HealthMedicine

Gene, genome, chromosome: definition, structure, functions

"Gene", "genome", "chromosome" - words that are familiar to every schoolboy. But the idea of this issue is quite generalized, since for the deepening into the biochemical jungle special knowledge and the desire to understand all this are required. And it, if 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 scientific and polar form.

What is a gene?

A gene is the smallest structural and functional particle of information about heredity in living organisms. In fact, it is a small piece of DNA, which contains knowledge of a certain sequence of amino acids for the construction of a protein or functional RNA (with which the protein will also be synthesized). The gene defines those attributes that will be inherited and transmitted by descendants further along the genealogical chain. In some unicellular organisms, there is a transfer of genes, which is not related to the reproduction of their own kind, it is called horizontal.

"On the shoulders" of genes is a huge responsibility for how each cell and organism will look and work as a whole. They control our life from conception to the last breath.

The first scientific step forward in the study of heredity was made by the Austrian monk Gregor Mendel, who in 1866 published his observations on the results when crossing peas. Hereditary material, which he used, clearly showed the patterns of the transmission of features, 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 a half of all their chromosomes. Thus, the signs of mom and dad, mixing, form a new combination of already existing characteristics. Fortunately, there are more options than living beings on the planet, and it is impossible to find two absolutely identical creatures.

Mendel showed that hereditary makings do not mix, but are transferred from parents to descendants in the form of discrete (isolated) units. These units, represented in pairs by pairs (alleles), remain discrete and are transmitted to subsequent generations in male and female gametes, each of which contains one unit from each pair. In 1909, the Danish botanist Johansen called these units genes. In 1912, a geneticist from the United States of America Morgan showed that they are in chromosomes.

Since then, more than one and a half centuries have passed, and research has progressed further than Mendel could have imagined. At the moment, scientists have settled on the opinion that information in the genes determines the growth, development and functions of living organisms. And maybe even their death.

Classification

The structure of the gene contains not only information about the protein, but also indications when and how to read it, as well as the empty areas necessary to separate information about different proteins and stop the synthesis of the information molecule.

There are two forms of genes:

  1. Structural - they contain information about the structure of proteins or RNA chains. The sequence of nucleotides corresponds to the arrangement of amino acids.
  2. Functional genes are responsible for the correct structure of all other parts of DNA, for the synchrony and sequence of its reading.

To date, scientists can answer the question: how many genes are on the chromosome? The answer will surprise you: about three billion pairs. And this is only in one of twenty-three. A genome is the smallest structural unit, but it can change a person's life.

Mutations

A random or targeted change in the sequence of nucleotides entering the DNA chain is called a mutation. It can practically not affect the structure of the protein, but can completely distort its properties. So, there will be local or global consequences of such a change.

By themselves, mutations can be pathogenic, that is, manifest themselves in the form of diseases, or lethal, not allowing the body to develop to a viable state. But most of the changes go unnoticed for a person. Deletions and duplications are constantly committed within DNA, but do not affect the course of life of each individual individual.

Deletion is the loss of a region of the chromosome that contains certain information. Sometimes such changes are beneficial to the body. They help him to protect himself from external aggression, such as the human immunodeficiency virus and the plague bacteria.

Duplication is the doubling of the chromosome region, which means that the set of genes that it contains also doubles. Because of the repetition of information, it is less susceptible to breeding, which means it can quickly accumulate mutations and change the body.

Properties of the gene

Each person has a huge DNA molecule. Genes are functional units in its structure. But even such small plots have their own unique properties that allow preserving the stability of organic life:

  1. Discreteness - the ability of genes not to mix.
  2. Stability - the preservation of structure and properties.
  3. Lability - the ability to change under the influence of circumstances, adapt to hostile conditions.
  4. Multiple allelicism is the existence within the DNA of genes, which, encoding the same protein, have a different structure.
  5. Allelicity - the presence of two forms of a single gene.
  6. Specificity is one attribute = one gene transmitted by inheritance.
  7. Pleiotropy is the multiplicity of the effects of one gene.
  8. Expressiveness - the degree of expression of the trait, which is encoded by this gene.
  9. Penetrence - the frequency of occurrence of a gene in a genotype.
  10. Amplification - the appearance of a significant number of copies of the gene in the DNA.

Genome

The human genome is all hereditary material that is in a single human cell. It contains instructions on the construction of the body, the work of organs, and physiological changes. The second definition of this term reflects the structure of a concept, not a function. A human genome is a collection of genetic material packed in a haploid set of chromosomes (23 pairs) and belonging to a particular species.

The basis of the genome is a molecule of deoxyribonucleic acid, well known as DNA. All genomes contain at least two types of information: coded information about the structure of intermediary molecules (so-called RNA) and protein (this information is contained in the genes), as well as instructions that determine the time and place of manifestation of this information in the development of the organism. The genes themselves occupy a small part of the genome, but they are the basis of it. The information recorded in the genes is a kind of instruction for making proteins, the main building blocks of our body.

However, for the complete characterization of the genome, there is not enough information about the structure of proteins in it. We also need data on the elements of the genetic apparatus that participate in the work of genes, regulate their manifestation at different stages of development and in different life situations.

But even this is not enough for a complete definition of the genome. After all, there are also elements in it that promote its self-reproduction (replication), compact packing of DNA in the nucleus, and some other incomprehensible areas, sometimes called "selfish" (that is, supposedly serving only for themselves). For all these reasons, at the present moment, when it comes to the genome, they usually mean the entire set of DNA sequences present in the chromosomes of the cell nuclei of a particular species of organism, including, of course, genes.

The size and structure of the genome

It is logical to assume that the gene, genome, and chromosome differ in different representatives of life on Earth. They can be both infinitesimal and huge and contain in themselves billions of pairs of genes. The structure of the gene will also depend on whose genome you are researching.

By the ratio between the size of the genome and the number of genes entering into it, two classes can be distinguished:

  1. Compact genomes that have no more than ten million bases. At them the set of genes strictly correlates with the size. The most characteristic for viruses and prokaryotes.
  2. Extensive genomes consist of more than 100 million base pairs that do not have a relationship between their length and the number of genes. More common in eukaryotes. Most nucleotide sequences in this class do not encode proteins or RNA.

Studies have shown that there are about 28,000 genes in the human genome. They are unevenly distributed across chromosomes, but the significance of this trait remains a mystery for scientists.

Chromosomes

Chromosomes are a way of packaging genetic material. They are in the nucleus of each eukaryotic cell and consist of one very long DNA molecule. They can easily be seen in a light microscope in the process of division. A karyotype is a complete set of chromosomes, which is specific for each individual species. Mandatory elements for them are centromeres, telomeres and replication points.

Changes in chromosomes in the process of cell division

The gene, the genome, the chromosome are the successive links in the information transfer chain, where each of the following includes the previous one. But they also undergo certain changes in the process of cell life. So, for example, in the interphase (the period between the divisions) the chromosomes in the nucleus are loosely located, taking up a lot of space.

When the cell prepares for mitosis (that is, to the division process in two), the chromatin is compacted and twisted into the chromosomes, and now it becomes visible in a light microscope. In metaphase, chromosomes resemble sticks closely located to each other and connected by a primary constriction, or by a centromere. It is she who is responsible for the formation of the spindle of division, when the groups of chromosomes line up. Depending on the location of the centromere, there is such a classification of chromosomes:

  1. Acrocentric - in this case the centromere is located polarly with respect to the center of the chromosome.
  2. Submetacentric, when the shoulders (that is, the areas before and after the centromere) are unequal in length.
  3. Metacentric, if the centromere divides the chromosome exactly in the middle.

This classification of chromosomes was proposed in 1912 and is used by biologists until today.

Anomalies of chromosomes

As with other morphological elements of a living organism, chromosomes can also undergo structural changes that affect their functions:

  1. Aneuploidy. This change in the total number of chromosomes in the karyotype by adding or removing one of them. The consequences of such a mutation can be lethal for an unborn fetus, as well as lead to birth defects.
  2. Polyploidy. It appears as an increase in the number of chromosomes, a multiple of half of their number. It is most often found in plants, such as algae, and fungi.
  3. Chromosomal aberrations, or perestroika, are changes in the structure of chromosomes under the influence of environmental factors.

Genetics

Genetics is a science that studies the laws of heredity and variability, and also provides biological mechanisms for them. Unlike many other biological sciences, it has since its inception sought to be an exact science. The whole history of genetics is the history of the creation and use of more and more accurate methods and approaches. Ideas and methods of genetics play an important role in medicine, agriculture, genetic engineering, microbiological industry.

Heredity is the ability of the organism to provide, in a series of generations, the continuity of morphological, biochemical and physiological signs and characteristics. In the process of inheritance, the main species-specific, group (ethnic, population) and family features of the structure and functioning of organisms, their ontogenesis (individual development) are reproduced. Inherited not only certain structural and functional characteristics of the body (facial features, certain features of metabolic processes, temperament, etc.), but also physicochemical features of the structure and functioning of the basic biopolymers of the cell. Variability is the diversity of characteristics among representatives of a particular species, and the property of descendants to acquire differences from parental forms. Variability, together with heredity, are two inseparable properties of living organisms.

Down Syndrome

Down's syndrome is a genetic disease in which a karyotype consists of 47 chromosomes in a human instead of the usual 46. This is one of the forms of aneuploidy mentioned above. In the twenty-first pair of chromosomes, an additional one appears, which introduces extra genetic information into the human genome.

The name of his syndrome was in honor of the doctor, Don Down, who discovered and described it in literature as a form of mental disorder in 1866. But the genetic background was discovered almost a hundred years later.

Epidemiology

At the moment, the karyotype in 47 chromosomes in humans occurs once per thousand newborns (previously the statistics was different). This became possible due to early diagnosis of this pathology. The disease does not depend on the race, the ethnicity of the mother or her social status. Influences age. The chances of having a baby with Down's syndrome increase after thirty-five years, and after forty the ratio of healthy children to the sick is 20 to 1. The age of the father over the age of forty also increases the chances of having a child with aneuploidy.

Forms of Down's Syndrome

The most frequent option is the appearance of an additional chromosome in the twenty-first pair along the non-hereditary pathway. It is due to the fact that during the meiosis this pair does not divide by the fission spindle. In five percent of cases, there is mosaicism (an additional chromosome is not contained in all cells of the body). Together they make up ninety-five percent of the total number of people with this congenital pathology. In the remaining five percent of cases, the syndrome is caused by hereditary trisomy of the twenty-first chromosome. However, the birth of two children with this disease in one family is insignificant.

Clinic

A person with Down syndrome can be recognized by their characteristic external features, here are some of them:

- flattened face;
- shortened skull (transverse dimension greater than longitudinal);
- skin fold on the neck;
- a fold of the skin that covers the inner corner of the eye;
- excessive mobility of the joints;
- decreased muscle tone;
- flattening of the occiput;
- short limbs and fingers;
- development of cataract in children older than eight years;
- anomalies in the development of teeth and hard palate;
- congenital heart diseases;
- the presence of an epileptic syndrome is possible;
- leukemia.

But unequivocally to diagnose, being based only on external displays, certainly, it is impossible. It is necessary to carry out karyotyping.

Conclusion

Gene, genome, chromosome - it seems that these are just words, the meaning of which we understand in a generalized and very distant manner. But in fact they strongly influence our life and, changing, make us change too. A person knows how to adapt to circumstances, whatever they turn out to be, and even for people with genetic abnormalities there is always time and place where they will be irreplaceable.

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