The Nature of the Genetic Material

  • Mendel (1866) - Particulate inheritance
  • Weismann (1892) - the germ line theory
  • DeVries et al. (1900) - rediscovery of Mendel
    • Leads to the chromosomal theory of inheritance
  • Morgan (1910) - showed that genes were on chromosomes
    • Sturtevant (1913) - linear arrangement of genes
  • Griffith (1928) - transformed bacteria
  • Avery, Mcleod and McCarty (1944) - DNA is the genetic material in transformations. This flash animation at "DNA from the Beginning" describes these experiments: A gene is made of DNA
    • Hershey and Chase (1952) - phage (T2) genes are made of DNA. The second half of this flash animation at "DNA from the Beginning" describes this experiment: Bacteria and viruses have DNA too.
    • Fraenkel-Conrat and Singer (1955) - reconstituted TMV - RNA is the genetic material of TMV
  • Chargaff (1950) - base composition does vary between species but A=T and G=C
  • Wilken's and Franklin (1953) - DNA is in a helix (X-ray crystallography)
  • Watson and Crick (1953) - a double helix with base pairing
    • strands are antiparallel (they run in opposite directions)
      • There is a free 5' carbon at one end of the chain and a free 3' carbon at the other end - in the double helix the 5' end of one chain is at the same end of the helix as the 3' end of the other chain
    • the bases stack on top of one another on the inside of the helix, the sugar and phosphate backbone is on the outside of the helix
    • the bases are 3.4 nm apart along the helix and there are 10.4 bases per one complete turn of the helix
    • a base on one strand hydrogen bonds to a base on the other strand (A to T and G to C)
      • two hydrogen bonds between A and T
      • three hydrogen bonds between G and C
    • a right-handed helix
    • there is a major groove and a minor groove
    • the diameter of the helix is 2nm (20 Angstroms)
    • This flash animation at "DNA from the Beginning" describes their work and how to interpret the X-ray crystal results (also has a nice picture of thier 6 foot tall model): The DNA molecule is shaped like a twisted ladder.

Here's a picture of two bases hydrogen bonded together. Note the distance between the two bases and how the bases are at right angles to the ribose sugar. Below the first base pair you can see the next one only with the purine on the left and rotated a little bit as we go around the helix

Here's a view down the center of the helix. Note that all of the nitrogenous bases (nitrogen is in blue) are in the middle while the phosphates (in yellow) are on the outside.

This is a view form the side of the helix, showing the major groove. Notice how easy it is to see or cantact the bases in the middle of the helix.

Here's a side view of the minor groove. Note how much closer together the sugar phosphate backbone is for this part of the helix.

This document is copyright of Jeff Bell
Last Update: Friday, October 27, 2000