In this article we will discuss about the subject-matter and structure of Ribosomes present in the cells of plant and animals.
Subject-Matter of Ribosomes:
In the cytoplasm of the cell there are found particles composed of RNA (ribonucleic acid) and proteins. These are called ribosomes. These particles occur in all the cells that synthesize proteins and they are involved in the synthesis of cellular proteins.
The sizes of ribosomes vary somewhat, being approximately 150 Å in bacteria, chloroplasts and mitochondria and 140 to 200 Å in the cytoplasm of eukaryotic plant and animal cells.
The ribosome particles are the integral part of rough surfaced endoplasmic reticulum and microsomes. However, it has now been shown that these particles exist in free state in the cytoplasmic matrix. In young meristematic cells, the ribosome granules lie freely in the cytoplasmic matrix and the young endoplasmic reticulum appears to be smooth surfaced.
In bacteria and other prokaryotes the ribosomes seem to exist freely in the cell. Ribosomes are often attached in rows or in clusters. These clusters or strings of ribosomes are called polyribosomes or polysomes (Fig. 5.7). The polysomes consist of such ribosomes as are actively engaged in protein synthesis and are held together by a string of messenger RNA (mRNA).
Structure of Ribosomes:
Ribosomes are generally defined by their sedimentation coefficients or Svedberg units or ‘S’ values (Svedberg unit is calculated on the basis of several factors). Essentially it represents sedimentation constant (in seconds) per unit field of force and is equal to 1 x 10-13).
On the basis of sedimentation coefficients there are two categories of ribosomes:
(1) Ribosomes found in bacteria, blue-green algae and those found in chloroplasts and mitochondria of higher organisms exist as 70 S particles (varying from 67 S to 73 S).
(ii) Ribosomes found in the cytoplasmic matrix of eukaryotic cells exist as 80 S particles (varying from 77 S to 81 S).
Each ribosome consists of two sub-units: a smaller sub-unit (molecular weight 8,00,000) and a bigger sub-unit (molecular wt. about 108 million).
70 S ribosome particles can be split into 50 S larger sub-unit and 30 S smaller sub-units (Figs. 5.8 and 5.9). Actually the two sub-units of ribosomes found in bacteria occur freely in cytoplasm and they unite only during protein synthesis. 80 S particles can be split into 60 S larger sub-unit and 40 S smaller sub-unit (Fig. 5.10).
The 40 S sub-unit occurs above the 60 S sub-unit forming a caplike structure and the 60 S sub-unit is dome-shaped and it remains attached to the membranes of canaliculae in fixed ribosomes. The structure of ribosomes is still less worked out.
According to Nauninga (1967), the 50 S sub- unit of 70 S ribosome is pentagonal campact particle of 160 to 180 Å with a round area of 40-60 Å in diameter. Florends (1968) has noticed a pore-like transparent area in 50 S sub-unit which does not allow entrance of the proteolytic enzymes or ribonuclease enzyme. Such pores also occur in 60 S sub-units of 80 S ribosomes.
It is not exactly known how the two sub-units come together to make up the whole ribosomes. The association and dissociation of these two sub-units depend on Mg++ ion concentration, low Mg++ concentration causes separation of two sub-units while the increased Mg++ ion concentration causes association of two sub-units into complete ribosomes.
At high Mg++ ion concentration in the cytoplasmic matrix two or more ribosomes become associated with one another to form dimer or polymer.
The chemical constitution of ribosomes is more or less the same in all the cases and they contain 40 to 60% of rRNA, the rest being all proteins of different kinds. The ribosomal proteins are mostly of basic nature and of about 20,000 mol. wt. Ribosomal RNA (r RNA) has a very high molecular weight, approximately 20,000.
The forces that hold ribosomal RNA to ribosomal proteins are either electrostatic in nature forming salt-like bonds between phosphate groups of r RNA and the amino groups of the basic amino acids in proteins or bonds involving magnesium complexing between the same groups; most likely they are combination of both.
RNA can be removed from protein by agents that break just such bonds as strong salt solutions or magnesium complexing reagents.
The rRNA of the ribosomes is thought to be almost entirely covered by the proteins. Spirin (1964) has shown that each of the two ribosomal sub-units has the form of a long RN A-protein filament which is intricately coiled to produce the globular form of mature ribosome.
When two sub-units of 70 S ribosomes dissociate, the inactive core particle and split proteins (SP) with different sedimentation co-efficient are obtained. The core particles can further be dissociated into RNA and core protein and the split proteins can further be fractionated into acidic proteins (SP-A) and basic split proteins (SP-B).
The larger subunit of ribosome is composed of one large RNA molecule and a small RNA molecule and several protein chains (nearly 30 different proteins) while the smaller sub-unit consists of just one RNA molecule and several (nearly 20) protein chains.
50 S sub-units of prokaryotic ribosome have a 23 S RNA (mol. wt. 1.1 million) and 5 S RNA (mol. wt. 30,000) while the 60 S sub-units of eukaryotic ribosomes have a 28 S RNA (mol. wt. 1.6x 106 daltons), 5.8 r RNA and 5 S RNA.
The function of 5 S RNA is unknown. 30 S (smaller subunit of 70 S ribosomes) and 40 S sub-units (small sub-unit of 80 S ribosomes) have 16 S RNA (mol. wt. 5,00,000) and 18 S RNA respectively (Fig. 5.11).
The protein synthesis is conducted by ribosomes. The protein synthesis conducted by 70 S ribosomes is inhibited by the antibiotic-chloromycetin whereas that conducted by 80 S ribosomes is inhibited by the antibiotic cycloheximide and the reverse does not hold. Thus the two different types of particles differ from each other.
Ribosomes are not self-replicating particles. Recent studies have established that the cytoplasmic ribosomal sub-units (large and small) are synthesized in the nucleolus from where these sub-units, along with residual accessory proteins are rapidly transported to the cytoplasm where discarding the accessory proteins, the ribosomal sub-units combine with m RNA to form polyribosomes (Fig. 5.12).
