DNA PROFILING UNIT1

• DNA molecule

  • The two strands are intertwined forming a "double helix" structure
  • The DNA backbone is an alternating sugar-phosphate sequence
  • The deoxyribose sugars are joined to phosphate groups by phosphodiester bonds at the 3'- and 5'-carbon
• DNA

  Deoxyribonucleic acid
• Nitrogenous Bases

  • Adenine
  • Guanine
  • Cytosine
  • Uracil
  • Thymine
• Base Pairs

  • Adenine forms 2 hydrogen bonds with Thymine on the opposite strand
  • Guanine forms 3 hydrogen bonds with Cytosine on the opposite strand
• DNA

  • Belongs to the class of macromolecules called nucleic acids
  • Polymer with monomer units called nucleotides
  • Known as a "polynucleotide" due to its structure
  • Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen-containing base, and a phosphate group
• DNA molecule was first described by James D. Watson and Francis Crick in 1953
• Nucleotides

  • Adenine: adenosine, adenosine triphosphate (ATP / dATP)
  • Guanine: guanosine, guanosine triphosphate (GTP / dGTP)
  • Cytosine: cytidine, cytidine triphosphate (CTP / dCTP)
  • Uracil: uridine, uridine triphosphate (UTP / dUTP)
  • Thymine: thymidine, thymidine triphosphate (TTP / dTTP)
• DNA molecule

  • Chain has a direction (polarity)
  • Two polynucleotide chains run in opposite directions - antiparallel
  • Sugar-phosphate backbone of DNA is polar and hydrophilic
  • Bases are relatively non-polar and hydrophobic
• Chargaff’s Rule: Amount of adenine equals thymine, amount of guanine equals cytosine in DNA
• Hydrostatic forces in DNA double helix

  • Have a stabilizing effect on the overall structure
  • Create a strong pressure holding the two strands together
• Cross pattern on DNA fibre diffraction indicates helical structure
• DNA Double Helix


  • Two DNA strands form a right-handed helical spiral
  • Sugar-phosphate backbones wind around the helix axis like a spiral staircase railing
  • Bases of nucleotides are stacked inside the helix like steps of a spiral staircase
  • Helix makes a turn every 3.4 nm
  • Distance between neighboring base pairs is 0.34 nm
  • About 10 pairs per turn
  • Intertwined strands create major and minor grooves of different widths
• Watson and Crick’s Model of DNA


  1. 1928 - Frederick Griffith's experiment on inheritance molecule using mice and bacteria

  2. Griffith's conclusion on transformation and inheritance molecule

  3. 1942 - Oswald Avery's experiment identifying DNA as the inheritance molecule

  4. 1940s - Erwin Chargaff's observation of base patterns in DNA

  5. Rosalind Franklin and Maurice Wilkins' x-ray pattern of DNA showing helical structure
• Ultra bright spots on either side of the center vertically suggest repetition in the helix every 3.4 units
• Source: 'www.who2.com/.../watson-v-franklin-round-27.html'
• Watson and Crick’s Model of DNA


  1953
• The pattern appeared to contain rungs, like those on a ladder between two strands that are side by side
• The distance of these spots from the centre suggested that the repetition is every 3.4 Angstroms
• Each side of DNA is a complete complement of the other
• Forms of DNA observed in cells

  • A-DNA
  • B-DNA
  • Z-DNA
• Rosalind Franklin did not receive the Nobel Prize because she had died of cancer by that time
• The cross pattern on the DNA fibre diffraction is an indication that the structure is helical
• Adenine and Guanine are purines having two carbon-nitrogen rings in their structures
• If purines and the pyrimidines were together, then DNA would look wobbly and crooked
• 1. DNA has a major and minor groove, with 10 base pairs per turn spanning 3.4 nm
• Watson and Crick used information from Avery, Chargaff, Griffith, and others to piece together the puzzle of DNA
• Two ultra bright spots on either side of the centre, vertically, indicate that there is repetition in the helix
• Ten lines from the Centre to the bright spots indicate that there are ten residues per turn
• The pattern showed an “X” shape indicating that DNA had a helix shape
• By using the picture of the crystallized DNA, Watson and Crick were able to put together the model of DNA
• If Thymine is paired with Adenine and Guanine with Cytosine, DNA would look uniform (Chargaff's rule)
• Thymine and Cytosine are pyrimidines having one carbon-nitrogen ring in their structure
• 1. DNA can form with higher salt concentrations or with alcohol added, having a wider right-handed spiral, a shallow, wider minor groove, a narrower, deeper major groove, 11 bases per turn, and one turn spanning 2.3 nm
• The Nobel Prize was awarded to Watson, Crick, and Maurice Wilkins for their work on DNA
• Watson and Crick made an accurate model of DNA - a double helix with little rungs connecting the two strands
• 1. DNA is the most common form of DNA under the conditions found in cells
• Watson and Crick (1953): '“It has not escaped our notice that the specific base pairing we have proposed immediately suggests a possible copying mechanism for the genetic material.”'
• In the extended chromosome, 30nm diameter threads form large coiled loops held together by a set of non-histone scaffolding proteins
• Genes determine traits and are mostly 1,000 to 4,000 nucleotides long
• Chromosomes are highly coiled condensed packages of DNA