Ribosomes

Palade discovered ribosomes in 1953 in animals. Robinson and Brown discovered ribosomes in plant cells. Ribosomes are non-membranous bounded structures. Ribosomes are the sites where the cell synthesizes proteins according to genetic instructions.

A bacterial cell may have a few thousand ribosomes. But a human liver cell has a few million ribosomes. Cells that have high protein synthesis have a particular great number of ribosomes. Cells active in protein synthesis also have prominent nucleoli. These nucleoli synthesize the ribosomes.

Chemical composition

Ribosomes contain equal amount of proteins and rRNA. So they are also known as ribonucleoprotein. The protein contains a large number lysine and arginine amino acids. These amino acids contain positive charges. It makes the ribosome strongly negative. There are four rRNAs molecules in eukaryotic ribosomes. 18S r RNA is present in small unit of ribosome, 20S, 2.8S and 18S rRNAs are present in larger subunits. The prokaryotic have only three molecules of r RNAs.

Types of Ribosomes

Ribosomes functions in two cytoplasmic locations.

Free Ribosomes: They are suspended in the cytoplasm. Most proteins synthesized by the free ribosomes functioned within the cytosol. The free ribosomes are abundant in the cells growing by the addition of cytoplasm.

Bound Ribosomes: They are attached to  the outside of the endoplasmic reticulum. The proteins of bound ribosomes are stored in the inclusion of membranes. Or these proteins are exported from the cell. Some cells specialize in the protein secretion like pancreas and other glands. These cells secrete digestive enzymes. They have a high proportion of bound ribosomes.

Structure of Ribosomes

The structure of ribosomes may be 23nm. Bound and free ribosomes are structurally identical and interchangeable. The cells can adjust the numbers of ribosomes. Each ribosome has two subunits. The larger subunit is 60S and smaller subunit is 40S in eukaryotic ribosomes. Both ribosomal units joins by Mg++ ions to form functional 80S ribosome. The prokaryotic ribosome is 70S with subunits. The larger subunit is 50S and the smaller subunit is 30S. The ribosomal subunits are constructed in  the nucleolus from RNA in eukaryotes. The rRNA is produced in the nucleus. Protein is imported from the cytoplasm. During protein synthesis, several ribosomes are attached to the same mRNA. It forms a structure called polysome. Thus single ribosome is translated many times.

Synthesis of Ribosomes

Eukaryotic ribosomes are synthesized in nucleolus. The 18S, 28S, and 5.8S RNAs are synthesized in nucleolus. 5S RNA is synthesized on the chromosome outside the nucleolus. The ribosomal proteins are synthesized in cytoplasm. All these components are then transferred in cytoplasm. All these components are then transferred to nucleolus. Nucleolus assemble the smaller and large subunits of ribosomes from these components.

Functions of Ribosomes

The ribosome plays a key role in translating the genetic message. This genetic message is carried by mRNA from the nucleus to the cytoplasm. This message is translated in the ribosomes. It forms specific primary structure (amino acid sequence) of a polypeptide chain.

The ribosomes of prokaryotes are smaller than eukaryotes. They are different in their molecular composition. This dissimilarity in a structure is fundamental. It differentiates the prokaryotes from eukaryotes. The difference in ribosomes is also medically significant.

Certain drugs (antibiotics) can effect the prokaryotic ribosomes. But they do not affect the eukaryotic ribosomes.