What does a chromosome consist of? The structure and functions of chromosomes. reproduction in the organic world. The structure of germ cells. Structure and functions of the cell nucleus
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 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 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.
The chromosome is an elongated, structured set of genes that carries information about heredity and is formed from a condensed one. Chromatin is made up of DNA and proteins that are tightly packed together to form chromatin fibers. Condensed chromatin fibers form chromosomes. Chromosomes are located in ours. Sets of chromosomes are joined together (one from the mother and one from the father) and are known as.
Diagram of the structure of the chromosome at the stage of metaphase
Non-duplicated chromosomes are single-stranded and consist of a region that connects the chromosome arms. The short arm is denoted by the letter p, and the long letter q. The terminal regions of chromosomes are called telomeres, which are made up of repetitive non-coding DNA sequences that shorten during cell division.
duplication of chromosomes
Chromosomal duplication occurs before division processes through or. DNA replication processes keep the correct number of chromosomes after the division of the parent cell. A duplicated chromosome consists of two identical chromosomes, called , which are linked at the centromere. The sister ones remain together until the end of the division process, where they are separated by spindle fibers and enclosed in. As soon as paired chromatids are separated from each other, each of them becomes.
Chromosomes and cell division
One of the most important elements of successful cell division is the correct distribution of chromosomes. In mitosis, this means that chromosomes must be distributed between two daughter cells. In meiosis, chromosomes are distributed among four daughter cells. The spindle is responsible for the movement of chromosomes during cell division.
This type of cell movement is associated with interactions between spindle microtubules and motor proteins working together to separate chromosomes. It is vital that daughter cells retain the correct number of chromosomes. Errors that occur during cell division can lead to unbalanced chromosome numbers, having too many or not enough chromosomes. This abnormality is known as aneuploidy and can occur on autosomal chromosomes during mitosis or on sex chromosomes during meiosis. Abnormalities in chromosome numbers can lead to birth defects, developmental disorders and death.
Chromosomes and protein production
Protein production is a vital cellular process that depends on DNA and chromosomes. DNA contains segments called genes that code for proteins. During protein production, DNA is unwound and its coding segments are transcribed into an RNA transcript. The RNA transcript is then translated to form a protein.
Chromosome mutation
Chromosome mutations are changes that occur in chromosomes and are usually the result of errors that occur during meiosis or when exposed to mutagens such as chemicals or radiation.
Breakage and duplication of chromosomes can lead to several types of structural changes in the chromosome that are usually harmful to humans. These types of mutations result in chromosomes with extra genes in the wrong sequence. Mutations can also produce cells with the wrong number of chromosomes. Abnormal chromosome numbers usually result from nondisjunction or disruption of homologous chromosomes during meiosis.
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 DNA strands 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 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 quite 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 to study 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.
Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?
We propose to understand this issue in order to dot the i's once and for all.
When looking at family photos, you might have noticed that members of the same kinship look alike: children look like parents, parents look like grandparents. This similarity is passed down from generation to generation through amazing mechanisms.
All living organisms, from single-celled to African elephants, have chromosomes in the cell nucleus - thin long threads that can only be seen with an electron microscope.
Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, its implementation and transmission.
How many chromosomes does a person have
As early as the end of the 19th century, scientists found that the number of chromosomes in different species is not the same.
For example, peas have 14 chromosomes, y - 42, and in humans - 46 (i.e. 23 pairs). Hence, it is tempting to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is not at all the case.
Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). Sexual have morphological and structural (composition of genes) differences.
In a female organism, a pair of gonosomes contains two X chromosomes (XX pair), and in a male organism, one X and one Y chromosome (XY pair).
It is on what will be the composition of the chromosomes of the twenty-third pair (XX or XY) that the sex of the unborn child depends. This is determined during fertilization and the fusion of the female and male reproductive cells.
This fact may seem strange, but in terms of the number of chromosomes, a person is inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80.
Chromosomes consist of a protein and a DNA (deoxyribonucleic acid) molecule, similar to a double helix. Each cell contains about 2 meters of DNA, and in total there are about 100 billion km of DNA in the cells of our body.
An interesting fact is that in the presence of an extra chromosome or in the absence of at least one of the 46, a person has a mutation and serious developmental abnormalities (Down's disease, etc.).
Chromosome is a DNA-containing thread-like structure in the cell nucleus that carries genes, the units of heredity, arranged in a linear order. Humans have 22 pairs of normal chromosomes and one pair of sex chromosomes. In addition to genes, chromosomes also contain regulatory elements and nucleotide sequences. They house DNA-binding proteins that control the functions of DNA. Interestingly, the word "chromosome" comes from the Greek word "chrome", meaning "color". Chromosomes got this name due to the fact that they have the peculiarity of being painted in different tones. The structure and nature of chromosomes vary from organism to organism. Human chromosomes have always been the subject of constant interest of researchers working in the field of genetics. The wide range of factors that are determined by human chromosomes, the anomalies they are responsible for, and their complex nature have always attracted the attention of many scientists.
Interesting facts about human chromosomes
Human cells contain 23 pairs of nuclear chromosomes. Chromosomes are made up of DNA molecules that contain genes. The chromosomal DNA molecule contains three nucleotide sequences required for replication. When staining chromosomes, the banded structure of mitotic chromosomes becomes apparent. Each strip contains numerous nucleotide pairs of DNA.
Man is a biological species that reproduces sexually and has diploid somatic cells containing two sets of chromosomes. One set is inherited from the mother, while the other is inherited from the father. Reproductive cells, unlike body cells, have one set of chromosomes. Crossing over (crossover) between chromosomes leads to the creation of new chromosomes. New chromosomes are not inherited from either parent. This is the reason for the fact that not all of us exhibit traits that we receive directly from one of our parents.
Autosomal chromosomes are numbered from 1 to 22 in descending order as their size decreases. Each person has two sets of 22 chromosomes, an X chromosome from the mother and an X or Y chromosome from the father.
An abnormality in the contents of a cell's chromosomes can cause certain genetic disorders in humans. Chromosomal abnormalities in humans are often responsible for the occurrence of genetic diseases in their children. Those who have chromosomal abnormalities are often only carriers of the disease, while their children have the disease.
Chromosomal aberrations (structural changes in chromosomes) are caused by various factors, namely, a deletion or duplication of a part of a chromosome, an inversion, which is a change in the direction of a chromosome to the opposite, or a translocation, in which a part of a chromosome breaks off and joins it to another chromosome.
An extra copy of chromosome 21 is responsible for a very well known genetic disorder called Down syndrome.
Trisomy 18 leads to Edwards syndrome, which can cause death in infancy.
A deletion of part of the fifth chromosome results in a genetic disorder known as 'cried cat' syndrome. People affected by this disease often have mental retardation, and their crying in childhood resembles a cat's cry.
Sex chromosome abnormalities include Turner syndrome, in which female sex characteristics are present but underdeveloped, and XXX syndrome in girls and XXY syndrome in boys, which cause dyslexia in affected individuals.
Chromosomes were first discovered in plant cells. Van Beneden's monograph on fertilized roundworm eggs led to further research. Later, August Weissman showed that the germline was different from the soma and found that the cell nuclei contained hereditary material. He also suggested that fertilization leads to the formation of a new combination of chromosomes.
These discoveries have become cornerstones in the field of genetics. Researchers have already accumulated a fairly significant amount of knowledge about human chromosomes and genes, but much remains to be discovered.
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