucleus, which can only be seen with a microscope, occupies a volume of about 110 microns, and the average human mitotic chromosome does not exceed 5-6 microns. Such compaction of the genetic material is possible due to the presence in eukaryotes of a highly organized system of packing DNA molecules both in the interphase nucleus and in the mitotic chromosome. It should be noted that in proliferating cells in eukaryotes there is a constant regular change in the degree of compaction of chromosomes. Before mitosis, chromosomal DNA is compacted 105 times compared to the linear length of DNA, which is necessary for successful segregation of chromosomes into daughter cells, while in the interphase nucleus, for successful transcription and replication processes, the chromosome must be decompacted. At the same time, DNA in the nucleus is never completely elongated and is always packed to some extent. Thus, the estimated size reduction between a chromosome in interphase and a chromosome in mitosis is only about 2 times in yeast and 4-50 times in humans.
One of the latest levels of packaging in the mitotic chromosome, some researchers consider the level of the so-called chromonemes, the thickness of which is about 0.1-0.3 microns. As a result of further compaction, the chromatid diameter reaches 700 nm by the time of metaphase. The significant thickness of the chromosome (diameter 1400 nm) at the metaphase stage allows, finally, to see it in a light microscope. The condensed chromosome looks like the letter X (often with unequal arms), since the two chromatids resulting from replication are interconnected at the centromere (for more on the fate of chromosomes during cell division, see the articles mitosis and meiosis).
Chromosomal abnormalities
Aneuploidy
With aneuploidy, a change in the number of chromosomes in the karyotype occurs, in which the total number of chromosomes is not a multiple of the haploid chromosome set n. In the case of the loss of one chromosome from a pair of homologous chromosomes, mutants are called monosomics, in the case of one extra chromosome, mutants with three homologous chromosomes are called trisomics, in case of loss of one pair of homologues - nullisomics. Autosomal aneuploidy always causes significant developmental disorders, being the main cause of spontaneous abortions in humans. One of the most famous aneuploidies in humans is trisomy 21, which leads to the development of Down syndrome. Aneuploidy is characteristic of tumor cells, especially of solid tumor cells.
polyploidy
Change in the number of chromosomes, a multiple of the haploid set of chromosomes ( n) is called polyploidy. Polyploidy is widely and unevenly distributed in nature. Polyploid eukaryotic microorganisms are known - fungi and algae, polyploids are often found among flowering plants, but not among gymnosperms. Whole-body polyploidy is rare in metazoans, although they often have endopolyploidy some differentiated tissues, for example, the liver in mammals, as well as intestinal tissues, salivary glands, Malpighian vessels of a number of insects.
Chromosomal rearrangements
Chromosomal rearrangements (chromosomal aberrations) are mutations that disrupt the structure of chromosomes. They can arise in somatic and germ cells spontaneously or as a result of external influences (ionizing radiation, chemical mutagens, viral infection and etc.). As a result of chromosomal rearrangement, a fragment of a chromosome can be lost or, conversely, doubled (deletion and duplication, respectively); a segment of a chromosome can be transferred to another chromosome (translocation) or it can change its orientation within the chromosome by 180° (inversion). There are other chromosomal rearrangements.
Unusual types of chromosomes
microchromosomes
B chromosomes
B chromosomes are extra chromosomes that are found in the karyotype only in certain individuals in a population. They are often found in plants and have been described in fungi, insects, and animals. Some B chromosomes contain genes, often rRNA genes, but it is not clear how functional these genes are. The presence of B chromosomes can affect the biological characteristics of organisms, especially in plants, where their presence is associated with reduced viability. It is assumed that B chromosomes are gradually lost in somatic cells as a result of their irregular inheritance.
Holocentric chromosomes
Holocentric chromosomes do not have a primary constriction, they have a so-called diffuse kinetochore, therefore, during mitosis, spindle microtubules are attached along the entire length of the chromosome. During chromatid divergence to the poles of division in holocentric chromosomes, they go to the poles parallel to each other, while in a monocentric chromosome, the kinetochore is ahead of the rest of the chromosome, which leads to a characteristic V-shaped diverging chromatids at the anaphase stage. During fragmentation of chromosomes, for example, as a result of exposure to ionizing radiation, fragments of holocentric chromosomes diverge towards the poles in an orderly manner, and fragments of monocentric chromosomes that do not contain centromeres are randomly distributed between daughter cells and may be lost.
Holocentric chromosomes are found in protists, plants, and animals. Nematodes have holocentric chromosomes C. elegans .
Giant forms of chromosomes
Polytene chromosomes
Polytene chromosomes are giant agglomerations of chromatids that occur in certain types of specialized cells. First described by E. Balbiani ( Edouard-Gerard Balbiani) in 1881 in the cells of the salivary glands of the bloodworm ( Chironomus), their study was continued already in the 30s of the XX century by Kostov, T. Paynter, E. Heitz and G. Bauer ( Hans Bauer). Polytene chromosomes have also been found in the cells of the salivary glands, intestines, trachea, fat body, and Malpighian vessels of Diptera larvae.
Lampbrush chromosomes
The lampbrush chromosome is a giant form of chromosome that occurs in meiotic female cells during the diplotene stage of prophase I in some animals, notably some amphibians and birds. These chromosomes are extremely transcriptionally active and are observed in growing oocytes when the processes of RNA synthesis leading to the formation of the yolk are most intense. At present, 45 animal species are known in whose developing oocytes such chromosomes can be observed. Lampbrush chromosomes are not produced in mammalian oocytes.
Lampbrush-type chromosomes were first described by W. Flemming in 1882. The name "lampbrush chromosomes" was proposed by the German embryologist I. Rückert ( J. Rϋckert) in 1892.
Lampbrush-type chromosomes are longer than polytene chromosomes. For example, the total length of the chromosome set in the oocytes of some caudate amphibians reaches 5900 µm.
Bacterial chromosomes
There is evidence of the presence of proteins associated with nucleoid DNA in bacteria, but no histones have been found in them.
human chromosomes
The normal human karyotype is represented by 46 chromosomes. These are 22 pairs of autosomes and one pair of sex chromosomes (XY in the male karyotype and XX in the female). The table below shows the number of genes and bases in human chromosomes.
| Chromosome | Total bases | Number of genes | Number of protein-coding genes |
|---|---|---|---|
| 249250621 | 3511 | 2076 | |
| 243199373 | 2368 | 1329 | |
| 198022430 | 1926 | 1077 | |
| 191154276 | 1444 | 767 | |
| 180915260 | 1633 | 896 | |
| 171115067 | 2057 | 1051 | |
| 159138663 | 1882 | 979 | |
| 146364022 | 1315 | 702 | |
| 141213431 | 1534 | 823 | |
| 135534747 | 1391 | 774 | |
| 135006516 | 2168 | 1914 | |
| 133851895 | 1714 | 1068 | |
| 115169878 | 720 | 331 | |
| 107349540 | 1532 | 862 | |
| 102531392 | 1249 | 615 | |
| 90354753 | 1326 | 883 | |
| 81195210 | 1773 | 1209 | |
| 78077248 | 557 | 289 | |
| 59128983 | 2066 | 1492 | |
| 63025520 | 891 | 561 | |
| 48129895 | 450 | 246 | |
| 51304566 | 855 | 507 | |
| X chromosome | 155270560 | 1672 | 837 |
| Y chromosome | 59373566 | 429 | 76 |
| Total | 3 079 843 747 | 36463 |
see also
Notes
- Tarantula V.Z. Explanatory biotechnological dictionary. - M.: Languages of Slavic cultures, 2009. - 936 p. - 400 copies. - ISBN 978-5-9551-0342-6.
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. fourteen. 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 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.
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 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 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. fifteen. 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.
Lecture #3
Theme: Flow organization genetic information
Lecture plan
1. Structure and functions of the cell nucleus.
2. Chromosomes: structure and classification.
3. Cellular and mitotic cycles.
4. Mitosis, meiosis: cytological and cytogenetic characteristics, significance.
Structure and functions of the cell nucleus
The main genetic information is contained in the nucleus of cells.
cell nucleus(lat. - nucleus; Greek - karyon) was described in 1831. Robert Brown. The shape of the nucleus depends on the shape and function of the cell. The sizes of the nuclei change depending on the metabolic activity of the cells.
Shell of the interphase nucleus (karyolemma) consists of outer and inner elementary membranes. Between them is perinuclear space. Membrane has holes pores. Between the edges of the nuclear pore are protein molecules that form pore complexes. The pore opening is covered with a thin film. With active metabolic processes in the cell, most of the pores are open. Through them there is a flow of substances - from the cytoplasm to the nucleus and vice versa. The number of pores in one nucleus
Rice. Scheme of the structure of the cell nucleus
1 and 2 - outer and inner membranes of the nuclear membrane, 3
- nuclear pore, 4 - nucleolus, 5 - chromatin, 6 - nuclear juice
reaches 3-4 thousand. The outer nuclear membrane connects to channels in the endoplasmic reticulum. It usually contains ribosomes. Squirrels inner surface nuclear envelope form nuclear plate. It maintains a constant shape of the nucleus, chromosomes are attached to it.
Nuclear juice - karyolymph, a colloidal solution in a gel state that contains proteins, lipids, carbohydrates, RNA, nucleotides, enzymes. nucleolus is a non-permanent component of the nucleus. It disappears at the beginning cell division and is restored at the end of it. The chemical composition of the nucleoli: protein (~90%), RNA (~6%), lipids, enzymes. Nucleoli are formed in the region of secondary constrictions of satellite chromosomes. Function of the nucleolus: assembly of ribosome subunits.
X romatin nuclei are interphase chromosomes. They contain DNA, histone proteins and RNA in a ratio of 1:1.3:0.2. DNA combines with protein to form deoxyribonucleoprotein(DNP). During mitotic division of the nucleus, DNP spiralizes and forms chromosomes.
Functions of the cell nucleus:
1) stores the hereditary information of the cell;
2) participates in cell division (reproduction);
3) regulates metabolic processes in the cell.
Chromosomes: structure and classification
Chromosomes(Greek - chromo- Colour, soma body) is a spiralized chromatin. Their length is 0.2 - 5.0 microns, diameter is 0.2 - 2 microns.
Rice. Chromosome types
Metaphase chromosome consists of two chromatids, which are connected centromere (primary constriction). She divides the chromosome into two shoulder. Individual chromosomes have secondary constrictions. The area they separate is called satellite, and such chromosomes are satellite. The ends of chromosomes are called telomeres. Each chromatid contains one continuous DNA molecule in combination with histone proteins. Intensely stained sections of chromosomes are areas of strong spiralization ( heterochromatin). Lighter areas are areas of weak spiralization ( euchromatin).
Chromosome types are distinguished by the location of the centromere (Fig.).
1. metacentric chromosomes- the centromere is located in the middle, and the arms are of the same length. The part of the shoulder near the centromere is called proximal, the opposite is called distal.
2. Submetacentric chromosomes- the centromere is displaced from the center and the arms have different lengths.
3. Acrocentric chromosomes- the centromere is strongly displaced from the center and one arm is very short, the second arm is very long.
In the cells of the salivary glands of insects (Drosophila flies) there are giant, polytene chromosomes(multistranded chromosomes).
For the chromosomes of all organisms, there are 4 rules:
1. The rule of constancy of the number of chromosomes. Normally, organisms of certain species have a constant number of chromosomes characteristic of the species. For example: a human has 46, a dog has 78, a fruit fly has 8.
2. pairing of chromosomes. In a diploid set, each chromosome normally has a paired chromosome - the same in shape and size.
3. Individuality of chromosomes. The chromosomes of different pairs differ in shape, structure and size.
4. Chromosome continuity. When the genetic material is duplicated, a chromosome is formed from a chromosome.
The set of chromosomes of a somatic cell, characteristic of an organism of a given species, is called karyotype.
Classification of chromosomes is carried out according to different criteria.
1. Chromosomes that are the same in the cells of male and female organisms are called autosomes. The human karyotype has 22 pairs of autosomes. Chromosomes that are different in male and female cells are called heterochromosomes, or sex chromosomes. In men, these are X and Y chromosomes; in women, X and X.
2. The arrangement of chromosomes in descending order is called idiogram. This is a systematic karyotype. Chromosomes are arranged in pairs (homologous chromosomes). The first pair are the largest, the 22nd pair are the smallest, and the 23rd pair are the sex chromosomes.
3. In 1960 The Denver classification of chromosomes was proposed. It is built on the basis of their shape, size, centromere position, presence of secondary constrictions and satellites. An important indicator in this classification is centromere index(CI). This is the ratio of the length of the short arm of the chromosome to its entire length, expressed as a percentage. All chromosomes are divided into 7 groups. Groups are designated by Latin letters from A to G.
Group A includes 1 - 3 pairs of chromosomes. These are large metacentric and submetacentric chromosomes. Their CI is 38-49%.
Group B. 4th and 5th pairs are large metacentric chromosomes. CI 24-30%.
Group C. Pairs of chromosomes 6 - 12: medium size, submetacentric. CI 27-35%. This group also includes the X chromosome.
Group D. 13 - 15th pairs of chromosomes. Chromosomes are acrocentric. CI about 15%.
Group E. Pairs of chromosomes 16 - 18. Relatively short, metacentric or submetacentric. CI 26-40%.
Group F. 19 - 20th pair. Short, submetacentric chromosomes. CI 36-46%.
Group G. 21-22 pairs. Small, acrocentric chromosomes. CI 13-33%. The Y chromosome also belongs to this group.
4. The Parisian classification of human chromosomes was created in 1971. With the help of this classification, it is possible to determine the localization of genes in a particular pair of chromosomes. Using special methods staining, in each chromosome, a characteristic order of alternation of dark and light stripes (segments) is revealed. Segments are designated by the name of the methods that reveal them: Q - segments - after staining with quinacrine mustard; G - segments - Giemsa staining; R - segments - staining after heat denaturation and others. The short arm of the chromosome is denoted by the letter p, the long arm by the letter q. Each chromosome arm is divided into regions and numbered from centromere to telomere. The bands within the regions are numbered in order from the centromere. For example, the location of the D esterase gene - 13p14 - is the fourth band of the first region of the short arm of the 13th chromosome.
Function of chromosomes: storage, reproduction and transmission of genetic information during the reproduction of cells and organisms.
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