DNA Double Helix
In 1953, two scientists, James Watson and Francis Crick, proposed a model
for the structure of DNA. They believed that DNA was made up of “two chains
that wrap around each other in the shape of a double helix.” This shape is very
similar to the shape of a winding spiral staircase. This was a remarkable discovery
in the biological sciences.
DNA Nucleotides
At the beginning of the year, we learned that DNA belongs to a group of
molecules called nucleic acids. These nucleic acids are made up of smaller units
called nucleotides. Each nucleotide consists of three parts: (1) five-carbon sugar,
(2) a phosphate group, (3) nitrogenous base.
(1) Five Carbon Sugar – this sugar molecule is shaped into a ring. The
five-carbon sugar found in DNA is deoxyribose (the “D” in DNA).
(2) Phosphate Group – the phosphate group forms a bond between 2
adjacent five-carbon sugars. The phosphate groups are responsible for
holding DNA together. The “backbone” of DNA is oftentimes referred to
as the “sugar-phosphate backbone” and it is what gives DNA its
characteristic shape.
(3) Nitrogenous Base - the nitrogenous base is attached to the other end of
the five-carbon sugar and sticks out into the middle of the DNA
molecule. There are four possible types of nitrogenous bases and these
are what specifically determine one’s characteristics.
Also helping give the DNA molecule shape are hydrogen bonds forming between
the nitrogenous bases on each side of the DNA. These bonds form the “steps” of
the staircase and can be seen in the middle of the molecule.
Nitrogenous Bases
DNA is made up of three parts (1) five-carbon sugar, (2) phosphate group and
(3) nitrogenous base. The sugar and phosphate groups are identical in all DNA.
Variety comes with the nitrogenous bases. There are four possible bases and which base
is present plays a large role in determining an individual’s characteristics.
The four nitrogenous bases are: Thymine, Cytosine, Adenine and Guanine. Biologists
usually shorten the names of these nitrogenous bases down to their first letter – “T” representing
thyamine, “C” representing cytosine and so forth.
Each of these bases is composed mostly of carbon (C), Nitrogen (N) and Hydrogen (H). All of
the bases are found in a circular shape, called a ring. If a base if composed of only one ring, it is
called a pyrimidine. Two bases are composed of a single ring: Thymine and Cytosine. If a
base if composed of a set of two rings, it is called a purine. The two purines are Adenine and
Guanine.
Complementary Bases
Important parts of DNA’s structure are the nitrogenous bases. These determine an
individual’s characteristics.
We have learned that hydrogen bonds form between the nitrogenous bases in the middle
of the DNA molecule. The formation of these bonds follow a specific pattern called the
base-pairing rules.
According to these rules, cytosine on one strand will only form bonds with guanine on
the opposite DNA strand. Likewise, thymine will only bond with an adenine on the
opposite strand.
Cytosine bonding with Guanine and Thymine bonding with Adenine are known as
complementary base pairs. Because of this rule, if someone has only one strand of
DNA, they can determine the opposite strand.
For example – if one strand of DNA is TAAG, we can use the base-pairing rules to
determine the opposite strand is ATTC.