The primary function of ribosomes is synthesis of proteins according to the sequence of amino acids as specified in the messenger RNA.
While studying the plant and animal cell, you might have come across many organelles that coordinate together to carry out the cell activities. One such important cell organelle is the ribosome, which is responsible for protein synthesis. While mitochondria are considered as powerhouse of the cell for energy production, ribosomes are popularly related as the site for protein synthesis in a cell.
Ribosomes Composition and Structure
Ribosomes are composed of ribonucleic acid (abbreviated as RNA) and proteins, in nearly equal amounts. The ribonucleic acid is derived from the nucleolus, where ribosomes are synthesized in a cell.
A simple prokaryotic cell (for example, bacteria) consists of a few thousands of ribosomes, whereas a highly developed eukaryotic cell (for example, human cell) has a few millions of ribosomes. Prokaryotic ribosomes are smaller in size as compared to the eukaryotic ones.
Coming to the ribosomal structure, a typical ribosome consists of two subunits, each of which contains RNA and proteins. These two subunits are categorized with respect to the rate of sedimentation in a specific medium.
For example, the two subunits in a eukaryotic cell are 40S (smaller subunit) and 60S (larger subunit), where ‘S’ stands for the density unit, Svedberg. Thus, the higher the value assigned to the subunit, the larger is the size of the subunit.
Ribosomes Function in a Cell
In a cell, ribosomes are located in two regions of the cytoplasm. Some ribosomes are found scattered in the cytoplasm (referred to as free ribosomes), while others are attached to the endoplasmic reticulum (bound ribosomes). Accordingly, the surface of endoplasmic reticulum when bound with ribosomes is called rough endoplasmic reticulum (RER). Both the free ribosomes and bound ribosomes have similar structure and are responsible for production of proteins.
Speaking about the main functions of ribosomes, they play the role of assembling amino acids to form specific proteins, which in turn are essential for carrying out the cell’s activities. As we all have a fair idea regarding production of proteins, the deoxyribonucleic acid (DNA) first produces RNA (messenger RNA or mRNA) by the process of DNA transcription, after which genetic message from the mRNA is translated into proteins during DNA translation.
To be more precise about protein synthesis by ribosomes, the sequence for assembling amino acids during protein synthesis are specified in the mRNA. The mRNA synthesized in the nucleus is then transported to the cytoplasm for further continuation of protein synthesis. In the cytoplasm, the two subunits of ribosomes bind around the mRNA polymers and synthesize proteins with the help of transfer RNA (tRNA), as per the genetic code. This whole process of protein synthesis is also referred to as central dogma.
Usually, the proteins synthesized by the free ribosomes are utilized in the cytoplasm itself, while the protein molecules produced by the bound ribosomes are transported outside the cell. Considering the primary function of ribosomes in constructing proteins, it is understandable that a cell cannot function without ribosomes.