Ribosomes | Definition, Structure, Types & its Functions
“Ribosomes are one of the most significant cell organelles composed of RNA and protein that transforms genetic code into chains of amino acids.”
It is discovered by George Palade in 1953 in the animal cell. They were observed under electron microscope as dense particles present in the cytoplasm. They are composed of rRNA & proteins & are not surrounded by any membrane.
Structure of Ribosomes
The robosomes are composed of two subunits. One ribosomal subunits is large in size & has a dome – like shape, while the other ribosomal subunits are smaller in size & occur above the larger subunits & forms a cap – like structure.
The two ribosomal subunits remain united with each other due to a specific concentration of the Mg2+ ions. When the concentration of Mg2+ ions reduces below a critical level both ribosomal subunits get separated. But, by raising the concentration of Mg2+ ions in the matrix, the two ribosomes become associated with each other & is known as the dimer. During protein synthesis, many ribosomes form a chain on a common messenger RNA & form the polyribosomes or polysomes, or ergasomes.
Types of Ribosomes
There are of two types – 70s & 80s, where S represents the sedimentation coefficient which indirectly measures the density & size of the robosomes. The ribosomes of prokaryotes are 70S while the eukaryotic ribosomes are 80S. In mitochondria & chloroplast, ribosomes are 70 S.
(a) 70 S ribosomes are made up of ribonucleoproteins in the ratio of 60 : 40 (RNA : Protein)
70 S ribosomes consisits of :
- 30 S smaller subunits – 21 protein molecules & 16 S rRNA.
- 50 S larger subunits – 34 proteins molecules & 23 S & 5 S rRNA.
(b) 80 S ribosomes are made up of ribonucleoproteins in the ratio of 40 : 60 (RNA : Protein)
80 S ribosomes consists of :
- 40 S smaller subunits – with 33 protein molecules & a single 18S – rRNA.
- 60 S larger subunits – about 40 protein molecules & three types of rRNAs – 28S, 5.8S & 5S.
Ribosomes are sites of protein synthesis. The free ribosomes synthesize non – secretory proteins while ER bound ribosomes synthesize secretory proteins. Free ribosomes synthesise structural & enzymatic protein for use inside the cell. The attached ribosomes synthesise proteins for transport. Thus, these organelles are also known as protein factories. Newly synthesized proteins are processed with the help of chaperons protein.