The Structure of DNA
Nucleic acids are
that is they
are made up of chains of many repeating units called
nucleotides (see bottom left of Figure 1 below). The DNA molecule actually
consists of two polynucleotide chains that spiral around an imaginary
axis to form a double helix (spiral.) Nucleic acid molecules are
incredibly complex, containing the code that guarantees the accurate
ordering of the 20 amino acids in all proteins made by living cells.
Surprisingly though there are only a few different nucleotides.
And, in fact, only four different nucleotide units comprise DNA,
nucleic acid of
interest to the genealogist.
1 is a diagram of a short stretch of a DNA molecule which is unwound and
flattened for clarity. The boxed area at the lower left encloses one
nucleotide. Each nucleotide is itself make of three subunits:
five carbon sugar
here for the
structural formula of deoxyribose)
(a phosphorous atom surrounded by four oxygen atoms.) (Labeled P)
And one of four
nitrogen-containing molecules called
nucleotide bases .
(Labeled A, T, C, or G)
Alternating sugar and phosphate units form the two sides of a
ladder-shaped arrangement with the rungs or steps each formed by a pair of
nucleotide bases (bps) The sides of the ladder are held together by weak electrostatic bonds
between base pairs called
(sort of the velcro of the molecular world.) The human genome has
about 3,000,000,000 bps!
Figure 2 below shows the structural formula of DNA in greater detail. The
nitrogen bases are ring compounds with their carbon and nitrogen atoms
arranged in single or double rings. Single ringed bases are called pyrimidines and double ringed bases are called purines
(and it is not necessary to remember that)
Only certain bases can pair together to form base pairs (bps.) In DNA,
Adenine (A) always pairs with
thymine (T), and
always pairs with
Notice that in figures 1 and 2, the two strands of a DNA molecule are
that is, they run in different directions. The side of the chain on the
left begins with a free phosphate group at the top and ends with a sugar
molecule at the bottom. In contrast, the complementary chain on the
right begins at the top with a sugar molecule and ends at the bottom with
a phosphate group.
phosphate end of the
DNA molecule is called the 5'
(pronounced five prime end.)
end is called the 3'
end (3 prime end.)
Happily, it is not necessary to hold the
details of DNA structure in your mind at all times! As the
sugar and phosphate sides of the molecule are constant they are frequently
represented by parallel lines. Even better, each of the nitrogen
bases is conveniently represented by the first letter of its name.
These conventions allow the simplified representation of the molecule
shown in Figure 3.
Or, even easier, a section
of a DNA molecule is often abbreviated to show the bases of just one
5'A T G G C T A C 3'
Knowing the base pairing
convention of A always pairing with T and G always pairing with C makes
the complementary strand of the molecule understood. It is this
feature of complementary base pairing that insures an exact duplicate
of each DNA molecule will be passed to its daughter cells when a cell
Determining the order
of the bases in a DNA chain is the goal of
DNA Sequencing. Determining the order of the bases in the entire
Human genome (the DNA in all human chromosomes) that is the goal of the
Human Genome Project. For more information about the many issues
addressed by the Human Genome Project, see
Human Genome Project Information.
to learn how DNA makes copies of itself.
Some illustrations on this page are from the website of
Office of Science Education and Outreach of the National Human Genome