Molecular Basis of Inheritance

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

• Total number of base pairs in a human DNA = 6.6 × 10⁹ bp

• Total length of human DNA = 0.34 × 10⁻⁹ × 6.6 × 10⁹

= ~ 2.2 m

• 2.2 m is too large to be accommodated in the nucleus (10⁻⁶ m).

• Organisation of DNA in prokaryotes:

  • They do not have nucleus. DNA is not scattered throughout the cell.

  • In certain regions called nucleoids, DNA (negatively charged) is organised in large loops and is held by some proteins (positively charged).

• Organisation of DNA in eukaryotes:

  • They have positively charged basic proteins called histones (positive and basic due to presence of positive and basic amino acid residues, lysine and arginine).

  • Histone octamer − Unit of eight molecules of histone

  • DNA (negatively charged) winds around histone octamer (positively charged) to form nucleosome.

    

  • 1 nucleosome has approx. 200 bp of DNA.

  • Nucleosomes in a chromatin resemble beads present on strings.

  • Beads on string structure in chromatin are further packaged to form chromatin fibres, which further coil and condense to form chromosomes during metaphase.

  • Non-histone chromosomal proteins − Additional set of proteins required for packaging of chromatin at higher level

  

Discovery of DNA as a Genetic Material

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