(2)

In prokaryotes, DNA being negatively charged is held with positively charged proteins in region termed as nucleoid.

(2)

In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin. Euchromatin is transcriptionally active chromatin whereas heterochromatin is inactive. Histone octamer is wrapped by negatively charged DNA to form nucleosome. Histones are rich in basic amino acid residues lysine and arginine. A typical nucleosome contains 200 bp of DNA helix.

(1)

Length of DNA double helix = Total number of bp × Distance between two consecutive bp.

So, total number of bp = $\frac{\mathrm{Length} \mathrm{of} \mathrm{DNA} \mathrm{double} \mathrm{helix}}{\mathrm{Distance} \mathrm{between} \mathrm{two} \mathrm{consecutive} \mathrm{bp}}$

                                   = $\frac{1.11 \mathrm{metres}}{0.34\times {10}^{-9} m/bp}=3.26\times {10}^9$bp

Therefore, approximate number of base pair will be $3.3\times {10}^9$ bp.

(4)

(4)

According to the Chargaff’s rule,
A = T and G = C
A + G = T + C
∴ If A = 30%, then T = 30%
⇒ A + T = 60%
G + C = 100 – 60 = 40%

G = C = 20%
T = 30%, G = 20%, C = 20%

(3)

Histones are positively charged basic proteins, thus, pH is alkaline.

(1)

Adenine pairs with thymine by forming two hydrogen bonds, A = T.

(3)

If the distance between two consecutive base pairs is 0.34 nm i.e.,$0.3\times {10}^{-9}$ m and the total number of base pairs of a DNA double helix in a typical mammalian cell is $6.6\times {10}^9$ bp, the length of DNA is calculated by multiplying the total number of base pairs with distance between two consecutive base pairs i.e., $6.6\times {10}^9$ bp × 0.34 nm = 2.2 m (approx.)

(3)

(2)

Histones help in packaging of DNA. In eukaryotes, DNA packaging is carried out with the help of positively charged basic proteins called histones. Histones are of five types – ${\mathrm{H}}_1, {\mathrm{H}}_2\mathrm{A}, {\mathrm{H}}_2\mathrm{B}, {\mathrm{H}}_3$ and ${\mathrm{H}}_4$. The association of ${\mathrm{H}}_1$ histone with nucleosome indicates that DNA remains in its condensed form. Nucleosome is the unit of compaction. ${\mathrm{H}}_1$ is attached over the linker DNA. The linker DNA, consisting of ${\mathrm{H}}_1$ histone, connects two adjacent nucleosomes. They together constitute chromatosome. It gives rise to a chromatin fibre after further condensation.