A chromosome consists of two components. Chromosome Observation

Today we will together examine an interesting question regarding the biology of the school course, namely: types of chromosomes, their structure, functions performed, and so on.

First you need to understand what it is, a chromosome? This is the common name for the structural elements of the nucleus in eukaryotic cells. It is these particles that contain DNA. The latter contains hereditary information that is transmitted from the parent organism to the descendants. This is possible with the help of genes (structural units of DNA).

Before we look at the types of chromosomes in detail, it is important to become familiar with some issues. For example, why are they called by this term? Back in 1888, the scientist V. Waldeyer gave them this name. If we translate from Greek, then literally we get color and body. What is this connected with? You can find out in the article. It is also very interesting that circular DNA in bacteria is called chromosomes. And this despite the fact that the structure of the latter and the chromosomes of eukaryotes is very different.

Story

So, it became clear to us that a chromosome is an organized structure of DNA and protein that is contained in cells. It is very interesting that one piece of DNA contains many genes and other elements that encode all the genetic information of the organism.

Before considering the types of chromosomes, we suggest talking a little about the history of the development of these particles. And so, the experiments that the scientist Theodore Boveri began to conduct back in the mid-1880s demonstrated the connection between chromosomes and heredity. At the same time, Wilhelm Roux put forward the following theory - each chromosome has a different genetic load. This theory was tested and proven by Theodore Boveri.

Thanks to the work of Gregor Mendel in the 1900s, Boveri was able to trace the connection between the rules of inheritance and the behavior of chromosomes. Boveri's discoveries were able to influence the following cytologists:

• Edmund Beecher Wilson.
• Walter Sutton.
• Theophilus Painter.

Edmund Wilson's work was to link the theories of Boveri and Sutton, which is described in the book The Cell in Development and Heredity. The work was published around 1902 and was devoted to the chromosomal theory of heredity.

Heredity

And one more minute of theory. In his works, researcher Walter Sutton was able to find out how many chromosomes are actually contained in the cell nucleus. It was already said earlier that the scientist considered these particles to be carriers of hereditary information. In addition, Walter found out that all chromosomes consist of genes, and they are precisely the culprits that parental properties and functions are passed on to offspring.

In parallel, work was carried out by Theodore Boveri. As mentioned earlier, both scientists explored a number of questions:

• transmission of hereditary information;
• formulation of the main provisions on the role of chromosomes.

This theory is now called the Boveri-Sutton theory. Its further development was carried out in the laboratory of the American biologist Thomas Morgan. Together, scientists were able to:

• establish patterns of gene placement in these structural elements;
• develop a cytological database.

Structure

In this section we propose to consider the structure and types of chromosomes. So, we are talking about structural cells that store and transmit hereditary information. What are chromosomes made of? From DNA and protein. In addition, the constituent parts of chromosomes form chromatin. Proteins play an important role in packaging DNA in the cell nucleus.

The diameter of the nucleus does not exceed five microns, and the DNA is completely packed into the nucleus. So, the DNA in the nucleus has a loop structure that is supported by proteins. At the same time, the latter recognize the nucleotide sequences to bring them closer together. If you are going to study the structure of chromosomes under a microscope, then the best time for this is metaphase of mitosis.

The chromosome has the shape of a small rod, which consists of two chromatids. The latter are held by the centromere. It is also very important to note that each individual chromatid consists of chromatin loops. All chromosomes can be in one of two states:

• active;
• inactive.

Forms

Now we will look at the existing types of chromosomes. In this section you can find out what forms of these particles exist.

All chromosomes have their own individual structure. A distinctive feature is the coloring features. If you are studying chromosome morphology, there are some significant things to pay attention to:

• centromere location;
• shoulder length and position.

So, there are the following main types of chromosomes:

• metacentric chromosomes (their distinctive feature is the location of the centromere in the middle, this form is also called equilateral);
• submetacentric (a distinctive feature is the displacement of the constriction to one side, another name is unequal shoulders);
• acrocentric (a distinctive feature is the presence of the centromere at almost one end of the chromosome, another name is rod-shaped);
• dotted (they received this name due to the fact that their shape is very difficult to determine, which is due to their small size).

Functions

Regardless of the type of chromosomes in humans and other creatures, these particles perform a lot of different functions. You can read what we are talking about in this section of the article.

• In storing hereditary information. Chromosomes are carriers of genetic information.
• In the transmission of hereditary information. Hereditary information is transmitted through the replication of a DNA molecule.
• In the implementation of hereditary information. Thanks to the reproduction of one or another type of mRNA, and, accordingly, one or another type of protein, control is exercised over all life processes of the cell and the entire organism.

DNA and RNA

We looked at what types of chromosomes exist. Now we move on to a detailed study of the role of DNA and RNA. It is very important to note that it is nucleic acids that make up about five percent of the cell’s mass. They appear to us as mononucleotides and polynucleotides.

There are two types of these nucleic acids:

• DNA, which stands for deoxyribonucleic acid;
• RNA, transcript - ribonucleic acids.

In addition, it is important to remember that these polymers consist of nucleotides, that is, monomers. These monomers in both DNA and RNA are basically similar in structure. Each individual nucleotide also consists of several components, or rather three, interconnected by strong bonds.

Now a little about the biological role of DNA and RNA. To begin with, it is important to note that three types of RNA can be found in a cell:

• informational (removing information from DNA, acting as a matrix for protein synthesis);
• transport (transports amino acids for protein synthesis);
• ribosomal (participates in protein biosynthesis and formation of the ribosome structure).

What is the role of DNA? These particles store heredity information. Parts of this chain contain a special sequence of nitrogenous bases, which are responsible for hereditary traits. In addition, the role of DNA is to transmit these characteristics during cell nuclear division. With the help of RNA, RNA synthesis is carried out in cells, due to which proteins are synthesized.

Chromosome set

So we're looking at types of chromosomes, sets of chromosomes. Let's move on to a detailed consideration of the issue regarding the chromosome set.

The number of these elements is a characteristic feature of the species. Let's take the Drosophila fly as an example. She has a total of eight, while primates have forty-eight. The human body has forty-six chromosomes. We immediately draw your attention to the fact that their number is the same for all cells of the body.

In addition, it is important to understand that there are two possible types of chromosome sets:

• diploid (characteristic of eukaryotic cells, is a complete set, that is, 2n, present in somatic cells);
• haploid (half of the complete set, that is, n, are present in the germ cells).

You need to know that chromosomes form pairs, the representatives of which are homologues. What does this term mean? Homologous are chromosomes that have the same shape, structure, centromere location, and so on.

Sex chromosomes

Now we will take a closer look at the next type of chromosomes - sex chromosomes. This is not one, but a pair of chromosomes, different in male and female individuals of the same species.

As a rule, one of the organisms (male or female) has two identical, fairly large X chromosomes, and the genotype is XX. An individual of the other sex has one X chromosome and a slightly smaller Y chromosome. In this case, the genotype is XY. It is also important to note that in some cases the formation of the male sex occurs in the absence of one of the chromosomes, that is, the X0 genotype.

Autosomes

These are paired particles in organisms with chromosomal sex determination that are the same in both males and females. To put it more simply, all chromosomes (except sex chromosomes) are autosomes.

Please note that the presence, copies and structure do not depend in any way on the sex of the eukaryote. All autosomes have a serial number. If we take a person, then twenty-two pairs (forty-four chromosomes) are autosomes, and one pair (two chromosomes) are sex chromosomes.

Encyclopedic YouTube

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Mysteries of the X Chromosome

Biology in Pictures: Chromosome Structure (Issue 70)

Is the Y chromosome vanishing | Voice acting DeeAFilm

Subtitles

Before diving into the mechanics of cell division, I think it will be helpful to talk about vocabulary related to DNA. There are many words, and some of them sound similar to each other. They can be confusing. First, I'd 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 section of DNA. I have A, G, T, let me have two Ts and then two Cs. Such a small area. It goes on like this. Of course it's a double helix. Each letter has its own. I'll 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 do. One of them has to do with your body cells - more of your skin cells need to be produced. Your DNA must 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 DNA structure. Replication. I'm making a general simplification, but the idea is that the two strands of DNA separate, and it doesn't happen on its own. This is facilitated by a lot of proteins and enzymes, but I will talk about microbiology in detail 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 chain will look something like this. They separate from each other. What can happen after this? I'll remove the extra bits 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 strand is on its own, now, all of a sudden, a thymine base can come and join here, and these nucleotides will start lining up. Thymine and cytosine, and then adenine, adenine, guanine, guanine. And so it goes on. And then, in this other part, on the green chain, which was previously attached to this blue one, the same thing will happen. There will be adenine, guanine, thymine, thymine, cytosine, cytosine. What happened just now? By dividing and attracting complementary bases, we 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 copies itself. Especially when we look at mitosis and meiosis, I can say, “This is the stage where replication occurs.” Now, another process that you'll hear a lot about. I talked about it in the video about DNA. This is a transcription. In the DNA video I didn't pay much attention to how DNA duplicates itself, but one of the great things about the double strand design is that it can easily duplicate itself. You simply separate 2 strips, 2 spirals, and then they become the template for another chain, and then a copy appears. Now for the transcription. This is what must happen to DNA in order for proteins to be made, but transcription is an intermediate step. This is the stage where you move from DNA to mRNA. This mRNA then leaves the cell nucleus and goes to the ribosomes. I'll talk about this in a few seconds. So we can do the same thing. These chains are again separated during transcription. One will separate here, and the other will separate... and the other will separate here. Wonderful. It might make sense to only use one half of the chain - I'll remove one. This 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 pairing with this DNA strand, you have ribonucleic acid, or RNA, pairing. I'll depict the RNA in purple. RNA will pair with DNA. Thymine, found in DNA, will form a pair with adenine. Guanine, now when we talk about RNA, instead of thymine we will have uracil, uracil, cytosine, cytosine. And this 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 occurs. Broadcast. Let's write down this term. Broadcast. It comes from mRNA... In the video about DNA, I had a small tRNA. Transfer RNA was like a truck carrying amino acids to the mRNA. This all happens in a part of the cell called the ribosome. Translation occurs from mRNA to protein. We've seen this happen. So, from mRNA to protein. You have this chain - I'll make a copy. I'll copy the entire chain at once. That strand separates, leaves the nucleus, and then you have these little tRNA trucks that actually pull 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. The amino acids then combine to form a long chain of amino acids, which is protein. Proteins form these strange complex shapes. To make sure you understand. We'll start with DNA. If we make copies of DNA, this is 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 it down. "Transcription". That is, you transcribe information from one form to another - transcription. Now that the mRNA is leaving the nucleus of the cell... I'll draw a picture of a cell to highlight this. We will deal with the structure of the cell in the future. If it is a whole cell, the nucleus is the center. This is where all the DNA is, all replication and transcription happens here. The mRNA then leaves the nucleus, and then translation occurs in the ribosomes, which we will discuss in more detail in the future, and the protein is formed. So, from mRNA to protein is translation. You translate 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 different processes. Now another part of DNA terminology. When I first met her, I found her to be extremely confusing. The word is "chromosome". I'll write down the words here - you can see for yourself how confusing they are: chromosome, chromatin and chromatid. Chromatid. So, the chromosome, we 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 make it clear that I drew this little green line here. This is a double helix. It wraps around proteins called histones. Histones. Let it turn out like this and somehow like that, and then somehow like that. Here you have substances called histones, which are proteins. Let's draw them like this. Like this. This is a structure, that is, DNA in combination with proteins that structure it, forcing it to wrap around further and further. Ultimately, depending on the stage of the cell's life, different structures will form. And when you talk about nucleic acid, which is DNA, and you combine it with proteins, you're talking about chromatin. This means that chromatin is DNA plus 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. Let's put it this way. This is the nucleus of the cell. I drew very clearly visible structures. This is one, this is another. Maybe it is shorter and has a homologous chromosome. I drew chromosomes, right? And each of these chromosomes, as I showed in the last video, are essentially long DNA structures, long strands of DNA wrapped tightly around each other. I drew it something like this. If we zoom in, we can see one chain, and it's actually wrapped around itself like this. This is its homologous chromosome. Remember, in the video on variability, I talked about a homologous chromosome, which encodes 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 be clear, DNA only takes this form at certain life stages when it reproduces itself, i.e. replicated. More precisely, not so... When a cell divides. Before a cell is capable of dividing, the DNA takes on this well-defined shape. Most of the life of a cell, when the DNA is doing its job, when it is making proteins, that is, proteins are transcribed and translated from the DNA, it does not fold in this 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 often, you cannot even see it with a regular light microscope. It is so thin that the entire DNA helix is ​​completely distributed in the nucleus. I'm drawing this here, another one might be here. And then you have a shorter chain like this. You can't even see it. 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 mess like this, made up 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 general usage is when you're talking about a well defined single strand of DNA, a well defined structure like that, it's a chromosome. The term chromatin can refer either to a structure like a chromosome, the combination of DNA and the proteins that structure it, or to the 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 chromatid? Just in case I haven't done this yet... I don't remember if I flagged this. These proteins that provide chromatin structure or make up chromatin and also provide structure are called "histones". There are different types that provide structure at different levels, which we will look at in more detail. So what is a chromatid? When 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. Dad's version looks like this at first. This is a large strand of DNA. It creates another version of itself that is identical if the system works correctly, 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, even though I have 2 chains here, fastened together. Two identical chains. One chain here, one here... Although let me depict it differently. In principle, this can be depicted in many different ways. This is one chain here, and this is another chain here. That is, we have 2 copies. They encode exactly the same DNA. So here it is. They're identical, so I still call it a chromosome. Let's write this down too. The whole thing 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 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, the strand of DNA that you once called a chromatid will now be called a separate chromosome. So this is one of them, and here's another one that might have separated in this direction. I'll circle this one in green. So, this one can go this way, and this one, which I circled in orange, for example, this... Now that they are separated and no longer connected by the centromere, what we originally called one chromosome with two chromatids, you now call two separate chromosomes. Or you could say that you now have two separate chromosomes, each consisting of a single chromatid. I hope this clears up some of the meaning of DNA related terms. I've always found them quite confusing, but they will be a useful tool when we start mitosis and meiosis and I talk about a chromosome becoming 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 have chosen another one, instead of calling it a chromosome and each of these separate chromosomes, but they decided to call it that way for us. You might be wondering where this word "lame" comes from. Maybe you know the old Kodak film called chromo 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 were stained with the dye. And we could see it with a light microscope. The "soma" part comes from the word "soma" which means "body", meaning we get a colored body. This is how the word “chromosome” appeared. Chromatin also stains... I hope this clears up the concepts of chromatid, chromosome, chromatin a little, and we are now prepared to study mitosis and meiosis.

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 priority for the discovery of chromosomes was given to different people. Among them are such names as I. D. Chistyakov (1873), A. Schneider (1873), E. Strassburger (1875), O. Buchli (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"(German) collected and organized information about them, supplementing them with 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 main dyes are well bound 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 behave during meiosis and fertilization exactly as expected from “particles of heredity.” 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 his discovery of the role of chromosomes in heredity.

Morphology of metaphase chromosomes

At the metaphase stage of mitosis, chromosomes consist of two longitudinal copies called sister chromatids, which are formed by replication. In metaphase chromosomes, sister chromatids are joined in the region primary constriction called a centromere. The centromere is responsible for the separation of 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 spindle microtubules - the movers of the chromosome in mitosis. The centromere divides the chromosomes into two parts called shoulders. In most species, the short arm of the chromosome is designated by the letter p, long shoulder - letter q. Chromosome length and centromere position are the main morphological characteristics of metaphase chromosomes.

Depending on the location of the centromere, three types of chromosome structure are distinguished:

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 identified telocentric chromosomes, that is, chromosomes with only one arm. However, according to modern concepts, there are no truly telocentric chromosomes. The second arm, even if very short and invisible in a regular microscope, is always present.

An additional morphological feature of some chromosomes is the so-called secondary constriction, which differs in appearance from the primary one by the absence of a noticeable angle between the chromosome segments. Secondary constrictions come in different lengths and can be located at different 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 chromosomal segments called satellites. Chromosomes that have a satellite are usually called SAT chromosomes (lat. SAT (Sine Acid Thymonucleinico)- without DNA).

Differential staining of metaphase chromosomes

With monochrome staining of chromosomes (aceto-carmine, aceto-orcein, Feulgen or Romanovsky-Giemsa staining), the number and size of chromosomes can be identified; their shape, determined primarily by the position of the centromeres, the presence of secondary constrictions and satellites. In the vast majority of cases, these characteristics are not enough to identify individual chromosomes in the chromosome set. In addition, monochromatically colored chromosomes are often very similar between different species. Differential staining of chromosomes, various techniques of which were developed in the early 70s of the 20th century, provided cytogeneticists with a powerful tool for identifying both individual chromosomes as a whole and their parts, thereby facilitating the procedure for genome analysis.

Differential staining methods are divided into two main groups:

Levels of chromosomal DNA compaction

The basis of a chromosome is a linear DNA macromolecule of considerable length. The DNA molecules of human chromosomes contain from 50 to 245 million nitrogen base pairs. 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 µm³, and a human mitotic chromosome on average does not exceed 5 - 6 µm. Such compaction of genetic material is possible due to the presence in eukaryotes of a highly organized system for laying down DNA molecules both in the interphase nucleus and in the mitotic chromosome. It should be noted that in eukaryotes in proliferating cells there is a constant, regular change in the degree of chromosome compaction. Before mitosis, chromosomal DNA is compacted 10 5 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 the successful processes of transcription and replication, the chromosome must be decompacted. At the same time, the DNA in the nucleus is never completely elongated and is always packaged to one degree or another. Thus, the estimated reduction in size between a chromosome in interphase and a chromosome in mitosis is only about 2 times in yeast and 4 to 50 times in humans.

Some researchers consider the level of the so-called chromonemas, 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 it to finally be seen under a light microscope. The condensed chromosome has the shape of the letter X (often with unequal arms), since the two chromatids resulting from replication are connected to each other at the centromere (for more information about 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 additional chromosome, mutants with three homologous chromosomes are called trisomic, in case of loss of one pair of homologs - nullisomics. Aneuploidy on autosomal chromosomes always causes significant developmental disorders, being the main cause of spontaneous abortions in humans. One of the most well-known aneuploidies in humans is trisomy 21, which leads to the development of Down syndrome. Aneuploidy is typical for tumor cells, especially for cells of solid tumors.

Polyploidy

Change in the number of chromosomes, a multiple of the haploid set of chromosomes ( n), 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. Polyploidy of cells of the whole organism in multicellular animals is rare, although it often occurs in them endopolyploidy some differentiated tissues, for example, the liver in mammals, as well as intestinal tissues, salivary glands, and 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 chromosome fragment may be lost or, conversely, doubled (deletion and duplication, respectively); a section 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 additional chromosomes that are present in the karyotype only in individual 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 influence 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 the irregularity of their inheritance.

Holocentric chromosomes

Holocentric chromosomes do not have a primary constriction; they have a so-called diffuse kinetochore, so during mitosis, spindle microtubules are attached along the entire length of the chromosome. During the divergence of chromatids to the division poles 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 the characteristic V-shape of diverging chromatids at the anaphase stage. When chromosomes fragment, for example, as a result of exposure to ionizing radiation, fragments of holocentric chromosomes diverge to 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. The nematode has holocentric chromosomes C. elegans .

Giant chromosome shapes

Polytene chromosomes

Polytene chromosomes are giant clusters of united chromatids that arise in certain types of specialized cells. First described by E. Balbiani ( Edouard-Gerard Balbiani) per year in the cells of the salivary glands of bloodworms ( Chironomus), however, their cytogenetic role was revealed later in the 30s of the 20th century by Kostov, T. Paynter, E. Heitz and G. Bauer ( Hans Bauer). Polytene chromosomes were also found in the cells of the salivary glands, intestines, tracheas, fat body and Malpighian vessels of dipteran larvae.

Lamp brush chromosomes

Lampbrush chromosomes are a giant form of chromosome that arises in meiotic female cells during the diplotene stage of prophase I in some animals, particularly 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 yolk are most intense. Currently, 45 animal species are known in whose developing oocytes such chromosomes can be observed. Lampbrush chromosomes are not produced in mammalian oocytes.

Lampbrush 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 chromosomes are longer than polytene chromosomes. For example, the total length of the chromosome set in the oocytes of some tailed amphibians reaches 5900 µm.

Bacterial chromosomes

There is evidence that bacteria have proteins associated with nucleoid DNA, but histones have not been found in them.

Human chromosomes

Each nucleated human somatic cell 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.

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.
2. Molecular biology of the cell: in 3 volumes / B. Alberts, A. Johnson, D. Lewis, etc. - M.-Izhevsk: Research Center “Regular and Chaotic Dynamics”, Institute of Computer Research, 2013. - Vol. I. - pp. 309-336. - 808 p. -

Heredity and variability in living nature exist thanks to chromosomes, genes, (DNA). It is stored and transmitted as a chain of nucleotides as part of DNA. What role do genes play in this phenomenon? What is a chromosome from the point of view of transmission of hereditary characteristics? Answers to questions like these provide insight into coding principles and genetic diversity on our planet. It largely depends on how many chromosomes are included in the set and on the recombination of these structures.

From the history of the discovery of “particles of heredity”

Studying plant and animal cells under a microscope, many botanists and zoologists in the middle of the 19th century drew attention to the thinnest threads and the smallest ring-shaped structures in the nucleus. More often than others, the German anatomist Walter Flemming is called the discoverer of chromosomes. It was he who used aniline dyes to treat nuclear structures. Flemming called the discovered substance “chromatin” for its ability to stain. The term “chromosomes” was introduced into scientific use in 1888 by Heinrich Waldeyer.

At the same time as Flemming, the Belgian Eduard van Beneden was looking for an answer to the question of what a chromosome is. A little earlier, German biologists Theodor Boveri and Eduard Strassburger conducted a series of experiments proving the individuality of chromosomes and the constancy of their number in different species of living organisms.

Prerequisites for the chromosomal theory of heredity

American researcher Walter Sutton found out how many chromosomes are contained in the cell nucleus. The scientist considered these structures to be carriers of units of heredity, characteristics of the organism. Sutton discovered that chromosomes consist of genes through which properties and functions are passed on to offspring from their parents. The geneticist in his publications gave descriptions of chromosome pairs and their movement during the division of the cell nucleus.

Regardless of his American colleague, work in the same direction was carried out by Theodore Boveri. Both researchers in their works studied the issues of transmission of hereditary characteristics and formulated the main provisions on the role of chromosomes (1902-1903). Further development of the Boveri-Sutton theory took place in the laboratory of Nobel laureate Thomas Morgan. The outstanding American biologist and his assistants established a number of patterns of gene placement on the chromosome and developed a cytological basis that explains the mechanism of the laws of Gregor Mendel, the founding father of genetics.

Chromosomes in a cell

The study of the structure of chromosomes began after their discovery and description in the 19th century. These bodies and filaments are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). Study under a microscope made it possible to establish what a chromosome is from a morphological point of view. It is a mobile filamentous body that is visible during certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. During other periods, chromosomes are distinguishable in the form of one or two chromatids.

These formations are better visible during cell division - mitosis or meiosis. In eukaryotic cells, large chromosomes with a linear structure can often be observed. In prokaryotes they are smaller, although there are exceptions. Cells often contain more than one type of chromosome, for example mitochondria and chloroplasts have their own small “particles of inheritance”.

Chromosome shapes

Each chromosome has an individual structure and differs from others in its coloring features. When studying morphology, it is important to determine the position of the centromere, the length and placement of the arms relative to the constriction. The set of chromosomes usually includes the following forms:

• metacentric, or equal arms, which are characterized by a median location of the centromere;
• submetacentric, or unequal arms (the constriction is shifted towards one of the telomeres);
• acrocentric, or rod-shaped, in which the centromere is located almost at the end of the chromosome;
• dotted with a difficult-to-define shape.

Functions of chromosomes

Chromosomes consist of genes - functional units of heredity. Telomeres are the ends of chromosome arms. These specialized elements serve to protect against damage and prevent fragments from sticking together. The centromere performs its tasks during chromosome doubling. It has a kinetochore, and it is to this that the spindle structures are attached. Each pair of chromosomes is individual in the location of the centromere. The spindle threads work in such a way that one chromosome at a time goes to the daughter cells, not both. Uniform doubling during division is provided by the origins of replication. Duplication of each chromosome begins simultaneously at several such points, which significantly speeds up the entire division process.

Role of DNA and RNA

It was possible to find out what a chromosome is and what function this nuclear structure performs after studying its biochemical composition and properties. In eukaryotic cells, nuclear chromosomes are formed by a condensed substance - chromatin. According to the analysis, it contains high-molecular organic substances:

Nucleic acids are directly involved in the biosynthesis of amino acids and proteins and ensure the transmission of hereditary characteristics from generation to generation. DNA is contained in the nucleus of a eukaryotic cell, RNA is concentrated in the cytoplasm.

Genes

X-ray diffraction analysis showed that DNA forms a double helix, the chains of which consist of nucleotides. They represent the carbohydrate deoxyribose, a phosphate group, and one of four nitrogenous bases:

Regions of helical deoxyribonucleoprotein strands are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary characteristics are transmitted from parents to offspring in the form of gene alleles. They determine the functioning, growth and development of a particular organism. According to a number of researchers, those sections of DNA that do not encode polypeptides perform regulatory functions. The human genome can contain up to 30 thousand genes.

Set of chromosomes

The total number of chromosomes and their features are a characteristic feature of the species. In the Drosophila fly their number is 8, in primates - 48, in humans - 46. This number is constant for the cells of organisms that belong to the same species. For all eukaryotes there is the concept of “diploid chromosomes”. This is a complete set, or 2n, as opposed to haploid - half the number (n).

Chromosomes in one pair are homologous, identical in shape, structure, location of centromeres and other elements. Homologues have their own characteristic features that distinguish them from other chromosomes in the set. Staining with basic dyes allows you to examine and study the distinctive features of each pair. is present in the somatic ones - in the reproductive ones (the so-called gametes). In mammals and other living organisms with a heterogametic male sex, two types of sex chromosomes are formed: the X chromosome and the Y. Males have a set of XY, females have a set of XX.

Human chromosome set

The cells of the human body contain 46 chromosomes. All of them are combined into 23 pairs that make up the set. There are two types of chromosomes: autosomes and sex chromosomes. The first form 22 pairs - common for women and men. What differs from them is the 23rd pair - sex chromosomes, which are non-homologous in the cells of the male body.

Genetic traits are associated with gender. They are transmitted by a Y and an X chromosome in men and two X chromosomes in women. Autosomes contain the rest of the information about hereditary traits. There are techniques that allow you to individualize all 23 pairs. They are clearly distinguishable in the drawings when painted in a certain color. It is noticeable that the 22nd chromosome in the human genome is the smallest. Its DNA, when stretched, is 1.5 cm long and has 48 million nitrogen base pairs. Special histone proteins from the composition of chromatin perform compression, after which the thread takes up thousands of times less space in the cell nucleus. Under an electron microscope, the histones in the interphase core resemble beads strung on a strand of DNA.

Genetic diseases

There are more than 3 thousand hereditary diseases of various types caused by damage and abnormalities in chromosomes. These include Down syndrome. A child with such a genetic disease is characterized by delays in mental and physical development. With cystic fibrosis, a malfunction occurs in the functions of the exocrine glands. Violation leads to problems with sweating, secretion and accumulation of mucus in the body. It makes it difficult for the lungs to function and can lead to suffocation and death.

Color vision impairment - color blindness - insensitivity to certain parts of the color spectrum. Hemophilia leads to weakened blood clotting. Lactose intolerance prevents the human body from digesting milk sugar. In family planning offices you can find out about your predisposition to a particular genetic disease. In large medical centers it is possible to undergo appropriate examination and treatment.

Gene therapy is a direction of modern medicine, identifying the genetic cause of hereditary diseases and eliminating it. Using the latest methods, normal genes are introduced into pathological cells instead of damaged ones. In this case, doctors relieve the patient not from the symptoms, but from the causes that caused the disease. Only correction of somatic cells is carried out; gene therapy methods are not yet applied en masse to germ cells.

Chromosome is a thread-like structure containing DNA in the cell nucleus, which carries genes, units of heredity, arranged in a linear order. Humans have 22 pairs of regular 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 DNA functions. Interestingly, the word "chromosome" comes from the Greek word "chrome", meaning "color". Chromosomes received this name because they have the ability to be colored in different tones. The structure and nature of chromosomes vary from organism to organism. Human chromosomes have always been a subject of constant interest to researchers working in the field of genetics. The wide range of factors that are determined by human chromosomes, the abnormalities for which they are responsible, 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 chromosomes are stained, the banded structure of mitotic chromosomes becomes apparent. Each strip contains numerous DNA nucleotide pairs.

Humans are a sexually reproducing species with 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 between chromosomes leads to the creation of new chromosomes. New chromosomes are not inherited from either parent. This accounts for the fact that not all of us exhibit traits that we receive directly from one of our parents.

Autosomal chromosomes are assigned numbers 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 people. Chromosomal abnormalities in people are often responsible for the development of genetic diseases in their children. Those who have chromosomal abnormalities are often only carriers of the disease, while their children develop the disease.

Chromosomal aberrations (structural changes in chromosomes) are caused by various factors, namely deletion or duplication of part of a chromosome, inversion, which is a change in the direction of a chromosome to the opposite, or translocation, in which part of a chromosome is torn off and attached to another chromosome.

An extra copy of chromosome 21 is responsible for a very well known genetic disorder called Down syndrome.

Trisomy 18 results in Edwards syndrome, which can cause death in infancy.

Deletion of part of the fifth chromosome results in a genetic disorder known as Cri-Cat Syndrome. People affected by this disease often have mental retardation and their crying in childhood resembles that of a cat.

Disorders caused by sex chromosome abnormalities include Turner syndrome, in which female sexual characteristics are present but characterized by underdevelopment, as well as 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. August Weissman later showed that the germ line was distinct from the soma and discovered that cell nuclei contained hereditary material. He also suggested that fertilization leads to the formation of a new combination of chromosomes.

These discoveries became cornerstones in the field of genetics. Researchers have already accumulated a significant amount of knowledge about human chromosomes and genes, but much remains to be discovered.

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For this reason, they reach large sizes, which is inconvenient during cell division. To prevent the loss of genetic information, nature came up with chromosomes.

Chromosome structure

These dense structures are rod-shaped. Chromosomes differ from each other in length, which ranges from 0.2 to 50 microns. The width usually has a constant value and does not differ between different pairs of dense bodies.

At the molecular level, chromosomes are a complex complex of nucleic acids and histone proteins, the ratio of which is respectively 40% to 60% by volume. Histones are involved in the compaction of DNA molecules.

It is worth noting that a chromosome is a non-permanent structure of the nucleus of a eukaryotic cell. Such bodies are formed only during the division period, when it is necessary to package all the genetic material to simplify its transmission. Therefore, we consider the structure of the chromosome at the time of preparation for mitosis/meiosis.

The primary constriction is a fibrillar body that divides the chromosome into two arms. Depending on the ratio of the lengths of these arms, chromosomes are distinguished:

1. Metacentric, when the primary constriction is exactly in the center.
2. Submetacentric: shoulder length differs slightly.
3. In acrocentric ones, the primary constriction is strongly shifted to one of the ends of the chromosome.
4. Telocentric, when one of the shoulders is completely absent (not found in humans).

Another feature of the structure of the chromosome of a eukaryotic cell is the presence of a secondary constriction, which is usually strongly displaced towards one of the ends. Its main function is to synthesize ribosomal RNAs on a DNA template, which then form the non-membrane cell organelles ribosomes. Secondary constrictions are also called nucleolar organizers. These formations are located at the distal part of the chromosome.

Several organizers form an integral structure - the nucleolus. The number of such formations in the nucleus can vary from 1 to several dozen, and they are usually visible even in a light microscope.

During the synthetic phase of mitosis, the structure of the chromosome changes as a result of DNA duplication during the replication process. In this case, a familiar shape is formed, reminiscent of the letter X. It is in this form that you can often catch chromosomes and take a high-quality picture on special microscopes.

It is worth noting that the number of chromosomes in different species does not in any way indicate the degree of their evolutionary development. Here are some examples:

1. Humans have 46 chromosomes.
2. The cat has 60.
3. The crucian carp has 100.
4. The rat has 42.
5. The bow has 16.
6. The Drosophila fly has 8.
7. The mouse has 40.
8. Corn has 20.
9. Apricot has 16.
10. The crab has 254.

Functions of chromosomes

The nucleus is the central structure of any eukaryotic cell because it contains all the genetic information. Chromosomes perform a number of important functions, namely:

1. Storing the genetic information itself in an unchanged form.
2. Transfer of this information by replication of DNA molecules during cell division.
3. Manifestation of characteristic features of an organism due to the activation of genes responsible for the synthesis of certain proteins.
4. Assembly of rRNA in nucleolar organizers to construct the small and large subunits of ribosomes.

An important role during cell division is played by the primary constriction, to the proteins of which the filaments of the spindle are attached in the metaphase of mitosis or meiosis. In this case, the X-structure of the chromosome is broken into two rod-shaped bodies, which are delivered to different poles and will subsequently be enclosed in the nuclei of daughter cells.

Levels of compaction

The first level is called nucleosomal. The DNA then wraps around histone proteins, forming “beads on a string.”

The second level is nucleomeric. Here the “beads” come together and form threads up to 30 nm thick.

The third level is called chromomeric. In this case, the strands begin to form loops of several orders, thereby shortening the initial length of DNA many times.

The fourth level is chromonemic. Compactization reaches its maximum, and the resulting rod-shaped formations are already visible in a light microscope.

Features of the genetic material of prokaryotes

A distinctive feature of bacteria is the absence of a nucleus. Genetic information is also stored using DNA, which is scattered throughout the cell as part of the cytoplasm. Among the nucleic acid molecules, one ring one stands out. It is usually located in the center and is responsible for all functions of the prokaryotic cell.

Sometimes this DNA is called the chromosome of a bacterium, the structure of which, of course, does not in any way coincide with that of a eukaryote. Therefore, such a comparison is relative and simply simplifies the understanding of some biochemical mechanisms.