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Medical Specializations


Microbiology => Bacteria => Ribosomes


Ribosomes


INTRODUCTION
Ribosome, cell structure that uses genetic instructions transported in ribonucleic acid (RNA) to link a specific sequence of amino acids into chains to form proteins. Ribosomes, which measure about 0.00025 mm (0.00001 in), are dispersed in the cytoplasm (the cell contents outside the nucleus) of all prokaryotic cells- archaebacteria and bacteria. They are also found in the cytoplasm of all eukaryotic cells-cells of protists, fungi, plants, and animals-where they either float free in the cytoplasm or are bound to networks of membrane-enclosed tubules in the cytoplasm, called the endoplasmic reticulum. In eukaryotic cells, two types of cell structures called mitochondria and chloroplasts also contain ribosomes.

STRUCTURE
Development of electron microscope techniques in the 1950s enabled scientists to detect tiny granules in the cell's cytoplasm. By separating the granules from the rest of the cell, scientists were able to study their composition. Because the granules are rich in ribonucleic acid (RNA), scientists named them ribosomes.

Ribosomes in the cytoplasm of eukaryotic cells differ slightly from ribosomes in prokaryotic cells. Eukaryotic ribosomes consist of four strands of RNA associated with 70 to 80 proteins. In contrast, prokaryotic ribosomes each contain only three strands of RNA and about 50 proteins. Prokaryotic ribosomes are smaller and less dense than eukaryotic ribosomes. The ribosomes located in the mitochondria and chloroplasts of eukaryotic cells more closely resemble prokaryotic ribosomes, a finding that suggests mitochondria and chloroplasts evolved from once free-living prokaryotes.

SYNTHESIS
Ribosomes are required for protein synthesis; when cells need large numbers of proteins, they must first build numerous-sometimes hundreds of thousands-of ribosomes. To ensure rapid production of the RNA portion of ribosomes, certain eukaryotic cells contain thousands of copies of genes that code for ribosomal RNA. The locations of all the genes that code for the protein component of ribosomes are yet to be determined.

In eukaryotic cells, three of the four ribosomal RNA strands are synthesized in the nucleolus, a dense, granular structure in the nucleus. The fourth ribosomal RNA strand is synthesized outside of the nucleolus and then transported into the nucleolus for ribosome assembly.

Ribosomal proteins enter the nucleolus and combine with the four ribosomal RNA strands to create two ribosomal structures: the large and small subunits. These two beadlike subunits leave the nucleus separately through special openings called nuclear pores. The two subunits unite outside of the nucleus just before the ribosome begins to manufacture proteins. Prokaryotic cells lack a nucleus or nucleolus, and ribosomal synthesis takes place in the cytoplasm. In these cells, only a few copies of genes coding for ribosomal RNA are present.

PROTEIN SYNTHESIS
Protein synthesis in a cell begins with initiation, when a chain of messenger RNA (mRNA) carrying genetic instructions from deoxyribonucleic acid (DNA) attaches to a ribosome. The mRNA instructs the ribosome how to assemble amino acids to form a protein. Two molecules of transfer RNA (tRNA), each carrying an amino acid, join the ribosome-messenger RNA complex at two positions called P-site and A-site. A chemical bond known as a peptide bond is formed between the first two amino acids.

During elongation the tRNA in the P-site detaches from its amino acid and floats away from the complex while the tRNA carrying the two bonded amino acids moves from the A-site over to the P-site. This leaves the A-site open for a new tRNA molecule carrying a third amino acid to attach to the ribosome. The new amino acid bonds to the second amino acid with another peptide bond. Again, one tRNA is released and the remaining tRNA molecule, now carrying a chain of three amino acids, moves over to the P-site. The ribosome coordinates this cycle repeatedly until termination occurs, when the ribosome encounters a stop signal on the mRNA. The completed protein, which may be a chain of hundreds of amino acids, is released from the ribosome.
In general, DNA carries the genetic instructions for making all cellular structures. Since all cells contain ribosomes, scientists are comparing DNA instructions for making ribosomes in different species to learn how closely the species are related.

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