All cells of all organisms as autotrophic and heterotrophic, able to carry out the synthesis of proteins — the main component of the plastic exchange. In a complex and multistep process of protein synthesis in a living cell (i.e., biosynthesis) will be considered only one step: the formation of polypeptide chains of the individual amino acids, each of which occupies a certain place in the protein molecule. In the most General form they can apply in the following way. In the cell nucleus DNA molecules “written” (encoded — word) code the sequence of amino acids in a protein. Information about the order of nuclear DNA is transmitted to the synthesized RNA. This process is called transcription. The RNA in the cytoplasm comes into connection with the ribosomes. The ribosomes of the cytoplasm do and amino acids. Them gives back the transfer RNA. Information and transport of RNA along strictly determine the sequence of amino acids during their synthesis by enzymes of the ribosomes into a protein molecule. This transfer code of RNA into amino acids of a protein molecule is called translation. After synthesis of the protein molecule detaches from the ribosome and goes through the endoplasmic reticulum into the cell.
let us now Consider these phases of protein biosynthesis in more detail.
In each DNA molecule encoded sequence of amino acids for many tens or hundreds of different proteins. The encoding method is as follows: the sequence of amino acids in a protein molecule is determined by the sequence of nucleotides in a DNA molecule. But because of amino acids into proteins, 20, and only 4 nucleotides, each amino acid has more than one nucleotide, and a combination of three nucleotides, called triplet. All of these combinations (from 4 to 3) can be 64, that is, even much more than amino acids.
Already decrypted the codes for all the amino acids composing the proteins. Thus, the amino acid cysteine is encoded in the DNA molecule this combination of nucleotides (a triplet): A—C—A1; the amino acid valine triplet C—A—A; amino acid leucine — triplet A—A—C; amino acid Proline — triplet G—G—G.
the following abbreviations are used: A — adenine, G — guanine, T — thymine, C — cytosine, U — uracil.
So if some part of the DNA molecule, the nucleotide sequence will be:
C – A – A – A – C – A – A – A – C – G – G – G
thus this part of the DNA molecule is encoded following link amino acids in a protein molecule:
valine — cysteine — leucine — Proline.
Since the length of DNA molecule greatly exceeds the length of the protein molecule along a single DNA molecule can be encoded amino acid sequence for many proteins. Segment of DNA molecule that carries information about a protein molecule, called a gene (more on this concept see the Chapter “Genetics”). The combination of all the DNA molecules of the cells comprises information about the structure of all the proteins that are able to synthesize this type of animal or plant. Transcription (rewriting) code information on protein synthesis from DNA molecules to RNA molecules occurs in the process of their synthesis.
RNA is synthesized in the nucleus. As in the case of replication of DNA molecules, RNA is synthesized from nucleotides by the principle of complementarity. The matrix of this synthesis is the DNA molecule. It is only necessary to note that RNA is diminovula nucleotide (T) has oralloy (U). Therefore, when synthesis of RNA against A(DNA) rise U(RNA) against T(DNA) — A(RNA) against the G(DNA) — C(RNA) vs C(DNA) — G(RNA) So the already above code for the sequence of four amino acids is “translated” from the language of DNA into the language of RNA to the following:
valine cystine leucine Proline
amino acid sequence
C — A — A — A — C — A — A — A — C — G — G — G
this code sequence
in one chain of DNA
G — U — U — U — G — U — U — U — G — C — C — C
transcription of this sequence
Thus, the same amino acid (e.g., valine) in the DNA molecule is encoded by the triplet C—A—A, and after transcription the RNA molecule is encoded complentary triplet G—U—U. This naturally follows from the method of synthesis of molecules of RNA. After completing the synthesis of these molecules from the nucleus out into the cytoplasm and come in contact with ribosomes. Place of protein synthesis are ribosomes. Each of them as if strung on a molecule of RNA (figure 8) and moving along it, “reading” the plan of the Assembly of protein molecules, triplet for triplet. The transformation of this plan into real protein molecules is carried out with the participation of another nucleic acid, transfer RNA.
transport molecules of RNA are enough crayons is a short single chain of nucleotides. Each molecule of transfer RNA is specific for a single amino acid, only it can deliver from the cytoplasm to the place of “Assembly” protein. Specificity of transfer RNA molecules is achieved by its structure: one end of a short chain carries a triplet, the corresponding code of the amino acid (e.g., valine — C—A—A) and the other end can chemically connect only with the same acid. It is in this pair (for example, Malinova transfer RNA and the valine), they fall on the ribosome. If at this point, the ribosome is “malinowa” triplet RNA (G—U—U), “malinowy” triplet free end of the transport RNA (C—A—A) according to the principle of complementarity immediately joins RNA. Thus, the amino acid valine is fixed just where it was first encoded in the DNA molecule (triplet C—A—A), then a molecule of RNA (the triplet G—U—U), and finally delivered to the transfer RNA with the triplet C—A—A. Thus, while the ribosome moves along the RNA molecule, different transfer RNA with its amino acids join the RNA. Enzyme systems of ribosomes sequentially cleaved amino acid from the transfer RNA and connects them together in a chain of protein molecules. The released RNA transport then move into the cytoplasm for the following portions of the amino acids. This establishes the specificity of a protein molecule at the level of its primary structure. Secondary and tertiary structure of a protein molecule are determined by its primary structure, and a number of other conditions. It should be borne in mind that each of the described elements of the biosynthesis is catalyzed by certain enzymes, and is supplied with energy by ATP molecules.
Even very schematically describes the process of biosynthesis surprised by its orderliness. And if we add to this that in the living cell the synthesis of one protein molecule only lasts 3-4 seconds, and that at the same time in different parts of the cell carried out the syntheses of a variety of proteins and along with that comes a lot of other biochemical processes, then the question is: in what way is it adjustable? Not all specific ways of regulation is discovered by science. But she opened the main principle of regulation in a living cell is autoregulation. It is very simple case is the following. If a protein synthesized in the cell in sufficient amounts, it prevents further synthesis the fact of the presence of this protein in the cell. Until he is removed from the cell (or not spent in some other way), it is chemically acts on proteins enzymes involved in its synthesis as a brake. Enzymes temporarily cease to operate. The synthesis is paused. But that’s the protein consumed. Thus, its inhibitory effect on enzymes disappeared. And synthesis again.
It is easy to understand how long and arduous was the path of evolution of living organisms, before they achieved such a perfect autoreguljatsii.