"Gene", "genome", "chromosome" - words that are familiar to every student. But the idea of this issue is quite generalized, as deepening in biochemical wilds requires special knowledge and a desire to understand all this. And if it is present at the level of curiosity, it 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 particle of information about heredity in living organisms. In fact, it is a small section of DNA that contains knowledge about a certain sequence of amino acids for constructing a protein or functional RNA (with which protein will also be synthesized). A gene determines those characters that will be inherited and transmitted by descendants further down the genealogical chain. In some unicellular organisms, there is a gene transfer that 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 the organism as a whole will look and work. They rule our lives from the moment of conception to the very 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 of pea crosses. 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 laws that formed the beginning of genetics as a science. 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 existing signs. Fortunately, there are more options than living creatures on the planet, and it is impossible to find two completely 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 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, Morgan, a geneticist from the United States of America, showed that they are on the chromosomes.
More than a century and a half have passed since then, and research has progressed further than Mendel could have imagined. At the moment, scientists have settled on the opinion that the information in the genes determines the growth, development and functions of living organisms. Or maybe even their death.
Classification
The structure of the gene contains not only information about the protein, but also instructions on when and how to read it, as well as blank areas necessary to separate information about different proteins and stop the synthesis of the information molecule.
There are two forms of genes:
- Structural - they contain information about the structure of proteins or RNA chains. The nucleotide sequence corresponds to the arrangement of amino acids.
- Functional genes are responsible for the correct structure of all other sections of DNA, for the synchronism and sequence of its reading.
Today, 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. The genome is called the smallest structural unit, but it can change a person’s life.
Mutations
A random or deliberate change in the sequence of nucleotides within a DNA strand 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.
Mutations themselves can be pathogenic, that is, manifest as diseases, or fatal, not allowing the body to develop to a viable state. But most of the changes pass unnoticed by humans. Deletions and duplications are constantly performed inside the DNA, but do not affect the course of life of each individual individual.
Deletion is the loss of a region of a chromosome that contains certain information. Sometimes such changes are useful for the body. They help him protect himself from external aggression, such as human immunodeficiency virus and plague bacteria.
Duplication is the doubling of a chromosome, and therefore the combination of genes that it contains also doubles. Due to the repetition of information, it is less prone to selection, which means it can accumulate mutations and change the body faster.
Gene properties
Every person has a huge DNA molecule. Genes are functional units in its structure. But even such small areas have their own unique properties, allowing to maintain the stability of organic life:
- Discreteness - the ability of genes to not mix.
- Stability - maintaining the structure and properties.
- Lability - the ability to change under the influence of circumstances, to adapt to hostile conditions.
- Multiple allelism is the existence of genes within DNA that, coding for the same protein, have a different structure.
- Allelicity - the presence of two forms of one gene.
- Specificity - one trait = one gene that is inherited.
- Pleiotropy is the multiplicity of effects of one gene.
- Expressivity is the severity of a trait that is encoded by a given gene.
- Penetrance - the frequency of occurrence of a gene in a genotype.
- Amplification is the appearance of a significant number of copies of a gene in DNA.
Genome
The human genome is all the hereditary material that is in a single human cell. It contains instructions on the construction of the body, the work of organs, physiological changes. The second definition of the term reflects the structure of a concept, not a function. The human genome is a combination of genetic material packed in a haploid set of chromosomes (23 pairs) and related to a specific 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: encoded information about the structure of intermediary molecules (the 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 during the development of the organism. Genes themselves occupy a small part of the genome, but at the same time are its basis. The information recorded in the genes is a kind of instruction for the manufacture of proteins, the main building blocks of our body.
However, for a complete characterization of the genome, information about the structure of proteins that is not inherent in it is not enough. We also need data on the elements of the genetic apparatus that take part in the work of genes, regulate their manifestation at different stages of development and in different life situations.
But even this is not enough to fully determine the genome. After all, it also contains elements that contribute to its self-reproduction (replication), compact packaging of DNA in the nucleus, and some still obscure areas that are sometimes called “selfish” (that is, as if serving only for themselves). For all these reasons, at the moment when it comes to the genome, they usually mean the whole set of DNA sequences represented in the chromosomes of the nuclei of cells of a certain type of organism, including, of course, genes.
Genome Size and Structure
It is logical to assume that the gene, genome, chromosome are different in different representatives of life on Earth. They can be either infinitely small or huge and contain billions of gene pairs. The structure of the gene will also depend on whose genome you are researching.
According to the ratio between the size of the genome and the number of genes included in it, two classes can be distinguished:
- Compact genomes with no more than ten million bases. In them, the totality of genes strictly correlates with size. Most characteristic of viruses and prokaryotes.
- Vast 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 about 28 thousand genes are in the human genome. They are unevenly distributed over the chromosomes, but the significance of this trait remains a mystery to scientists.
Chromosomes
Chromosomes are a way of packaging genetic material. They are located in the nucleus of each eukaryotic cell and consist of one very long DNA molecule. They can easily be seen in a light microscope during the division process. A karyotype is a complete set of chromosomes that is specific for each individual species. Mandatory elements for them are centromere, telomeres and replication points.
Changes in chromosomes during cell division
A gene, a genome, a chromosome are sequential links in a chain of information transfer, where each 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 divisions), the chromosomes in the nucleus are loose, occupy a lot of space.
When the cell prepares for mitosis (i.e., the process of separation in two), chromatin is compacted and twisted into chromosomes, and now it becomes visible under a light microscope. In the metaphase, chromosomes resemble sticks closely spaced to each other and connected by a primary constriction, or centromere. It is she who is responsible for the formation of the spindle of division, when groups of chromosomes line up. Depending on the location of the centromere, there is such a classification of chromosomes:
- Acrocentric - in this case, the centromere is located polar to the center of the chromosome.
- Submetacentric, when the shoulders (that is, the areas located before and after the centromere) are of unequal length.
- 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.
Chromosome abnormalities
As with other morphological elements of a living organism, structural changes can also occur with chromosomes that affect their functions:
- Aneuploidy. This is a change in the total number of chromosomes in the karyotype due to the addition or removal of one of them. The consequences of such a mutation can be fatal for an unborn fetus, as well as lead to birth defects.
- Polyploidy. It manifests itself in the form of an increase in the number of chromosomes, a multiple of half their number. Most often found in plants, such as algae, and fungi.
- Chromosomal aberrations, or rearrangements, 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, as well as the biological mechanisms that provide them. Unlike many other biological sciences, since its inception, it has 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 - the ability of an organism to ensure the continuity of morphological, biochemical and physiological signs and features in a series of generations . 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, some features of metabolic processes, temperament, etc.), but also the physico-chemical characteristics of the structure and functioning of the main biopolymers of the cell. Variability - a variety of characters among representatives of a certain species, as well as the ability of descendants to acquire differences from parental forms. Variability together with heredity are two inseparable properties of living organisms.
Down Syndrome
Down syndrome is a genetic disease in which the karyotype consists of 47 chromosomes in humans instead of the usual 46. This is one of the forms of aneuploidy, which was mentioned above. In the twenty-first pair of chromosomes, an additional one appears that brings extra genetic information to the human genome.
The syndrome was named after the doctor, Don Down, who discovered and described it in the 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 of 47 chromosomes in humans occurs once per thousand newborns (previously statistics were different). This was made possible thanks to the early diagnosis of this pathology. The disease does not depend on the race, ethnicity of the mother or her social status. Affects age. The chances of having a baby with Down Syndrome increase after thirty-five years, and after forty the ratio of healthy children to sick is already 20 to 1. The age of a father older than forty years also increases the chances of having a baby with aneuploidy.
Forms of Down Syndrome
The most common option is the appearance of an additional chromosome in the twenty-first pair along a non-hereditary path. It is due to the fact that during meiosis this pair does not diverge along the division spindle. Five percent of patients have mosaicism (an additional chromosome is not found 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 the same family is insignificant.
Clinic
A person with Down syndrome can be recognized by characteristic external signs, here are some of them:
- flattened face;
- shortened skull (transverse dimension greater than longitudinal);
- skin fold on the neck;
- a fold of skin that covers the inner corner of the eye;
- excessive mobility of the joints;
- reduced muscle tone;
- flattening of the nape;
- short limbs and fingers;
- development of cataracts in children over eight years old;
- Anomalies in the development of teeth and hard palate;
- congenital heart defects;
- the presence of an epileptic syndrome is possible;
- leukemia.
But a definite diagnosis, based only on external manifestations, of course, 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 way. But in fact, they strongly affect our lives and, changing, make us change too. A person knows how to adapt to circumstances, whatever they may be, and even for people with genetic anomalies, there will always be time and a place where they will be indispensable.