Describe the molecular 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).

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.