NCERT Class 12 Biology: Chapter – 6 Molecular Basis of Inheritance Part 10 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 3:

Give an account of post transcriptional modifications of a eukaryotic mRNA.


Post transcriptional modifications are the chemical alterations done to primary transcript of RNA. These modifications convert a gene into functional RNA.

1. In prokaryotes the primary transcript (hn-RNA) has both the non-coding introns inside the gene and the coding exons but they are non-functional.

2. Hence, Splicing takes place. In which introns are removed and exons are joined in definite order.

3. In Eukaryotes (hn-RNA) or primary transcript is absent so, slicing is not needed.

4. In such cases capping and tailing process takes place.

5. In capping Methyl Guanosine Triphosphate nucleotide is added to the 5՚ end of hn-RNA known as cap structure.

6. In tailing Acetylate residues are added at the 3՚ end of the RNA.

7. After the whole process hn-RNA is processed to form m-RNA or messenger RNA.

8. This m-RNA is transported out of the nucleus for translation.

Question 4:

Discuss the process of translation in detail.


Translation is the decoding of m-RNA to form amino acid chain with the help of ribosome in the cytosol of the cell.

Molecules Involved In Translation:

1. Amino acids: These are the structural unit of proteins which are joined together to form a protein chain.

2. m-RNA: The m-RNA or messenger RNA has codons, each codon codes for one amino acid.

3. t-RNA: The t-RNA or transfer RNA binds with amino acid carrier and carry the amino acid sequence.

4. Ribosome: These are required for the catalysis of the process.

Steps Involved In Translation:


1. At the 5 ‘end of the m-RNA the smaller 40S subunit of the ribosome with methionyl-tRNA scans the m-RNA to find the start codon (5’ AUG) .

2. This codon (AUG) is specific to methionine.

3. Now, the larger 60S subunit of the ribosome binds to the m-RNA. This 60S ribosomal subunit has two t-RNA binding sites.

Site P: This site can hold the peptide chain.

Site A: This site can hold the t-RNA.


1. Now, as the Met-tRNA binds to the P site of the 60S subunit of the RNA. Another aminoacyl- tRNA which has an anticodon complementary to the next codon occupies the site A within the 60S subunit of ribosome.

2. The enzyme peptidyl transferase forms a peptide bond between Methionyl-tRNA and Aminoacyl-tRNA.

3. This makes the t-RNA molecule at the P site to become uncharged and it leaves the ribosome.

4. The ribosome moves along the m-RNA molecule to the next codon which opens the Site A for next aminoacyl-tRNA.

5. The polypeptide chain is built in the direction of N to C terminal.


1. Now, as the A site is vacant the stop codon enters the site.

2. None of the t-RNA molecule binds to this codon.

3. The peptide bond of Methionyl-tRNA and Aminoacyl-tRNA become hydrolysed releasing the polypeptide into the cytoplasm.

4. The ribosomal subunit now disassociates.