Ribosomes (cell biology)

RIBOSOMES 

1. Ribosomes were discovered by Palade in 1955 and the term 'ribosome' has been introduced by Roberts in 1958.  

2. Ribosomes are so called because of a higher content of ribonucleic acid.  

3. Ribosomes are the cell organelles discovered with the help of electron microscope.  

4. Ribosomes are found in all types of cells.  

5. In prokaryotes, they are found in the cytoplasm in free form and are called monosomes.  

6. In eukaryotes, ribosomes are usually associated with the membrane of endoplasmic reticulum.  

7. Ribosomes are attached to endoplasmic reticulum through ribophorins.  

8. Many ribosomes may be associated with m - RNA to form polyribosomes or polysomes.  

9. Ribosomes are the smallest organelles in the cell.  

10. The average size of ribosome ranges from 150 Å to 230 Å.  

11. Unlike other cell organelles, ribosomes are not bounded by membranes.  

12. Ribosomes, in addition to cytoplasm, also occur within mitochondria, chloroplasts and nuclei.  

13. So ribosomes are called as 'organelles within an organelle'.  

Physical Characteristics of Ribosomes

Source               Ribosome Type         Subunits

Prokaryotes     705                               30S  50S 

Eukaryotes (cytosol)   80S                  40S  60S 

Mitochondria  55S                               30S  40S 

Chloroplast      705                               335  50S 

14. Ribosomes of bacteria, mitochondria and chloroplasts are of 70S type.  

15. In prokaryotes 70S ribosome is found;  it is composed of two subunits 30S and SOS.  

16. Eukaryotic cytosol ribosomes are called 80S and composed of 40S and 60S subunits.  

17. Ribosomes are made up of r - RNA and proteins are

18. Ribosomes have almost equal amount of RNA and protein.  

Function of Ribosomes 

19. Ribosomes are the protein factories of the cell.  

20. The proteins synthesized on membrane - bound ribosomes pass through protein - translocating channels in the ER membrane and they give rise to secreted protein, integral membrane protein and proteins of lysosomes.  

21. The proteins synthesized by free cytoplasmic ribosomes give rise to soluble cytosol proteins, peripheral membrane proteins, mitochondrial proteins, chloroplast proteins, peroxisomal proteins and nuclear proteins.  

22. The proper folding of proteins following sythesis is assisted by specific proteins called chaperons which also assist in transport of proteins into organelles such as mitochondria.  

Biogenesis 

23. Biogenesis of ribosome involves the synthesis of ribosomal RNA and ribosomal proteins.  

24. In eukaryotes, the site of synthesis of most ribosoma!  (r - RNA) RNA is the nucleolus.  

25. The chief role of nucleolus concerns ribosome synthesis.