Nucleic acids and DNA

Nucleic acids are chemical compounds of great biological importance; all living organisms contain nucleic acids in the form of DNA and RNA (respectively deoxyribonucleic acid and ribonucleic acid). Nucleic acids are very important molecules because they exert primary control over vital vital processes in all organisms.

Everything suggests that nucleic acids have played an identical role since the first forms of primitive life that could survive (like bacteria).

In the cells of living organisms, DNA is present above all in chromosomes (in dividing cells) and in chromatin (in intercinetic cells).

It is also present outside the nucleus (in particular in the mitochondria and in the plastids, where it fulfills its function as an information center for the synthesis of part or all of the organelle).

RNA, instead, is present both in the nucleus and in the cytoplasm: in the nucleus it is more concentrated in the nucleolus; in the cytoplasm it is more concentrated in polysomes.

The chemical structure of nucleic acids is quite complex; they are formed by nucleotides, each of which (as we have seen) is made up of three components: carbon hydrate (pentose), nitrogen base (purine or pyrimidine) and phosphoric acid.

Nucleic acids are therefore long polynucleotides, resulting from the concatenation of units called nucleotides. The difference between DNA and RNA lies in the pentose and the base. There are two types of pentose, one for each type of nucleic acid:

1) Ribose in RNA;

2) Dessosiribosio in DNA.

Also with regard to the bases we must repeat the distinction; pyrimidine bases include:

1) Cytosine;

2) Thymine, present only in DNA;

3) Uracil, present only in RNA.

The purine bases are, instead, made up of:

1) Adenine

2) Guanina.

In summary, in the DNA we find: Cytosine - Adenine - Guanina - Timina (CAGT); while in the RNA we have: Cytosine - Adenine - Guanine - Uracil (CAGU).

All nucleic acids have the polynucleotide linear chain structure; the specificity of the information is given by the different sequence of the bases.

DNA structure

The nucleotides of the DNA chain are bonded together with ester between phosphoric acid and pentose; the acid is found bound to the carbon 3 of the nucleotide pentose and to the carbon 5 of the next; in these bonds it uses two of its three acid groups; the remaining acid group gives the acid character to the molecule and allows to form bonds with basic proteins.

DNA has a double helix structure: two complementary chains, one of which "goes down" and the other "goes up". This concept corresponds to the concept of "antiparallel" chains, that is, parallel but in opposite directions. Starting from one side, one of the chains begins with a bond between phosphoric acid and carbon 5 of the pentose and ends with a free carbon 3; while the direction of the complementary chain is opposite. We also see that the hydrogen bonds between these two chains occur only between a purine base and a pyrimidine base and vice versa, ie between Adenina and Timina and between Cytosine and Guanine, and vice versa; there are two hydrogen bonds in the AT pair, while in the GC pair there are three bonds. This means that the second pair has greater stability.

DNA reduplication

As already mentioned in connection with the intercinetic nucleus, DNA can be found in the "autosynthetic" and "allosynthetic" phases, ie respectively committed to synthesizing pairs of itself (autosynthesis) or another substance (RNA: allosynthesis). Intercinetic activity in this regard it is divided into three phases, called G1, S, G2 . In the G1 phase (where G can be taken as initial growth, growth) the cell synthesizes, under the control of nuclear DNA, all that is necessary for one's metabolism. In the S phase (where S stands for synthesis, ie synthesis of new nuclear DNA) DNA reduplication takes place. In phase G2 the cell resumes growth, preparing for the next division.

WE SHOULD SEE THE PHENOMENA IN THE STAGE S

First of all we can represent the two antiparallel chains as if they were already "despiralized". Starting from one end the bonds between base pairs (A - T and G - C) are broken, and the two complementary chains move away (the comparison of the opening of a "lightning" is suitable). At this point an enzyme ( DNA-polymerase ) "flows" along each single chain, favoring the formation of bonds between the nucleotides that compose it and new nucleotides (previously "activated" with energy given by ATP) prevalent in the karyoplasm. A new timína is necessarily bound to each adenine, and so on, forming each time a new double chain.

The polymers DNA seems to act in vivo indifferently on the two chains, whatever the "direction" (from 3 to 5 or vice versa). In this way, when all the original double DNA chain has been traveled, there will be the presence of two double chains, exactly equal to the original. The term that defines this phenomenon is "semiconservativ reduplication", where "reduplication" concentrates the meanings of quantitative and exact copy doubling, while "semi-conservative" recalls the fact that, for each new double chain of DNA, a single chain is neosítetico.

DNA contains genetic information, which it transmits to RNA; the latter in turn transmits it to proteins, thus regulating the metabolic functions of the cell. Consequently the whole metabolism is directly or indirectly under the control of the nucleus.

The genetic heritage that we find in DNA is intended to give specific proteins to the cell.

If we take them in pairs, the four bases will give 16 possible combinations, ie 16 letters, not enough for all amino acids. If instead we take them in triplets, there will be 64 combinations, which may seem too many, but which, in reality, are all in use since science has discovered that different amino acids are encoded by more than one triplet. Thus, there is the translation from the 4 letters of the nucleotide nitrogenous bases to the 21 of the amino acids; however, before the «translation», there is the «transcription», still in the context of the four letters, that is the passage of the genetic information from the 4 letters of the DNA to the 4 letters of the RNA, taking into account that, instead of timid (DNA), there is uracil (RNA).

The transcription process occurs when, in the presence of ribonucleotides, enzymes (RNA-polymerase) and energy contained in the ATP molecules, the DNA chain is opened and RNA is synthesized, which is a faithful reproduction of genetic information contained in that stretch of open chain.

There are three main types of RNA and all originate from nuclear DNA: