The structure and chemical composition of chromosomes. Human chromosomes Chromosome structure
Lecture #3
Topic: Organizing the flow of 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. Proteins on the inner surface of the 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 of 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 centromeric 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 staining methods, a characteristic order of alternation of dark and light stripes (segments) is revealed in each chromosome. 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.
Similar information.
Chromosomes are the nucleoprotein structures of a eukaryotic cell that store most of the hereditary information. Due to their ability to self-reproduce, it is the chromosomes that provide the genetic link between generations. Chromosomes are formed from a long DNA molecule, which contains a linear group of many genes, and all the genetic information, whether it be about a person, animal, plant, or any other living being.
The morphology of chromosomes is related to the level of their spiralization. So, if during the interphase stage the chromosomes are maximally deployed, then with the onset of division, the chromosomes actively spiralize and shorten. They reach their maximum shortening and spiralization during the metaphase stage, when new structures are formed. This phase is most convenient for studying the properties of chromosomes and their morphological characteristics.
The history of the discovery of chromosomes
Back in the middle of the nineteenth century before last, many biologists, studying the structure of plant and animal cells, drew attention to thin filaments and the smallest ring-shaped structures in the nucleus of some cells. And now the German scientist Walter Fleming, using aniline dyes to process the nuclear structures of the cell, what is called "officially" opens the chromosomes. More precisely, the discovered substance was called “chromatid” by him for its ability to stain, and the term “chromosomes” was introduced into use a little later (in 1888) by another German scientist, Heinrich Wilder. The word "chromosome" comes from the Greek words "chroma" - color and "somo" - body.
Chromosomal theory of heredity
Of course, the history of the study of chromosomes did not end with their discovery, so in 1901-1902, American scientists Wilson and Saton, independently of each other, drew attention to the similarity in the behavior of chromosomes and Mendeleian factors of heredity - genes. As a result, scientists came to the conclusion that genes are located on chromosomes and it is through them that genetic information is transmitted from generation to generation, from parents to children.
In 1915-1920, the participation of chromosomes in the transmission of genes was proved in practice in a whole series of experiments made by the American scientist Morgan and his laboratory staff. They managed to localize several hundred hereditary genes in the chromosomes of the Drosophila fly and create genetic maps of the chromosomes. Based on these data, the chromosome theory of heredity was created.
The structure of chromosomes
The structure of chromosomes varies depending on the species, so the metaphase chromosome (formed in the metaphase stage during cell division) consists of two longitudinal threads - chromatids, which are connected at a point called the centromere. The centromere is the part of the chromosome that is responsible for the separation of sister chromatids into daughter cells. She also divides the chromosome into two parts, called the short and long arms, she is also responsible for the division of the chromosome, since it contains a special substance - the kinetochore, to which the division spindle structures are attached.
Here the picture shows a visual structure of the chromosome: 1. chromatids, 2. centromere, 3. short arm of chromatids, 4. long arm of chromatids. At the ends of chromatids are telomeres, special elements that protect the chromosome from damage and prevent fragments from sticking together.
Shapes and types of chromosomes
The sizes of chromosomes of plants and animals vary considerably: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes range from 1.5 to 10 microns. Depending on the type of chromosome, its ability to stain also differs. Depending on the location of the centromere, the following forms of chromosomes are distinguished:
- Metacentric chromosomes, which are characterized by a median location of the centromere.
- Submetacentric, they are characterized by an uneven arrangement of chromatids, when one shoulder is longer and the second is shorter.
- Acrocentric or rod-shaped. Their centromere is located almost at the very end of the chromosome.
Functions of chromosomes
The main functions of chromosomes, both for animals and for plants and in general for all living beings, are the transfer of hereditary, genetic information from parents to children.
Set of chromosomes
The value of chromosomes is so great that their number in cells, as well as the characteristics of each chromosome, determine the characteristic feature of a particular biological species. So, for example, the fruit fly has 8 chromosomes, the y - 48, and the human chromosome set is 46 chromosomes.
In nature, there are two main types of chromosome sets: single or haploid (contained in germ cells) and double or diploid. The diploid set of chromosomes has a paired structure, that is, the entire set of chromosomes consists of chromosome pairs.
Human chromosome set
As we wrote above, the cells of the human body contain 46 chromosomes, which are combined into 23 pairs. Together they make up the human chromosome set. The first 22 pairs of human chromosomes (they are called autosomes) are common for both men and women, and only 23 pairs - sex chromosomes - differ in different sexes, it also determines the gender of a person. The totality of all pairs of chromosomes is also called a karyotype.
This species has a human chromosome set, 22 pairs of double diploid chromosomes contain all our hereditary information, and the last pair is different, in men it consists of a pair of conditional X and Y sex chromosomes, while in women there are two X chromosomes.
All animals have a similar structure of the chromosome set, only the number of non-sex chromosomes in each of them is different.
Genetic diseases associated with chromosomes
Violation of the chromosomes, or even their very wrong number is the cause of many genetic diseases. For example, Down syndrome appears due to the presence of an extra chromosome in the human chromosome set. And such genetic diseases as color blindness, hemophilia are caused by malfunctions of existing chromosomes.
Chromosomes, video
And in conclusion, an interesting educational video about chromosomes.
This article is available in English - .
As part of the capsid.
Encyclopedic YouTube
1 / 5
✪ 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 mechanics of cell division, I think it would be helpful to talk about the vocabulary associated with 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 get into the microbiology of this in the future, this is just to get a general idea of how DNA replicates 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 life stage of the cell, different structures will form. 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 clarifies a bit the meaning of DNA related terms. 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 of the 19th century, and the priority of discovering chromosomes is given to 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" literally means "colored body", since 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, T. Morgan received the Nobel Prize in Physiology or Medicine for the discovery of the role of chromosomes in heredity.
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 concepts, truly telocentric chromosomes do not exist. 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, 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.
). Chromatin is heterogeneous, and some types of such heterogeneity are visible under a microscope. The fine structure of chromatin in the interphase nucleus, determined by the nature of DNA folding and its interaction with proteins, plays an important role in the regulation of gene transcription and DNA replication and, possibly, cellular differentiation.
The DNA nucleotide sequences that form genes and serve as a template for mRNA synthesis are distributed along the entire length of chromosomes (individual genes, of course, are too small to be seen under a microscope). By the end of the 20th century, for about 6,000 genes, it was established on which chromosome and in which part of the chromosome they are located and what is the nature of their linkage (that is, their position relative to each other).
The heterogeneity of metaphase chromosomes, as already mentioned, can be seen even with light microscopy. Differential staining of at least 12 chromosomes revealed differences in the width of some bands between homologous chromosomes (Fig. 66.3). Such polymorphic regions are composed of non-coding highly repetitive DNA sequences.
The methods of molecular genetics have made it possible to identify a huge number of smaller and therefore polymorphic DNA regions that are not detected by light microscopy. These regions are identified as restriction fragment length polymorphisms, tandem repeats varying in number, and short tandem repeat polymorphisms (mono-, di-, tri-, and tetranucleotides). Such variability usually does not appear phenotypically.
However, polymorphism serves as a convenient tool for prenatal diagnosis due to the linkage of certain markers to disease-causing mutant genes (for example, in Duchenne myopathy), as well as in establishing twin zygosity, establishing paternity, and predicting transplant rejection.
It is difficult to overestimate the importance of such markers, especially highly polymorphic short tandem repeats that are widespread in the genome, for mapping the human genome. In particular, they make it possible to establish the exact order and nature of the interaction of loci, which play an important role in ensuring normal ontogeny and cell differentiation. This also applies to those loci, mutations in which lead to hereditary diseases.
Microscopically visible areas on the short arm of acrocentric autosomes (Fig. 66.1) provide rRNA synthesis and nucleolus formation, therefore they are called regions of the nucleolar organizer. In metaphase, they are uncondensed and do not stain. The regions of the nucleolar organizer are adjacent to the condensed sections of chromatin - satellites located at the end of the short arm of the chromosome. Satellites do not contain genes and are polymorphic regions.
In a small part of the cells, it is possible to identify other areas decondensed in metaphase, the so-called fragile areas, where "complete" breaks of the chromosome can occur. Of clinical importance are disorders in the only such site located at the end of the long arm of the X chromosome. Such disorders cause fragile X syndrome.
Other examples of specialized regions of chromosomes are telomeres and centromeres.
The role of heterochromatin, which accounts for a significant part of the human genome, has not yet been precisely established. Heterochromatin is condensed during almost the entire cell cycle, it is inactive and replicates late. Most of the sites are condensed and inactive in all cells (), although others, such as the X chromosome, can be either condensed and inactive, or decondensed and active (facultative heterochromatin). If, due to chromosomal aberrations, genes are close to heterochromatin, then the activity of such genes can change or even be blocked. Therefore, manifestations of chromosomal aberrations, such as duplications or deletions, depend not only on the affected loci, but also on the type of chromatin in them. Many non-lethal chromosomal abnormalities affect inactive or inactivated regions of the genome. Perhaps this explains why trisomy for some chromosomes or monosomy for the X chromosome is compatible with life.
Manifestations of chromosomal abnormalities also depend on the new arrangement of structural and regulatory genes in relation to each other and to heterochromatin.
Fortunately, many structural features of chromosomes can be reliably detected by cytological methods. Currently, there are a number of methods for differential staining of chromosomes (Fig. 66.1 and Fig. 66.3). The location and width of the bands are identical in each pair of homologous chromosomes, with the exception of polymorphic regions, so staining can be used in clinical cytogenetics to identify chromosomes and detect structural abnormalities in them.
Chromosomes are an intensely colored body, consisting of a DNA molecule associated with histone proteins. Chromosomes are formed from chromatin at the beginning of cell division (in the prophase of mitosis), but they are best studied in the metaphase of mitosis. When the chromosomes are located in the plane of the equator and are clearly visible in a light microscope, the DNA in them reaches maximum helicity.
Chromosomes consist of 2 sister chromatids (doubled DNA molecules) connected to each other in the region of the primary constriction - the centromere. The centromere divides the chromosome into 2 arms. Depending on the location of the centromere, chromosomes are divided into:
the metacentric centromere is located in the middle of the chromosome and its arms are equal;
submetacentric centromere is displaced from the middle of the chromosomes and one arm is shorter than the other;
acrocentric - the centromere is located close to the end of the chromosome and one arm is much shorter than the other.
In some chromosomes, there are secondary constrictions that separate from the shoulder of the chromosome a region called the satellite, from which the nucleolus is formed in the interphase nucleus.
Chromosome Rules
1. The constancy of the number. The somatic cells of the body of each species have a strictly defined number of chromosomes (in humans -46, in cats - 38, in fruit flies - 8, in dogs -78, in chickens -78).
2. Pairing. Each chromosome in somatic cells with a diploid set has the same homologous (same) chromosome, identical in size, shape, but unequal in origin: one from the father, the other from the mother.
3. Individuality. Each pair of chromosomes differs from the other pair in size, shape, alternation of light and dark stripes.
4. Continuity. Before cell division, the DNA is doubled and the result is 2 sister chromatids. After division, one chromatid enters the daughter cells and, thus, the chromosomes are continuous - a chromosome is formed from the chromosome.
All chromosomes are divided into autosomes and sex chromosomes. Autosomes - all chromosomes in cells, with the exception of sex chromosomes, there are 22 pairs of them. Sexual - this is the 23rd pair of chromosomes, which determines the formation of the male and female body.
In somatic cells there is a double (diploid) set of chromosomes, in sex cells - haploid (single).
A certain set of chromosomes of a cell, characterized by the constancy of their number, size and shape, is called karyotype.
In order to understand a complex set of chromosomes, they are arranged in pairs as their size decreases, taking into account the position of the centromere and the presence of secondary constrictions. Such a systematized karyotype is called an idiogram.
For the first time, such a systematization of chromosomes was proposed at the Congress of Geneticists in Denver (USA, 1960)
In 1971, in Paris, chromosomes were classified according to color and alternation of dark and light bands of hetero- and euchromatin.
To study the karyotype, geneticists use the method of cytogenetic analysis, in which a number of hereditary diseases associated with a violation of the number and shape of chromosomes can be diagnosed.
1.2. The life cycle of a cell.
The life of a cell from its inception as a result of division to its own division or death is called the cell life cycle. Throughout life, cells grow, differentiate, and perform specific functions.
The life of a cell between divisions is called interphase. Interphase consists of 3 periods: presynthetic, synthetic and postsynthetic.
The presynthetic period immediately follows the division. At this time, the cell grows intensively, increasing the number of mitochondria and ribosomes.
During the synthetic period, replication (doubling) of the amount of DNA occurs, as well as the synthesis of RNA and proteins.
During the post-synthetic period, the cell stores energy, achromatin spindle proteins are synthesized, and preparations for mitosis are in progress.
There are different types of cell division: amitosis, mitosis, meiosis.
Amitosis is a direct division of prokaryotic cells and some cells in humans.
Mitosis is an indirect cell division during which chromosomes are formed from chromatin. Somatic cells of eukaryotic organisms divide by mitosis, as a result of which the daughter cells receive exactly the same set of chromosomes as the daughter cell had.
Mitosis
Mitosis consists of 4 phases:
Prophase is the initial phase of mitosis. At this time, DNA spiralization and shortening of chromosomes begin, which from thin invisible chromatin threads become short thick ones, visible in a light microscope, and arranged in the form of a ball. The nucleolus and the nuclear envelope disappear, and the nucleus disintegrates, the centrioles of the cell center diverge along the poles of the cell, and the fission spindle threads stretch between them.
Metaphase - chromosomes move towards the center, spindle threads are attached to them. Chromosomes are located in the plane of the equator. They are clearly visible under a microscope and each chromosome consists of 2 chromatids. In this phase, the number of chromosomes in a cell can be counted.
Anaphase - sister chromatids (appeared in the synthetic period when DNA is duplicated) diverge towards the poles.
Telophase (telos Greek - end) is the opposite of prophase: chromosomes from short thick visible ones become thin long ones invisible in a light microscope, the nuclear envelope and nucleolus are formed. Telophase ends with the division of the cytoplasm with the formation of two daughter cells.
The biological significance of mitosis is as follows:
daughter cells receive exactly the same set of chromosomes that the mother cell had, so a constant number of chromosomes is maintained in all cells of the body (somatic).
all cells divide except sex cells:
the body grows in the embryonic and postembryonic periods;
all functionally obsolete cells of the body (epithelial cells of the skin, blood cells, cells of the mucous membranes, etc.) are replaced by new ones;
processes of regeneration (recovery) of lost tissues occur.
Diagram of mitosis
When exposed to unfavorable conditions on a dividing cell, the spindle of division can unevenly stretch the chromosomes to the poles, and then new cells are formed with a different set of chromosomes, a pathology of somatic cells (autosomal heteroploidy) occurs, which leads to diseases of tissues, organs, body.