Both DNA and RNA are known as nucleic acids. They have been given this name for
the simple reason that they are made up of structures called nucleotides. Those
nucleotides, themselves comprising a number of components, bond together to form
the double-helix first discovered by the scientists James Watson and Francis
Crick in 1956. This discovery won the two scientists the Nobel Prize. For now,
when we discuss nucleic acids you should assume we are discussing DNA rather
than RNA, unless otherwise specified.
Nucleotides
A nucleotide consists of three things:
A nitrogenous base, which can be either adenine, guanine, cytosine, or
thymine (in the case of RNA, thymine is replaced by uracil).
A five-carbon sugar, called deoxyribose because it is lacking an oxygen
group on one of its carbons.
One or more phosphate groups.
The nitrogen bases are pyrimidine in structure and form a bond between their 1'
nitrogen and the 1' -OH group of the deoxyribose. This type of bond is
called a glycosidic bond. The phosphate group forms a bond with the
deoxyribose sugar through an ester bond between one of its negatively
charged oxygen groups and the 5' -OH of the sugar ().
Figure %: A Nucleotide
Nucleic Acids
Nucleotides join together through phosphodiester linkages between the 5'
and 3' carbon atoms to form nucleic acids. The 3' -OH of the sugar group forms
a bond with one of the negatively charged oxygens of the phosphate group
attached to the 5' carbon of another sugar. When many of these nucleotide
subunits combine, the result is the large single-stranded polynucleotide or
nucleic acid, DNA ()
Figure %: The Nucleic Acid DNA
If you look closely, you can see that the two sides of the nucleic acid strand
shown above are different, resulting in polarity. At one end of the large
molecule, the carbon group is unbound and at the other end, the -OH is unbound.
These different ends are called the 5'- and 3'-ends, respectively.
The Helical Structure of DNA
shows a single strand of DNA. However, as stated earlier,
DNA exists as a double-helix, meaning two strands of DNA bind together.
Figure %: Double-helical DNA
As seen above, one strand is oriented in the 5' to 3' direction while the
complementary strand runs in the 3' to 5' direction. Because the two strands
are oppositely oriented, they are said to be anti-parallel to each other.
The two strands bond through their nitrogen bases (marked A, C, G, or T for
adenine, cytosine, and guanine). Note that adenine only bonds with thymine, and
cytosine only bonds with guanine. The nitrogen bases are held together by
hydrogen bonds: adenine and thymine form two hydrogen bonds; cytosine and
guanine form three hydrogen bonds.
An important thing to remember about the structure of the DNA helix is that as a
result of anti-parallel pairing, the nitrogen base groups face the inside of the
helix while the sugar and phosphate groups face outward. The sugar and
phosphate groups in the helix therefore make up the phosphate backbone of
DNA. The backbone is highly negatively charged as a result of the phosphate
groups.
