Adenine and guanine each have two rings of carbon and nitrogen atoms.
Remember, DNA has two strands that fit together something like a zipper.
The teeth are the nitrogenous bases but why do they stick together?
The bases attract each other because of hydrogen bonds.
Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.
The bonds between cytosine and guanine are shown here with dotted lines
Hydrogen Bonds, cont.
When making hydrogen bonds, cytosine always pairs up with guanine
Adenine always pairs up with thymine
Adenine is bonded to thymine here
Adenine and Thymine always join together
Cytosine and Guanine always join together
DNA by the Numbers
Each cell has about 2 m of DNA.
The average human has 75 trillion cells.
The average human has enough DNA to go from the earth to the sun more than 400 times.
DNA has a diameter of only 0.000000002 m.