The Structure of DNA   


Nucleic acids 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 such 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: only four different nucleotide units comprise DNA, the nucleic acid of interest to the genealogist.  



This figure 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:

A five carbon sugar called deoxyribose (Labeled S)

A phosphate group (a phosphorous atom surrounded by four oxygen atoms.) (Labeled P)

And one of four nitrogen-containing molecules called nucleotides . (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. 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.  Only certain bases can pair together to form base pairs.  In DNA, Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).  

Notice that in the two figures above, the two strands of a DNA molecule are antiparallel, 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. 

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 the figure below

Or, even easier, a section of a DNA molecule is often abbreviated to show the bases of just one strand:


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 divides.

Some illustrations on this page are from the website of the
Office of Science Education and Outreach of the National Human Genome Research Institute
Figure 1)
Figure 2)