What is a ribosome?
The ribosome is made up of two words, “ribo” meaning “ribonucleic acid” and “somes” from the Greek word “soma” meaning “body”. Ribosomes are small granular structures made up of ribonucleic acid (RNA) and protein, present in both eukaryotic and prokaryotic cells. The RNA present in ribosomes is known as ribosomal RNA, while proteins present in ribosomes are called ribosomal proteins.
It was first studied by a cell biologist named George Emil Palade in 1955, while Richard B. Roberts propsed the term “ribosome” in the 1950s. The ribosome is the site for the synthesis of protein, where amino acids are joined together in a specific order by following the instructions encoded in mRNA.
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Location of ribosomes
Ribosomes are present in two forms inside a cell. They may float freely inside the cytoplasm and the proteins synthesized by these ribosomes are released into the cytoplasm. The ribosomes may also be found attached to the rough endoplasmic reticulum, where they synthesize a variety of proteins for the cell including secretory proteins. Ribosomes are also found inside the mitochondrial matrix and chloroplast.
How ribosomes are formed?
In eukaryotic cells, the site for the synthesis of ribosomes is the nucleolus. Ribosomal RNA and proteins are assembled in the nucleolus to form ribosomes. The newly formed ribosomes are then transported out of the nucleus into the cytoplasm through nuclear pores. The nucleolus is known as the “factory of ribosomes”, while ribosomes are known as the “factory of proteins”. Bacterial cells lack a nucleus so the synthesis of ribosomes takes place in the cytoplasm.
Structure of ribosomes
The ribosome is made up of two parts known as large and small subunits. Each subunit is made of some ribosomal RNA and many ribosomal proteins. The size of the ribosome is different in prokaryotic and eukaryotic cells.
Ribosomes in prokaryotic cell
The ribosome in a prokaryotic cell is 70S, which is made up of a small subunit of 30S and a large subunit of 50S. It is made up of 35% ribosomal proteins and 65% ribosomal RNA having a diameter of about 20nm
Ribosomes in eukaryotic cell
A eukaryotic cell on the other hand has 80S ribosomes, which are made up of a small subunit of 40S and a large subunit of 60S. The 60S subunit is furthure made up of 28S RNA, 5S RNA and 5.8S RNA, while the small subunit is composed of 18S RNA.
“S” is a Svedberg unit used to measure the size of a particle in ultracentrifugation based on its sedimentation rate. The attachment of both subunits is controlled by magnesium ions.
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| A simple representaion of two subunits of ribosmone. Image created in BioRender.com |
Functions of ribosomes
Functions of ribosomes
Ribosomes are known as “factory of protein” because it is involved in the synthesis of proteins. In ribosomes, a small subunit is attached with mRNA, and a large subunit bound with tRNA (transfer RNA) to produce protein.
In a eukaryotic cell, millions of ribosomes may attach to mRNA to synthesis protein. The attachment of a large number of ribosomes to mRNA is called a polysome. Altogether two main functions of ribosomes are:
- Translation of encoded messages present in the form of mRNA. The genes present in the DNA contain important information that is transcribed into mRNA and then this mRNA. The series of codons present in mRNA is sent to the ribosome for decoding and to make protein.
- Attachment of individual amino acids with each other to form polypeptides, which are later converted into proteins. A polypeptide is a chain of amino acids. The amino acids are brought to the ribosomes by tRNA.
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| Synthesis of protein by ribosome by forming polysome. Image created in BioRender.com |
Some questions and answers
1. Who discovered ribosome?
A. Palade
B. Robert Brown
C. Robert Hooke
D. Golgi Brown
Correct Answer: A
2. What does “some” mean in ribosome?
A. Vesicles
B. Body
C. Cell
D. Network
Correct Answer: B
3. Which of the following is not the function of the ribosome?
A. Modification of protein
B. Decoding of mRNA
C. Attachment of amino acids into a polypeptide
D. Synthesis of protein
Correct Answer: A
Explanation: Modification of proteins takes place in the Golgi apparatus, while the rest of the functions are performed in the ribosome.
4. …… is known as protein factory.
A. Nucleolus
B. Golgi apparatus
C. Ribosome
D. Endoplasmic reticulum
Correct Answer: C
5. How many forms of ribosomes are present in a eukaryotic cell?
A. 2
B. 3
C. 4
D. None of the above
Correct Answer: A
Explanation: Ribosomes are present in two forms i.e., freely floating and attached to the organelles.
6. Ribosomes are made up of ……..
A. tRNA and proteins
B. rRNA and proteins
C. mRNA and proteins
D. None of the above
Correct Answer: B
Explanation: Ribosome is madeup of ribosomal RNA and ribosomal proteins. mRNA brings information about the sequence of a protein to the ribosome, while individual amino acids are carried by tRNA to the ribosome during the translation process.
7. What is the size of a small subunit of the prokaryotic ribosome?
A. 20S
B. 30S
C. 40S
D. 50S
Correct Answer: B
Explanation: The ribosome in a prokaryotic cell is 70S, which is made up of a small subunit of 30S and a large subunit of 50S.
8. Which unit is used to measure ribosome size?
A. Sazhen
B. Sabin
C. Svedberg
D. None of the above
Correct Answer: C
Explanation: Svedberg unit or in short “S” is used to measure the size of a particle in ultracentrifugation based on its sedimentation rate.
9. Which process takes place in the ribosome?
A. Mutation
B. Transcription
C. Replication
D. Translation
Correct Answer: D
Explanation: Translation is the process of decoding the information of mRNA for the synthesis of protein. It takes place in the ribosome.
10. The ribosomes present in ……….. are generally larger compared to ………
A. Prokaryotic cell, eukaryotic cell
B. Eukaryotic cell, prokaryotic cell
C. Archaeal cell, Eukaryotic cell
D. Archaeal cell, Prokaryotic cell
Correct Answer: B
Explanation: The ribosome in an archeal and prokaryotic cell is 70S, while a eukaryotic cell has 80S ribosomes.
References
- Wilson, D. N., & Cate, J. H. D. (2012). The structure and function of the eukaryotic ribosome. Cold Spring Harbor perspectives in biology, 4(5), a011536.
- Doudna, J. A., & Rath, V. L. (2002). Structure and function of the eukaryotic ribosome: the next frontier. Cell, 109(2), 153-156.
- Jobe, A., Liu, Z., Gutierrez-Vargas, C., & Frank, J. (2019). New insights into ribosome structure and function. Cold Spring Harbor perspectives in biology, 11(1), a032615.
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