Answer:
1. DNA molecule is a double helix of 2 complementary polynucleotide strands;;
2. The two strands coil around each other to form a right-handed double helix;;
3. The strands are antiparallel, i.e. run in opposite directions (one strand runs in
the 5’ to 3’ direction while the complementary strand runs in the 3’ to 5’
direction);;
4. Each (polynucleotide) strand consists of very long chain of nucleotides, with
each nucleotide comprising of a deoxyribose sugar, a phosphate group and one
of the four nitrogenous bases - Adenine, Thymine, Cytosine or Guanine;;
5. Each strand contains a sugar-phosphate backbone – comprising of nucleotides
arranged in sequence, held together by phosphodiester bonds between C3 of
the (deoxyribose) sugar of one nucleotide and C5 of the (deoxyribose) sugar of
the adjacent nucleotide;;
6. The nitrogenous bases are arranged as side groups of the polynucleotide
strands (oriented toward the central axis);;
7. The width between the 2 sugar-phosphate backbones is constant at 2nm, this is
equal to the width of 1 base pair, i.e. 1 purine + 1 pyrimidine;;
8. One complete turn of the double helix measures 3.4nm in length and comprises
10 base pairs;;
9. The double helical nature of DNA results in the surface of the DNA molecule to
have major grooves and minor grooves;;
10. The nitrogenous bases of 1 strand pair with nitrogenous bases of the opposite
strand via hydrogen bonds. There are 2 hydrogen bonds between adenine and
thymine, and 3 hydrogen bonds between cytosine and guanine;;
11. Base pairing is complementary, i.e. adenine pairs with thymine, guanine pairs with cytosine. The base-pairing is specific and the 2 strands are said to be
complementary (i.e. no. of A = no. of T, no. of G = no. of C,);;
12. (extra pt) The DNA molecule is further stabilised by hydrophobic interactions between the stacked nitrogenous bases;;
13. The nucleosomes are connected by linker DNA and associates with H1 forming the nucleohistone complex;
14. Interactions between the H1 histones (histone tails of octamers and linker DNA) results in further coiling into a 30nm chromatin fibre;
15. The 30nm chromatin fibre form looped domains that are attached to a scaffold of non-histone proteins, giving rise to a 300nm fibre;
