Hydrogen bond which binds the nucleotides in a DNA.

The name of the segment of DNA is gene.

The both alleles are identical in homozygous condition.

Structure of a Chromosome:
(i) The chromosomes are thin, long and thread like structures consisting of two identical strands called sister chromatids.
(ii) They are held together by the centromere.
(iii) Each chromatid is made up of spirally coiled thin structure called chromonema.
(iv) The chromonema has number of bead-like structures along its length which are called chromomeres.
(v) The chromosomes are made up of DNA, RNA, chromosomal proteins (histones and non-histones) and certain metallic ions.
(vi) These proteins provide structural support to the chromosome.
A chromosome consists of the following regions:
Primary constriction:
(i) The two arms of a chromosome meet at a point called primary constriction or centromere.
(ii) The centromere is the region where spindle fibres attach to the chromosomes during cell division.
Secondary constriction:
(i) Some chromosomes possess secondary constriction at any point of the chromosome.
(ii) They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).
Telomere:
(i) The end of the chromosome is called telomere. Each extremity of the chromosome has a polarity and prevents it from joining the adjacent chromosome.
(ii) It maintains and provides stability to the chromosomes.
Satellite:
(i) Some of the chromosomes have an elongated knob-like appendage at one end of the chromosome known as satellite.
(ii) The chromosomes with satellites are called as the sat-chromosomes

(i) Allosomes are chromosomes which are responsible for determining the sex of an individual.
(ii) They are also called as sex chromosomes or hetero-chromosomes.
 

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.