Heredity and variability in wildlife exist thanks to chromosomes, genes, deoxyribonucleic acid (DNA). Genetic information is stored and transmitted in the form of a chain of nucleotides in DNA. What is the role of genes in this phenomenon? What is a chromosome in terms of the transmission of hereditary traits? Answers to such questions allow us to understand the principles of coding and genetic diversity on our planet. In many ways, it depends on how many chromosomes are included in the set, on the recombination of these structures.
From the history of the discovery of "heredity particles"
Studying the cells of plants and animals 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 W alter Flemming is called the discoverer of chromosomes. It was he who used aniline dyes to process nuclear structures. Flemming called the discovered substance "chromatin" for its ability to stain. The term "chromosomes" was coined by Heinrich Waldeyer in 1888.
At the same time as Flemming, I was looking for an answer to the question of what ischromosome, Belgian Edouard van Beneden. A little earlier, German biologists Theodor Boveri and Eduard Strasburger conducted a series of experiments proving the individuality of chromosomes, the constancy of their number in different types of living organisms.
Prerequisites for the chromosome theory of heredity
American researcher W alter Sutton found out how many chromosomes are contained in the cell nucleus. The scientist considered these structures to be carriers of units of heredity, signs of an organism. Sutton discovered that chromosomes are made up of genes that transmit properties and functions from parents to offspring. The geneticist in his publications gave descriptions of chromosome pairs, their movement in the process of division of the cell nucleus.
Regardless of the American colleague, Theodore Boveri led the work in the same direction. Both researchers in their writings studied the issues of transmission of hereditary traits, 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. An outstanding American biologist and his assistants established a number of regularities in the placement of genes in a chromosome, developed a cytological base 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 threads are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). Study undermicroscope made it possible to establish what a chromosome is from a morphological point of view. This is a mobile thread-like body, which is distinguishable during certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. In other periods, chromosomes are distinguishable in the form of one or two chromatids.
These formations are better seen during cell divisions - mitosis or meiosis. In eukaryotic cells, large linear chromosomes are more often observed. They are smaller in prokaryotes, although there are exceptions. Cells often contain more than one type of chromosome, for example, mitochondria and chloroplasts have their own small "heredity particles".
Shapes of chromosomes
Each chromosome has an individual structure, differs from other staining 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 (constriction is shifted towards one of the telomeres);
- acrocentric, or rod-shaped, in them the centromere is located almost at the end of the chromosome;
- dot with hard-to-define shape.
Chromosome Functions
Chromosomes consist of genes - the functional units of heredity. Telomeres are the ends of the chromosome arms. These specialized elements serve to protect against damage, preventsticking of fragments. The centromere performs its tasks when the chromosomes are duplicated. It has a kinetochore, it is to it that the fission spindle structures are attached. Each pair of chromosomes is individual in the location of the centromere. The spindle fibers of division work in such a way that one chromosome leaves for daughter cells, and not both. Uniform doubling in the process of division is provided by the points of origin of replication. The duplication of each chromosome begins simultaneously at several such points, which significantly speeds up the entire process of division.
The role of DNA and RNA
It was possible to find out what a chromosome is, 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:
- deoxyribonucleic acid (DNA);
- ribonucleic acid (RNA);
- Histone proteins.
Nucleic acids are directly involved in the biosynthesis of amino acids and proteins, ensure the transmission of hereditary traits 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 are a deoxyribose carbohydrate, a phosphate group, and one of four nitrogenous bases:
- A- adenine.
- G - guanine.
- T - thymine.
- C - cytosine.
Segments of helical deoxyribonucleoprotein filaments are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary traits are passed 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 30,000 genes.
Set of chromosomes
The total number of chromosomes, their characteristics - a characteristic feature of the species. In fruit flies, their number is 8, in primates - 48, in humans - 46. This number is constant for 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, in contrast to the 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 consider, study the distinctive features of each pair. The diploid set of chromosomes is present in somatic cells, while the haploid set is present in the sex (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 Y. Males haveset XY, females - 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. The 23rd pair differs from them - the sex chromosomes, which are non-homologous in the cells of the male body.
Genetic traits are linked to gender. They are transmitted by a Y and an X chromosome in men, two Xs 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 well 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 stretched DNA is 1.5 cm long and has 48 million 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 nucleus 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 the chromosomes. Down syndrome is one of them. A child with such a genetic disease is characterized by a lag in mental and physical development. With cystic fibrosis, there is a malfunction in the functions of the external secretion glands. Violation leads to problems with sweating, excretion and accumulationmucus in the body. It makes it difficult for the lungs to work, and can lead to suffocation and death.
Violation of color vision - color blindness - immunity to certain parts of the color spectrum. Hemophilia leads to a weakening of blood clotting. Lactose intolerance prevents the human body from absorbing milk sugar. In family planning offices, you can find out about the predisposition to a particular genetic disease. In large medical centers, it is possible to undergo appropriate examination and treatment.
Genotherapy is a direction of modern medicine, finding out the genetic cause of hereditary diseases and its elimination. Using the latest methods, normal genes are introduced into pathological cells instead of disturbed ones. In this case, doctors relieve the patient not of the symptoms, but of the causes that caused the disease. Only correction of somatic cells is carried out, methods of gene therapy are not yet applied en masse in relation to germ cells.
