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(ribosomes)

Ribosomes are supramolecular complexes responsible for synthesizing proteins from genetic information DNA that comes in the form of transcribed messenger RNA (mRNA). They are visible only by electron microscopy because of its small size (29 nm in prokaryotic and eukaryotic 32 nm). Under the electron microscope are seen as round structures, dense to electrons. Under the microscope are seen to be responsible for the basophilia exhibited by several cells. They are in every cell (except sperm).

In eukaryotic cells, ribosomes are produced in the nucleus but perform their function of synthesis in the cytosol. They consist of ribosomal RNA (rRNA) and protein. Structurally, they have two subunits. In cells, these organelles appear in various states of dissociation. When complete, may be isolated or forming groups (polysomes), the proteins synthesized by them primarily active in the cytosol, they can also appear associated with rough endoplasmic reticulum or nuclear membrane, and proteins are synthesized mainly for export.

Both rRNA and ribosomal subunits are usually appointed by their sedimentation coefficient in Svedberg units. In eukaryotes, ribosomes the cytoplasm are called 80 S. In mitochondria and plastids of eukaryotes and prokaryotes, there are 70 S.

prokaryotic ribosomes:

The ribosomes of prokaryotes are the most studied. Are 70 S and its molecular weight is 2,500 kilodaltons. RRNA molecules are the 65% of the ribosome and the proteins represent 35%. Ribosomal RNA molecules are rich in adenine and guanine to form a helix around proteins. Ribosomes are composed of two subunits:

subunit: is 50 S. It consists of two molecules of RNA, a 23 S and another 5 S. In addition there are 34 basic proteins of which only one is repeated in the small subunit. Subunit
minor: S is 30 and has a molecule of 16 S rRNA as well as 21 proteins.

Eukaryotic Ribosome:

In eukaryotes, ribosomes are 80 S. Its molecular weight is 4,200 Kd. Contain 40% rRNA and 60% protein. As prokaryotes are divided into two subunits of different sizes: large subunit
: is 60 S. It has three types of rRNA, 5 S, 28 S and 5.8 S and has 49 proteins, each different from the small subunit. Subunit
child: is 40 S. It has a single molecule of 18 S rRNA and contains 33 proteins. Depending on which eukaryotic organism is, the 18 S rRNA can be disrupted.

Ribosome of plastids:

Ribosomes displayed in plastids are similar to prokaryotes. They, like prokaryotes, 70 S, but in the large subunit rRNA is a 4-S which is equivalent to 5 S prokaryote.

Functions:

Ribosomes are the organelles responsible for protein synthesis in a process known as translation. The information required for this synthesis is in the messenger RNA (mRNA), whose sequence of nucleotides determines the amino acid sequence of the protein, in turn, the mRNA sequence from transcription of a gene's DNA. Transfer RNA carries amino acids to ribosomes where incorporated into the growing polypeptide.

Translation:

The ribosome reads the messenger RNA and assembles the amino acids supplied by the transfer RNA to the protein in growth, a process known as translation or protein synthesis.
All proteins are composed of amino acids. Among the living beings have been discovered so far 20 amino acids. In the genetic code, each amino acid is encoded by one or more codons. In total there are 64 codons encoding 20 amino acids and 3 stop signs translation. This makes the code is redundant and there are several different codons for the same amino acid.
The translation generally begins at the AUG codon encoding the amino acid methionine. At the end of the sequence is located one codon that signals the end of the protein is the stop codon. The genetic code is universal because each codon encoding the same amino acid for most organisms (not all).
The ribosome consists of two parts, the large subunit and a smaller, they exit the nucleus separately. The subunits are held together by loads, and experimentally by reducing the concentration of Mg +2, subunits tend to separate.