Nucleic Acids - DNA and RNA Structure - UNSOLVED PRACTICE SET

Q7. Distinguish between DNA and RNA with respect to:

(a) Sugar

(b) Nitrogenous bases

(c) Structure

(d) Location in the cell

Q8. What is a nucleoside? How does it differ from a nucleotide?

Q9. Describe the complementary base pairing in DNA. Why is A-T pairing different from G-C pairing in terms of hydrogen bonds?

Q10. What is meant by the 5' → 3' directionality of a DNA strand? Why is this directionality important?

Q11. Draw a rough sketch of a nucleotide, labeling the three components.

Q12. Why is DNA more stable than RNA? Explain based on the sugar component and the presence of thymine instead of uracil.

Q13. (a) Describe the structure of a nucleotide. Draw a diagram showing the three components and how they are linked.

(b) Explain the difference between a nucleoside and a nucleotide with examples.

(c) Name the four nitrogenous bases found in DNA and classify them as purines and pyrimidines.

Q14. (a) Describe the double helix structure of DNA as proposed by Watson and Crick. Include:

(i) The nature of the backbone

(ii) The base pairing rules

(iii) The type of bonds stabilizing the structure

(iv) The major and minor grooves

(b) What experimental evidence supported the double helix model? (Mention at least two key findings.)

Q15. (a) Compare the structures of DNA and RNA in a tabular form covering:

- Sugar

- Bases

- Strands

- Stability

- Function

(b) Describe the three types of RNA (mRNA, tRNA, rRNA) and their functions in protein synthesis.

Q16. A segment of DNA contains 1000 nucleotide pairs.

(a) If the number of adenine bases is 300, calculate the number of thymine, guanine, and cytosine bases.

(b) Calculate the total number of hydrogen bonds in this DNA segment. (A-T = 2 H-bonds, G-C = 3 H-bonds)

(c) If the length of one complete turn of the DNA helix is 3.4 nm and contains 10 base pairs, calculate the total length of this DNA segment.

(d) The molecular mass of one nucleotide pair is approximately 660 Da. Calculate the molecular mass of this DNA segment.

(e) If this DNA replicates, how many new strands are formed? How many hydrogen bonds must be broken during replication?

Q17. The following data relates to Chargaff's rules for DNA from different organisms:

(a) Complete the table using Chargaff's rules (%A = %T, %G = %C).

(b) Verify that %A + %T + %G + %C = 100% for each organism.

(c) Calculate the A+T/G+C ratio for each organism. What does this ratio tell you?

(d) Which organism has the highest G-C content? What does higher G-C content imply about the stability of DNA?

(e) A student claims that the A+T/G+C ratio is the same for all species. Is this correct? Use the data to support your answer.

Q18. In a school biology project, students build a model of DNA using coloured beads.

(a) They use white beads for deoxyribose, red beads for phosphate, and four different colours for the nitrogenous bases. If they make a strand with 20 nucleotides, how many white and red beads do they need?

(b) They pair the bases according to Chargaff's rules: blue (A) with green (T), and yellow (G) with orange (C). If the first strand has 5 blue, 5 green, 5 yellow, and 5 orange beads, how many of each colour are on the complementary strand?

(c) The students twist their model into a double helix. What does the twisting represent in real DNA? What forces hold the two strands together?

(d) One student accidentally uses a ribose bead (instead of deoxyribose) in the model. What type of nucleic acid would this represent? How would its structure differ?

(e) The teacher asks: "If you separate the two strands and use each as a template to build a new complementary strand, how many DNA molecules will you have?" Explain the process and its biological significance.