Chromosomes are nucleoprotein structures located in the nuclei of eukaryotic cells. They store almost all the hereditary information, and it is they who carry the function of its storage, transmission and implementation. Chromosomes are practically invisible even under a light microscope, but they can be clearly seen during periods of cell division, during mitosis and meiosis.
Karyotype and chromosome rules
The karyotype is the set of all chromosomes (diploid set) located in the cell. It is species-specific, that is, it is unique for each type of living creature on the planet, its level of variability is relatively low, but it may have certain characteristics in some individuals. For example, representatives of different sexes have basically the same chromosomes (autosomes), the difference in karyotypes is only one pair of chromosomes - sex chromosomes, or heterochromosomes.
The rules of chromosomes are simple: their number is constant (only a strict number of chromosomes can be contained in somatic cells, for example, in cats - 38, in the fruit fly Drosophila - 8, in chicken - 78, and in humans 46).
Chromosomes are paired, each of them has a homologous pair, identical in all respects, including shape and size. Only the origin differs: one from the father, the other from the mother.
Homologous pairs of chromosomes are individual: each of the pairs differs from the others not only in appearance - shape and size - but also in the arrangement of light and dark stripes.
Continuity is another rule of chromosomes. Cell DNA doubles before division, resulting in a pair of sister chromatids. After dividing, each daughter cell receives one chromatid, i.e., a chromosome is formed from the chromosome.
Required Items
The chromosome, whose structure is relatively simple, is formed from a DNA molecule with a long length. It contains linear groups of many genes. Each chromosome has a centromere and telomeres, replication initiation points - these are its necessary functional elements. Telomeres are at the ends of chromosomes. Due to them and the origin of replication (they are also called initiation sites), the DNA molecule can replicate. In centromeres, sister DNA molecules attach to the mitotic spindle of division, which allows them to accurately disperse into daughter cells during the mitosis process.
About viruses
The term "chromosome" was originally proposed as a designation of structures characteristic of eukaryotic cells, but scientists are increasingly mentioning viral and bacterial chromosomes. Their composition and functions are almost similar, therefore D.E. Koryakov and I.F. Zhimulev believe that the concept has long been necessary to expand and define the chromosome as a structure containing nucleic acid and having the function of storing, implementing and transmitting information about genes. In eukaryotes, chromosomes are contained in the nucleus, as well as plastids and mitochondria. Prokaryotes (non-nuclear) also contain DNA, but there is no nucleus in the cell. In viruses, chromosomes have the appearance of an RNA or DNA molecule located in a capsid. Regardless of the presence of a nucleus in a cell, the composition of chromosomes includes organic substances, metal ions and many other substances.
Discovery story
Scientists have come a long way before exploring chromosomes. They were first described in the seventies of the century before last: different authors mentioned them in their articles, books and scientific papers, so the discovery of chromosomes is attributed to different people. In this list, the names of I. D. Chistyakov, A. Schneider, O. Buchley, E. Strasburger and many others, however, by most scientists, 1882 is recognized as the year of the discovery of chromosomes, and V. Fleming, the German anatomist who collected and ordered information, is called the discoverer. about chromosomes in his book Zellsubstanz, Kern und Zelltheilung, adding his own studies to the already available information. The term itself was proposed in 1888 by the histologist G. Valdeyer. Translated, the chromosome literally means “colored body”. The name is due to the fact that the chemical composition of the chromosome allows it to easily bind the basic dyes.
Mendel’s laws were "rediscovered" in 1900, and very soon, within two years, scientists came to the conclusion that chromosomes behave like "particles of heredity" during the processes of meiosis and fertilization, the behavior of which was theoretically described earlier. In 1902, independently of each other, T. Boveri and W. Setton put forward the hypothesis that the chromosome, the structure of which was still unknown, carries the function of transmitting and storing hereditary information.
Drosophila and genetics
The first quarter of the last century was marked by experimental confirmation of the idea that chromosomes have a genetic role. American scientists T. Morgan, A. Störtevant, K. Bridges and G. Möller worked on studies whose objects were the structure and classification of chromosomes, as well as their functions. The experiments were conducted on D.melanogaster, perhaps known to all fruit flies. The obtained data served as the foundation for the chromosomal theory of heredity, which is relevant even now, after almost a hundred years. According to her, chromosomes are associated with hereditary information, and the genes in them are localized linearly, in a clear sequence, but the chemical composition and morphology of chromosomes are still being studied by scientists.
For the work T. Morgan was awarded the Nobel Prize in physiology and medicine in 1933.
The chemical composition of chromosomes
It can be briefly described that the hereditary material on the chromosomes appears as a nucleo-protein complex. After studying the chemical organization of chromosomes in eukaryotic cells, scientists can say that they are composed mostly of DNA and proteins, which form a nucleoprotein complex called chromatin.
Proteins that make up the composition of chromosomes, this is a significant part of the entire substance in the chromosomes, about 65% of the total mass of structures falls on them. Chromosomal proteins are subdivided into non-histone proteins and histones. Histones are strongly basic, their alkaline nature is determined by the presence of lysine and arginine, the main amino acids.
The chemical and structural composition of chromosomes is diverse. Histones represent five fractions: Hl, H2A, H2B, H3 and H4. All but the first fraction, in approximately equal amounts, are present in the cells of all species belonging to higher mammals. Hl proteins are half as much.
The synthesis of histones occurs on the polysomes of the cytoplasm. These are the main proteins that have a positive charge, due to which they can firmly bind to DNA molecules and thus do not allow reading the enclosed hereditary information. This is the regulatory role of histones, but in addition to it there is also a structural function, due to which the spatial organization of DNA in the chromosomes is ensured.
The characteristic chemical composition of interphase chromosomes also includes non-histone proteins, which, in turn, are subdivided into more than one hundred fractions. This series includes enzymes responsible for RNA synthesis, and enzymes that trigger DNA repair and reduction. As well as basic, acid chromosomal proteins have regulatory and structural functions.
However, the chemical composition of the chromosome does not end there: in addition to proteins and DNA, RNA, metal ions, lipids and polysaccharides are present in the composition. Partially, chromosomal RNA is present as transcription products that have not yet left the site of synthesis.
In metaphase
The morphological features of the metaphase chromosome are as follows: during the first half of mitosis, they consist of a pair of sister chromatids, which are interconnected in the centromere region (primary constriction, or kinetochore) - this is the chromosome region common to both chromatids. The chemical composition of the chromosome also changes. The second half of mitosis is characterized by the separation of chromatids, after which the formation of single-stranded daughter chromosomes occurs, which are distributed into daughter cells. The question of how much DNA is part of the metaphase chromosome is often found in biology tests and confuses students. In the last period of interphase, as well as in prophase and metaphase, chromosomes are two-chromatid, so their set corresponds to the formula 2n4c.
Chromosome classification
According to the position of the centromeres and the length of the shoulders located on either side of it, the chromosomes are classified into metacentric (equal arms), if the centromere is located in the middle, and submetacentric (unequal arms), if the centromere is shifted to one of the ends. There are also acrocentric, or rod-shaped chromosomes (their centromere is located almost at the very end) and point chromosomes, which got their name for their small size, so it is almost impossible to determine their shape. In telecentric chromosomes, it is also difficult to determine the location of the primary constriction.
Compaction
Any somatic cell contains 23 pairs of chromosomes, each of which consists of one DNA molecule. The total length of all 46 molecules is about two meters! These are more than three billion pairs of nucleotides, and all of them fit in one cell, while the chromosomes during the interphase period are almost indistinguishable even with an electron microscope. The reason for this is the supramolecular organization of chromosomes, or compaction. Upon transition to another phase of the cell cycle, chromatin can change its organization.
The structure and chemical composition of interphase chromosomes and the structure of metaphase chromosomes are regarded by scientists as polar variants of the structure, which are interconnected by mutual transitions during the mitosis process.
The first level of compaction is represented by a nucleosome thread, which is also called "beads on a thread." The characteristic size is 10-11 nm, which does not allow them to be examined under a microscope.
The chemical composition of the chromosome determines the presence of this level of organization: it is provided by four types of histones - the main proteins (H2A, H2B, NZ, N4). They form crusts - bodies of protein molecules in the form of washers. Each cortex consists of eight molecules (a pair of molecules from each of the histones).
A complete set of the DNA molecule occurs, it is spirally wound onto the cortex. Each protein body is contacted by a piece of DNA molecule, numbering 146 nucleotide pairs. There are also areas not involved in the contact, called linker, or binders. Their size varies, but on average is 60 pairs of nucleotides (bp).
A nucleosome is a portion of DNA that is 196 bp in length. and including protein cortex. However, the nucleosome strand, similar to a bead strand, also has regions that do not contain bark.
Similar sites that distinguish non-histone proteins perfectly, due to the presence of certain nucleotide sequences, are found quite uniformly with an interval of several thousand nucleotide pairs. Their presence is important for further chromatin compaction.
Further chromatin packaging
Chromatin fibril - the second level of compaction - is also called the solenoid or nucleomeric level. The size is 30 nm. Provided by histone HI. It combines with the linker site of DNA, as well as with two neighboring cores and “pulls” them together. The result of the process is the formation of a much more compact structure, reminiscent in structure of a solenoid. Such a fibril, in addition to chromatin, is called elementary.
The chronomeric level follows. The characteristic size of this level of compaction is 300 nm. An additional spiralization no longer occurs, however, transverse loops are formed that coincide with the size of one replicon and are combined by means of non-histonic (acidic)
At the chromonemal level (700 nm), the loops come together, and chromatin becomes even more compacted. The formed chromosome strands are already visible under a light microscope.
Chromosomal level (1400 nm) is observed during the metaphase.
Mutations and their role in medicine
Mutation of chromosomes is not uncommon, however, it can have a different degree and mechanisms of occurrence. Changes in the structural form of chromosomes are usually based on an initial integrity violation. If gaps are present in the chromosome, the body has to rearrange them, as a result of which a chromosomal mutation, or aberration, occurs.
In the process of crossing over, homologous chromosomes exchange corresponding regions, and it is at this time that breaks usually occur. If during the crossover there was an exchange of unequal parts of the genes, new linking groups appear.
Types of Mutations
There are several types of mutations based on the mechanism of their origin. The mutation of division appears due to the loss of regions of genes. If any parts of the genome have been doubled - this is duplication. During inversion, the section of the chromosome between the breaks rotates 180 °.
A translocation is a transition of a region from one chromosome to another, and if the movement occurs between non-homologous chromosomes, the translocation is called reciprocal, and if the fragment was attached to the same chromosome, the mutation is called transposition. During the Robertson translocation , two two non-homologous structures merge into one.
There are also pericentric and paracentric mutations.
RNA
Depending on the phase in which the cell is located, the chemical composition, characteristics of the morphology of the chromosomes and their size change, but the genetic material carries not only the DNA and chromosomes in the nucleus.
Ribonucleic acid (RNA) is another structure involved in the transfer and storage of genetic information.
There is mRNA, or mRNA (matrix, or informational), it is involved in the synthesis of proteins with desired properties. For this, it is necessary that an “instruction” is received at the place of “construction”, which will indicate in what order the amino acids should be included in the chain of peptides. This instruction is the information encoded in the nucleotide sequence of mRNA (mRNA). Transcription is called the process of synthesis of messenger RNA.
The process of reading information from DNA can be compared with a computer program. First, RNA polymerase must detect the promoter - a special section of the DNA molecule that marks the region where transcription begins. RNA polymerase binds to the promoter and begins to unwind the adjacent strand of the DNA helix. At this point, two DNA strands are disconnected from each other, after which the enzyme begins the formation of mRNA on one of them (codogenic, facing the enzyme at the 3`-end). Ribonucleotides are assembled in a chain according to the rule of complementarity with DNA nucleotides, and antiparallel to the template DNA strand.
Transcription process
Thus, as it moves along the DNA chain, the enzyme accurately reads all the information, continuing the process until it again encounters a special sequence of nucleotides. It is called the transcription terminator, and signals that RNA polymerase must be separated from both the DNA template strand and the newly synthesized mRNA. The sum of the regions from the promoter to the terminator, including the transcribed region, is called the transcription unit - transcripton.
As the RNA polymerase moves along the codogenic chain, the transcribed single-stranded sections of DNA combine again and take the form of a double helix. The formed mRNA carries an exact copy of the data copied from the DNA site. Nucleotides of mRNA encoding amino acid sequences are grouped in three and are called codons. Each codon of mRNA corresponds to a specific amino acid.
Properties and functions of genes
A gene is considered an elementary indivisible functional unit of a hereditary material. It has the form of a portion of a DNA molecule that encodes the structure of at least one peptide.
A gene has certain properties, the first of which is discrete action. This means that variously localized genes control the development of individual traits.
The property of constancy is determined by the fact that the gene is unchanged during hereditary transmission, unless, of course, a mutation has occurred. It follows that the gene cannot be changed during life.
The specificity of the action lies in the conditionality of the development of a trait or group of traits, however, genes can also have multiple actions - this is called pleiotropy.
The dosed property of an action determines the limit to which a trait due to the gene can develop.
They are also characterized by an allelic state, that is, almost all genes are in alleles, the number of which begins with two.