Now that we've looked at the general structure of DNA, we should take a closer
look at the structures that make up nucleotides.
The Bases of DNA
The four nitrogen bases found in DNA are adenine, cytosine, guanine, and
thymine. Each of these bases are often abbreviated a single letter: A (adenine),
C (cytosine), G (guanine), T (thymine). The bases come in two categories:
thymine and cytosine are pyrimidines, while adenine and guanine are
purines ().
Figure %: DNA Bases
The pyrimidine structure is produced by a six-membered, two-nitrogen molecule;
purine refers to a nine-membered, four-nitrogen molecule. As you can see, each
constituent of the ring making up the base is numbered to help with specificity
of identification.
Base Pairing in DNA
The nitrogen bases form the double-strand of DNA through weak hydrogen
bonds. The nitrogen bases, however, have specific shapes and hydrogen bond
properties so that guanine and cytosine only bond with each other, while adenine
and thymine also bond exclusively. This pairing off of the nitrogen bases is
called complementarity. In order for hydrogen bonding to occur at all, a
hydrogen bond donor must have a complementary hydrogen bond acceptor in
the base across from it. Common hydrogen bond donors include primary and
secondary amine groups or hydroxyl groups. Common acceptor groups are carbonyls
and tertiary amines ().
Figure %: Common Hydrogen Bond Donors and Acceptors
There are three hydrogen bonds in a G:C base pair. One hydrogen bond forms
between the 6' hydrogen bond accepting carbonyl of the guanine and the 4'
hydrogen bond accepting primary amine of the cytosine. The second between the 1'
secondary amine on guanine and the 3' tertiary amine on cytosine. And the third
between the 2' primary amine on guanine and the 2' carbonyl on cytosine
().
Figure %: Guanine : Cytosine Base Pair
Between an A:T base pair, there are only two hydrogen bonds. One is found
between the 6' primary amine of adenine and the 4' carbonyl of thymine. The
other between the 1' tertiary amine of adenine and the 2' secondary amine of
thymine ().
Figure %: Adenine : Thymine Base Pair
The Deoxyribose Sugar
The deoxyribose sugar in DNA is a pentose, a five-carbon sugar.
Four
carbons and an oxygen make up the five-membered ring; the other carbon branches
off the ring. Similar to the numbering of the purine and pyrimidine rings (seen
in ), the carbon constituents of the sugar ring are
numbered 1'-4' (pronounced "one-prime carbon"), starting with the carbon to the
right of the oxygen going clockwise (). The fifth
carbon (5') branches from the 4' carbon.
