Gene : It's Nature, Expression and Regulation

Structure of DNA

DNA

• DNA − Polymer of deoxyribonucleotides

• Nucleoside = Nitrogenous base + Pentose sugar (linked through N − glycosidic bond)
  Example − adenosine, deoxyadenosine, cytidine, etc.

• Nucleotide = Nucleoside + Phosphate group (linked through phosphodiester bond)

• Many nucleotides link together through 3′ − 5′ phosphodiester bond to form polynucleotide chain (as in DNA and RNA).

• In course of formation of polynucleotide chain, a phosphate moiety remains free at 5′ end of ribose sugar (5′ end of polymer chain) and one -OH group remains free at 3′ end of ribose (3′ end of polymer chain).

Double Helix Model for the Structure of DNA

• Scientists involved

  • Friedrich Meischer − First identified DNA as an acidic substance present in nucleus and named it as ‘Nuclein’

  • Wilkins and Franklin − Produced X-ray diffraction data for DNA structure

  • Watson and Crick − Proposed double helix structure model for DNA based on X-ray diffraction data

  • Erwin Chargaff − Proposed that in ds DNA, ratios A:T and C:G remain same and are equal to one

Features of double helix structure of DNA:

• In a DNA, two polynucleotide chains are coiled to form a helix. Sugar-phosphate forms backbone of this helix while bases project in wards to each other.
  

• Complementary bases pair with each other through hydrogen bond. Purines always pair with their corresponding pyrimidines. Adenine pairs with thymine through two hydrogen bonds while guanine pairs with cytosine through three hydrogen bonds.

  
  • The helix is right-handed.
    Pitch − 3.4 nm
    10 bp in each turn
  • The plane of one base pair stacks over the other in a double helix. This provides stability to the helix along with hydrogen bonding.

Packaging of DNA Helix

• Distance between two consecutive base pairs in a DNA = 0.34 nm = 0.34 × 10⁻⁹ m

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