The Structure of DNA

Question 1

Base Composition of DNA from different species + different tissues within same species

Answer 1

G-C content is often referenced to compare base comparisons between species
- Different tissues within a single species have the same base composition
- Different organisms have different (A+T)/(C+G)

Question 2

The Structure of DNA - How it was discovered

Answer 2

• Franklin + Wilkins obtained experimental data from x-ray diffraction pattern - gives info on dimensions
• Shows wire frame view of double helix proposed by Watson + Crick - structural info created the model

Question 3

What the structure of DNA is like

Answer 3

• Two-stranded helical structure
• Two strands run in opposite directions
• Helix is right-handed w/ a major and minor groove

Question 4

Structure of DNA - How molecules are arranged

Answer 4

• Bases on inside & sugar + phosphate residues on outside
• Sugar-phosphate backbone is regular
• Planes of bases are almost perpendicular to helix + are stacked
• Planes of sugar are nearly at right angles to the bases
• 36 degrees rotation relative to previous base pair

Question 5

The Structural Stability of DNA

Answer 5

Due to:
- Van der Waals contacts between bases on the same strand (hydrophobic interactions)
- Hydrogen bonding between opposing bases

Question 6

How bases are paired

Answer 6

• Adenine always pairs with Thymine, using 2 H bonds
• Guanine always pairs with Cytosine, using 3 H bonds
  (amount of A = T, amount of C = G)
• purine always bonding to a pyrimidine insures uniformity of the helix

Question 7

Structure of DNA - Major + Minor Grooves

Answer 7

• C-1’ carbons of opposing nucleotides don’t lie directly opposite each other
• So, the two sugar-phosphate backbones are not equally spaced along the helical axis - resulting in grooves
• size/width of major groove can fit an alpha helix of protein - side chains of alpha protein with interact with bases in DNA - thus can read for DNA sequences