(i) DNA molecule consists of two polynucleotide chains.
(ii) These chains form a double helix structure with two strands which run anti-parallel to one another.
(iii) Nitrogenous bases in the centre are linked to sugar-phosphate units which form the backbone of the DNA.
(iv) Pairing between the nitrogenous bases is very specific and is always between purine and pyrimidine linked by hydrogen bonds.
a) Adenine (A) links Thymine (T) with two hydrogen bonds (A = T)
b) Cytosine (C) links Guanine (G) with three hydrogen bonds (C ≡ G). This is called complementary base pairing.
(v) Hydrogen bonds between the nitrogenous bases make the DNA molecule stable.
(vi) Each turn of the double helix is 34 A° (3.4 nm). There are ten base pairs in a complete turn.
(vii) The nucleotides in a helix are joined together by phosphodiester bonds.

Parental generation:
i) Pure breeding tall plant and a pure breeding dwarf plant (tt).
F₁ generation:
i)  Plants raised from the seeds of pure breeding parental cross in F₁ generation were tall and monohybrids.
F₂ generation:
i) Selfing of the F₁ monohybrids resulted in tall and dwarf plants respectively in the ratio of 3:1.
ii) The actual number of tall and dwarf plants obtained by Mendel was 787 tall and 277 dwarf.
iii) External expression of a particular trait is known as phenotype. So the phenotypic ratio is 3:1.
In the F₂ generation 3 different types were obtained:
i) Tall Homozygous - TT (Pure) - 1.
ii) Tall Heterozygous - Tt - 2.
iii) Dwarf Homozygous - tt -1.
So the genotypic ratio 1:2:1.
(A genotype is the genetic expression of an organism).

(i) Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes.
(ii) Tetraploidy plants are advantageous as they often result in increased fruit and flower size.

(i) In the DNA replication after the formation of RNA primer two strands are formed.
(ii) In one strand short segments of DNA are synthesised.
(iii) This strand is called lagging strand. The short segments of DNA are called okazaki fragments.

(i) External expression of a particular trait is known as phenotype.
(ii) A genotype is the genetic expression of an organism.