IGCSE Biology Paper-4: Specimen Questions with Answers 189 - 189 of 279

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Question 189

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Draw and annotate a diagram to show the structure of DNA.

Explanation

Important Features of Watson and Crick Double Helical Model of DNA

There are important features of DNA double helix:

  • The double helix comprises of two polynucleotide chains.
  • The two strands (polynucleotide chains) of double helix are anti-parallel due to phosphodiester bond.
  • Each polynucleotide chain has a sugar-phosphate ‘backbone’ with nitrogenous bases directed inside the helix.
  • The nitrogenous bases of two antiparallel polynucleotide strands are linked through hydrogen bonds. There are two hydrogen bonds between A and T, and three between G and C. The hydrogen bonds are the only attractive forces between the two polynucleotides of double helix. These serve to hold the structure together.
  • The two polynucleotides in a double helix are complementary. The sequence of nitrogenous bases in one determines the sequence of the nitrogenous bases in the other. Complementary base pairing is of fundamental importance in molecular genetics.
  • Erwin Chargaff (1950) made quantitative analysis of DNA and proposed “base equivalence rule” starting that molar concentration of or & which is constant for a species.
  • Ten base pairs occur per turn of helix (abbreviated ) . The spacing between adjacent base pairs is . The helix is in diameter and DNA molecule found in a clockwise.
DNA Double Helical Structure

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