When we observe a living cell, we can see a variety of living bodies of definite structures and functions suspended in the cytoplasm, known as organoides or organelles. These organelles are the main sites for the various cytoplasmic activities. Some of these are concerned with the chemical activities or metabolism of the cytoplasm. Ribosome is one of them. Ribosomes are particles composed of ribonucleic acid (RNA) and proteins. These particles are the integral part of rough endoplasmic reticulum and microsomes. But they also exist freely in cytoplasm, e.g. in young meristematic cells.
Ribosomes are often attached in rows or clusters. These clusters or strings of ribosomes are called polyribosomes or polysomes. They consist of such ribosomes that are very active in protein synthesis and are held together by a string of messenger RNA (mRNA).
- Hanstien and Altmann (1882) found some very small granules in the cytoplasm of cell. They termed this granules as microsome.
- Claude (1954) got the similar types of particles after centrifuging the liver cells and termed this particles as microsome like Hanstien and Altmann.
- In 1955, George Emil Palade discovered ribosomes and described them as small particles in the cytoplasm that mainly remain with endoplasmic reticulum. He, along with other scientists, discovered that ribosomes performed protein synthesis. For his work, Palade won the Noble prize in 1974.
- Later, Richard B. Roberts first proposed the term “ribosome” in the end of 1950s.
- Sevetz (1965) observed that kind of particles under electron microscope and described microsome as a small portion of rough walled endoplasmic reticulum. It consists of –
- membrane of E.R.
- many small particles i.e. ribosome
- Generally, they are found on the rough-walled endoplasmic reticulum.
- In case of eukaryotic organism, ribosomes are found within the mitochondria, chloroplast and also in cytoplasm freely.
- In case of prokaryotic organism, ribosomes are found in cytoplasm freely.
- The ribosome granules in young meristematic cells lie freely in the cytoplasmic matrix.
Classification on the basis of S-value
Swedish biochemist Svedberg observed that after keeping ribosomes in less concentration of Mg (0.001M) solution, if it is centrifuged then the ribosomes are sedimented. Sedimentation rate is constant for similar types of ribosomes and it is called as “Svedberg constant” or “S-value”.
On the basis of S-value, ribosomes are two types:
- 70S (found in prokaryotic organisms like bacteria, blue-green algae etc.)
- 80S (found in eukaryotic organisms)
Each ribosome consists of two sub-units: a smaller sub-unit and a larger sub-unit. Both sub-units composed of long strands of RNA.
70S ribosome particles split into 50S larger sub-unit and 30S smaller sub-units. Actually this two sub-units occur freely in bacteria cytoplasm and unite only during protein synthesis. Similarly, 80S particles split into 60S and 40S sub-units. The 40S sub-unit occurs above the 60S sub-unit forming a cap-like structure. The 60S sub-unit is dome-shaped and attaches to the membranes of canaliculae in fixed ribosomes.
The association or dissociation of the two sub-units depends on Mg2+ ion concentration. Low Mg2+ concentration causes separation of two sub-units, while high Mg2+ concentration causes association of them into complete ribosome. Moreover, at high concentration of Mg2+ ion in the cytoplasmic matrix, two or more ribosomes become associated with one another to form dimer or polymer.
D. Watson (1958) isolated ribosome and analyzed from E. coli. He observed that there are two main components:
- 40% RNA (specially r-RNA)
- 60% Protein
- In E. coli, ribosome is about 22% of their total dry weight.
- r-RNA is about 98% of the total RNA.
Spirin (1964) has shown that each of the two ribosomal sub-units has the form of a long RNA-protein filament coiling to produce the globular form of mature ribosome. The larger sub-unit has one large RNA molecule as well as a small RNA molecule, along with several protein chains. On the other hand, the smaller sub-unit consists of just one RNA molecule and several protein chains.
- At first, people believed that ribosome has a uniform structure. Nauringa (1967) proved that it has two components. The larger one ( 50S sub-unit of 70S ribosome) is a pentagonal structure of 100Å that attach with a globular structure of 40-60Å in diameter.
- In bacteria and other prokaryotes, chloroplast and mitochondria of eukaryotes, ribosomes are 150Å in diameter. However, ribosomes in the cytoplasm of plant and animal cells are about 140 to 200Å in diameter.
- Ribosome is bounded by a membrane (thickness 70Å) that could not be dissolved by ribonuclease, however, dissolve with protease. It means, the wall is made up of protein. There are many soluble protein present in ribosome which are basically different enzymes.
- Ribosomes have a groove at the junction of large and small sub-units.
- The m-RNA remains in the gap between both ribosomal sub-units, where the ribosome protects a stretch of some 25 nucleotides of m-RNA from degradation by ribonuclease.
- From this groove, a canal or tunnel extends through the large sub-unit and opens into the lumen of the endoplasmic reticulum.
- The groove between the two ribosomal subunits is the site for the synthesis of polypeptides. After that, they pass through the tunnel of the large subunit into the endoplasmic reticulum.
Functions of Ribosome
Ribosomes are the protein factories of cell. The one & only function of them is to synthesize protein by the process of Translation.
Attached Ribosomes: Attached to endoplasmic reticulum and other organelles.
- These ribosomes provide space and enzymes for the synthesis of proteins in the cell. The ribosomes bound to the ER membranes synthesize:
- integral proteins for cellular membranes
- lysosomal proteins and
- secretary proteins for export as secretions.
Free Ribosomes: Found freely in cytoplasm.
- The free ribosomes produce structural and enzymatic proteins for use in the cell itself.
References & Other Links
- Class lecture of Chandan Kumar Dash Sir (Lecturer, Dept. of Botany, University of Dhaka).
- Book- “Cytogenetics, Evolution, Biostatistics & Plant Breeding” by R.S. Shukla, P.S. Chandel.
- Ribosome from Scitable by Nature Education .
- Ribosome from Slideshare.