DNA is a large molecule consisting of millions of nucleotides. Hence, it is also called a polynucleotide. Each nucleotide consists of three components.
(i) A sugar molecules - Deoxyribose sugar.
(ii) A nitrogenous base.
There are two types of nitrogenous bases in DNA.
They are
(a) Purines (Adenine and Guanine)
(b) Pyrimidines (Cytosine and Thymine)
(iii) A phosphate group
Nucleoside and Nucleotide:
Nucleoside = Nitrogen base + Sugar
Nucleotide = Nucleoside + Phosphate
The nucleotides are formed according to the purines and pyrimidines present in them.
Watson and Crick model of DNA:
(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^o (3.4 nm). There are ten base pairs in a complete turn.
(vii) The nucleotides in a helix are joined together by phosphodiester bonds.
Significance of DNA:
(i) DNA is responsible for the transmission of hereditary information from one generation to next generation.
(ii) It contains information required for the formation of proteins.
(iii) It controls the developmental process and life activities of an organism.

a) The F₁ generation plants are tall.
b) In the F₁ generation there were 3 tall plants and a dwarf plant. The ratio is 3 : 1.
c) The dwarf plants were missing in F₁ generation but reappeared in F₂ generation.

(i) Flowers of the garden pea are bisexual and self-pollinated.
(ii) During hybridization experiment, Mendal simply cut off anther in bud (Emasculation) and did the cross pollination of selected plant's pollen on the stigma of the flower.
(iii) So self pollination will be avoided.
(iv) And there by he followed the effects of crossing for several generation.

(i) In the body cells of sexually reproducing organisms, the chromosomes generally occur in pairs. This condition is called diploid (2n).
(ii) The gametes produced by the organisms contain a single set of chromosomes. Hence, the gametes are said to be haploid (n).

(i) Addition or deletion in the number of chromosomes in a cell. This is called ploidy. It is of two types Euploidy and Aneuploidy.
(ii) Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). Triploid plants and animals are typically sterile. If it has four haploid sets of chromosomes, the condition is called tetraploidy (4n). Tetraploid plants are advantageous as they often result in addition or deletion in the number of chromosomes in a cell. This is called ploidy. It is of two types Euploidy and Aneuploidy. Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). Triploid plants and animals are typically sterile. If it has four haploid sets of chromosomes, the condition is called tetraploidy (4n). Tetraploid plants are advantageous as they often result in increased fruit and flower size reased fruit and flower size.

During Replication of DNA:
(i) In one strand, the daughter strand is synthesized as a continuous strand which is called leading strand.
(ii) In the other strand, short segments of DNA are synthesized. This strand is called lagging strand. The short segment of DNA are called Okazaki fragments.

(i) Acquired traits are the characters that cannot be passed from one generation to the next generation.
(ii) Characters which are inherited by the offspring from the parents and carried from one generation to another generation are called Inherited traits.

(i) The term mutation was introduced by Hugo De Vries in 1901 when he observed phenotypic changes in the evening Primrose plant, Oenothera lamarckiana.
(ii) Mutation is an inheritable sudden change in the genetic material (DNA) of an organism.
(iii) Mutations are classified into two main types, namely chromosomal mutation and gene mutation. Chromosomal mutation: The sudden change in the structure or number of chromosomes is called chromosomal mutation. This may result in
Changes in the structure of chromosomes:
Structural changes in the chromosomes usually occurs due to errors in cell division. Changes in the number and arrangement of genes takes place as a result of deletion, duplication, inversion and translocation in chromosomes. Changes in the number of chromosomes: They involve addition or deletion in the number of chromosomes present in a cell. This is called ploidy. There are 2 types of ploidy:
Euploidy in plants and animals:
(a) Euploidy: It is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). If it has four haploid sets of chromosomes. It is called Tetraploidy. Tetraploid plants are advantageous as they often result in increased fruit and flower size.
(b) Aneuploidy: It is the loss or gain of one or more chromosomes in a set. It is of 3 types:
(i) Monosomy (2n-1),
(ii) Trisomy (2n+1)
(iii) Nullisomy (2n-2). In man, Down's syndrome is one of the commonly known aneuploid condition.
Gene or point mutation:
(i) Gene mutation is the changes occurring in nucleotide sequence of a gene.
(ii) It involves substitution, deletion, insertion or inversion of a single or more than one nitrogenous base.
(iii) Gene alteration results in abnormal protein formation in an organism.

(i) DNA molecule produces exact copies of its own structure during replication process.
(ii) The two strands of a DNA molecule have complementary base pairs. The nucleotides of each strand provide the information needed to produce its new strand.
Origin of replication: The specific points on the DNA, where the replication begins. It is the site of origin of replication. The two strands open and separate at this point forming the replication fork.
Unwinding of DNA molecule:
(i) The enzyme called helicase, bind to the origin of replication site.
(ii) They separate the two strands of the DNA. The enzyme called topoisomerase seperates the double helix above the replication fork and removes the twists formed during the unwinding process.
(iii) Each of the separated DNA strands function as template.
Formation of RNA primer: An RNA primer is a short segment of RNA nucleotides. The primer is synthesized by the DNA template close to the origin of replication.
Synthesis of new complimentary strand from the parent strand :
(i) After the formation of RNA primer, nucleotides are added with the help of an enzyme DNA polymerase and new complimentary strand of DNA is formed from each of the parent strand. The synthesis is unidirectional.
(ii) In one strand the daughter strand is synthesized as a continuous strand which is called leading strand.
(iii) In the order stand, short segment of DNA are synthesized. This strand is called lagging strand.
(iv) The short segments of DNA are called Okazaki fragments. The fragments are joined together by the enzyme DNA ligase.
(v) The replication stops when the replication fork of the two sides meet at a site called terminus which is situated opposite to origin of replication site

Based on the position of centromere, the chromosomes are classified as Telocentric, Acrocentric, Submetacentric andMetacentric.
(i) Telocentric - The centromere is found on the proximal end. They are rod shaped chromosomes.
(ii) Acrocentric - The centromere is found at the one end with a short arm and long arm. They are also rod - shaped chromosomes.
(iii) Submetacentric - The centromere is found near the centre of the chromosome. Thus forming two unequal arms. They are J

(iv) Metacentric - The centromere occurs in the centre of the chromosome and form two equal arms. They are V shaped chromosomes.