The synthesis of the proteins is the process by which a cell assembles a proteinic chain by combining amino-acid isolated present in its Cytoplasme, guided by the contained information in DNA. It is held in two stages at least: the transcription of DNA in ARN messenger and the translation of the ARN messenger in a protein.

At the Eucaryote S, there exists an intermediate stage, the maturation of the ARN prémessager, which occurs in the core. The ARN prémessager undergoes the addition of a cap of 7-méthylguanosine triphosphate at end 5 ' and of a tail poly-A (50 to 250 nucleotides of adénine) at end 3 '. Thereafter, the ARN prémessager undergoes an excision of its will introns (parts of gene which does not code a polypeptide) and the épissage of its exons (bits coding). The ARN prémessager is now with maturity and takes the name of ARN messenger. The transcription proceeds in the core, the translation, in the endoplasmic Réticulum. A last stage of Glycosylation (covalent bond of oses to proteins) takes place in the Appareil of Golgi. At the Procaryotic S, the two stages take place in the cytoplasm and can be simultaneous, the translation beginning whereas the transcription is not completed yet. This simultaneity gives place to an important type of regulation of the translation.

First approach: total mechanism

Highlighted of the total mechanism of synthesis (pulsate-drives out)

It is legitimate to wonder how the biologists discovered the succession of stages which leads to the completion of proteins. The principal technique is that of the Pulsate-drives out or pulsate-drives out, which is held in four principal stages.

  1. One cultivates cells in-vitro, in an acid medium.

  2. One adds in the medium of the amino-acids, constituent of the proteins, which are marked radioactively. It is the stage of pulsates, which lasts a few minutes, the time of the incorporation of the amino-acids in the biosynthetic chain.
  3. One places then the cells in a medium where the amino-acids are not marked radioactively. It is the stage of hunting.
  4. One takes with regular intervals of the cells of this new medium; one then has two techniques of exploitation of this experiment. The first is autoradiography; the second master key by an ultracentrifugation and gives a better precision in the results.
  5. * autoradiography: one prepares a cut of the cell, by fixing then cutting with the microtome. On the plate obtained, one deposits a photographic film containing of the money grains, and one lets the whole rest a few weeks with the darkness. The electrons resulting from the disintegration of the radioactive cores assimilated by the cell reduce the Ag+ ions in black money grains, thus giving a “photography” of the localization of the cellular radioactivity. One can thus recall, by observation of thin blades at different times of “hunting”, the cellular way of proteins during their synthesis. However the electrons of the thin blade can mark enough the photographic plate far from their zone of emission (precision about the half-micrometer). Thus, one cannot know for example, if the radioactivity in an organoid is inside or possibly just outside its walls.
  6. * ultracentrifugation: One centrifuges at high-speed each taking away of cells of the second medium. One obtains thus, after several successive centrifugations with the increasing accelerations, various fractions of cell, classified according to their mass. One knows the correspondence between the various organoids (endoplasmic Réticulum, Appareil of Golgi, core) and the fractions after centrifugations. Thus, if the radioactivity of each fraction is measured, one can know in which organoid the marked amino-acids were at the time of the taking away. One from of deduced from the curves distribution of radioactivity according to time for each cellular compartment, which makes it possible to find the way of the amino-acids lately assembled out of proteins.

Complementarity of the nitrogenized bases

It is important to know, before proceeding, that there exists a complementarity between the nitrogenized bases of the DNA and the ARN. It is this complementarity, due to hydrogen bonds, which allows the replication, the transcription and the translation of the nucleic acids.

There exist 5 nitrogenized Bases:

With and G are Purine S (2 cycles)
T, C and U are Pyrimidine S (1 cycle)

With and T are complémentaires
G and C are complémentaires

With and U are complémentaires

The transcription and the translation use this complementarity during the creation of ARN and of the approach of ARNt.

Transcription of the DNA

See also: Transcription (biology)

The first stage of the synthesis of proteins is the transcription of a Gène of DNA in a molecule of ARN messenger (ARNm or ribonucleic acid messenger). The stage proceeds inside even core of a cell Eucaryote and in the Cytoplasme of the Procaryote S. This difference will have important consequences on the treatment of the synthesized ARN. The DNA is a molecule made up of a succession of Nucléotide S. No characteristic physical does not differentiate a Gène from a portion not coding of the organization. The location of genes along the molecule of DNA (and generally any information of regulation along the chain of DNA) will be made in the same way that of the location of the files on a magnetic band: markings consisting of a particular sequence of Nucléotide S will indicate the beginning of gene. These special marks are called sequences consensus , it does not act indeed of exact sequences but of approximate sequences of an average sequence but differing only by a few pairs from bases. Both used to locate the beginning of a gene are the boxes CAAT and TOUCHED , of the name of nucleotides forming the heart of the sequence average and located in a zone preceding gene called promoter because it initiates the transcription of gene.

The transcription consists in making a “copy of work” of the DNA. The DNA is a single molecule in the cell. In addition to the fact that the synthesis of an intermediate ARN makes it possible to limit the damage of the DNA in consequence of too much handling, that makes it possible to multiply the copies available for the phase of translation and thus to synthesize the Protéine much more quickly.

The synthesis of the ARN utilizes a very complex proteinic unit, the ARN polymerase. The first stage of the transcription is the recognition of gene to be transcribed. This stage utilizes varied mechanisms which depend on protein to transcribe, but which rests all on the principle of a specific protein genes to be transcribed, which is fixed in a precise place of the DNA, located in the promoter. This protein will be used as point of anchoring with the system ARN polymerase, this phase taking place naturally only if two boxes CAAT and TOUCHED are present. This complex will traverse the Molécule DNA to read it. It first of all will unroll the molecule of DNA, then to separate the two bits, then to assemble the bases nitrogenized while making use of the complementary bit like stamps to lead to the molecule of ARN. Behind it, the two bits are reassembled and the DNA is rewound. When the ARN polymerase meets the site of gene termination, it separates from the DNA, and the ARN is released from the chain of DNA.

At the Procaryotic S, the ARN can be used directly. The translation besides will start before even as the transcription is completed. At the Eucaryote S on the other hand, the ARN is not directly usable. It must undergo a maturation. The Gène S of the eucaryotes are not sequences of DNA coding continuous beginning at the end of gene but consisted of pieces of coding sequences (the Exon S) separated by zones not coding (the Intron S). To obtain a molecule of functional ARN, it is thus necessary first of all to remove will introns them and to connect let us exons them the ones with the others. This phase is called the épissage and brings into play mechanisms still not very clear. One can note with the passage which the cells can decide not to keep that some exons on the unit available, this choice differing according to the cellular type within the same organization. A single gene thus makes it possible to synthesize several proteins according to exons preserved in ARNm. In addition, the ARN is preceded by a molecular cap in position 5 ' and ends in a polyadénylée tail; these modifications have as an aim of protection the ARN of a too fast degradation in the cytoplasm. ARNm now could be exported core towards the Cytoplasme via the nuclear pores for the following phase of the proteinic synthesis: translation …

An example of transcription

A molecule of DNA consists of two bits:

the bit of DNA not transcribed : WITH T G C G T T A.C.G HAS HAS C T G HAS T HAS C G HAS T HAS the bit of DNA transcribed : T HAS T C G A.C. HAS G T C T T G HAS C T HAS T G C T HAS T T the bit of ARN : HAS U HAS G C G U U.A. HAS G HAS C U G HAS U HAS C G HAS U HAS

Translation of ARNm

See also: Translation (biology)

Once the bit of ARNm reached the Cytoplasme, where takes place the translation, it is fixed at a Ribosome, composed of a small under-part and a great under-part, which will assemble an amino-acid sequence of according to the “instructions” of the genetic Code: each Codon (group of 3 nucleotides of ARNm) corresponds to an amino-acid, except 3 code, called code-stop, which causes the stop of the translation. Codon AUG, called codon-initiator, will make it possible to begin the translation, as its name indicates it, by forming the amino-acid methionine, which will be detached later from the polypeptide chain.

Ribosome will traverse the bit of ARNm codon by codon and (ARNt) will add an amino-acid via a ARN of transfer to protein in the course of manufacture according to the codon read. Once the Codon-stop reached, the protein is complete: ribosome is detached from protein and the bit of ARNm, and the protein is released in the organization.

The same filament of ARNm can be used with simultaneous manufacture several molecules as proteins, when several ribosomes take care some. Before being destroyed, this molecule takes part indeed in the manufacture from 10 to 20 proteins.

As soon as the chain of amino-acids is finished, it is detached from the ribosome which is then available for a new synthesis. Then start the Transport of the proteins, which carry out them out of the cell and in the blood system.

Let us take again our example

the bit of ARN messenger is HAS U HAS G C G U u.a. HAS G HAS C U G HAS U HAS C G U Has the different ones let us code are thus AUA - GCG - UUC - AGA - ACU - GAU - ACG - UAA the ARN of transfer set UAU CGC AAG UCU UGA CUA UGC AUU by complementarity and brings | | | | | | | | the suitable amino-acids Île Went Phe Arg Thr Asp Thr X

The ARN of transfer arrive successively with for anti-codon:

  • UAU : it is formed the amino-acid Isoleucine (ISO) (Island)

  • CGC: it is formed the amino-acid Alanine (WENT) (Went)
  • AAG: it is formed the amino-acid Phénylalanine (PHE) (Phe)
  • UCU: it is formed the amino-acid Arginine (ARG) (Arg)
  • UGA: it is formed the amino-acid Thréonine (THR) (Thr)
  • CUA: it is formed the amino-acid Aspartic acid (ASP) (Asp)
  • UGC: it is formed the amino-acid Thréonine (THR) (Thr)
  • AUU: codon UAA is a codon stop (which thus does not code any amino-acid). The polypeptide chain is detached from ribosome and the translation is finished.

Sources

  • Neil A. Campbell and Jane B. Reece, Biology , translated by Richard Mathieu, ED. Editions of Teaching Revival Inc., the St. Lawrence (Quebec), March 3rd, 2004, 1400 pages, ISBN 2-7613-1379-8.

See too

Internal bonds

External bonds

  • Baptist Deleplace (2003), '' translation (a ludic approach of the translation of the ARN m in polypeptide chain).

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