Q4. How is a nucleoside joined to a phosphate, and how are two nucleotides joined?
The phosphate is linked to the OH of the 5′C of the nucleoside by a phosphoester linkage; two nucleotides are joined by a 3′–5′ phosphodiester linkage (NCERT page 80).
Q5. A stretch of double-helical DNA is 34 nm long. How many base pairs and how many complete turns does it hold?
34 nm ÷ 0.34 nm per bp = 100 bp; 100 ÷ 10 bp per turn = 10 turns (equivalently 34 ÷ 3.4 = 10) (NCERT pages 81–82).
Q6. Which part of chromatin is transcriptionally active, and how does it stain?
Euchromatin, which is loosely packed and stains light (NCERT page 84).
Q7. Whose X-ray diffraction data did Watson and Crick use, and what observation of Chargaff supported base pairing?
Maurice Wilkins and Rosalind Franklin’s; Chargaff observed that in double-stranded DNA the ratios of A to T and of G to C are constant and equal one (NCERT page 81).
Q8. The haploid human genome has 3.3 × 10 bp. How long would this DNA be if stretched out?
3.3 × 10 × 0.34 × 10 m ≈ 1.1 m, half the 2.2 m of the 6.6 × 10 bp in NCERT’s typical mammalian cell (NCERT pages 80 and 83).