DNA (Deoxyribonucleic Acid)

 DNA

What is DNA?

DNA (Deoxyribonucleic Acid) is the genetic material that carries hereditary information in almost all organisms. The concept of DNA as genetic material was proven by experiments like that of Oswald Avery and Hershey and Chase.

Structure of DNA

Proposed by James Watson and Francis Crick (1953):

  • DNA has a double helix structure
  • Made of two polynucleotide chains
  • Backbone: sugar (deoxyribose) + phosphate
  • Nitrogen bases:
    • Purines: Adenine (A), Guanine (G)
    • Pyrimidines: Thymine (T), Cytosine (C)
  • Base pairing (Chargaff’s rule):
    • A = T (2 hydrogen bonds)
    • G ≡ C (3 hydrogen bonds)
  • Strands are antiparallel (5’ → 3’ and 3’ → 5’)

Components of DNA

Each nucleotide consists of:

  1. Nitrogenous base
  2. Pentose sugar
  3. Phosphate group

Functions of DNA

  • Stores genetic information
  • Controls protein synthesis
  • Transfers hereditary traits
  • Regulates cell activities

DNA Replication

  • Occurs in S-phase of cell cycle
  • Semi-conservative (proved by Meselson and Stahl)
  • Enzymes involved:
    • DNA helicase (unwinds DNA)
    • DNA polymerase (adds nucleotides)
    • Ligase (joins fragments)

Types of DNA (NEET Important)

  • B-DNA → most common
  • A-DNA and Z-DNA → less common

Location of DNA

  • Eukaryotes: nucleus, mitochondria, chloroplast
  • Prokaryotes: nucleoid

Important Points for NEET

  • DNA is negatively charged due to phosphate group
  • Length in humans ≈ 2 meters per cell
  • Human genome project led by Francis Collins


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