What is a chromosome definition. Interesting facts about human chromosomes

First, let's agree on terminology. Human chromosomes were finally counted a little more than half a century ago - in 1956. Since then we have known that somatic, that is, not germ cells, there are usually 46 of them - 23 pairs.

Chromosomes in a pair (one received from the father, the other from the mother) are called homologous. They contain genes that perform the same functions, but often differ in structure. The exception is the sex chromosomes - X and Y, the gene composition of which does not completely match. All other chromosomes except the sex chromosomes are called autosomes.

Number of sets of homologous chromosomes - ploidy- in germ cells it is equal to one, and in somatic cells, as a rule, two.

So far, B chromosomes have not been found in humans. But sometimes an additional set of chromosomes appears in cells - then they talk about polyploidy, and if their number is not a multiple of 23 - about aneuploidy. Polyploidy occurs in certain types of cells and contributes to their increased work, while aneuploidy usually indicates violations in the work of the cell and often leads to its death.

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Most often, the wrong number of chromosomes is the result of an unsuccessful cell division. In somatic cells, after DNA duplication, the maternal chromosome and its copy are linked together by cohesin proteins. Then protein complexes of kinetochore sit on their central parts, to which microtubules are later attached. When dividing along microtubules, kinetochores disperse to different poles of the cell and pull chromosomes along with them. If the cross-links between copies of the chromosome are destroyed ahead of time, then microtubules from the same pole can attach to them, and then one of the daughter cells will receive an extra chromosome, and the second will remain deprived.

Meiosis also often passes with errors. The problem is that the construction of linked two pairs of homologous chromosomes can twist in space or separate in the wrong places. The result will again be an uneven distribution of chromosomes. Sometimes the sex cell manages to track this so as not to transmit the defect by inheritance. Extra chromosomes are often misfolded or broken, which triggers the death program. For example, among spermatozoa there is such a selection for quality. But the eggs were less fortunate. All of them are formed in humans even before birth, prepare for division, and then freeze. Chromosomes are already doubled, tetrads are formed, and division is delayed. In this form, they live until the reproductive period. Then the eggs mature in turn, divide for the first time and freeze again. The second division occurs immediately after fertilization. And at this stage, it is already difficult to control the quality of the division. And the risks are greater, because the four chromosomes in the egg remain cross-linked for decades. During this time, breakdowns accumulate in cohesins, and chromosomes can spontaneously separate. Therefore, the older the woman, the greater the likelihood of incorrect chromosome divergence in the egg.

Aneuploidy in germ cells inevitably leads to aneuploidy of the embryo. When a healthy egg with 23 chromosomes is fertilized by a sperm with an extra or missing chromosome (or vice versa), the number of chromosomes in the zygote will obviously be different from 46. But even if the germ cells are healthy, this does not guarantee healthy development. In the first days after fertilization, the cells of the embryo actively divide in order to quickly gain cell mass. Apparently, in the course of rapid divisions, there is no time to check the correctness of chromosome segregation, so aneuploid cells can arise. And if an error occurs, then further fate embryo depends on the division in which it happened. If the balance is disturbed already in the first division of the zygote, then the whole organism will grow aneuploid. If the problem arose later, then the outcome is determined by the ratio of healthy and abnormal cells.

Some of the latter may die further, and we will never know about their existence. Or he can take part in the development of the body, and then he will succeed mosaic- different cells will carry different genetic material. Mosaicism causes a lot of trouble for prenatal diagnosticians. For example, at the risk of having a child with Down syndrome, sometimes one or more embryonic cells are removed (at the stage when this should not be dangerous) and the chromosomes are counted in them. But if the embryo is mosaic, then this method becomes not particularly effective.

Third wheel

All cases of aneuploidy are logically divided into two groups: deficiency and excess of chromosomes. The problems that arise with a deficiency are quite expected: minus one chromosome means minus hundreds of genes.

If the homologous chromosome is working normally, then the cell can get away with only an insufficient amount of proteins encoded there. But if some of the genes remaining on the homologous chromosome do not work, then the corresponding proteins will not appear in the cell at all.

In the case of an excess of chromosomes, everything is not so obvious. There are more genes, but here - alas - more does not mean better.

First, extra genetic material increases the load on the nucleus: an additional strand of DNA must be placed in the nucleus and served by information reading systems.

Scientists have found that in people with Down syndrome, whose cells carry an extra 21st chromosome, the work of genes located on other chromosomes is mainly disrupted. Apparently, an excess of DNA in the nucleus leads to the fact that there are not enough proteins that support the work of chromosomes for everyone.

Secondly, the balance is disturbed in the quantity cellular proteins. For example, if activator proteins and inhibitor proteins are responsible for some process in the cell, and their ratio usually depends on external signals, then an additional dose of one or the other will cause the cell to stop responding adequately to the external signal. Finally, an aneuploid cell has an increased chance of dying. When duplicating DNA before division, errors inevitably occur, and the cellular proteins of the repair system recognize them, repair them, and start doubling again. If there are too many chromosomes, then there are not enough proteins, errors accumulate and apoptosis is triggered - programmed cell death. But even if the cell does not die and divides, then the result of such division is also likely to be aneuploids.

You will live

If even within a single cell, aneuploidy is fraught with disruption and death, then it is not surprising that it is not easy for an entire aneuploid organism to survive. On the this moment only three autosomes are known - 13, 18 and 21, trisomy for which (that is, an extra, third chromosome in cells) is somehow compatible with life. This is probably due to the fact that they are the smallest and carry the fewest genes. At the same time, children with trisomy on the 13th (Patau syndrome) and 18th (Edwards syndrome) chromosomes live at best up to 10 years, and more often live less than a year. And only trisomy on the smallest in the genome, the 21st chromosome, known as Down syndrome, allows you to live up to 60 years.

It is very rare to meet people with general polyploidy. Normally, polyploid cells (carrying not two, but four to 128 sets of chromosomes) can be found in the human body, for example, in the liver or red bone marrow. These are usually large cells with enhanced protein synthesis, which do not require active division.

Additional set chromosomes complicates the task of their distribution among daughter cells, so polyploid embryos, as a rule, do not survive. Nevertheless, about 10 cases have been described when children with 92 chromosomes (tetraploids) were born and lived from several hours to several years. However, as in the case of other chromosomal anomalies, they lagged behind in development, including mental development. However, for many people with genetic abnormalities, mosaicism comes to the rescue. If the anomaly has developed already during the fragmentation of the embryo, then a certain number of cells may remain healthy. In such cases, the severity of symptoms decreases and life expectancy increases.

Gender injustices

However, there are also such chromosomes, the increase in the number of which is compatible with human life or even goes unnoticed. And this, surprisingly, the sex chromosomes. The reason for this is gender injustice: about half of the people in our population (girls) have twice as many X chromosomes as others (boys). At the same time, the X chromosomes serve not only to determine sex, but also carry more than 800 genes (that is, twice as many as the extra 21st chromosome, which causes a lot of trouble for the body). But girls come to the aid of a natural mechanism to eliminate inequality: one of the X chromosomes is inactivated, twisted and turns into a Barr body. In most cases, the selection occurs randomly, and in some cells the maternal X chromosome is active, while in others the paternal X chromosome is active. Thus, all girls are mosaic, because different copies of genes work in different cells. Tortoiseshell cats are a classic example of such mosaicity: on their X chromosome there is a gene responsible for melanin (a pigment that determines, among other things, coat color). Different copies work in different cells, so the color is spotty and is not inherited, since inactivation occurs randomly.

As a result of inactivation, only one X chromosome always works in human cells. This mechanism allows you to avoid serious trouble with X-trisomy (XXX girls) and Shereshevsky-Turner syndromes (XO girls) or Klinefelter (XXY boys). About one in 400 children is born this way, but vital functions in these cases are usually not significantly impaired, and even infertility does not always occur. It is more difficult for those who have more than three chromosomes. This usually means that the chromosomes did not separate twice during the formation of germ cells. Cases of tetrasomy (XXXXX, XXYY, XXXY, XYYY) and pentasomy (XXXXX, XXXXY, XXXYY, XXYYY, XYYYY) are rare, some of which have been described only a few times in the history of medicine. All of these variants are compatible with life, and people often live to advanced years, with abnormalities manifesting themselves in abnormal skeletal development, genital defects, and mental decline. Tellingly, the extra Y-chromosome itself has little effect on the functioning of the body. Many men with the XYY genotype do not even know about their features. This is due to the fact that the Y chromosome is much smaller than the X and carries almost no genes that affect viability.

The sex chromosomes have another interesting feature. Many mutations in genes located on autosomes lead to abnormalities in the functioning of many tissues and organs. At the same time, most gene mutations on the sex chromosomes manifest themselves only in mental impairment. It turns out that, to a significant extent, the sex chromosomes control the development of the brain. Based on this, some scientists hypothesize that it is they who are responsible for the differences (however, not fully confirmed) between the mental abilities of men and women.

Who benefits from being wrong

Despite the fact that medicine has been familiar with chromosomal abnormalities for a long time, in Lately aneuploidy continues to attract the attention of scientists. It turned out that more than 80% of tumor cells contain an unusual number of chromosomes. On the one hand, the reason for this may be the fact that proteins that control the quality of division are able to slow it down. In tumor cells, these very control proteins often mutate, so division restrictions are removed and chromosome checking does not work. On the other hand, scientists believe that this may serve as a factor in the selection of tumors for survival. According to this model, tumor cells first become polyploid, and then, as a result of division errors, they lose different chromosomes or parts of them. It turns out a whole population of cells with a wide variety of chromosomal abnormalities. Most of them are not viable, but some may accidentally succeed, for example, if they accidentally get extra copies of genes that start division, or lose genes that suppress it. However, if the accumulation of errors during division is additionally stimulated, then the cells will not survive. The action of taxol, a common cancer drug, is based on this principle: it causes systemic nondisjunction of chromosomes in tumor cells, which should trigger their programmed death.

It turns out that each of us can be a carrier of extra chromosomes, at least in individual cells. but modern science continues to develop strategies to deal with these unwanted passengers. One of them proposes to use the proteins responsible for the X chromosome and incite, for example, the extra 21st chromosome of people with Down syndrome. It is reported that in cell cultures this mechanism was able to be brought into action. So, perhaps in the foreseeable future, dangerous extra chromosomes will be tamed and rendered harmless.

Polina Loseva

Today we propose to consider in as much detail as possible an interesting question from school course biology - what is a chromosome? In biology, this term occurs quite often, but what does it mean? Let's figure it out.

Let's start, perhaps, with the concept of "period of cell life". This is the period of time that begins with its very occurrence and until death. It is also customary to call this time interval the life cycle. Even within the same organism, cycle times vary by species. For example, let's take a cell of epithelial tissue and liver, the life cycle of the first is only about fifteen hours, and the second is a year. It is also important to note the fact that the entire period of cell life is divided into two intervals:

• interphase;
• division.

Chromosomes play an important role in the life cycle of a cell. Let's move on to the definition of what is a chromosome in biology? It is a complex of DNA molecules and proteins. We will talk about their functions in more detail later in the article.

A bit of history

What is a chromosome in biology was known as early as the middle of the nineteenth century, thanks to the research of the German botanist W. Hofmeister. The scientist at that time became interested in studying cell division in a plant called tradescantia. What did he discover new? To begin with, it became clear that before cell division, nuclear division also occurs. But this is not the most interesting! Even before two daughter nuclei are formed, the main one splits into very thin threads. They can only be seen under a microscope, stained with a special dye.

Then the Chamberlain gave them a name - chromosomes. What is a chromosome in biology? If we translate the term into Russian literally, then we get “painted bodies”. A little later, scientists noticed that these filamentous particles are in the nucleus of absolutely any plant or animal cell. But once again we draw your attention to the fact that their number varies depending on the type of cell and organism. If we take a person, then his cells contain only forty-six chromosomes.

Theory of heredity

We have already defined what a chromosome is in biology. Now we propose to move on to genetics, namely, to the transfer of genetic material from parents to offspring.

Thanks to the work of Walter Sutton, the number of chromosomes in cells became known. In addition, the scientist argued that these tiny particles are the carriers of units of heredity. Sutton also found that chromosomes are made up of genes.

At the same time there were similar works and Theodore Boveri. It is important to note that both scientists studied this issue and came to the same conclusion. They studied and formulated the main provisions of the role of chromosomes.

Cells

After the discovery and description of chromosomes in the middle of the nineteenth century, scientists began to be interested in their structure. It became clear that these little bodies are located in absolutely any cell, regardless of whether the prokaryotic or eukaryotic cell is in front of us.

Microscopes helped in the study of the structure. Scientists managed to establish several facts:

• chromosomes are threadlike bodies;
• they can be observed only in certain phases of the cycle;
• if you study in interphase, you can see that the nucleus consists of chromatin;
• during other periods, chromosomes consisting of one or two chromatids can be distinguished;
• the best time to study is mitosis or meiosis (the thing is that in the process of cell division, these little bodies are better visible);
• in eukaryotes, large chromosomes with a linear structure are most common;
• very often cells have several types of chromosomes.

Forms

We dealt with the question - what is a chromosome in biology, but did not say anything about possible varieties. We propose to fill this gap immediately.

So, in total it is customary to distinguish four forms:

• metacentric (if the centromere is in the middle);
• submetacentric (centromere shift to one of the ends);
• acrocentric, another name is rod-shaped (if the centromere is located at either end of the chromosome);
• telocentric (they are also commonly called point ones, since it is very difficult to see the shape due to their small size).

Functions

The chromosome is supramolecular level organization of genetic material. The main component is DNA. It has a number of important features:

• storage of genetic material;
• its transfer;
• its implementation.

Genetic material is presented in the form of genes. It is important to note that there are many (from several hundred to thousands) of genes in one chromosome, it has the following features:

• the chromosome represents only one linkage group;
• arranges the arrangement of genes;
• ensures the joint inheritance of all genes.

Each individual cell has a diploid set of chromosomes. Biology is a very fascinating subject that, if taught correctly, will interest many students. Now let's take a closer look at DNA and RNA.

DNA and RNA

What are chromosomes made of? If we are talking about eukaryotes, these particles in cells are formed with the help of chromatin. The latter includes:

• deoxyribonucleic acid (abbreviated as DNA);
• ribonucleic acid (abbreviation - RNA);
• proteins.

All of the above are macromolecular organic matter. In terms of location, DNA can be found in the nucleus in eukaryotes, while RNA can be found in the cytoplasm.

Genes and chromosomes

Biology considers the issue of genetics in some detail, starting from the school bench. Let's refresh our memory, what is a gene anyway? It is the smallest unit of all genetic material. A gene is a section of DNA or RNA. The second case occurs in viruses. It is he who encodes the development of some trait.

It is also important to note that the gene is responsible only for any one trait, it is functionally indivisible. Now let's move on to X-ray diffraction analysis of DNA. So, the latter forms a double helix. Its chains are made up of nucleotides. The latter are the carbohydrate deoxyribose, a phosphate group, and a nitrogenous base. And here it is a little more interesting, there can be several types of nitrogenous bases:

• adenine;
• guanine;
• thymine;
• cytosine.

Chromosomal set

The species depends on the number of chromosomes and their features. For example, let's take:

• fruit flies (eight chromosomes each);
• primates (48 chromosomes each);
• people (forty-six chromosomes each).

And this number is constant for a particular type of organism. All eukaryotic cells have a diploid set of chromosomes (2n), and haploid is half of it (that is, n). In addition, a pair of chromosomes is always homologous. What does homologous chromosomes mean in biology? These are those that are completely identical (in shape, structure, location of centromeres, and so on).

It is also very important to note that the diploid set is inherent in somatic cells, and the haploid set is inherent in sexual ones.

As part of the capsid.

Encyclopedic YouTube

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✪ Chromosomes, chromatids, chromatin, etc.

✪ Genes, DNA and chromosomes

✪ The most important terms of genetics. loci and genes. homologous chromosomes. Coupling and crossing over.

✪ Chromosomal diseases. Examples and reasons. Biology video lesson Grade 10

✪ Cellular technologies. DNA. Chromosome. Genome. Program "In the first approximation"

Subtitles

Before diving into the 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, for example, 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. Wonderful. 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. There 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 streaming from 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, then 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? Every time I draw 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" Thus, words can be very ambiguous and very confusing, but general use when you talk about a well-defined single strand of DNA, well-defined structure like this, it's a 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 by the set different ways. This is one chain here, and here is another chain here. So we have 2 copies. They code for exactly the same DNA. So. They are identical, which is why I still call it a chromosome. Let's write it down too. All this together is called a chromosome, but now each individual copy is called a chromatid. So this is one chromatid and this is the other. They are sometimes called sister chromatids. They can also be called twin chromatids because they share the same genetic information. So this chromosome has 2 chromatids. Now, before replication, or before DNA duplication, you can say that this chromosome right here has one chromatid. You can call it a chromatid, but it doesn't have to be. People start talking about chromatids when two of them are present on a chromosome. We learn that in mitosis and meiosis these 2 chromatids separate. When they separate, there is a strand of DNA that you once called a chromatid, now you will call a single chromosome. So this is one of them, and here's another one that could have branched off in that direction. I'll circle this one in green. So this one can go to this side, and this one that I circled in orange, for example, to this ... Now that they are separated and no longer connected by a centromere, what we originally called one chromosome with two chromatids, now you call two separate chromosomes. Or you could say that you now have two separate chromosomes, each consisting of one chromatid. I hope this clears things up a bit meaning of terms associated with DNA. I have always found them rather confusing, but they will be a useful tool when we start mitosis and meiosis and I will talk about how a chromosome becomes a chromatid. You will ask how one chromosome became two chromosomes, and how a chromatid became a chromosome. It all revolves around vocabulary. I would choose another instead of calling it a chromosome and each of these individual chromosomes, but that's what they decided to call for us. You might be wondering where the word "chromo" comes from. Maybe you know an old Kodak film called "chrome color". Basically "chromo" means "color". I think it comes from the Greek word for color. When people first looked at the nucleus of a cell, they used a dye, and what we call chromosomes was stained with the dye. And we could see it with a light microscope. The part "soma" comes from the word "soma" meaning "body", that is, we get a colored body. Thus the word "chromosome" was born. Chromatin also stains... I hope this clarifies a little the concepts of "chromatid", "chromosome", "chromatin", and now we are prepared for the study of mitosis and meiosis.

The history of the discovery of chromosomes

The first descriptions of chromosomes appeared in articles and books by various authors in the 70s. years XIX century, and priority is given to the discovery of chromosomes different people. Among them are such names as I. D. Chistyakov (1873), A. Schneider (1873), E. Strasburger (1875), O. Büchli (1876) and others. Most often, the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming, who in his fundamental book "Zellsubstanz, Kern und Zelltheilung" collected and streamlined information about them, supplementing the results of his own research. The term "chromosome" was proposed by the German histologist G. Waldeyer in 1888. "Chromosome" 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, for the discovery of the role of chromosomes in heredity, T. Morgan received Nobel Prize in Physiology and Medicine.

Morphology of metaphase chromosomes

In the metaphase stage of mitosis, chromosomes consist of two longitudinal copies called sister chromatids, which are formed during replication. At metaphase chromosomes sister chromatids are joined together primary constriction called the centromere. The centromere is responsible for separating sister chromatids into daughter cells during division. At the centromere, the kinetochore is assembled - a complex protein structure that determines the attachment of the chromosome to the microtubules of the spindle division - the movers of the chromosome in mitosis. The centromere divides chromosomes into two parts called shoulders. In most species, the short arm of the chromosome is denoted by the letter p, long shoulder - letter q. Chromosome length and centromere position are the main morphological features of metaphase chromosomes.

Three types of chromosome structure are distinguished depending on the location of the centromere:

This classification of chromosomes based on the ratio of arm lengths was proposed in 1912 by the Russian botanist and cytologist S. G. Navashin. In addition to the above three types, S. G. Navashin also singled out telocentric chromosomes, that is, chromosomes with only one arm. However, according to modern ideas There are no truly telocentric chromosomes. The second arm, even if very short and invisible in a conventional microscope, is always present.

Additional morphological feature 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 in representatives different types. 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 connected to each other in the centromere region (more on the fate of chromosomes during cell division see articles mitosis and meiosis).

Chromosomal abnormalities

Aneuploidy

With aneuploidy, a change in the number of chromosomes in the karyotype occurs, in which total number 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. When chromosome fragmentation occurs, 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 can 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.

see also

Notes

1. Tarantula V.Z. Explanatory biotechnological dictionary. - M.: Languages ​​of Slavic cultures, 2009. - 936 p. - 400 copies. - ISBN 978-5-9551-0342-6.

The history of the discovery of chromosomes

Drawing from the book of W. Flemming, depicting the different stages of cell division of the salamander epithelium (W. Flemming. Zellsubstanz, Kern und Zelltheilung. 1882)

In different articles and books, the priority of discovering chromosomes is given to different people, but most often the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming. However, it would be more fair to say that he did not discover chromosomes, but in his fundamental book "Zellsubstanz, Kern und Zelltheilung" (German) he collected and streamlined information about them, supplementing the results of his own research. The term "chromosome" was proposed by the German histologist Heinrich Waldeyer in 1888, "chromosome" literally means "colored body", since the basic dyes are well connected by chromosomes.

Now it is difficult to say who made the first description and drawing of chromosomes. In 1872, the Swiss botanist Carl von Negili published a work in which he depicted some little bodies that appear in place of the nucleus during cell division during the formation of pollen in a lily ( Lilium tigrinum) and Tradescantia ( Tradescantia). However, his drawings do not allow us to unequivocally state that K. Negili saw exactly the chromosomes. In the same 1872, the botanist E. Russov brought his images of cell division during the formation of spores in a fern from the genus Uzhovnik ( Ophioglossum) and lily pollen ( Lilium bulbiferum). In his illustrations, it is easy to recognize individual chromosomes and stages of division. Some researchers believe that the German botanist Wilhelm Hoffmeister was the first to see chromosomes long before K. Negili and E. Russov, back in 1848-1849. At the same time, neither K. Negili, nor E. Russov, and even more so V. Hofmeister did not realize the significance of what they saw.

After the rediscovery in 1900 of Mendel's laws, it took only one or two years for it to become clear that chromosomes behave exactly as expected from "heredity particles". In 1902 T. Boveri and in 1902-1903 W. Setton ( Walter Sutton) were independently the first to put forward a hypothesis about the genetic role of chromosomes. T. Boveri discovered that the embryo sea ​​urchin Paracentrotus lividus can develop normally only if there is at least one, but a complete set of chromosomes. He also found that different chromosomes are not identical in composition. W. Setton studied gametogenesis in acridoids Brachystola magna and realized that the behavior of chromosomes during meiosis and during fertilization fully explains the patterns of divergence of Mendelian factors and the formation of their new combinations.

Experimental confirmation of these ideas and the final formulation of the chromosome theory was made in the first quarter of the 20th century by the founders of classical genetics, who worked in the USA with the fruit fly ( D. melanogaster): T. Morgan, C. Bridges ( C.B. Bridges), A. Sturtevant ( A. H. Sturtevant) and G. Möller. Based on their data, they formulated the "chromosome theory of heredity", according to which the transmission hereditary information associated with chromosomes, in which genes are localized linearly, in a certain sequence. These findings were published in 1915 in The mechanisms of mendelian heredity.

In 1933, T. Morgan received the Nobel Prize in Physiology or Medicine for the discovery of the role of chromosomes in heredity.

eukaryotic chromosomes

The basis of the chromosome is a linear (not closed in a ring) macromolecule of deoxyribonucleic acid (DNA) of considerable length (for example, in DNA molecules of human chromosomes, there are from 50 to 245 million pairs of nitrogenous bases). In a stretched form, the length of a human chromosome can reach 5 cm. In addition to it, the chromosome includes five specialized proteins - H1, H2A, H2B, H3 and H4 (the so-called histones) and a number of non-histone proteins. The amino acid sequence of histones is highly conserved and practically does not differ in various groups of organisms.

Primary constriction

Chromosomal constriction (X. p.), in which the centromere is localized and which divides the chromosome into arms.

Secondary constrictions

A morphological feature that allows you to identify individual chromosomes in a set. They differ from the primary constriction in the absence of a noticeable angle between the segments of the chromosome. Secondary constrictions are short and long and are localized at different points along the length of the chromosome. In humans, these are 9, 13, 14, 15, 21 and 22 chromosomes.

Types of chromosome structure

There are four types of chromosome structure:

• telocentric(rod-shaped chromosomes with a centromere located at the proximal end);
• acrocentric(rod-shaped chromosomes with a very short, almost imperceptible second arm);
• submetacentric(with shoulders of unequal length, resembling the letter L in shape);
• metacentric(V-shaped chromosomes with arms of equal length).

The chromosome type is constant for each homologous chromosome and may be constant in all members of the same species or genus.

Satellites (satellites)

Satellite- this is a rounded or elongated body, separated from the main part of the chromosome by a thin chromatin thread, equal in diameter or slightly smaller than the chromosome. Chromosomes that have a companion are commonly referred to as SAT chromosomes. The shape, size of the satellite and the thread connecting it are constant for each chromosome.

nucleolus zone

Zones of the nucleolus ( nucleolus organizers) are special areas associated with the appearance of some secondary constrictions.

Chromonema

A chromoneme is a helical structure that can be seen in decompacted chromosomes through an electron microscope. It was first observed by Baranetsky in 1880 in the chromosomes of Tradescantia anther cells, the term was introduced by Veydovsky. Chromonema may consist of two, four or more threads, depending on the object under study. These threads form spirals of two types:

• paranemic(elements of the spiral are easy to separate);
• plectonemic(the threads are tightly intertwined).

Chromosomal rearrangements

Violation of the structure of chromosomes occurs as a result of spontaneous or provoked changes (for example, after irradiation).

• Gene (point) mutations (changes at the molecular level);
• Aberrations (microscopic changes visible with a light microscope):

giant chromosomes

Such chromosomes, which are characterized by huge sizes, can be observed in some cells at certain stages of the cell cycle. For example, they are found in the cells of some tissues of dipteran insect larvae (polytene chromosomes) and in the oocytes of various vertebrates and invertebrates (lampbrush chromosomes). It was on preparations of giant chromosomes that it was possible to reveal signs of gene activity.

Polytene chromosomes

The Balbiani were first discovered in th, but their cytogenetic role was identified by Kostov, Paynter, Geitz, and Bauer. Contained in the cells of the salivary glands, intestines, trachea, fat body and malpighian vessels of Diptera larvae.

Lampbrush 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

In each nucleus somatic cell human contains 23 pairs of linear chromosomes, as well as numerous copies of mitochondrial DNA. The table below shows the number of genes and bases in human chromosomes.

What each human chromosome is responsible for, you will learn from this article.

Chromosomes is the genetic material found in a cell of an organism. Each of them contains a DNA molecule in a twisted helix. The complete set of chromosomes is called a karyotype. It consists of 46 units, forming 23 pairs.

What is chromosome 1 responsible for?

Each chromosome contains genes that are responsible for the individuality of a person from birth - for appearance, temperament, tendency to certain ailments and the like.

What is the first pair of chromosomes responsible for?

The first pair of chromosome set determines the future sex of a person. The fact is that women contain a set of two identical units - XX, and men with different ones - X and Y. In the rest of the cells of the body, there are twice as many chromosomes as in the egg and sperm. After the fusion of the latter, a new cell is formed, which already contains a complete genetic set of 46 chromosomes.

So how can one pair of chromosomes determine the sex of a baby? Each egg contains a set of 22 normal and 1 sex (X) chromosomes. But the spermatozoon has the same set, only the sex chromosome can be both X and Y.

The conclusion suggests itself - the sex of the baby depends only on the male factor, or rather on the chromosome that the sperm brought. The egg plays a neutral role in this process - it equally gives rise to life for both a girl and a boy. The content of chromosomes X and Y in the spermatozoon is an equal percentage: 50% to 50%.