Gene, genome, chromosome: definition, structure, functions. Biology What is a chromosome? sex chromosomes
What is the human genome? How long has this term been used in science and, and why is this concept so important in our time?
human genome- the totality of hereditary material contained in the cell. It consists of 23 pairs.
Genes are separate parts of DNA. Each of them is responsible for some sign or part of the body: height, eye color, etc.
When scientists manage to completely “decipher” the information recorded on DNA, people will be able to fight those diseases that are inherited. Moreover, perhaps then it will be possible to solve the problem of aging.
Previously, it was believed that the number of genes in our body is more than a hundred thousand. However, recent international studies have confirmed that there are approximately 28,000 genes in our body. To date, only a few thousand of them have been studied.
Genes are unevenly distributed across chromosomes. Why this is so, scientists do not yet know.
The cells of the body read the information that is stored in DNA all the time. Each of them does its job: it carries oxygen through the body, destroys viruses, and so on.
But there are also special cells - sex cells. In men, these are spermatozoa, and in women, they are eggs. They do not contain 46 chromosomes, but exactly half - 23.
When the sex cells merge, the new organism has a complete set of chromosomes: half from the father and half from the mother.
That is why children are somewhat similar to each of their parents.
Several genes are usually responsible for the same trait. For example, our growth depends on 16 units of DNA. At the same time, some genes affect several traits at once (for example, owners of redheads have a fair skin tone and freckles).
Eye color in humans is determined by two genes, and the one responsible for brown eyes is dominant. This means that it is more likely to show up when it "meets" another gene.
Therefore, for a brown-eyed dad and a blue-eyed mom, the baby is likely to be brown-eyed. Dark hair, thick eyebrows, dimples on the cheeks and chin are also dominant features.
But the gene responsible for blue eyes is recessive. Such genes appear much less frequently if both parents have them.
We hope that now you know what the human genome is. Of course, in the near future, science may surprise us with new discoveries in this area. But this is a matter for the future.
If you like interesting facts about everything - subscribe to in any social network. It's always interesting with us!
Liked the post? Press any button.
8.1. Gene as a discrete unit of heredity
One of the fundamental concepts of genetics at all stages of its development was the concept of the unit of heredity. In 1865, the founder of genetics (the science of heredity and variability), G. Mendel, based on the results of his experiments on peas, came to the conclusion that hereditary material is discrete, i.e. represented by individual units of heredity. Units of heredity, which are responsible for the development of individual traits, G. Mendel called "inclinations". Mendel argued that in the body, for any trait, there is a pair of allelic inclinations (one from each of the parents), which do not interact with each other, do not mix and do not change. Therefore, during sexual reproduction of organisms, only one of the hereditary inclinations in a "pure" unchanged form enters the gametes.
Later, G. Mendel's assumptions about the units of heredity received complete cytological confirmation. In 1909, the Danish geneticist W. Johansen called Mendel's "hereditary inclinations" genes.
Within the framework of classical genetics, a gene is considered as a functionally indivisible unit of hereditary material that determines the formation of some elementary trait.
Various variants of the state of a particular gene, resulting from changes (mutations), are called "alleles" (allelic genes). The number of alleles of a gene in a population can be significant, but in a particular organism the number of alleles of a particular gene is always equal to two - according to the number of homologous chromosomes. If in a population the number of alleles of any gene is more than two, then this phenomenon is called "multiple allelism".
Genes are characterized by two biologically opposite properties: the high stability of their structural organization and the ability to hereditary changes (mutations). Thanks to these unique properties, it is ensured: on the one hand, the stability of biological systems (immutability in a number of generations), and on the other hand, the process of their historical development, the formation of adaptations to environmental conditions, i.e. evolution.
8.2. Gene as a unit of genetic information. Genetic code.
More than 2500 years ago, Aristotle suggested that gametes are by no means miniature versions of the future organism, but structures containing information about the development of embryos (although he recognized only the exceptional importance of the egg to the detriment of the spermatozoon). However, the development of this idea in modern research became possible only after 1953, when J. Watson and F. Crick developed a three-dimensional model of the structure of DNA and thereby created the scientific prerequisites for revealing the molecular foundations of hereditary information. Since that time, the era of modern molecular genetics began.
The development of molecular genetics has led to the disclosure of the chemical nature of genetic (hereditary) information and has filled with concrete meaning the idea of a gene as a unit of genetic information.
Genetic information is information about the signs and properties of living organisms, embedded in the hereditary structures of DNA, which is realized in ontogenesis through protein synthesis. Each new generation receives hereditary information, as a program for the development of an organism, from its ancestors in the form of a set of genome genes. The unit of hereditary information is a gene, which is a functionally indivisible section of DNA with a specific nucleotide sequence that determines the amino acid sequence of a particular polypeptide or RNA nucleotides.
Hereditary information about the primary structure of a protein is recorded in DNA using the genetic code.
The genetic code is a system for recording genetic information in a DNA (RNA) molecule in the form of a specific sequence of nucleotides. This code serves as a key for translating the nucleotide sequence in mRNA into the amino acid sequence of the polypeptide chain during its synthesis.
Properties of the genetic code:
1. Tripletity - each amino acid is encoded by a sequence of three nucleotides (triplet or codon)
2. Degeneracy - most amino acids are encrypted by more than one codon (from 2 to 6). There are 4 different nucleotides in DNA or RNA, which theoretically can form 64 different triplets (4 3 = 64) to code for 20 amino acids that make up proteins. This explains the degeneracy of the genetic code.
3. Non-overlapping - the same nucleotide cannot be part of two adjacent triplets at the same time.
4. Specificity (uniqueness) - each triplet encodes only one amino acid.
5. The code has no punctuation marks. Reading information from mRNA during protein synthesis always goes in the direction 5, - 3, in accordance with the sequence of mRNA codons. If one nucleotide falls out, then when reading it, the nearest nucleotide from the neighboring code will take its place, which will change the amino acid composition in the protein molecule.
6. The code is universal for all living organisms and viruses: the same triplets encode the same amino acids.
The universality of the genetic code indicates the unity of the origin of all living organisms
However, the universality of the genetic code is not absolute. In mitochondria, the number of codons has a different meaning. Therefore, sometimes one speaks of the quasi-universality of the genetic code. Features of the genetic code of mitochondria indicate the possibility of its evolution in the process of historical development of living nature.
Among the triplets of the universal genetic code, three codons do not code for amino acids and determine the end of the synthesis of a given polypeptide molecule. These are the so-called "nonsens" codons (stop codons or terminators). These include: in DNA - ATT, ACT, ATC; in RNA - UAA, UGA, UAG.
The correspondence of nucleotides in a DNA molecule to the order of amino acids in a polypeptide molecule is called collinearity. Experimental confirmation of collinearity played a decisive role in deciphering the mechanism for the realization of hereditary information.
The meaning of the codons of the genetic code are given in table 8.1.
Table 8.1. Genetic code (mRNA codons for amino acids)
Using this table, mRNA codons can be used to determine amino acids. The first and third nucleotides are taken from the vertical columns located on the right and left, and the second - from the horizontal. The place where the conditional lines cross contains information about the corresponding amino acid. Note that the table lists mRNA triplets, not DNA triplets.
Structural - functional organization of the gene
Molecular biology of the gene
The modern idea of the structure and function of the gene was formed in line with a new direction, which J. Watson called the molecular biology of the gene (1978)
An important stage in the study of the structural and functional organization of the gene was the work of S. Benzer in the late 1950s. They proved that a gene is a nucleotide sequence that can change as a result of recombinations and mutations. S. Benzer called the unit of recombination a recon, and the unit of mutation a muton. It has been experimentally established that the muton and recon correspond to one pair of nucleotides. S. Benzer called the unit of genetic function the cistron.
In recent years, it has become known that the gene has a complex internal structure, and its individual parts have different functions. In a gene, the nucleotide sequence of the gene can be distinguished, which determines the structure of the polypeptide. This sequence is called a cistron.
A cistron is a sequence of DNA nucleotides that determines a particular genetic function of a polypeptide chain. A gene may be represented by one or more cistrons. Complex genes containing several cistrons are called polycistronic.
Further development of the theory of the gene is associated with the identification of differences in the organization of the genetic material in organisms taxonomically distant from each other, which are pro- and eukaryotes.
Gene structure of prokaryotes
In prokaryotes, of which bacteria are typical representatives, most of the genes are represented by continuous informative DNA sections, all of which information is used in the synthesis of the polypeptide. In bacteria, genes occupy 80-90% of DNA. The main feature of prokaryotic genes is their association into groups or operons.
An operon is a group of successive structural genes controlled by a single regulatory region of DNA. All linked operon genes code for enzymes of the same metabolic pathway (eg lactose digestion). Such a common mRNA molecule is called polycistronic. Only a few genes in prokaryotes are individually transcribed. Their RNA is called monocistronic.
An operon-type organization allows bacteria to quickly switch metabolism from one substrate to another. Bacteria do not synthesize enzymes of a particular metabolic pathway in the absence of the required substrate, but are able to start synthesizing them when a substrate is available.
Structure of eukaryotic genes
Most eukaryotic genes (unlike prokaryotic genes) have a characteristic feature: they contain not only regions encoding the structure of the polypeptide - exons, but also non-coding regions - introns. Introns and exons alternate with each other, which gives the gene a discontinuous (mosaic) structure. The number of introns in genes varies from 2 to tens. The role of introns is not completely clear. It is believed that they are involved in the processes of recombination of genetic material, as well as in the regulation of expression (implementation of genetic information) of the gene.
Thanks to the exon-intron organization of genes, the prerequisites for alternative splicing are created. Alternative splicing is the process of “cutting out” different introns from the primary RNA transcript, as a result of which different proteins can be synthesized based on one gene. The phenomenon of alternative splicing occurs in mammals during the synthesis of various antibodies based on immunoglobulin genes.
Further study of the fine structure of the genetic material further complicated the clarity of the definition of the concept of "gene". Extensive regulatory regions have been found in the eukaryotic genome with various regions that can be located outside the transcription units at a distance of tens of thousands of base pairs. The structure of a eukaryotic gene, including transcribed and regulatory regions, can be represented as follows.
Fig 8.1. Structure of a eukaryotic gene
1 - enhancers; 2 - silencers; 3 – promoter; 4 - exons; 5 - introns; 6, exon regions encoding untranslated regions.
A promoter is a section of DNA for binding to RNA polymerase and the formation of a DNA-RNA polymerase complex to start RNA synthesis.
Enhancers are transcription enhancers.
Silencers are transcription attenuators.
Currently, the gene (cistron) is considered as a functionally indivisible unit of hereditary mastery, which determines the development of any trait or property of the organism. From the standpoint of molecular genetics, a gene is a section of DNA (in some viruses, RNA) that carries information about the primary structure of a polypeptide, a molecule of transport and ribosomal RNA.
Diploid human cells have approximately 32,000 gene pairs. Most of the genes in every cell are silent. The set of active genes depends on the type of tissue, the period of development of the organism, and the received external or internal signals. It can be said that in each cell its own chord of genes “sounds”, determining the spectrum of synthesized RNA, proteins and, accordingly, the properties of the cell.
Gene structure of viruses
Viruses have a gene structure that reflects the genetic structure of the host cell. Thus, bacteriophage genes are assembled into operons and do not have introns, while eukaryotic viruses have introns.
A characteristic feature of viral genomes is the phenomenon of "overlapping" genes ("gene within a gene"). In "overlapping" genes, each nucleotide belongs to one codon, but there are different frames for reading genetic information from the same nucleotide sequence. Thus, the phage φ X 174 has a segment of the DNA molecule, which is part of three genes at once. But the nucleotide sequences corresponding to these genes are read each in its own frame of reference. Therefore, it is impossible to talk about "overlapping" the code.
Such an organization of the genetic material ("gene within a gene") expands the information capabilities of a relatively small virus genome. The functioning of the genetic material of viruses occurs in different ways depending on the structure of the virus, but always with the help of the enzyme system of the host cell. The various ways in which genes are organized in viruses, pro- and eukaryotes are shown in Figure 8.2.
Functionally - genetic classification of genes
There are several classifications of genes. So, for example, allelic and non-allelic genes, lethal and semi-lethal, “housekeeping” genes, “luxury genes”, etc. are isolated.
Housekeeping Genes- a set of active genes necessary for the functioning of all cells of the body, regardless of the type of tissue, the period of development of the body. These genes encode enzymes for transcription, ATP synthesis, replication, DNA repair, etc.
"luxury" genes are selective. Their functioning is specific and depends on the type of tissue, the period of development of the organism, and the received external or internal signals.
Based on modern ideas about the gene as a functionally indivisible unit of hereditary material and the systemic organization of the genotype, all genes can be fundamentally divided into two groups: structural and regulatory.
Regulatory genes- encode the synthesis of specific proteins that affect the functioning of structural genes in such a way that the necessary proteins are synthesized in the cells of different tissue affiliation and in the required quantities.
Structural called genes that carry information about the primary structure of a protein, rRNA or tRNA. Protein-coding genes carry information about the amino acid sequence of certain polypeptides. From these DNA regions, mRNA is transcribed, which serves as a template for the synthesis of the primary structure of the protein.
rRNA genes(4 varieties are distinguished) contain information about the nucleotide sequence of ribosomal RNA and determine their synthesis.
tRNA genes(more than 30 varieties) carry information about the structure of transfer RNAs.
Structural genes, the functioning of which is closely related to specific sequences in the DNA molecule, called regulatory regions, are divided into:
independent genes;
Repetitive genes
gene clusters.
Independent genes are genes whose transcription is not associated with the transcription of other genes within the transcription unit. Their activity can be regulated by exogenous substances, such as hormones.
Repetitive genes present on the chromosome as repeats of the same gene. The ribosomal 5-S-RNA gene is repeated many hundreds of times, and the repeats are arranged in tandem, i.e., following closely one after another without gaps.
Gene clusters are groups of different structural genes with related functions localized in certain regions (loci) of the chromosome. Clusters are also often present in the chromosome in the form of repeats. For example, a cluster of histone genes is repeated in the human genome 10-20 times, forming a tandem group of repeats. (Fig. 8.3.)
Fig.8.3. Cluster of histone genes
With rare exceptions, clusters are transcribed as a whole, as one long pre-mRNA. So the pre-mRNA of the histone gene cluster contains information about all five histone proteins. This accelerates the synthesis of histone proteins, which are involved in the formation of the nucleosomal structure of chromatin.
There are also complex gene clusters that can code for long polypeptides with multiple enzymatic activities. For example, one of the NeuraSpora grassa genes encodes a polypeptide with a molecular weight of 150,000 daltons, which is responsible for 5 consecutive steps in the biosynthesis of aromatic amino acids. It is believed that polyfunctional proteins have several domains - conformationally limited semi-autonomous formations in the polypeptide chain that perform specific functions. The discovery of semifunctional proteins gave reason to believe that they are one of the mechanisms of the pleiotropic effect of one gene on the formation of several traits.
In the coding sequence of these genes, non-coding ones, called introns, can be wedged. In addition, between the genes there may be sections of spacer and satellite DNA (Fig. 8.4).
Fig.8.4. Structural organization of nucleotide sequences (genes) in DNA.
Spacer DNA is located between genes and is not always transcribed. Sometimes the region of such DNA between genes (the so-called spacer) contains some information related to the regulation of transcription, but it can also be simply short repetitive sequences of excess DNA, the role of which remains unclear.
Satellite DNA contains a large number of groups of repeating nucleotides that do not make sense and are not transcribed. This DNA is often located in the heterochromatin region of the centromeres of mitotic chromosomes. Single genes among satellite DNA have a regulatory and reinforcing effect on structural genes.
Micro- and minisatellite DNA are of great theoretical and practical interest for molecular biology and medical genetics.
microsatellite DNA- short tandem repeats of 2-6 (usually 2-4) nucleotides, which are called STR. The most common are nucleotide CA repeats. The number of repetitions can vary significantly from person to person. Microsatellites are found predominantly in certain regions of DNA and are inherited according to the laws of Mendel. Children receive one chromosome from their mother, with a certain number of repeats, another from their father, with a different number of repeats. If such a cluster of microsatellites is located next to the gene responsible for a monogenic disease, or inside the gene, then a certain number of repeats along the length of the cluster can be a marker of the pathological gene. This feature is used in the indirect diagnosis of gene diseases.
Minisatellite DNA- tandem repeats of 15-100 nucleotides. They were called VNTR - tandem repeats variable in number. The length of these loci is also significantly variable in different people and can be a marker (label) of a pathological gene.
Micro- and macrosatellite DNA use:
1. For the diagnosis of gene diseases;
2. In forensic medical examination for personal identification;
3. To establish paternity and in other situations.
Along with structural and regulatory repeating sequences, the functions of which are unknown, migrating nucleotide sequences (transposons, mobile genes), as well as the so-called pseudogenes in eukaryotes, have been found.
Pseudogenes are non-functioning DNA sequences that are similar to functioning genes.
They probably occurred by duplication, and the copies became inactive as a result of mutations that violated any stages of expression.
According to one version, pseudogenes are an "evolutionary reserve"; in another way, they represent "dead ends of evolution", a side effect of rearrangements of once functioning genes.
Transposons are structurally and genetically discrete DNA fragments that can move from one DNA molecule to another. First predicted by B. McClintock (Fig. 8) in the late 40s of the XX century based on genetic experiments on corn. Studying the nature of the color of corn grains, she made the assumption that there are so-called mobile ("jumping") genes that can move around the cell genome. Being next to the gene responsible for the pigmentation of corn grains, mobile genes block its work. Subsequently, transposons were identified in bacteria and it was found that they are responsible for the resistance of bacteria to various toxic compounds.
Rice. 8.5. Barbara McClintock was the first to predict the existence of mobile ("jumping") genes capable of moving around the genome of cells.
Mobile genetic elements perform the following functions:
1. encode proteins responsible for their movement and replication.
2. cause many hereditary changes in cells, as a result of which a new genetic material is formed.
3. leads to the formation of cancer cells.
4. integrating into different parts of chromosomes, they inactivate or enhance the expression of cellular genes,
5. is an important factor in biological evolution.
Current State of Gene Theory
Modern gene theory was formed due to the transition of genetics to the molecular level of analysis and reflects the fine structural and functional organization of units of heredity. The main provisions of this theory are as follows:
1) gene (cistron) - a functional indivisible unit of hereditary material (DNA in organisms and RNA in some viruses), which determines the manifestation of a hereditary trait or property of an organism.
2) Most genes exist in the form of two or more alternative (mutually exclusive) variants of alleles. All alleles of a given gene are localized on the same chromosome in a certain section of it, which is called a locus.
3) Changes in the form of mutations and recombinations can occur inside the gene; the minimum sizes of a muton and a recon are equal to one pair of nucleotides.
4) There are structural and regulatory genes.
5) Structural genes carry information about the sequence of amino acids in a particular polypeptide and nucleotides in rRNA, tRNA
6) Regulatory genes control and direct the robot of structural genes.
7) The gene is not directly involved in protein synthesis, it is a template for the synthesis of various types of RNA that are directly involved in protein synthesis.
8) There is a correspondence (colinearity) between the arrangement of triplets of nucleotides in structural genes and the order of amino acids in the polypeptide molecule.
9) Most gene mutations do not manifest themselves in the phenotype, since DNA molecules are capable of repair (restoring their native structure)
10) The genotype is a system that consists of discrete units - genes.
11) The phenotypic manifestation of a gene depends on the genotypic environment in which the gene is located, the influence of factors of the external and internal environment.
With the development of the natural sciences, which occurred at the beginning of the 20th century, it was possible to identify the principles of heredity. In the same period, new terms arose to describe what genes and the human genome are. A genome is a unit of hereditary information responsible for the formation of a carrier of any property in the body. In wildlife, it is the transmission of this information that is the basis of the entire process of reproduction. This term, like the very definition of what genes are, was first used by the botanist Wilhelm Johansen in 1909.
Gene structure
To date, it has been established that genes are separate sections of DNA - deoxyribonucleic acid. Each gene is responsible for the transmission of information about the structure of RNA (ribonucleic acid) or protein in the human body. As a rule, a gene contains several sections of DNA. The structures that take over the transmission of hereditary information are called coding sequences. But at the same time, there are also structures in DNA that affect the expression of a gene. These areas are called regulatory. That is, genes include coding and regulatory sequences that are located separately from each other in DNA.
human genome
In 1920, Hans Winkler introduced the concept of the genome. At first, this term was used to refer to the gene set of an unpaired single set of chromosomes, which is inherent in a biological species. There was an opinion that the genome completely replenishes all the properties of an organism of a certain species. But in the future, the meaning of this term has changed slightly, since the studies have shown that such a definition is not entirely true.
genetic information
It was found what genes are and that in the DNA of many organisms there are sequences that do not code for anything. In addition, some of the genetic information is contained in DNA, which is located outside the cell nucleus. Some of the genes responsible for encoding the same trait can differ significantly in their structure. That is, the genome is called a collective set of genes that are contained in the chromosomes and beyond. It characterizes the properties of a certain population of individuals, but the genetic set of each individual organism has significant differences from its genome.
What is the basis of heredity
In an attempt to define what genes are, a wide variety of studies have been carried out. Therefore, it is impossible to answer this question unambiguously. According to the biological definition of this term, a gene is a DNA sequence containing information about a particular protein. And until recently, such an explanation of this term was quite enough. But it has now been established that the sequence in which the protein is encoded is not always continuous. It can be interrupted by sections interspersed in it that do not carry any information.
Gene identification
A gene can be identified by a group of mutations, each of which prevents the creation of the corresponding protein. Nevertheless, this statement can be considered correct with respect to discontinuous genes. The properties of their clusters in this case turn out to be much more complicated. But this statement is rather controversial, since many genes with a discontinuous chain are found in situations where it is impossible to conduct a thorough genetic analysis. It was believed that the genome is fairly constant, and any changes in its overall structure occur only in extreme cases. And specifically, only in the extended evolutionary-time scale. But such a judgment contradicts recent evidence that certain rearrangements occur periodically in DNA and that there are relatively variable components of the genome.
Properties of genes identified in the work of Mendel
In the work of Mendel, namely in his first and second laws, it is precisely formulated what genes are and what their properties are. The first law deals with the features of an individual gene. In the body there are two copies of each gene, that is, in the language of modernity, it is diploid. One of the two copies of the gene passes to the offspring from the parent through gametes, that is, it is inherited. The gametes combine to form a fertilized egg (zygote), which carries one copy from each parent. Therefore, the organism receives one maternal copy of the gene and one paternal copy.
The two-faced aging gene
As you know, human aging is explained not only by the accumulation of malfunctions in the body, but also by the work of certain genes that carry information about aging. The question immediately arises as to why this gene has been preserved in the process of evolution. Why is it needed in the body and what role does it play? Research on this topic was based on breeding mice without the characteristic p66Shc protein. Individuals that lacked this protein were not prone to the accumulation of body fat, aged more slowly, suffered less from metabolic shifts, cardiovascular diseases and diabetes. It turns out that this protein is a gene that accelerates the aging process. But such results were given only by laboratory studies. Then the animals were transferred to natural habitats, and as a result, the population of mutant individuals began to decline. For this reason, a decision was made to further research, and as a result, the fact was confirmed that the “aging gene” is of great importance in the adaptation processes of the body and is responsible for the natural energy metabolism in the animal body.
Richard Dawkins - evolutionary biologist and his "Selfish Gene"
The book written by Richard Dawkins (The Selfish Gene) is the most popular book on evolution. The book sets a not quite typical viewing angle, showing that evolution, or rather natural selection, occurs primarily at the level of genes. Of course, today this fact is no longer in doubt, but in 1976 such a statement was very innovative. We are created by our genes. All living beings are necessary in order to preserve genes. The world of the selfish gene is a world of ruthless exploitation, fierce competition, and deceit.
“Gene”, “genome”, “chromosome” are words that are familiar to every schoolchild. But the idea of this issue is rather generalized, since deepening into the biochemical jungle requires special knowledge and a desire to understand all this. And it, if it is present at the level of curiosity, then quickly disappears under the weight of the presentation of the material. Let's try to understand the intricacies of hereditary information in a scientific polar form.
What is a gene?
A gene is the smallest structural and functional piece of information about heredity in living organisms. In fact, it is a small section of DNA that contains knowledge about a specific amino acid sequence for building a protein or functional RNA (from which a protein will also be synthesized). The gene determines those traits that will be inherited and passed on to descendants further along the genealogical chain. Some unicellular organisms have gene transfer that is not related to the reproduction of their own kind, it is called horizontal.
“On the shoulders” of genes lies a huge responsibility for how each cell and the organism as a whole will look and work. They govern our lives from conception to our very last breath.
The first scientific advance in the study of heredity was made by the Austrian monk Gregor Mendel, who in 1866 published his observations on the results of crossing peas. The hereditary material that he used clearly showed the patterns of transmission of traits, such as the color and shape of peas, as well as flowers. This monk formulated the laws that formed the beginning of genetics as a science. The inheritance of genes occurs because parents give their child half of all their chromosomes. Thus, the signs of mom and dad, mixing, form a new combination of already existing signs. Fortunately, there are more options than living creatures on the planet, and it is impossible to find two absolutely identical creatures.
Mendel showed that hereditary inclinations do not mix, but are transmitted from parents to descendants in the form of discrete (isolated) units. These units, represented in individuals by pairs (alleles), remain discrete and are passed on to subsequent generations in male and female gametes, each of which contains one unit from each pair. In 1909, the Danish botanist Johansen named these units genes. In 1912, Morgan, a geneticist from the United States of America, showed that they are in the chromosomes.
Since then, more than a century and a half have passed, and research has advanced further than Mendel could have imagined. At the moment, scientists have settled on the opinion that the information contained in the genes determines the growth, development and functions of living organisms. Or maybe even their death.
What is a chromosome? sex chromosomes
The totality of an individual's genes is called the genome. Naturally, the entire genome cannot be packed into a single DNA. The genome is divided into 46 pairs of DNA molecules. One pair of DNA molecules is called a chromosome. So it is precisely these chromosomes that a person has 46 pieces. Each chromosome carries a strictly defined set of genes, for example, the 18th chromosome contains genes encoding eye color, etc. Chromosomes differ from each other in length and shape. The most common forms are in the form of X or Y, but there are also others. A person has two chromosomes of the same shape, which are called paired (pairs). In connection with such differences, all paired chromosomes are numbered - there are 23 pairs. This means that there is a pair of chromosomes #1, pair #2, #3, and so on. Each gene responsible for a particular trait is located on the same chromosome. In modern manuals for specialists, the localization of the gene may be indicated, for example, as follows: chromosome 22, long arm.
What are the differences between chromosomes?
How else do chromosomes differ from each other? What does the term long arm mean? Let's take X-shaped chromosomes. The crossing of DNA strands can occur strictly in the middle (X), or it can occur not centrally. When such an intersection of DNA strands does not occur centrally, then relative to the point of intersection, some ends are longer, others, respectively, are shorter. Such long ends are commonly called the long arm of the chromosome, and short ends, respectively, the short arm. Y-shaped chromosomes are mostly occupied by long arms, and short ones are very small (they are not even indicated on the schematic image).
The size of the chromosomes fluctuates: the largest are the chromosomes of pairs No. 1 and No. 3, the smallest chromosomes of pairs No. 17, No. 19.
In addition to shapes and sizes, chromosomes differ in their functions. Out of 23 pairs, 22 pairs are somatic and 1 pair is sexual. What does it mean? Somatic chromosomes determine all the external signs of an individual, the characteristics of his behavioral reactions, hereditary psychotype, that is, all the features and characteristics of each individual person. A pair of sex chromosomes determines the sex of a person: male or female. There are two types of human sex chromosomes - X (X) and Y (Y). If they are combined as XX (X - X) - this is a woman, and if XY (X - Y) - we have a man in front of us.
Hereditary diseases and chromosome damage
However, there are "breakdowns" of the genome, then genetic diseases are detected in people. For example, when there are three chromosomes in 21 pairs of chromosomes instead of two, a person is born with Down syndrome.
There are many smaller "breakdowns" of the genetic material that do not lead to the onset of the disease, but, on the contrary, give good properties. All "breakdowns" of the genetic material are called mutations. Mutations that lead to disease or deterioration of the properties of the organism are considered negative, and mutations that lead to the formation of new beneficial properties are considered positive.
However, in relation to most of the diseases that people suffer today, it is not a disease that is inherited, but only a predisposition. For example, in the father of a child, sugar is absorbed slowly. This does not mean that the child will be born with diabetes, but the child will have a predisposition. This means that if a child abuses sweets and flour products, then he will develop diabetes.
Today, the so-called predictive medicine is developing. As part of this medical practice, predispositions are identified in a person (based on the identification of the corresponding genes), and then recommendations are given to him - what diet to follow, how to properly alternate work and rest regimens so as not to get sick.
Sources of human diversity
Genes carry plans (or "drawings") of both common traits inherent in all people, and numerous individual differences. They determine the specific characteristics that distinguish a person from other living beings in such areas as the size and shape of the body, behavior and aging, at the same time determining those unique features that distinguish us from each other. Based on this, a blue-eyed blond weighing 80 kilograms with slightly protruding ears and an infectious smile, masterfully playing jazz on the trombone, can be considered one of a kind.
Human life begins with a single fertilized cell - the zygote. After the sperm enters the egg, the pronucleus of the egg, containing 23 chromosomes (literally, “painted bodies”), moves to its center in a few hours. Here it fuses with the sperm pronucleus, which also contains 23 chromosomes. Thus, the formed zygote contains 23 pairs of chromosomes (46 chromosomes in total), half from each of the parents, the amount necessary for a normal child to be born.
Zygote- the first cell of a human being, appearing as a result of - fertilization.
After the formation of a zygote, the process of cell division begins. As a result of the first crushing, two daughter cells appear, identical in their organization to the original zygote. In the course of further cell division and differentiation, each newly formed cell contains exactly the same number of chromosomes as any other, that is, 46. Each chromosome consists of many genes arranged in a chain. According to experts, the number of genes in one chromosome reaches tens of thousands, which means that in all 16 chromosomes there are about a million of them (Kelly, 1986). Nine months after conception, the zygote develops into a newborn baby with ten trillion cells organized into organs and systems. Upon reaching adulthood, there are already more than 300 trillion cells in his body. Each 13 of them contains the complete genetic code of the individual.
Genes are built from DNA (deoxyribonucleic acid) - a huge molecule consisting of carbon, hydrogen, oxygen, nitrogen and phosphorus atoms. “There are so many DNA molecules in the human body that if you stretch them in a line, its length will exceed twice the distance from the Earth to the Moon by 20 thousand times” (Rugh & Shettles, 1971, p. 199). The structure of DNA resembles a long spiral staircase, the side railings of which are made of alternating phosphates and sugars, and the steps are made of four types of nitrogenous bases, connected in pairs in a regular way. The order of these paired bases changes, and it is these variations that cause one gene to differ from another. A single gene is part of this DNA ladder, which can be up to 2,000 steps long in its helix (Kelly, 1986).
Watson and Crick (1953) suggested that at the moment when the cell is ready to divide, the DNA helix unwinds, and two long chains diverge in different directions, separating from each other due to the breaking of bonds between paired nitrogenous bases. Then each chain, attracting new material from the cell, synthesizes a second chain and forms a new molecule, changing the amount or structure of DNA. Mutations, or rearrangements, can occur from time to time in these long strands of nucleic acid. In most cases, such rearrangements lead to the death of the protein (and, consequently, the cell), but a small number of mutants survive and further affect the body.
Mutation- a change in the amount or structure of DNA, and hence the genetic code.
DNA contains the genetic code, or blueprint, that governs how an organism functions and develops. However, this plan, listing all the objects and the exact dates for their construction, is locked in the nucleus of the cell and is inaccessible to those of its elements that are assigned to build the body. RNA (ribonucleic acid) - a substance formed from and similar to DNA - acts as a messenger between the nucleus and the rest of the cell. If DNA is the "what" and "when", then RNA is the "how" of development. Shorter RNA chains, which are mirror images of sections of the DNA molecule, move freely inside the cell and serve as a catalyst for the formation of new tissue.
Viruses
About 1% of the human genome is occupied by built-in retrovirus genes (endogenous retroviruses). These genes usually do not benefit the host, but there are exceptions. So, about 43 million years ago, retroviral genes that served to build the envelope of the virus got into the genome of the ancestors of monkeys and humans. In humans and monkeys, these genes are involved in the work of the placenta.
Most retroviruses integrated into the genome of human ancestors over 25 million years ago. Among the younger human endogenous retroviruses, no useful ones have been found so far.
Loading...