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 with the possibility of variability in some frameworks is carried out. The synthesis of all the proteins necessary for a living system would not be possible without a DNA template. Below we consider the structure, formation, basics of functioning and the role of DNA in protein biosynthesis.
DNA molecule structure
Deoxyribonucleic acid is a macromolecule consisting of two strands. Its structure has several levels of organization.
- The primary structure of a DNA chain is a sequence of nucleotides containing each one of four nitrogenous bases: adenine, guanine, cytosine, or thymine. Chains arise when deoxyribose sugar of one nucleotide is combined with 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). The bases are able to connect with each other as follows: adenine and thymine form a double hydrogen bond, and guanine and cytosine form a triple bond. This feature underlies the principle of complementarity of bases, according to which the chains are connected to each other. In this case, a helical (often right) twisting of the double chain occurs.
- The tertiary structure is a complex conformation of a huge molecule arising through additional hydrogen bonds.
- The quaternary structure is formed in complex with special proteins and RNA and is a way of packaging DNA in the cell nucleus.
DNA functions
Consider what role DNA plays in living systems. This biopolymer is a matrix containing a record of the structure of various proteins, RNA, and various regulatory sites that are necessary for the body. In general, all these components make up the body’s genetic program.
Through DNA biosynthesis, the genetic program is passed on to the next generation, ensuring the inheritance of life-critical information. DNA is capable of mutating, due to which there is a variability of living organisms of one biological species and, as a result, the process of natural selection and the evolution of living systems are possible.
During sexual reproduction, the DNA of a descendant organism is formed by combining paternal and maternal hereditary information. When combining it, various options arise, which also contributes to variability.
How is the genetic program reproduced?
Due to the complementary structure, matrix self-reproduction of a DNA molecule is possible. In this case, the information contained in it is copied. Doubling a molecule to form two daughter "double helices" is called DNA replication. This is a complex process involving many components. But with a certain simplification, it can be represented in the form of a diagram.
Replication is initiated by a special complex of enzymes in certain DNA regions. At the same time, the double chain unwinds, forming a replication fork, where the DNA biosynthesis takes place - building up 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 - replicoma, the main role in which is played by DNA polymerase.
One of the chains during DNA biosynthesis is the leading one and is formed continuously. The formation of the lagging chain occurs by attaching short sequences - Okazaki fragments. These fragments are crosslinked using DNA ligase. This process flow 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 maternal and the second is the daughter.
DNA replication is one of the key steps in cell division. This process underlies the transmission of hereditary information to a new generation, as well as the growth of the body.
What are proteins
Protein is an essential 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 species with certain characteristics of (canonical) amino acids used by living systems to build a huge number of a wide variety of proteins. On its own, protein, as a rule, is not synthesized. 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 on which information about the proteins necessary for the body for growth and vital activity is stored. Proteins are built from amino acids, DNA (and RNA) from nucleotides. Certain nucleotide sequences of a DNA molecule correspond to certain amino acid sequences of certain proteins.
The structural units of the protein - canonical amino acids - in the cell are 20 species, and the nucleotides in the DNA are 4 species. So, each amino acid is written on a DNA matrix as a combination of three nucleotides - a triplet, the key components of which are nitrogenous bases. This correspondence principle is called the genetic code, and base triplets are called codons. A gene is a sequence of codons containing a record of a protein and some service combinations of bases - start codon, stop codon, and others.
Some properties of the genetic code
The genetic code is practically universal - with very few exceptions, it is the same in all organisms, from bacteria to humans. This indicates, firstly, the kinship of all life forms on Earth, and secondly, the antiquity of the code itself. Probably, in the early stages of the existence of primitive life, various versions of the code formed quite quickly, but only one gained an evolutionary advantage.
In addition, it is specific (unique): 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.
Reading the genetic record is continuous; triplets of bases also perform punctuation marks. As a rule, there are no overlapping entries in the genetic "text", but here, too, is not without exceptions.
Functional units in DNA
The totality of all the genetic material of the body is called the genome. Thus, DNA is the carrier of the genome. The structure of the genome includes not only structural genes encoding certain proteins. A substantial part of the DNA contains regions having different functional purposes.
So, the DNA contains:
- regulatory sequences encoding specific RNAs, for example, genetic switches and regulators of expression of structural genes;
- elements that regulate the transcription process - the initial stage of protein biosynthesis;
- pseudogenes - a kind of “fossil genes” that have lost due to mutations the ability to encode a protein or transcribe;
- mobile genetic elements - areas that can move within the genome, such as transposons (“jumping genes”);
- telomeres are special areas at the ends of chromosomes, thanks to which the DNA in the chromosomes is protected from shortening with each replication event.
DNA involvement in protein biosynthesis
DNA is capable of forming a stable structure, the key element of which is a complementary compound of nitrogenous bases. The double DNA strand 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 portion of DNA containing a specific gene is unwrapped, and an RNA molecule is synthesized on one of the chains — the template — 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, utility DNA regions and the RNA polymerase enzyme participate in the synthesis . RNA already serves as a matrix for protein synthesis, and DNA is not involved in the further process.
Reverse transcription
For a long time, it was believed that matrix copying of genetic information can go only in one direction: DNA → RNA → protein. This scheme is called the central dogma of molecular biology. However, during the studies it was found that in some cases copying from RNA to DNA is possible - the so-called reverse transcription.
The ability to transfer genetic material from RNA to DNA is characteristic of retroviruses. A representative representative of such RNA-containing viruses is the human immunodeficiency virus. The incorporation 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 the RNA matrix. Revertase is also part of the viral particle. The newly formed molecule integrates into cellular DNA, where it serves to produce new viral particles.
What is human DNA
The human DNA contained in the cell nucleus is packaged in 23 pairs of chromosomes and contains about 3.1 billion paired nucleotides. In addition to nuclear, in human cells, like other eukaryotic organisms, there is mitochondrial DNA - a factor of heredity of the mitochondrial cell organelles.
The coding genes of nuclear DNA (they number from 20 to 25 thousand) make up only a small part of the human genome - approximately 1.5%. The rest of the DNA was previously called “junk,” but numerous studies reveal a significant role for non-coding regions of the genome, which were discussed a little higher. It is extremely important, in addition, to study the processes of reverse transcription in human DNA.
Science has already formed a fairly clear understanding of what human DNA is structurally and functionally, but further work by scientists in this area will bring new discoveries and new biomedical technologies.