Q3. Give two chemical differences between DNA and RNA, and one difference in structure.
Sugar: DNA has β-D-2-deoxyribose, RNA β-D-ribose. Base: DNA has thymine, RNA uracil. Structure: DNA is a double helix of two complementary strands, while RNA is a single-stranded helix that sometimes folds back on itself (NCERT pages 297–299).
Q4. One strand of a short DNA piece reads 5′-GACTTC-3′. Write the complementary strand from its own 5′ end, and find how many hydrogen bonds join the two strands.
Pair A with T and G with C, and run the second strand the opposite way: 3′-CTGAAG-5′, which is 5′-GAAGTC-3′ read from its own 5′ end. The piece has three G–C and three A–T pairs, so 3 × 3 + 3 × 2 = 15 hydrogen bonds (pairing from NCERT page 299; opposite directions and the bond counts from NCERT Class 12 Biology, page 81).
Q5. About how long is a stretch of double-stranded DNA with 1200 base pairs? Give the length in nanometres and in micrometres.
Consecutive base pairs are about 0.34 nm apart (NCERT Class 12 Biology, page 82), so 1200 × 0.34 = 408 nm, which is 0.408 μm, about 0.41 μm (reasoned).
Q6. About how many base pairs, and how many nucleotides, are there in three full turns of the DNA double helix?
About 10 base pairs per turn (NCERT Class 12 Biology, page 81) gives 3 × 10 = 30 base pairs. Each pair is two nucleotides, one on each strand, so 60 nucleotides (reasoned).