The nucleic acid are Macromolécule S, i.e. large relatively complex molecules. They enter the family of the Biomolécule S since they are of very great importance in the reign of the life, “ bio ” meaning life in Greek.

The nucleic acid are Polymère S of which the basic unit, or Monomère, is the Nucléotide. These nucleotides are related the ones to the others by connections phosphodiester.

Types of nucleic acid

There exist two types of nucleic acids: the desoxyribonucleic Acid ( DNA ) and the Ribonucleic acid ( ARN ). The DNA contains genetic information. The ARN as for him is the copy of the DNA (often in only one bit whereas the DNA is a double helix = two bits). The difference between the DNA and the ARN is that the DNA contains the genome, all that is necessary to the formation of proteins, but cannot leave the core, therefore the ARN copies the genetic information of the DNA and leaves the core by the nuclear pores to provide information and to thus allow the direct synthesis of proteins (by ribosomes in general).

Localization

One finds nucleic acids (DNA or ARN) in the cells of almost each organization. Any cell Procaryotic Eucaryote or , is the animal cells, the vegetable cells, the Bactérie S, the mycètes (or Champignons) and even the Mitochondrie S and the Chloroplaste S contain the two types of nucleic acid. However, the Virus can contain DNA or ARN, but never both at the same time.

The DNA is in the cellular core at the Eucaryote S. It joins Protéine S like Histone S. This fitting of DNA and proteins forms the Chromatine which one finds in the form of linear Chromosome S at the Eucaryote S (quite visible during the Mitose) and in the form of single helicoid chromosome at the Procaryote S. For its part, the ARN is found as much on the level of the core as on the level of the Cytosol.

Composition

The nucleic acids are clusters of Nucléotide S. the nucleotides forming the DNA are called Désoxyribonucléotide S while those forming the ARN are called Ribonucléotide S. the composition of these Nucléotide S varies according to whether they are in the DNA or the ARN. However, that they are found in one or the other of these nucleic acids, the nucleotides always have three fundamental substances:
  • a sugar
  • a group phosphates
  • a nitrogenized base .

Connections

To support of such Molecule S and to maintain them “in a piece”, chemical bonds are necessary. These connections must be strong to avoid breakings and to remain stable, but must also be weak to a certain extent where these nucleic acids must constantly be handled by varied Protéine S, amongst other things of the Enzyme S, in processes like the replication, the transcription or the translation.

Connections phosphodiester

In the nucleic acids, different the Nucléotide S is placed end to end and bound the ones to the others by bonds 3' - 5' (delivery 3 precedes - 5 premium) phosphodiester. These figures give the direction of the connection; the Phosphate binds to carbon 5 of the sugar of the first Nucléotide and to carbon 3 of the sugar of following nucleotide. The connections phosphodiester are covalent bonds, i.e. there is division of electrons between the atoms. The Phosphate is thus the bond (or the bridge) between each sugar.

Covalent bonds

The nitrogenized bases are attached on carbon 1 ' of sugars by covalent bonds.

The connections phosphodiester are covalent bonds . (One can say that all the connections phophodiester are covalent bonds, but all the covalent bonds are not connections phophodiester).

Put aside, hydrogen bonds, that one often names hydrogen interactions because makes of it, it does not have “connection” itself there, any other connection present in the nucleic acids are covalent connections .

Creation of the skeleton

The alternation of the Phosphate S and the Sucre S produces the skeleton of the nucleic acid on which the nitrogenized bases stick . The Polymère formed names a bit and with the diagrammatic pace of a “cord”.

The skeleton is a relatively rigid part since it is composed of covalent bonds, of the very strong chemical bonds.

Hydrogen bonds

In the case of the DNA, the two bits (two cords) are laid out so that all the nitrogenized bases are found in the center of the structure. This structure called Double helix is maintained by hydrogen bonds (weak bonds, which retain little) which is formed between the complementary nitrogenized bases; the adénine always joining the Thymine (in the DNA) or the Uracil (in the ARN) using two hydrogen bonds and the Guanine always joining the Cytosine using three hydrogen bonds .

Creation of the helicoid structure

The two bits (more often found in the DNA rare in the ARN) take the form of a Double helix (helicoid structure). This structure is made possible thanks to the flexibility of the hydrogen bonds.

This flexible structure is ideal to allow the Protéine S the such Polymérase S, the Primase S and the Ligase S, to translate or duplicate the DNA.

Roles

Together, the DNA and the ARN play a fundamental role: they store, maintain and translate genetic information. They ensure the maintenance of the Génotype and the Phénotype by synthesizing Protéines thanks to the Gène S.

Role of the DNA

the DNA constitutes genetic information and determines the biological identity of the organization (plant, frog or human). The safeguarding of this genetic information is done thanks to a duplication of the molecules of DNA before the Mitose (creation of two cells identical girls).

Role of the ARN

the ARN has many roles. In fact, there exist several types different of ARN and each one of them plays a specific part.

  • the ARN messenger (ARNm) : is the product of the épissage of the ARN prémessager (ARNpm), which is to him the product of the transcription operated on the DNA. the ARN prémessager is also called heterogeneous ARN nuclear (ARNnh) because it is found strictly in the core and is composed of Intron S and of Exon S. the épissage of ARNpm consists in removing will introns them and to connect let us exons them the ones following the others. This chain of let us exons then constitutes the ARN messenger “finished product”. Contrary to the ARN prémessager, the ARN messenger leaves the core and ultimement is ultimement translated into Peptide in the cytosol or in the endoplasmic Réticulum. ARNm is the “plan of construction” of a Protéine. There is no épissage at the Procaryote S where the ARN produced by the transcription is directly ARNm (indeed these organizations do not have a core and ribosomes are fixed on the molecule of ARN while it is synthesized).

  • the ARN of transfer (ARNt) : is implied at the time of the translation of the ARN messenger in Peptide. It is charged to bring the amino-acid goods by deciphering the language which constitute code them and to translate them into sequence Amino-acid . A codon consists of three Nucléotide S adjacent. A codon corresponds to only one Amino-acid, but the same amino-acid can be specified by different code.

See genetic Code to know which amino-acids are associated with which code.

  • |The ribosomal ARN (ARNr) : constitute the Ribosome after maturation and association with Protéine S. the Ribosome S are manufacturing planies of Protéine S. ribosome joins the ARN messenger and “reads” the Codon S which are found there. It manages then the entry and the exit of the ARN of transfer which transport the amino-acid . The birth of a Peptide follows which will be possibly, after several stages of maturation and assembly, transformed into Protéine.

  • the microARN (miARN) : discovered in 1993 by Victor Ambros in the worm Caenorhabditis elegans. They have a structure simple bit and are long of 19 to 25 nucleotides. They play a part in the cellular metabolism by preventing the translation some ARN messenger in Peptide S. While binding to ARN messengers of which they are partially complementary, the microARN involve the blocking of the translation of ARNm by ribosomes.

The miARN can control the expression of several genes (perhaps a hundred for some of between-them).

  • the silencingRNA (siRNA) are small ARN of 21-22 nucleotides perfectly complementary to their ARNm targets. Contrary to the miRNA, the siRNA are not coded by the genome of the host cell but rather not brought by a possible invader such as the viruses. Moreover, they have a structure in double bit, and their action consists in degrading ARNm. It is carried out in collaboration with proteins called RISC (RNA Induced Silencing Complex). These last set on the bit antisens (complementary to the coding bit) siARN, the bit direction is abandoned, and the complex (RISC + ARN simple bit antisens) thus formed can recognize the fragment of corresponding ARNm and destroy it, thus preventing the form of associated gene.

The siARN are more specific than the miARN: they are conceived to recognize only one gene.

These short ARN became a tool very much used in molecular biology to one by one extinguish the genes whose one wishes to determine the metabolic role. Their specificity of action makes siARN a way very studied in the fight against cancer and the viral diseases.

  • snRNA (small nuclear RNA), snoRNA (small nucleolus RNA), scaRNA (small cajal bodies RNA): they are short chains of ribonucléotides (which is found exclusively in the core and more precisely in compartments of the core like the nucleole Pr the snoRNA and the bodies of Cajal for the scaRNA. These ARN not coding join proteins to form named complexes small ribonucleoproteins nuclear (RNPpn), essential during the process of épissage of ARN prémessagers and during the process of maturation of ARNr and ARNtm

Nucleic acids in the viruses

In the cells Eucaryote S and Procaryotic S, the DNA and the ARN is present. However, at the Virus, there is one nucleic acid present. It can be is the DNA or the ARN, but never both at the same time. One can find there DNA with simple or double chain or of the ARN to simple or double chain.

Let us note that one separates the viruses in several classes, according to the form under which is presented the Genetic material virus. Thus the HIV, the virus transmitting the AIDS, are a Rétrovirus, or virus with ARN because its genetic material is presented in the form of ARN inside its Capside. In a general way, there is no particular name for the viruses with DNA.

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