After the discovery of the principle of molecular organization of such a substance as DNA in 1953, molecular biology began to develop. Further, in the process of research, scientists found out how DNA is recombined, its composition and how our human genome works.
Every day at the molecular level, complex processes occur. What is the structure of a DNA molecule? What does it consist of? And what role do DNA molecules play in the cell? We will tell in detail about all the processes occurring inside the double chain.
What is hereditary information?
So where did it all start? Even in 1868, nucleic acids were found in the nuclei of bacteria. And in 1928, N. Koltsov put forward the theory that all genetic information about a living organism is encrypted in DNA. Then J. Watson and F. Crick found a model for the now-famous DNA helix in 1953, for which they deservedly received recognition and award - the Nobel Prize.
What is DNA in general? This substance consists of 2 combined threads, more precisely spirals. A section of such a chain with certain information is called a gene.
All information about what proteins will be formed and in what order is stored in DNA. A DNA macromolecule is a material carrier of incredibly voluminous information, which is recorded by a strict sequence of individual bricks - nucleotides. There are 4 nucleotides in total; they complement each other chemically and geometrically. This principle of complementation, or complementarity, in science will be described later. This rule plays a key role in encoding and decoding genetic information.
Since the DNA strand is incredibly long, there are no repetitions in this sequence. Each living creature has its own unique DNA strand.
DNA functions
The functions of deoxyribonucleic acid include the storage of hereditary information and its transmission to offspring. Without this function, the genome of a species could not be preserved and developed over millennia . Organisms that have undergone serious gene mutations often do not survive or lose their ability to produce offspring. This is how natural protection against degeneration of the species occurs.
Another essential function is the implementation of stored information. A cell cannot create a single vital protein without instructions that are stored in a double chain.
The composition of nucleic acids
Now it is already reliably known what the nucleotides themselves are made of - the bricks of DNA. They include 3 substances:
- Orthophosphoric acid.
- Nitrogen base. Pyrimidine bases - which have only one ring. These include thymine and cytosine. Purine bases, which contain 2 rings. These are guanine and adenine.
- Sucrose. DNA contains deoxyribose, RNA contains ribose.
The number of nucleotides is always equal to the number of nitrogenous bases. In special laboratories, the nucleotide is cleaved and a nitrogenous base is isolated from it. So study the individual properties of these nucleotides and possible mutations in them.
Levels of organization of hereditary information
There are 3 levels of organization: gene, chromosomal and genomic. All the information needed for the synthesis of a new protein is contained in a small section of the chain - the gene. That is, the gene is considered the lowest and easiest level of information encoding.
Genes, in turn, are assembled into chromosomes. Thanks to such an organization of the carrier of the hereditary material, the groups of characters, according to certain laws, alternate and are transmitted from one generation to another. It should be noted that there are an incredible amount of genes in the body, but information is not lost, even when it is recombined many times.
Several types of genes are shared:
- 2 types are distinguished according to their functional purpose: structural and regulatory sequences;
- according to the influence on the processes occurring in the cell, they distinguish: supervital, lethal, conditionally lethal genes, as well as mutator and antimutator genes.
Genes are located along the chromosome in a linear order. In chromosomes, information is not focused randomly; there is a certain order. There is even a map in which the positions or loci of genes are displayed. For example, it is known that in chromosome No. 18 data on the color of the eyes of a child is encrypted.
But what is a genome? This is the name of the entire set of nucleotide sequences in the cell of the body. The genome characterizes the whole species, not a single individual.
What is the human genetic code?
The fact is that the whole tremendous potential of human development is already in place at the time of conception. All hereditary information that is necessary for the development of the zygote and the growth of the baby after birth is encrypted in the genes. Sections of DNA are the most basic carriers of hereditary information.
A person has 46 chromosomes, or 22 somatic pairs, plus one sex-determining chromosome from each parent. This diploid set of chromosomes encodes the entire physical appearance of a person, his mental and physical abilities, and his predisposition to diseases. Somatic chromosomes are outwardly indistinguishable, but they carry different information, since one of them is from the father, the other from the mother.
The male code differs from the female one in the last pair of chromosomes - XU. The female diploid set is the last pair, XX. Men get one X-chromosome from a biological mother, and then it is passed on to daughters. The sexual U-chromosome is passed on to sons.
Human chromosomes vary widely in size. For example, the smallest pair of chromosomes is No. 17. And the largest pair is 1 and 3.
The diameter of the double helix in humans is only 2 nm. DNA is so tightly twisted that it fits in the small nucleus of the cell, although its length will reach 2 meters if it is untwisted. The length of the helix is hundreds of millions of nucleotides.
How is the genetic code transmitted?
So, what role do DNA molecules play in the cell during fission? Genes - carriers of hereditary information - are inside every cell in the body. In order to transfer their code to a subsidiary organism, many creatures divide their DNA into 2 identical helices. This is called replication. In the process of DNA replication, it unwinds and special “machines” complete each chain. After the genetic spiral bifurcates, the nucleus and all the organelles begin to divide, and then the whole cell.
But a person has a different process of gene transfer - sexual. The signs of the father and mother are mixed, the new genetic code contains information from both parents.
Storage and transmission of hereditary information is possible due to the complex organization of the DNA helix. After all, as we said, the structure of proteins is encrypted precisely in the genes. Once created during conception, this code will copy itself throughout life. A karyotype (a personal set of chromosomes) does not change during organ cell renewal. The transmission of information is carried out using sex gametes - male and female.
Only viruses containing one strand of RNA are not capable of transmitting their information to offspring. Therefore, in order to reproduce, they need human or animal cells.
Implementation of hereditary information
Important processes are constantly taking place in the cell nucleus. All information recorded on the chromosomes is used to construct proteins from amino acids. But the DNA strand never leaves the nucleus, so the help of another important compound = RNA is needed here. Just RNA is able to penetrate through the membrane of the nucleus and interact with the DNA chain.
Through the interaction of DNA and 3 types of RNA, all encoded information is realized. At what level is the implementation of hereditary information? All interactions occur at the nucleotide level. Informational RNA copies a portion of the DNA strand and brings this copy to the ribosome. Here begins the synthesis of a new molecule from the nucleotides.
In order for mRNA to copy the necessary part of the chain, the helix unfolds, and then, upon completion of the transcoding process, is restored again. Moreover, this process can occur simultaneously on 2 sides of 1 chromosome.
Principle of complementarity
DNA helices are composed of 4 nucleotides - this is adenine (A), guanine (G), cytosine (C), thymine (T). They are connected by hydrogen bonds according to the rule of complementarity. The work of E. Chargaff helped establish this rule, since the scientist noticed some patterns in the behavior of these substances. E. Chargaff discovered that the molar ratio of adenine to thymine is unity. And in the same way, the ratio of guanine to cytosine is always equal to unity.
Based on his work, genetics formed the rule of nucleotide interaction. The rule of complementarity states that adenine binds only to thymine, and guanine to cytosine. During the decoding of the helix and the synthesis of a new protein in the ribosome, this alternation rule helps to quickly find the necessary amino acid that is attached to the transport RNA.
RNA and its types
What is hereditary information? This is a nucleotide sequence in a double strand of DNA. What is RNA? What is her job? RNA, or ribonucleic acid, helps to extract information from DNA, decode it and, based on the principle of complementarity, create proteins necessary for cells.
In total, 3 types of RNA are isolated. Each of them strictly fulfills its function.
- Information (mRNA) , or else it is called matrix. She goes straight to the center of the cell, to the nucleus. Finds in one of the chromosomes the necessary genetic material for building protein and copies one of the sides of the double chain. Copying occurs again on the principle of complementarity.
- A transport molecule is a small molecule that has decoder nucleotides on one side and amino acids corresponding to the main code on the other side. The task of tRNA is to deliver it to the “workshop”, that is, to the ribosome, where it synthesizes the necessary amino acid.
- rRNA is ribosomal. It controls the amount of protein that is produced. Consists of 2 parts - amino acid and peptide sites.
The only difference when decoding is that RNA does not have thymine. Instead of thymine, there is uracil. But then, in the process of protein synthesis, with TNRNA it still correctly sets all the amino acids. If there are any malfunctions in the decoding of information, then a mutation occurs.
Repair damaged DNA molecule
The process of repairing a damaged double chain is called repair. During the repair process, damaged genes are removed.
Then the necessary sequence of elements is exactly reproduced and cut back into the same place on the chain from where it was extracted. All this happens thanks to special chemicals - enzymes.
Why do mutations occur?
Why do some genes begin to mutate and cease to fulfill their function - the storage of vital hereditary information? This is due to a decoding error. For example, if adenine is accidentally replaced by thymine.
There are also chromosomal and genomic mutations. Chromosomal mutations occur if parts of the hereditary information drop out, double, or even be transferred and integrated into another chromosome.
Genomic mutations are the most serious . Their reason is a change in the number of chromosomes. That is, when instead of a pair - diploid set, a triploid set is present in the karyotype.
The most famous example of triploid mutation is Down syndrome, in which a personal set of chromosomes is 47. In such children, 3 chromosomes are formed in place of the 21st pair.
A mutation such as polyplodia is also known. But polyplodia is found only in plants.