Ribosome: A Short Introduction
Ribosomes might be small in size but they do a big job. Ribosomes are tiny factories to make proteins and help the cells function properly. These macromolecules are present in every living cell whether prokaryotic cell or eukaryotic cell. Mammalian cells can be as many as 10 million ribosomes.
In prokaryotic cells, ribosomes are assembled in the cytoplasm, and in the eukaryotic cell, they are produced in nucleolus from where they are transported to the cytoplasm via the nuclear pores in the nuclear membrane. The process of making ribosomes is called Ribosome biogenesis. Ribosomes can be found alone or in groups within the cytoplasm. In eukaryotic cells, ribosomes are also attached to the endoplasmic reticulum (ER) to form a rough endoplasmic reticulum (RER). RER works with ribosomes in the preparation of protein.
Ribosomes are composed of RNA (ribonucleic acid) and protein. Therefore, it is also called ribonucleoprotein particles. The protein present in the ribosome is called ribosomal protein. Similarly, the RNA present in ribosomes is called ribosomal RNA.
Eukaryotic 80S ribosomes are larger than prokaryotic 70S ribosomes. The 80S and 70S are the sedimentation coefficients which are expressed in the Svedberg units. The sedimentation coefficient is calculated by the mass, shape, density, and velocity of the particle.
Ribosomes consist of two major components of different sizes, large ribosomal subunit and small ribosomal subunit. 80S ribosomes are composed of a 60S large subunit and a 40S small subunit whereas 70S ribosomes are composed of a 50S large subunit and a 30S small subunit. Both large and small subunits combine together to form protein.
Messenger RNA (mRNA) carries the information of specific protein synthesis copied from DNA. A small ribosomal subunit reads the mRNA and the large subunit joins the amino acids to form a polypeptide chain of protein. The attachment of the subunit is controlled by the presence of Mg2+ ions. A group of ribosomes attached to mRNA is known as polysome.
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