THE WATSON-CRICK MODEL BASE-PAIRING IN DNA
Experiments have shown that DNA samples taken from different cells of the same species have the same proportions of the four bases. For example, human DNA contains about 30% each of adenine and thymine, and 20% each of guanine and cytosine. The figure is different for other organisms, but the amounts of A and T are always the same, as are the amounts of C and G!
Why is this the case?
In 1953, James Watson and Francis Crick proposed a structure for DNA that not only accounts for this pairing of bases but also explains how relatively simply the system of storing and transferring genetic information is. According to the Watson-Crick model, a DNA molecule consists of two polynucleotide strands coiled around each other in a helical "twisted ladder" structure. As mentioned earlier in the tutorial, the sugar-phosphate backbone is on the outside of the double helix, and the bases are on the inside, so that a base on one strand points directly toward a base on the second strand. When using the twisted ladder analogy, think of the sugar-phosphate backbones as the two sides of the ladder and the bases in the middle as the rungs of the ladder. In effect, each strand of DNA is one-half of the double helix. The two halves come together to form the double helix structure.
The two strands of the DNA double helix run in opposite directions, one in the 5' to 3' direction, the other in the 3' to 5' direction. The term that describes how the two strands relate to each other is known as antiparallel.
So what holds the two strands together at the bases?
The strands are held together by hydrogen bonds between the nitrogenous bases. In the double helix, adenine and thymine form two hydrogen bonds to each other but not to cytosine or guanine. Similarly, cytosine and guanine form three hydrogen bonds to each other in the double helix, but not to adenine or thymine.
If you clicked on the links above, you may have noticed the exact nature of the hydrogen bonds. Hydrogen bonds occur only between a Hydrogen atom on one base and either an oxygen or nitrogen atom on the other base. This explains why only two hydrogen bonds can form between A's and T's and three can form between G's and C's, because a hydrogen bond can only form where a H atom comes in close proximity to an Oxygen or Nitrogen atom of a base on the opposite strand.
You may also have noticed that every base pair contains one purine and one pyrimidine ALWAYS. Again, this is related to the structure of each base and how a proper "fit" (both in base size and chemical makeup) allows the DNA helix to exist in a physically and chemically stable structure. This type of base pairing is called complementary rather than identical. Identical base pairing would mean that A's bond with A's, G's with G's, and so on, which (of course) isn't the case. The figure below shows a generalized structure of the DNA helix with all its components and hydrogen bonds.
This complementary base pairing in the two strands explains why A/T and G/C always occur in equal amounts. A helpful way to remember the base pairing in DNA is to memorize the phrase "Pure silver taxi."
"Pure Silver Taxi"
Pur AG TC
The purines A and G pair with T and C.
I'm ready to try Practice Problem #2.