Historically, chemistry and biology developed as different sciences. But as biology and genetics delve deeper and deeper into the study of molecules, the boundary lines are beginning to blur.

Nowadays, it is common to hear a term like "molecular biology" or "molecular genetics" -- but those terms would have been incomprehensible to scientists 50 years ago.

According to Erwin Chargaff, the "father" of the rules of base pairing, the bridge between biology and chemistry only began to be understood in the l940's with the discovery by Osward T. Avery that the units of heredity - genes - were composed of DNA (which of course is composed of chemicals: purines and pyrimidines).

Purines are double-ring molecules (bases), while pyrimidines are single-ring molecules (bases), and all DNA is made up of just four chemicals:

• (A) Adenine (purine)
   
• (T) Thymine (pyrimidine)
   
• (C) Cytosine (pyrimidine)
   
• (G) Guanine (purine)
   

Chargaff's Rules
http://fig.cox.miami.edu/~cmallery/150/gene/chargaff.htm 

Here are Chargaff's base pairing rules:

A always pairs with T

C always pairs with G

You already know that purines bond with pyrimidines, but why can't purines bond with purines or pyrimidines bond with pyrimidines?

It's because there is not enough space for two purines to fit within the helix and too much space for two pyrimidines to get close enough together for hydrogen bonds to form between them. (As you may remember, the molecules in DNA are held together by hydrogen bonding.)

Bonding
http://www.visionlearning.com/library/module_viewer.php?mid=55 

To understand chemical bonding, you must remember that atoms have a nucleus of positively-charged protons and neutrally-charged neutrons. Negatively-charged electrons arranged outside the nucleus.

It's how these electrons are arranged and how they react with other atoms that determines the nature of the chemical bonds that form between them.

There are three different kinds of chemical bonds.

1. Ionic Bonds (also called electrovalent bonds) are formed between atoms when one or more electrons are completely transferred from one atom to the other.
    
2. Covalent Bonds are formed between atoms when two atoms share one or more pairs of electrons.
    
3. Hydrogen Bonds are formed between the positively-charged hydrogen atom in one covalently-bonded molecule and the negatively-charged area of another covalently-bonded molecule.

As we said, DNA bases are held together by hydrogen bonds. There are two hydrogen bonds between A and T and three hydrogen bonds between C and G.

The bonds between the anticodon of a tRNA molecule and the complementary codon of mRNA are catalyzed by aminoacyl-tRNA synthetase.

The rules of base pairing tell us that if we can "read" the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand.

These rules also explain the phenomenon that whatever the amount of Adenine (A) in the DNA of an organism, the amount of Thymine (T) is the same.

Similarly, whatever the amount of Guanine (G) in the DNA of an organism, the amount of Cytosine (C) is the same.

These base pair relationships are called Chargaff's Rules after the Columbia University professor whose team first observed the equal concentrations of A and T - and C and G - in most DNA molecules.

Here are the relative proportions of bases in the DNA of a few organisms.

Here are a few more things to remember about basic pairing and coding.

A base deletion early in the coding sequence of a gene may result in a nonsense mutation, a frameshift mutation, multiply missense mutations and a nonfuctional protein.

Base pair substitutions may have little effect on the resulting protein. That's because the redundancy of the code my result in a silent mutation, the missense mutation may not occur in a critical part of the protein, the new amino acid may have similar ties to the replaced amino acid and the wobble phenomenon would result in no change in translation.

Experiments for Home and Classroom

In this activity, called "Have Your DNA and Eat It Too," students build an edible model of DNA while learning basic DNA structure and the rules of base pairing.

The structure of DNA is a double helix, much like a ladder that is twisted into a spiral shape. The bases of the DNA are found in pairs that make up the rungs of the ladder. The uprights of the ladder are the structural backbone of the DNA -- they don't carry information, they just hold the bases in their proper order. DNA bases pair G with C and A with T. In this activity, students are invited to "Build a DNA Molecule" online.

Students are invited to demonstrate the bonding properties of DNA by forming a living DNA molecule. Each wears a colored headband to represent one of the four base pairs and all join hands to illustrate the bonding of base pairs to a rope, which represents the outside hydrogen bonds. This activity is designed for younger students but is very illuminating. Scroll down to "Science."
http://www.escueladelsol.org/elementary.html 

In this activity, students are invited to read "Theory" and then play the "Thrillionaire Game", an interactive quiz about chemical bonding.
http://www.syvum.com/cgi/online/tgamem.cgi/squizzes/chem/bonds1.tdf?0