RIBOSOME site of protein synthesis
- Ribosomes are round, granular and membraneless cell organelle which are chemically nucleoprotein and found enormously in all the prokaryotic and eukaryotic cells.
- These were discovered first by Claude in 1943 and were named as ‘microsomes’.
- Robinson and Brown isolated ribosomes from root cells of broad beans.
- Palade coined the term ‘ribosomes’ and isolated it from animal cells.
- After his name, ribosomes are also called ‘Palade granules.’
- Ribosomes may be defined as “The smallest known electron microscopic, ribonucleoprotein particles attached to the on RER or floating freely in the cytoplasm and are the sites of protein biosynthesis”.
OCCURRENCE:
- Ribosomes are generally found in all known prokaryotic and eukaryotic organisms except mature RBCs.
- In prokaryotes these are found only in free form in the cytoplasm while in eukaryotes these are found both in the cytoplasm and on the surface of RER.
- The former is called cytoplasmic and later is called a bound form of ribosomes.
- These may also be found on the surface of nuclear membranes.
- Some organelles like mitochondria and chloroplast contain ribosomes in the matrix.
- These are called organellar ribosomes and referred to as ‘mitoribosomes’ and ‘plastidoribosomes’.
- The ribosomes found on the surface of RER are bound with the membrane with special proteins called ribo-phorines.
Number:
- The number of ribosomes in a cell depends on the content of RNA.
- These are more in number in metabolically active cells like plasma cells, livercells, nissl’s granules of nerve cells, meristemati c cells, cancer cells, endocrine cells etc.
- In a cell of E.coli, the number of ribosomes vary from 10,000-20,000.
Structure:
- Ribosomes are globular structures having the diameter of 150-250 Å.
- Each ribosome is made up of two subunits one is smaller and another is larger in size.
- The latter is dome shaped and is covered by a cap like a smaller unit.
- In the 70S type of ribosome the larger and smaller units are 50S and 30S type.
- On the other hand, in 80S type, these are of 60S and 40S type, respectively.
- The two subunits of ribosomes are freely distributed in the cytoplasm.
- The two subunits unite to form a complete ribosome.
- Likewise, the two subunits dissociate with each other when the concentration of Mg++ ion decreases in the cytoplasm.
- During protein synthesis many ribosomes become attached with mRNA forming a peculiar structure called polyribosome.
Ultrastructure of Ribosomes
- The last point about the ultrastructure of ribosomes has not been said till date.
- The credit of giving the present knowledge of the ultrastructure of ribosomes goes to Nauninga.
- According to him, the size of the larger (50S) subunit of the 70S type of ribosome is 160 to 180 Å which is pentagonal in shape.
- This unit has a groove of 40-60 Å size in which the smaller subunit is attached during association.
- The smaller subunit has a platform, cleft, head, base and also a binding site for nRNA.
- The smaller unit of 70S and 80S type of ribosomes does not have a definite shape.
- Florendo in 1968 reported a pore-like transparent area on the larger unit of 50S of 70S ribosomes.
- In between two subunits of ribosomes, mRNA is found. t-RNA molecule is found in the side of nRNA.
- The newly-formed polypeptide chain being synthesized on the ribosome mRNA complex has been seen passing through the transparent pore on the larger unit.
- It also has a protuberance, a ridge and stalk.
- Two binding sites, peptidyl and amino acyl sites are found on the larger units
- The 50S and 30S subunits have been reported to have the molecular weight of 1.8 X 106 Daltons and 0.9 X 106 Daltson, respectively.
- It must be noted here that size and type of ribosomes and their subunit are determined on the basis of their sedimentation coefficient.
Types of Ribosomes
- On the basis of their sedimentation coefficient, ribosomes have been classified into two main types:
70S ribosomes:
- These are found in prokaryotes and mitochondria and plastids of eukaryotes.
- Each 70S ribosome is about 200-290 Å in size and 2.7 X 106 Daltons in its mol.
- Weight.
- It consists of two subunits of 50S and 30S size. Both of these units are made up of ribosomal RNA and ribosomal proteins.
- The 50S subunits again consist of 23S and 5S rRNA and 30 types of proteins. Similarly, the smaller unit is made up of 16S types of rRNA and 20 types of proteins.
80S ribosomes:
- These are the characteristics of eukaryotic cells and found. In their cytoplasm.
- It consists of two subunits.
- The size of larger subunits is 60S and that of smaller subunits is 40S.
- It is also made up of rRNA and proteins.
- The 60S subunit consists of 28S rRNA, 5.8 S rRNA and 5S rRNA and about 50 types of proteins.
- The smaller subunits are similarly made up of 18S rRNA and 30 different proteins.
Polyribosomes or Polysomes:
- When many ribosomes are attached to the same mRNA strand, it is called polyribosomes or polysomes.
- It is formed when a simple protein is required in high quantities.
- The number of ribosomes in a polysome depends on the length of mRNA.
- The distance between two adjacent ribosomes is about 90 nucleotides.
Origin of ribosomes:
- We have studied that ribosomes are solely made up of rRNA and proteins.
- The former is formed inside the nucleus and the later is produced in the cytoplasm. Therefore, these are partly nuclear and partly cytoplasmic in nature.
- However, in prokaryotes, since there is no nucleus, ribosomes are totally cytoplasmic in nature.
Functions of Ribosomes.
- Ribosomes are called factories of proteins or engineers of the cell because these are the side of protein synthesis.
- Sometimes rRNA of ribosomes has been found to function as enzymes controlling the cellular functions. These are called ribozymes.
- The process of translation of genetic language into the language of enzymes or protein take place at ribosomes. It takes place with the help of nRNA, which, is produced during transcription of nuclear DNA.
- In general the ribosomes bound on RER synthesise enzymes for extracellular use e.g., pancreatic cells, chief cells of gastric glands, liver cells etc.
- Ribosomes temporarily store proteins.
- Ribosomes keep the mRNA molecules functionally alive.
RIBOSOMAL RNA or rRNA
- The RNA, which is found in ribosomes, is called ribosomal RNA.
- Ribosomes are chemically ribonucleoprotein as they consist of RNA and proteins.
- It is known as soluble RNA.
- Its quantity in a cell is much higher than that of mRNA and tRNA.
- It constitutes about 80% of total RNA.
On the basis of their sedimentation coefficient or rate of sedimentation, rRNA molecules may be classified into following categories:
- 28S-rRNA: It has molecular weight more than 10,00000. Sedimentation coefficient is between 21S and 29S. It is found in 60S subunit of eukaryotic ribosomes.
- 18s-rRNA: It molecular weight is less than a millions. Sedimentaion varies between 12S to 18S. It is found in 40S subunit of ribosomes.
- 5S-rRNA: It has much lower molecular weight and is found in 30S unit of ribosomes.
Structure of rRNA
- Ribosomal RNA molecules are single stranded but in the solution of high ionic concentration, irregular spiral coiling of rRNA is formed.
- As the ionic concentration of the solution increases, the degree of irregular coiling of rRNA also increases.
- In this coiling the intramolecular bases show base pairing.
- The pairing is normal as A pairs with U and C pairs with G.
Function of rRNA
The main function of rRNA may be summarized as below:
- In many viruses specially in plant viruses, RNA function as genetic material and carry genetic information from generation to generation.
- Different RNAs function as structural component of a cells
- mRNA is the site of protein synthesis where polymerization of amino acids takes place through peptide bond formation between amino acid molecules during the translation process.
- A tRNA molecule has an anticodon site and has the capacity of attachment with the complementary codon on mRNA.
- tRNAs further carry activated amino acids to the mRNA and catalyze peptide bond formation between two amino acid molecules.
- Ribosomal RNA(rRNA) is the constituent unit of ribosomes.
1. Who first discovered ribosomes and what were they initially called?
A. Claude; Microsomes
B. Palade; Ribosomes
C. Robinson and Brown; Ribosomes
D. Nauninga; Ribosomal units
✅ Answer: A. Claude; Microsomes
Explanation: Ribosomes were first discovered by Claude in 1943 and were initially called microsomes.
2. Ribosomes are found in all cells except: A. Prokaryotes
B. Eukaryotes
C. Mature RBCs
D. Mitochondria
✅ Answer: C. Mature RBCs
Explanation: Ribosomes are not found in mature RBCs because they lack a nucleus and do not synthesize proteins.
3. What is the size of a typical 70S ribosome? A. 100-150 Å
B. 150-250 Å
C. 200-290 Å
D. 300-350 Å
✅ Answer: B. 150-250 Å
Explanation: Ribosomes are typically 150-250 Å in diameter.
4. Which of the following subunits are present in 70S ribosomes? A. 50S and 30S
B. 60S and 40S
C. 28S and 5S
D. 23S and 5S
✅ Answer: A. 50S and 30S
Explanation: 70S ribosomes consist of a larger 50S subunit and a smaller 30S subunit.
5. What is the primary function of ribosomes?
A. Protein synthesis
B. DNA replication
C. Energy production
D. Cell division
✅ Answer: A. Protein synthesis
Explanation: Ribosomes are the sites where protein synthesis (translation) occurs.
6. Where are ribosomes found in prokaryotes? A. On the surface of the RER
B. In the cytoplasm
C. In the mitochondria
D. In the nucleus
✅ Answer: B. In the cytoplasm
Explanation: In prokaryotes, ribosomes are found freely in the cytoplasm.
7. What is a polyribosome or polysome? A. A type of RNA molecule
B. A ribosome bound to the nucleus
C. A structure formed by multiple ribosomes attached to a single mRNA
D. A protein complex
✅ Answer: C. A structure formed by multiple ribosomes attached to a single mRNA
Explanation: Polyribosomes are formed when multiple ribosomes are attached to a single mRNA molecule.
8. What does the ribosome consist of? A. RNA and proteins
B. Lipids and carbohydrates
C. Only RNA
D. DNA and proteins
✅ Answer: A. RNA and proteins
Explanation: Ribosomes are composed of ribosomal RNA (rRNA) and proteins.
9. Which ribosomal subunit is larger in the 80S ribosome? A. 30S
B. 50S
C. 60S
D. 40S
✅ Answer: C. 60S
Explanation: In 80S ribosomes, the larger subunit is 60S.
10. What is the function of ribosomal RNA (rRNA)? A. To store genetic information
B. To catalyze protein synthesis
C. To replicate DNA
D. To transport proteins
✅ Answer: B. To catalyze protein synthesis
Explanation: rRNA plays a critical role in catalyzing the synthesis of proteins by providing the site for peptide bond formation.
11. Which of the following ribosomal subunit types is found in mitochondria? A. 70S
B. 80S
C. 50S
D. 60S
✅ Answer: A. 70S
Explanation: Mitochondria contain 70S ribosomes similar to those in prokaryotes.
12. What protein complex helps bind ribosomes to the endoplasmic reticulum? A. Ribophorins
B. RNA polymerase
C. DNA helicase
D. DNA polymerase
✅ Answer: A. Ribophorins
Explanation: Ribophorins are proteins that help attach ribosomes to the rough endoplasmic reticulum (RER).
13. What type of RNA is found in ribosomes? A. mRNA
B. tRNA
C. rRNA
D. snRNA
✅ Answer: C. rRNA
Explanation: Ribosomal RNA (rRNA) is a key component of ribosomes and aids in protein synthesis.
14. Which rRNA is part of the 60S subunit in eukaryotic ribosomes? A. 18S
B. 5S
C. 23S
D. 28S
✅ Answer: D. 28S
Explanation: The 28S rRNA is a component of the 60S subunit of eukaryotic ribosomes.
15. What is the role of the 5S rRNA in ribosomes? A. It is part of the smaller ribosomal subunit
B. It helps bind the ribosome to the mRNA
C. It catalyzes peptide bond formation
D. It stabilizes the ribosome structure
✅ Answer: D. It stabilizes the ribosome structure
Explanation: The 5S rRNA helps stabilize the overall structure of the ribosome.
16. Which of the following is a characteristic of the 70S ribosome? A. Found in eukaryotic cytoplasm
B. Composed of 60S and 40S subunits
C. Found in prokaryotes and organelles like mitochondria
D. Contains 18S rRNA
✅ Answer: C. Found in prokaryotes and organelles like mitochondria
Explanation: 70S ribosomes are found in prokaryotes and some organelles like mitochondria.
17. What is the sedimentation coefficient of the smaller subunit in 70S ribosomes? A. 50S
B. 30S
C. 40S
D. 60S
✅ Answer: B. 30S
Explanation: The smaller subunit of 70S ribosomes is the 30S subunit.
18. What happens to ribosomal subunits in the absence of magnesium ions? A. They form a complex
B. They dissociate
C. They bind to mRNA
D. They catalyze protein synthesis
✅ Answer: B. They dissociate
Explanation: In the absence of magnesium ions, the ribosomal subunits dissociate.
19. What is a key feature of the structure of the larger 50S ribosomal subunit in 70S ribosomes? A. It is spherical
B. It is pentagonal in shape
C. It is cylindrical
D. It lacks rRNA
✅ Answer: B. It is pentagonal in shape
Explanation: The larger 50S subunit of 70S ribosomes is pentagonal in shape.
20. What is the molecular weight of the 50S subunit of 70S ribosomes? A. 1.8 × 10⁶ Daltons
B. 2.5 × 10⁶ Daltons
C. 1.2 × 10⁶ Daltons
D. 3.0 × 10⁶ Daltons
✅ Answer: A. 1.8 × 10⁶ Daltons
Explanation: The molecular weight of the 50S subunit of 70S ribosomes is 1.8 × 10⁶ Daltons.
21. What does the presence of poly-A binding proteins indicate? A. It signifies the maturation of rRNA
B. It indicates the ribosomal subunits are formed
C. It shows that mRNA is ready for translation
D. It suggests that tRNA is being synthesized
✅ Answer: C. It shows that mRNA is ready for translation
Explanation: Poly-A binding proteins indicate that the mRNA is mature and ready for translation by the ribosome.
22. Which of the following describes the function of ribosomes in protein synthesis? A. They transcribe genetic material into RNA
B. They facilitate peptide bond formation between amino acids
C. They replicate DNA during cell division
D. They translate RNA into lipids
✅ Answer: B. They facilitate peptide bond formation between amino acids
Explanation: Ribosomes facilitate the formation of peptide bonds between amino acids during protein synthesis.
23. In which form of ribosomes does translation primarily occur in eukaryotic cells? A. 70S ribosomes
B. 80S ribosomes
C. 60S ribosomes
D. 30S ribosomes
✅ Answer: B. 80S ribosomes
Explanation: Translation in eukaryotic cells primarily occurs on 80S ribosomes, consisting of 60S and 40S subunits.
24. What is the primary role of tRNA during translation? A. To store genetic information
B. To carry amino acids to the ribosome
C. To catalyze peptide bond formation
D. To transcribe mRNA
✅ Answer: B. To carry amino acids to the ribosome
Explanation: tRNA molecules carry amino acids to the ribosome for protein synthesis.
25. What structure does mRNA adopt when multiple ribosomes are translating it simultaneously? A. Polyribosome
B. Ribosomal subunit
C. Polypeptide chain
D. Exon-intron structure
✅ Answer: A. Polyribosome
Explanation: When multiple ribosomes are translating a single mRNA, it forms a structure known as a polyribosome.