Exercises

1. Group the following as nitrogenous bases and nucleosides:

• Nitrogenous Bases: Adenine, Thymine, Uracil, Cytosine.
• Nucleosides: Cytidine, Guanosine.

2. If a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.

• According to Chargaff's rule, C = G and A = T.
• Given C = 20%, therefore G = 20%.
• C + G = 40%.
• Remaining percentage (A + T) = 100% - 40% = 60%.
• Since A = T, A = 30% and T = 30%.

3. If the sequence of one strand of DNA is written as follows: 5'-ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of complementary strand in 5'→3' direction.

• Complementary sequence (3'→5'): 3'-TACGTACGTACGTACGTACGTACGTACG-5'
• In 5'→3' direction: 5'-GCATGCATGCATGCATGCATGCATGCAT-3'

4. If the sequence of the coding strand in a transcription unit is written as follows: 5'-ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of mRNA.

• The mRNA sequence is identical to the coding strand, except Thymine (T) is replaced by Uracil (U).
• mRNA sequence: 5'-AUGCAUGCAUGCAUGCAUGCAUGCAUGC-3'

5. Which property of DNA double helix led Watson and Crick to hypothesise semi-conservative mode of DNA replication? Explain.

• The property of base pairing and complementarity between the two strands led to this hypothesis.
• Since each strand is a complement of the other, they suggested that the two strands would separate and act as templates for the synthesis of new complementary strands.
• Each daughter DNA molecule would then consist of one parental and one newly synthesized strand.

6. Depending upon the chemical nature of the template (DNA or RNA) and the nature of nucleic acids synthesised from it (DNA or RNA), list the types of nucleic acid polymerases.

• DNA-dependent DNA polymerase: Uses DNA template to synthesize DNA (Replication).
• DNA-dependent RNA polymerase: Uses DNA template to synthesize RNA (Transcription).
• RNA-dependent DNA polymerase: Uses RNA template to synthesize DNA (Reverse Transcription).
• RNA-dependent RNA polymerase: Uses RNA template to synthesize RNA (found in some viruses).

7. How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?

• They used radioactive isotopes: 32P to label DNA (phosphorus is present in DNA but not in protein) and 35S to label protein (sulfur is present in protein but not in DNA).
• They observed that radioactive 32P entered the bacterial cells infected by labeled bacteriophages, while 35S remained in the viral coats outside.
• This proved that DNA, and not protein, is the genetic material.

8. Differentiate between the following:

• (a) Repetitive DNA and Satellite DNA: Repetitive DNA refers to sequences repeated many times in the genome. Satellite DNA is a subset of repetitive DNA that forms distinct peaks during density gradient centrifugation.
• (b) mRNA and tRNA: mRNA (messenger RNA) carries genetic code from DNA to ribosome; tRNA (transfer RNA) acts as an adapter that reads the mRNA code and brings the specific amino acids.
• (c) Template strand and Coding strand: The template strand (3'→5') is the one actually copied into RNA; the coding strand (5'→3') has the same sequence as the RNA (with T replaced by U).

9. List two essential roles of ribosome during translation.

1. Provides the site for binding of mRNA and tRNA.
2. Acts as a catalyst (ribozyme) for the formation of peptide bonds between amino acids.

10. In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?

• The lac operon produces enzymes like β-galactosidase that break down lactose into glucose and galactose.
• Once all the lactose in the medium is consumed, there is no more inducer to bind to the repressor.
• The repressor becomes free to bind to the operator again, shutting down the operon.

11. Explain (in one or two lines) the function of the followings:

• (a) Promoter: A DNA sequence that provides a binding site for RNA polymerase to initiate transcription.
• (b) tRNA: An adapter molecule that brings specific amino acids to the ribosome based on the mRNA codon.
• (c) Exons: The coding sequences in eukaryotic genes that are expressed in mature mRNA.

12. Why is the Human Genome project called a mega project?

• Due to its enormous scale: sequencing 3 billion base pairs, estimated cost of US $9 billion, 13 years of international collaboration, and the requirement for high-speed computational tools for data storage and analysis.

13. What is DNA fingerprinting? Mention its application.

• A technique to identify variations in DNA sequences (polymorphism) between individuals.
• Applications: Forensic science (solving crimes), paternity testing, and determining genetic diversity in populations.

14. Briefly describe the following:

• (a) Transcription: Process of copying genetic information from DNA to RNA.
• (b) Polymorphism: Genetic variation observed at a high frequency in a population.
• (c) Translation: Process of synthesizing a polypeptide chain from mRNA.
• (d) Bioinformatics: Use of computational tools for biological data analysis and storage.