Tuesday, 31 January 2017

Biology notes for Competitive Exam


Deoxyribo nucleic Acid (DNA) :

  • DNA stands for Deoxyribo nucleic Acid , it was discovered by Frederic Maischer.
  • DNA is a double stranded molecule made up of elongated chain of sub-units called nucleotides.
  • DNA is mainly found in nucleus and in small amount it is also found in mitochondria an chloroplast.
  • Chemically a nucleotide has three components.
(1) Nitrogenous base
(2) Pentose Sugar
(3) Phosphate group.
  • Nitrogenous  base are of two type i.e.Purine & Pyrimidines. 
  • Purines contain two nitrogen base i.e. Adinine and Guanine.
  • Pyrimidine nitrogen base are Thymine and Cytosine, thus there are four kinds of nucleotide present in DNA i.e. Adinine, Guanine, Thymine, Cytosine.

Structure of DNA:

  • Watson and Crick give the structural model of DN
  • DNA molecule is consists of two polynucleotide strand, forming a double helix structure.
  • Each strand has a backbone of sugar and phosphate, nitrogen base is attached to the sugar.
  • Two strands are joined together by hydrogen bonds between the bases.
  • Adenine pairs with thymine whereas guanine pairs with cytosine.
  • Adenine and thymine are complementary to each other and cytosine is complementary to guanine.

Function of DNA:

  • DNA is genetic material and contains genetic information in coded form.
  • DNA has two main function replication and expression.

2.Ribonucleic Acid (RNA) : 

  • RNA stands for Ribonucleic acid.
  • RNA is single stranded nucleic acid made up of phosphate, ribose sugar and nitrogen base uracil, adinine, guanine and cytosine.
  • It is found in nucleus as well as cytoplasm.
  • Its main function is to synthesis the protein.
  • RNA is of three kind.
    1.Messenger RNA (mRNA): It brings the massage from DNA found in the nucleus to cytoplasm in the coded          form.
     2.Ribosomal RNA (rRNA): Present in ribosome which is the site of protein synthesis.
     3.Transfer RNA (t RNA): It is the carrier of amino acid and transfers it to the ribosome.
Difference between RNA and DNA


  • Virus was discovered by Russian Scientist Iavanosky during the test of Mosaic disease in Tobacco.
  • Virus acts as a link between the living and non-living.
  • Virus is dead but when it comes in contact with the living cell it gets activated.
  • ‘The virus in which RNA  is found as the genetic material are called Retrovirus.
  • Virus is of three types:
  1. Plant Virus: RNA is present as its nucleic acid
  2. Animal Cell: DNA and sometimes RNA is found in it.
  3. Bacteriophage: They kill the bacteria e.g. T-2 phage.


  • Bacteria was discovered by Antony von Lecuwenhoek and its study is known as bacteriology.
  •  Bacteria is of different types on the basis of shape:
  1. Bacillus: This is a rod like or cylindrical.
  2. Round or Cocus: These are round and smallest bacteria.
  3. Comma shape or vibrio
  4. Sprillum: Spring or Screw shaped.
  • Anabaena and Nostoc cyanobacteria fixed the atmospheric nitrogen into soil.