DNA (deoxyribonucleic acid) is one of the most important components of living matter. Through it, the preservation and transmission of hereditary information from generation to generation is carried out with the possibility of variability within certain limits. The synthesis of all proteins necessary for a living system would be impossible without a DNA matrix. Below we will consider the structure, formation, basic functioning and the role of DNA in protein biosynthesis.
The structure of the DNA molecule
Deoxyribonucleic acid is a macromolecule consisting of two strands. Its structure has several levels of organization.
The primary structure of the DNA chain is a sequence of nucleotides, each containing one of the four nitrogenous bases: adenine, guanine, cytosine or thymine. Chains arise when the deoxyribose sugar of one nucleotide is joined to the phosphate residue of another. This process is carried out with the participation of a protein-catalyst - DNA ligase
- The secondary structure of DNA is the so-called double helix (more precisely, a double screw). Grounds are capableconnect with each other as follows: adenine and thymine form a double hydrogen bond, and guanine and cytosine form a triple. This feature underlies the principle of base complementarity, according to which chains are connected to each other. In this case, a helical (more often right) twisting of the double chain occurs.
- A tertiary structure is a complex conformation of a huge molecule that occurs through additional hydrogen bonds.
- The quaternary structure is formed in combination with specific proteins and RNA and is the way DNA is packaged in the cell nucleus.
DNA functions
Let's consider the role DNA plays in living systems. This biopolymer is a matrix containing a record of the structure of various proteins, RNA needed by the body, as well as various kinds of regulatory sites. In general, all these components make up the genetic program of the body.
Through DNA biosynthesis, the genetic program is passed on to the next generations, ensuring the heredity of information fundamental to life. DNA is able to mutate, due to which the variability of living organisms of one biological species arises and, as a result, the process of natural selection and the evolution of living systems is possible.
During sexual reproduction, the DNA of an organism-descendant is formed by combining paternal and maternal hereditary information. When combined, there are various variations, which also contributes to variability.
How the genetic program is reproduced
Due to the complementary structure, matrix self-reproduction of the DNA molecule is possible. In this case, the information contained in it is copied. The duplication of a molecule to form two daughter "double helixes" is called DNA replication. This is a complex process that involves many components. But with a certain simplification, it can be represented as a diagram.
Replication is initiated by a special complex of enzymes in certain areas of DNA. At the same time, the double chain unwinds, forming a replication fork, where the process of DNA biosynthesis takes place - the buildup of complementary nucleotide sequences on each of the chains.
Features of the replication complex
Replication also proceeds with the participation of a complex set of enzymes - replisomes, in which DNA polymerase plays the main role.
One of the chains in the course of DNA biosynthesis is the leader and is continuously formed. The formation of a lagging strand occurs by attaching short sequences - Okazaki fragments. These fragments are ligated using DNA ligase. Such a process is called semi-continuous. In addition, it is characterized as semi-conservative, since in each of the newly formed molecules one of the chains is the parent, and the second is the daughter.
DNA replication is one of the key steps in cell division. This process underlies the transfer of hereditary information to a new generation, as well as the growth of the organism.
What are proteins
Protein isthe most important functional element in the cells of all living organisms. They perform catalytic, structural, regulatory, signaling, protective and many other functions.
A protein molecule is a biopolymer formed by a sequence of amino acid residues. It, like nucleic acid molecules, is characterized by the presence of several levels of structural organization - from primary to quaternary.
There are 20 distinct (canonical) amino acids used by living systems to build a huge variety of proteins. As a rule, protein is not synthesized on its own. The leading role in the formation of a complex protein molecule belongs to nucleic acids - DNA and RNA.
The essence of the genetic code
So, DNA is an information matrix that stores information about the proteins necessary for the body to grow and live. Proteins are built from amino acids, DNA (and RNA) from nucleotides. Certain nucleotide sequences of the DNA molecule correspond to certain amino acid sequences of certain proteins.
There are 20 types of protein structural units - canonical amino acids - in a cell, and 4 types of nucleotides in DNA. So each amino acid is written on the DNA matrix as a combination of three nucleotides - a triplet, the key components of which are nitrogenous bases. This principle of correspondence is called the genetic code, and base triplets are called codons. Gene isa sequence of codons containing a record of a protein and some service combinations of bases - a start codon, a stop codon, and others.
Some properties of the genetic code
The genetic code is almost universal - with very few exceptions, it is the same in all organisms, from bacteria to humans. This testifies, firstly, to the relationship of all forms of life on Earth, and secondly, to the antiquity of the code itself. Probably, in the early stages of the existence of primitive life, different versions of the code formed quite quickly, but only one received an evolutionary advantage.
Besides, it is specific (unambiguous): different amino acids are not encoded by the same triplet. Also, the genetic code is characterized by degeneracy, or redundancy - several codons can correspond to the same amino acid.
Genetic record is read continuously; the functions of punctuation marks are also performed by triplets of bases. As a rule, there are no overlapping records in the genetic "text", but here too there are exceptions.
Functional units of DNA
The totality of all the genetic material of an organism is called the genome. Thus, DNA is the carrier of the genome. The composition of the genome includes not only structural genes encoding certain proteins. A significant part of DNA contains regions with different functional purposes.
So, DNA contains:
- regulatorysequences encoding specific RNAs, such as genetic switches and regulators of structural gene expression;
- elements that regulate the process of transcription - the initial stage of protein biosynthesis;
- pseudogenes are a kind of "fossil genes" that have lost their ability to encode a protein or be transcribed due to mutations;
- mobile genetic elements - regions that can move within the genome, such as transposons ("jumping genes");
- telomeres are special regions at the ends of chromosomes, thanks to which the DNA in chromosomes is protected from shortening with each replication event.
Involvement of DNA in protein biosynthesis
DNA is able to form a stable structure, the key element of which is the complementary compound of nitrogenous bases. The double strand of DNA provides, firstly, the complete reproduction of the molecule, and secondly, the reading of individual sections of DNA during protein synthesis. This process is called transcription.
During transcription, a section of DNA containing a certain gene is untwisted, and on one of the chains - the template one - an RNA molecule is synthesized as a copy of the second chain, called the coding one. This synthesis is also based on the property of bases to form complementary pairs. Non-coding, service regions of DNA and the enzyme RNA polymerase take part in the synthesis. RNA already serves as a template for protein synthesis, and DNA is not involved in the further process.
Reverse transcription
For a long time it was believed that the matrixcopying of genetic information can only go in one direction: DNA → RNA → protein. This scheme has been called the central dogma of molecular biology. However, in the course of research, it was found that in some cases it is possible to copy from RNA to DNA - the so-called reverse transcription.
The ability to transfer genetic material from RNA to DNA is characteristic of retroviruses. A typical representative of such RNA-containing viruses is the human immunodeficiency virus. The integration of the viral genome into the DNA of an infected cell occurs with the participation of a special enzyme - reverse transcriptase (revertase), which acts as a catalyst for DNA biosynthesis on an RNA template. Revertase is also part of the viral particle. The newly formed molecule is integrated into the cellular DNA, where it serves to produce new viral particles.
What is human DNA
Human DNA, contained in the cell nucleus, is packed into 23 pairs of chromosomes and contains about 3.1 billion paired nucleotides. In addition to nuclear DNA, human cells, like other eukaryotic organisms, contain mitochondrial DNA, a factor in the heredity of mitochondrial cell organelles.
Coding genes of nuclear DNA (there are from 20 to 25 thousand of them) make up only a small part of the human genome - about 1.5%. The rest of the DNA was previously called "junk", but numerous studies reveal the significant role of non-coding regions of the genome, which were discussed above. It is also extremely important to study the processesreverse transcription in human DNA.
Science has already formed a fairly clear understanding of what human DNA is in structural and functional terms, but further work of scientists in this area will bring new discoveries and new biomedical technologies.
