The Structure of DNA ~ Reading
Directions: Read and highlight the following passage. Use the information included in the passage to
help you color the drawing of DNA attached. When you are finished, answer the questions on the back of this
page. The diagram attached shows the structure of DNA. DNA is in the shape of a double helix, or twisted
ladder. The double helix structure of DNA was proposed by James Watson and Francis Crick in 1953 and is
widely accepted today as correct. The upper end of the drawing shows DNA in a simplified form. This part of
the drawing uses simple shapes to represent each part of the DNA molecule. The lower portion of the drawing
is much more complex. It shows the structural formula of each part of the DNA molecule, with all of the
individual atoms and their bonds included.
DNA stands for deoxyribonucleic acid. It is composed of smaller units (monomers) called nucleotides.
Each nucleotide contains a sugar called deoxyribose, a phosphate group, and one of four different nitrogenous
bases. Nitrogenous bases are molecules that contain nitrogen and have the properties of a base. The 4
different nitrogenous bases found in DNA are: adenine (A), thymine (T), guanine (G), and cytosine (C).
Adenine and guanine are larger bases, known as purines, while thymine and cytosine are smaller and are
known as pyrimidines.
DNA consists of two strands of nucleotides that are lined up next to each other in a very specific way.
The overall structure of the DNA molecule is often compared to that of a ladder that has been twisted. The
sides of the ladder, often called the “backbone”, are made up of the sugar deoxyribose and phosphate groups.
The deoxyribose sugars and phosphate groups alternate all the way down the sides of the ladder. On the
drawing attached, color the deoxyribose sugars brown and the phosphates orange. Start ONLY with the
simplified end of the drawing. You will move onto the more complicated end of the drawing later. The “rungs” of
the ladder, or the horizontal lines that connect the sides of the ladder, are made up of pairs of bases – a base
on one side of the ladder matches up with a base on the other side of the ladder. The bases pair up according
to specific base pairing rules - adenine always matches with thymine and guanine always matches with
cytosine (A to T and G to C). On the simplified end of the drawing only, color adenine red, thymine blue,
guanine green, and cytosine yellow. Base pairs are held together by weak bonds called hydrogen bonds. Color
the hydrogen bonds black. Again, color the simplified end of the drawing first.
The structural formula (the bottom, more complicated, portion of the diagram) shows the exact
arrangement of the atoms in each part of the DNA molecule. Each nucleotide contains a deoxyribose sugar, a
phosphate, and one of the four bases that are all made up of specific arrangements of atoms. Use the same
colors as stated above to color this portion of the drawing. Each part of the DNA molecule is outlined with a
solid black line, and the atoms that make up that part are shown as letters inside the black outlines. The
hydrogen bonds are represented as small circles between the bases.
In 1958, Watson, Crick and another scientist named Maurice Wilkins, received the Nobel Prize in
physiology and medicine for discovering the structure that you see in this diagram. It was an extremely
important step for science because Watson and Crick not only helped to discover the structure of DNA, but
they also realized that this structure lends itself well to self-replication so that the cell can easily make exact
copies of its own DNA.
