Unit – II B

Structure of DNA

  • Each cell in a particular living organism contains the exact same DNA.
  • The size of the DNA polymer is directly related to the complexity of the organism; more complex organisms tend to have larger molecules of DNA, while less complex organisms have smaller DNA.  
  • For e.g. The DNA in simple bacteria contains about 8 million nucleotides, whereas human DNA contains up to 500 million nucleotides.

Primary Structure:

  • The primary structure of DNA is simply the sequence of nucleotides. The sugar-phosphate chain is called the DNA backbone, and it is constant throughout the entire DNA molecule. The variable portion of DNA is the sequence of nitrogenous bases
  • The phosphate groups link the 3rd carbon of one sugar (of deoxyribose or ribose) to the 5th carbon of the next sugar (of deoxyribose or ribose).

  • A strand of DNA has two distinct terminals or ends, one will be a 5-phosphate end and the other will be a 3-hydroxyl end. By convention, a nucleic acid sequence is always read in the 5 to 3 direction, that is, from the sugar with the free 5-phosphate to the sugar with a 3-hydroxyl group. The order of nucleotides is generally
    written using the capitalized first letter of the name of a base. Thus the following structure is written ACGT (in the 5
     to 3 direction)

Secondary Structure (Double Helix Structure):

Double Helix Structure:

  • The secondary structure of DNA was proposed by James Watson and Francis Crick in 1953. This was considered to be the greatest discovery of modern biology and hence they got Nobel prize for the same.
  • They proposed that DNA is a double helix containing two polynucleotide strands wound as if around a central axis.

    Complementary Base Pairing:

  • The two polynucleotide strands are connected by hydrogen bonds formed between a purine on one strand and a pyrimidine on the other. In DNA, adenine is always paired with thymine and guanine is always paired with cytosine. The pairs A-T and G-C are called complementary base pairs. According to base pairing rules discovered by Watson and Crick, each A is bound to T and each G is bound to C.

    Purine Pyrimidine Ratio:

  • Therefore, the total number of A’s in any molecule of DNA must be equal to a total number of T’s (the same is true of G and C). Thus, the % of A in DNA must equal the % of T (the same is true of G and C). The total percent of A, T, G, and C must be equal to 100. Human DNA contains 30% adenine, 30% thymine, 20% cytosine, and 20% guanine.
  • Due to complementary base pairing, the total number of purine bases is equal to a total number of pyrimidine bases. Thus the ratio is 1:1. This relation is called Chargaff’s rule.

Polarity  of Strands (Antiparallel):

  • The two strands of DNA run antiparallel to one another, that is, the two strands run in opposite directions. One in the 5 to 3 direction, the other in the 3 to 5 direction. Therefore, both ends of the double helix contain the 5 end of one strand (5 phosphate) and the 3 end of the other (3 OH).

Major and Minor Grooves:

  • The helix is right-handed and it results in the formation of deep groove called a major groove and another shallow groove called a minor groove.
  • The two groves are alternate.


  • The diameter of DNA molecule is 2 nm. The deflection pitch angle of a helix is 36º. The helix completes its one turn after covering a distance of 3.4 nm. There are 10 base pairs in one complete spiral.


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