Heredity and variability in wildlife exist due to chromosomes, genes, deoxyribonucleic acid (DNA). Genetic information in the form of a chain of nucleotides in DNA is stored and transmitted. What role does genes play in this phenomenon? What is a chromosome in terms of transmission of hereditary traits? Answers to such questions make it possible to understand the principles of coding and genetic diversity on our planet. In many respects, it depends on how many chromosomes are included in the set, on the recombination of these structures.
From the history of the discovery of "particles of heredity"
Studying under the microscope the cells of plants and animals, many botanists and zoologists as early as the middle of the 19th century drew attention to the finest threads and the smallest ring-shaped structures in the nucleus. More often than not, the German anatomist Walter Fleming is called the discoverer of chromosomes. It was he who applied aniline dyes to treat nuclear structures. Fleming called the discovered substance "chromatin" for its ability to stain. The term "chromosomes" in 1888 was introduced into the scientific circulation by Heinrich Waldeyer.
At the same time as Fleming, he was looking for the answer to the question of what is the chromosome, the Belgian Eduard van Beneden. A little earlier, German biologists Theodor Boveri and Eduard Strasburger conducted a series of experiments proving the identity of chromosomes, the constancy of their number in different species of living organisms.
Background of the chromosome theory of heredity
American researcher Walter Sutton found out how many chromosomes are in the cell nucleus. The scientist considered these structures as carriers of units of heredity, signs of the body. Sutton discovered that chromosomes are made up of genes through which properties and functions are passed on to offspring from their parents. The geneticist in his publications gave descriptions of chromosome pairs, their movements in the process of cell nucleus fission.
Regardless of the American colleague, Theodore Boveri worked in the same direction. Both researchers in their works studied the transmission of hereditary traits, formulated the main provisions on the role of chromosomes (1902-1903). Further development of the Bowery-Sutton theory took place in the laboratory of Nobel Laureate Thomas Morgan. An outstanding American biologist and his assistants established a number of patterns of gene placement on the chromosome, developed a cytological base that explains the mechanism of the laws of Gregor Mendel, the founding father of genetics.
Cell chromosomes
The study of the structure of chromosomes began after their discovery and description in the XIX century. These bodies and threads are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in the nuclei). A microscopic study revealed what a chromosome is from a morphological point of view. This is a mobile filiform body, which is distinguishable in certain phases of the cell cycle. In interphase, the entire core volume is occupied by chromatin. In other periods, chromosomes in the form of one or two chromatids are distinguishable.
These formations are better visible during cell divisions - mitosis or meiosis. In eukaryotic cells , large chromosomes of a linear structure can often be observed. Prokaryotes have fewer, although there are exceptions. Cells often include more than one type of chromosome, for example, their own small “particles of heredity” are present in mitochondria and chloroplasts.
Chromosome forms
Each chromosome has an individual structure, differs from other features of staining. When studying morphology, it is important to determine the position of the centromere, the length and placement of the shoulders relative to the constriction. A set of chromosomes usually includes the following forms:
- metacentric, or equal shoulders, which are characterized by the middle location of the centromere;
- submetacentric, or unequal shoulders (hauling is shifted towards one of the telomeres);
- acrocentric, or rod-shaped, in them the centromere is located almost at the end of the chromosome;
- point with a difficult to define shape.
Chromosome functions
Chromosomes consist of genes - functional units of heredity. Telomeres are the ends of the shoulders of the chromosome. These specialized elements are used to protect against damage and prevent fragments from sticking together. The centromere performs its tasks when doubling chromosomes. There is a kinetochore on it, it is to it that the structures of the division spindle are attached. Each pair of chromosomes is individual at the location of the centromere. The threads of the fission spindle work in such a way that one chromosome leaves for daughter cells, and not both. Uniform doubling during the division process provides the starting point for replication. Duplication of each chromosome begins simultaneously at several such points, which significantly accelerates the entire process of division.
The role of DNA and RNA
It was possible to find out what the 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, its composition includes high molecular weight organic substances:
- deoxyribonucleic acid (DNA);
- ribonucleic acid (RNA);
- histone proteins.
Nucleic acids take a direct part 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 carbohydrate deoxyribose, a phosphate group and one of four nitrogenous bases:
- A is adenine.
- G is guanine.
- T - thymine.
- Ts is cytosine.
Sections of helical deoxyribonucleoprotein filaments are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary traits from parents are transmitted to offspring in the form of gene alleles. They determine the functioning, growth and development of a particular organism. According to some researchers, those DNA sections that do not encode polypeptides perform regulatory functions. The human genome can number up to 30 thousand genes.
Chromosome set
The total number of chromosomes, their features - a characteristic feature of the species. In Drosophila flies, their number is 8, in primates - 48, in humans - 46. This number is constant for the cells of organisms that belong to the same species. For all eukaryotes, the concept of "diploid chromosomes" exists. This is a complete set, or 2n, in contrast to the haploid - half amount (n).
Chromosomes in one pair are homologous, identical in shape, structure, location of centromeres and other elements. Homologists have their own characteristics that distinguish them from other chromosomes in the set. Staining with basic dyes allows you to consider, explore the distinctive features of each pair. A diploid set of chromosomes is present in somatic cells, while a haploid set is found in reproductive (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 possess a set of XY, females have XX.
Human Chromosome Set
The cells of the human body contain 46 chromosomes. All of them are combined in 23 pairs that make up the set. There are two types of chromosomes: autosomes and genital. The first form 22 pairs - common for women and men. The 23rd pair - sex chromosomes, which in the cells of the male body are not homologous, differs from them.
Genetic traits are related to gender. For their transmission are the Y and X chromosome in men, two X in women. Autosomes contain the rest of the information on hereditary traits. There are techniques to individualize all 23 pairs. They are clearly 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 has a length of 1.5 cm and totals 48 million pairs of nitrogenous bases. Special histone proteins from chromatin perform compression, after which the filament occupies thousands of times less space in the cell nucleus. Under an electron microscope, the histones in the interphase nucleus resemble beads strung on a DNA strand.
Genetic diseases
There are more than 3 thousand hereditary diseases of various types, caused by damage and abnormalities in the chromosomes. These include Down syndrome. A child with such a genetic disease is characterized by a lag in mental and physical development. With cystic fibrosis, a malfunction in the functions of the endocrine glands occurs. Violation leads to problems with sweating, secretion and accumulation of mucus in the body. It complicates the work of the lungs, can lead to suffocation and death.
Color vision impairment - color blindness - immunity to some parts of the color spectrum. Hemophilia leads to a weakening of blood coagulation. Lactose intolerance does not allow the human body to absorb milk sugar. In family planning rooms, you can learn about a predisposition to a particular genetic disease. In large medical centers there is an opportunity to undergo appropriate examination and treatment.
Gene therapy - the 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 broken ones. In this case, doctors do not relieve the patient of the symptoms, but of the causes that caused the disease. Only the correction of somatic cells is carried out, the methods of gene therapy have not yet been applied massively to sex cells.