A pyrine binds with a pyrimidene in DNA to form a basepair. Adenine and thymine
bind together to form the A-T basepair. Likewise, guanine and cytosine
come together to form the G-C basepair. The bases are joined together by
weak hydrogen bonds, and it is this hydrogen bonding that produces DNA's
familiar double helix shape. An image illustrating the how two bases pair with hydrogen bonding is shown below (The
blue lines are the hydrogen bonds.)
is a five carbon sugar, and to fully understand many of the concepts that
are presented later on, one must know the structure of deoxyribose. A visual
representation of the sugar and how it relates to the other two components
of a nucleotide is shown below in figure 1.
The carbons of deoxyribose sugar are numbered sequentially from right to left.
The first carbon is 1' (read as one prime), the second is 2' (two prime),
and so on. The nitrogenous base attaches to the 1' carbon, and the phosphate
group attaches to the 5' carbon. The nucleotide below is covalently bonded
to the 3' carbon. This allows for a long strand to be built. An example
of a single strand of DNA is shown below.
Instead of always seeing a huge molecular diagram of a DNA strand, what one often
sees in a string of letters, such as "ATCTTAG". This string represents
the what bases are in a certain side of a strand of DNA. The above string
(ATCTTAG) represents the string "adenine-thymine-cytosine-thymine-thymine-adenine-guanine."
DNA has two strands. Whatever nucleotides are in one strand, they rigidly fix
the sequence of nucleotides in the other strand due to the way base pairing
occurs (A with T, G with C). The two strands are complementary. They aren't
identical, but fit together just right.
In addition, it must be noted that the two strands are antiparallel.
That means that they run in opposite directions. One strand goes in a 5'
to 3' direction while the other goes in a 3' to 5' direction. By convention,
the strand which goes in the 5' to 3' direction is placed on the left in
2-dimensional drawing. Figure 2 gives a visual example of this concept
as well as showing how the strands are complementary.
In this next image, the double-helix shape of DNA is shown. The two strands
are clearly visible, one being coloured blue, and the other red.