Ribosomes

 Ribosomes

Ribosomes are important granular structures found in millions in eukaryotic cells and in thousands in prokaryotic cells either inthe free form or bound with endoplasmic reticulum. They have a very important function in protein synthesis that’s why they exist in a large number of cells. Ribosomes are not regarded as organelles due to the absence of membranes and their presence in prokaryotic cells.

Discovery of Ribosomes

Ribosomes were first distinguished by Claude in 1941 and named microsomes. Palade in 1953 named them ribosomes.

Ramakrishnan, Steitz and Ada E. Yonath described the structure and functions of ribosomes and for their work, they were given the Nobel Prize in 2009.

Structure of Ribosomes

Ribosomes are structures composed of protein and ribosomal RNA. Hence, they are also knon as ribonucleoprotein particles. Almost equal amounts of proteins and ribosomal RNA combine to make the structure of ribosomes. 

In eukaryotic cells, Each ribosome has two sub-units. There is a large subunit of the 60s and a small subunit of the 40s. The complete size of large and small subunits is the 80s due to the merging of both subunits. The size of the prokaryotic ribosome is 70s.

Each ribosome has three binding sites, namely, A-site, P-site and E-site.

• A-site is for the fitting of RNA containing activated amino acids.
• P-site is for the fitting of RNA-containing polypeptide chain.
• The E-site is the exit site where the release of deacylated tRNA occurs.

In eukaryotes, only two sites are present, and E- the site is absent.

How are ribosomes assembled?

Ribosomes are continuously formed inside the cells. They are assembled in the nucleolus. The mRNA synthesized from DNA inside the nucleus carrying the code for ribosomal protein travels out of the nucleus through the nuclear pore. This mRNA attaches to the already present ribosome in the cytoplasm and the ribosome will decode the message in the form of protein and will make ribosomal protein in the form of a large and a small subunit of the ribosome. Both of these subunits enter the nucleus through nuclear pores and then to the nucleolus. The nucleolus contains ribosomal RNA in large amounts and it combines the ribosomal RNA and subunits of ribosomes to make a complete assembly of ribosomes. The ribosome will go to the cytoplasm to perform the function of protein synthesis.

Functions of Ribosomes

• The main function of ribosomes is protein synthesis that’s why ribosomes are also known as protein factories.
• The free ribosomes make non-secretory proteins. On the other hand, the bound ribosomes make secretory proteins.
• Ribosomes move along mRNA to read the codes.
• Ribosomes recruit the correct tRNA-carrying amino acids.
• They link the amino acids by peptide bonds.