Chromosome – Basics of Hereditary

Chromosomes are thread like structure that occur in the nuclear of plants and animals and carry heredity information from generation to generation.

There are seen only during nuclear division and occur in identical pair called homologous pair. The body cell (somatic) of man has 46 chromosomes (23pairs). The sex cell (sperm, ovum) have 23 chromosomes. The 23pairs of chromosomes are called diploid number (2N) and 23 chromosomes are referred is as haploid number (N).chromosome are made up of two threads ‘chromatids’. Chromosome carry genes that are hereditary unit for inherited traits

Structure of chromosome:

During the early cell division chromosome become condensed and visible, each, then ,duplicate or replicate to produce a compact twins connected by a centromere. Each member of a bond pair is called a chromatid which tread like in appearance

Chemical analysis: of chromosomes show that they are composed of proteins combined with nucleic acids which are of two  kinds.

  1. De oxyribonucleic acid (DNA) Which is confined to the nuclear material of the cell which the primary heredity material making up the gene.
  2. Ribonucleic acid (RNA) in some viruses.

The DNA consists of a double chains formed by repeating small chemical units known as Nucleotides. Each nucleotide is composed of

  1. deoxyribo sugar (s)
  2. Phosphate group (p)

iii.        Nitrogeneous bases which may be purine (Adenine and Guanine)or

pyrimidine (cytocine and Thymine).

The two chain of nucleotides of DNA are coiled like a spring to give a structure called a double helix. The nitrogenous base of the nucleotide pairs along the DNA chain.

Adenine (A) pairs with thymine (T) A-T while Guanine (G) pairs with cytocine

(C) G-C

Role of chromosome in transmitting hereditary characters.

  1. Formation of gametes – gametes are formed by meiotic divisions of cells. Each sperm or egg producing cell in reproductive organ is diploid that is having two sets of chromosome.

During meiosis the number of pair homologous separate first, then the twin chromatids separate The outcome of the cell division result into four daughter cells.


2          It allows for reshuffle of gene by allowing crossing over. It happens during the prophase of meiosis. It is important because it leads to new combination of alleles on a chromosome which leads to more types of allele combination in gametes.

  1. Homozygous chromosome pair up, each chromosome has already replicated
  2. Exchange of genetic material takes place between chromatids of homologous pair of chromosomes
  3. crossing over result in re arrangement of alleles A and

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