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, complex processes take place at the molecular level. How is the DNA molecule arranged, what does it consist of? What role do DNA molecules play in a cell? Let's talk in detail about all the processes taking place inside the double chain.
What is hereditary information?
So how did it all start? As early as 1868, nucleic acids were found in the nuclei of bacteria. And in 1928, N. Koltsov put forward the theory that it is in DNA that all genetic information about a living organism is encrypted. Then J. Watson and F. Crick found a model for the now well-known DNA helix in 1953, for which they deserved recognition and an award - the Nobel Prize.
What is DNA anyway? This substance is made up of 2combined threads, more precisely spirals. A section of such a chain with certain information is called a gene.
DNA stores all the information about what kind of proteins will be formed and in what order. A DNA macromolecule is a material carrier of incredibly voluminous information, which is recorded in a strict sequence of individual building blocks - 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.
Because the DNA strand is incredibly long, there are no repetitions in this sequence. Every living thing 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 major gene mutations are more likely to not survive or lose their ability to produce offspring. So there is a natural protection against the degeneration of the species.
Another essential function is the implementation of stored information. The cell cannot make any vital protein without the instructions stored in the double strand.
Composition of nucleic acids
Now it is already reliably known what they themselves consist ofNucleotides are the building blocks of DNA. They contain 3 substances:
- Orthophosphoric acid.
- Nitrogen base. Pyrimidine bases - which have only one ring. These include thymine and cytosine. Purine bases containing 2 rings. These are guanine and adenine.
- Sucrose. In DNA - deoxyribose, In RNA - ribose.
The number of nucleotides is always equal to the number of nitrogenous bases. In special laboratories, a nucleotide is cleaved and a nitrogenous base is isolated from it. So they 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 simplest level of encoding information.
Genes, in turn, are assembled into chromosomes. Thanks to such an organization of the carrier of hereditary material, groups of traits alternate according to certain laws and are transmitted from one generation to another. It should be noted that there are incredibly many genes in the body, but information is not lost even when it is recombined many times.
Separate several types of genes:
- there are 2 types according to their functional purpose: structural and regulatory sequences;
- by the influence on the processes occurring in the cell, there are: supervital, lethal, conditionally lethal genes, as well as mutator genesand antimutators.
Genes are located along the chromosome in a linear order. In chromosomes, information is not randomly focused, there is a certain order. There is even a map showing positions, or gene loci. For example, it is known that data on the color of the eyes of a child is encrypted in chromosome number 18.
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 all the enormous potential of human development is laid down already in the period of conception. All hereditary information that is necessary for the development of the zygote and the growth of the child after birth is encrypted in the genes. Sections of DNA are the most basic carriers of hereditary information.
A human 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 predisposition to diseases. Somatic chromosomes are outwardly indistinguishable, but they carry different information, since one of them is from the father, the other is from the mother.
The male code differs from the female one by the last pair of chromosomes - XY. The female diploid set is the last pair, XX. Males get one X chromosome from their biological mother, and then it is passed on to their daughters. The sex Y chromosome is passed on to sons.
Human chromosomes significantlyvary in size. For example, the smallest pair of chromosomes is 17. And the biggest pair is 1 and 3.
Diameter of the human double helix is only 2 nm. The DNA is so tightly coiled that it fits in the small nucleus of the cell, although it will be up to 2 meters long if unwound. 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 a cell during division? Genes - carriers of hereditary information - are inside every cell of the body. In order to pass on their code to a daughter organism, many creatures divide their DNA into 2 identical helices. This is called replication. In the process of replication, DNA unwinds and special "machines" complete each chain. After the genetic helix bifurcates, the nucleus and all 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 in genes. Once created at the time of conception, this code will copy itself throughout life. The karyotype (personal set of chromosomes) does not change during the renewal of organ cells. The transmission of information is carried out with the help of sex gametes - male and female.
Only viruses containing a single strand of RNA are unable to transmit their information to their offspring. Therefore, in order toto reproduce, they need human or animal cells.
Implementation of hereditary information
In the cell nucleus, important processes are constantly taking place. All information recorded in chromosomes is used to build proteins from amino acids. But the DNA chain never leaves the nucleus, so another important compound, RNA, is needed here. Just RNA is able to penetrate the nuclear membrane 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. Messenger RNA copies a segment of the DNA chain and brings this copy to the ribosome. Here begins the synthesis of a new molecule from the nucleotides.
In order for the mRNA to copy the necessary part of the chain, the helix unfolds, and then, upon completion of the recoding process, is restored again. Moreover, this process can occur simultaneously on 2 sides of 1 chromosome.
Principle of complementarity
DNA helices consist of 4 nucleotides - these are adenine (A), guanine (G), cytosine (C), thymine (T). They are connected by hydrogen bonds according to the rule of complementarity. The works of E. Chargaff helped to 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 equal to one. Similarly, the ratio of guanine to cytosine is always one.
Based on his work, geneticists have formed a rule of interactionnucleotides. The rule of complementarity states that adenine combines only with thymine, and guanine with 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 transfer RNA.
RNA and its types
What is hereditary information? This is the sequence of nucleotides in the 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 performs strictly its function.
- Information (mRNA), or else it is called matrix. It goes right into the center of the cell, into the nucleus. It finds in one of the chromosomes the necessary genetic material for building a protein and copies one of the sides of the double chain. Copying occurs again according to the principle of complementarity.
- Transport is a small molecule that has nucleotide decoders on one side, and on the other side corresponding to the main amino acid code. 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 site.
The only difference when decoding is that RNA does not have thymine. Instead of thymine, uracil is present here. But then, in the process of protein synthesis, with tRNA it is still correctsets all amino acids. If there are any failures in the decoding of information, then a mutation occurs.
Repair of a damaged DNA molecule
The process of repairing a damaged double chain is called reparation. During the repair process, damaged genes are removed.
Then the required sequence of elements is reproduced exactly and crashes back into the same place on the chain from where it was extracted. All this happens thanks to special chemicals - enzymes.
Why do mutations happen?
Why do some genes begin to mutate and cease to perform their function - the storage of vital hereditary information? This is due to a decoding error. For example, if adenine is accidentally replaced with thymine.
There are also chromosomal and genomic mutations. Chromosomal mutations occur when pieces of hereditary information fall out, are duplicated, or even transferred and integrated into another chromosome.
Genomic mutations are the most serious. Their cause is a change in the number of chromosomes. That is, when instead of a pair - a diploid set, a triploid set is present in the karyotype.
The most famous example of a 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.
Also known is such a mutation as polyploidy. But polyplodiafound only in plants.
