These are dense granular nucleoprotein structures occurring in cytoplasm, matrix of mitochondria and chloroplasts. In many instances ribosomes are attached to the ER.
Observed first in plant cells in 1953 by Robinson and Brown while studying bean roots, these were later discovered in animal cells also. Ranging in diameter from 150 – 200A they have RNA and protein in equal quantities.
Ribosomes are isolated by differential centrifugation depending on sedimentation coefficient. The sedimentation coefficient is expressed in terms of Svedberg units. The S units are related with the size and weight of the ribosome molecules.
Image Source: online.science.psu.edu
Types of ribosome’s:
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Two types of ribosomes have been identified based on the sedimentation coefficient which shows how soon a cell organelle cans sediment under centrifugation.
If the organelle is heavier, its sedimentation coefficient is more. The two types of ribosome are – 70s ribosomes and 80s ribosomes.
The 70s ribosomes are comparatively smaller and have a molecular weight of 2.7 x 106 dal tons (Dalton is a unit of molecular weight; one Dalton = weight of one H atom = 1) According to Huxley and Zubay (1960) the size of a 70s particle is 170 x 170 xx 200A.
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These ribosomes occur in prokaryotic cells as well as in chloroplasts op eukaryotic cells. The 70s ribosomes have two sub units – a large 50s sub unit and a small 30s sub unit.
The 80s ribosomes have a moleculer weight of 4 x 106 daltons, and are predominantly found in eukaryotic cells. There are two sub units in a 80s ribosome. The:se are – 60 and 40s.
Ribosomeis may occur singly as isolated units when they are called monosomes. “When they occur in clusters or groups, they are called polyribosomes-. Tbe polyribosomes may have a sedimentation coefficient of 100s-600s.
The number of ribosomes per cell varies, it may be 10,000 (bacterial cell) or up to 10 million (eukaryotic cell).
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Ribosomes of chloroplasts and mitochondria have their own protein synthesis. They have a sedimentation coefficient of 55s with two sub units 40s and 30s.
Ultra structure of ribosome’s:
Ribosomes are oblate or spherical structures having two sub units (a large and a small). The large sub unit is dome like and the smaller subunit is placed above like a cap. The 70s ribosome has two units 50s and 30s as already mentioned.
The 50s sub unit has a diameter of 140- 160A°andis pentagonal in shape. In the center of this sub unit occurs a round area of 40 – 60A°. According to Fernando (1968), there is a pore like transparent area in the centre which prohibits the entry’ of proteolytic enzymes.
The pores have been found in 50s sub units of 80s ribosome’s also. The smaller sub limits of both the types of Ribosomes are often divided into two portions which remain connected to each other by a strand of 30 – 69A° Thickness-
Models for Ribosomes structure:
There are two models, viz., Wittmann’s quasi symmetrical model and Lake’s asymmetrical model. Accordinng to Wittmann’s model, the 30s sub unit is an elongate slightly bent structure,, having a head and a larger body with a cleft in between; with two parts protrudiHng uniequally. It appears like a telephone receiver.
The 50s sub unit is variously shaped – round shaped, maple leaf .shaped etc. The 50s sub unit has thread protruberances. The two s sub units of 70s ribosome’s have four areas of contact. The mRNA binding, regioon is located on the right side of the 30s sub unit.
Lake’s rrrnodel regards the 30s sub unit to be totally asymmetrical. The sub unit is divideod into two parts (1/3 and 2/3) unequally. Extending from the lower 2/3 region is sa portion called the platform.
This cleft is important in the sense of location of codon – anticodon interaction. The major difference between the Wittmanns nnodel and Lake’s model for the 30s sub unit is that the latter regards the 30s sub unit as totally asymmetrical. According to Lake, even the 50s subunit is asymmetrical.
Biogenesis of Ribosomes:
The synthesis of eukaryotic ribosome is a complicated process involving several areas of the cell. The genes coding for 18s, 28s and; 58s RNA are in the nucleolar organizing part of the chromosomes; the 5s genes are localized elsewhere in the chromosome.
The ribosomal proteins are synthesized in the cytoplasm. The proteins and 5s gene products move towards the rnucleolus, where they are assembled into ribosomes. Evidence for nucleolus being the site of ribosome biogenesis has been obtained by observations on the oocytes of Xenopus laevis
In bactesria, the ribosomes (70s) are synthesized in the cytoplasm as the cistron porti on produces a ribosomal RNA which is immediately released
Functions of ribosome:
As is very well known, Ribosomes (poiyribosonmes) are the sites of protein systheses. The polyribosomes serve as a platform for the assembly of amino acids (on a mRNA template) brought together by a specific tRNA from cytoplasm.