Biomolecules / Macromolecules – Nucleic Acids
Nucleotides
Our final major category of biological molecules is the class known as nucleotides, which received this name because
they were first discovered in the nuclei of cells. We will deal with them in more detail later, but any discussion of the
molecules of life would be incomplete without at least an introduction to this important group.
Color the headings Nitrogenous Bases, Purines, and Pyrimidines. Using pale colors so as not to obscure the
structural formulae, color titles A through E and the five nitrogenous bases illustrated.
A nucleotide consists of at least three components: a nitrogenous base, a pentose (five-carbon sugar), and a phosphate
ion. The nitrogenous bases receive their name because they all contain nitrogen and chemically act as weak bases
(proton acceptors). The five shown here are the most important ones, although there are others with lesser roles. The two
purines (adenine and guanine) consist of double rings of carbon and nitrogen atoms, while the three pyrimidines
(cytosine, thymine, and uracil) consist of a single ring. They differ from one another in the groups of atoms attached to the
rings.
Color the heading Pentoses, titles F and G, and the related illustrations. Color title H and the phosphate ion.
Two different pentoses are found in nucleotides. Most nucleotides contain ribose, but some contain deoxyribose, which is
exactly like ribose except that it is missing the oxygen atom from the hydroxyl (OH) group on carbon 2. Phosphate ion is
the negative ion of phosphoric acid (H 3PO4). It is shown here with a single negative charge where its proton (hydrogen
ion) has dissociated away, but each of the hydroxyl groups remaining can also dissociate a proton, producing phosphate
ions with two or three negative charges. (In any group of phosphate ions, a few have two or three negative charges, but
most have only one.)
Color the heading Ribonucleotide and the adenine, ribose, and phosphate components of the ribonucleo-tide,
using the colors you used in the top part of the plate. Then color title I and the broken ring labeled I (which is for
illustrative purposes only). Color the heading Functions in a Cell, titles J through M using light colors, and the
related diagrams.
We see at the bottom of this plate one particular kind of nucleotide called adenosine monophosphate. (Ribose plus a
base, without the phosphate, is called a nucleoside.) Adenosine monophosphate is classified as a ribonucleotide because
it has ribose as part of its structure. The ribose has a molecule of adenine and a phosphate ion attached to it Like
polysaccharides, proteins, and lipids, the nucleotides are assembled from their subunits by dehydration condensation. In
this case it occurs in several steps too complicated to go into here, but the net result is that a molecule of water is formed
when a base and a pentose are joined, and another water molecule results when a pentose and a phosphate are joined.
As we will see in various later plates, nucleotides can perform a number of functions in a living cell: they can be used as
chemical messengers to regulate cell activities; they can function as coenzymes, essential parts of certain enzymes; with
one or two additional phosphates added, they can serve to carry energy from one part of a cell to another; or they can
become components of a nucleic acid. Nucleic acids are of two types: deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA). Both consist of very long chains of nucleotides, usually double-stranded in DNA and single-stranded in RNA. DNA
is made up of nucleotides containing deoxyribose combined with one of the four bases, adenine, guanine, cytosine, and
thymine. RNA nucleotides contain ribose combined with the bases, with uracil substituted for thymine.
The sequence of the bases in DNA forms the hereditary code, which is passed from generation to generation. RNA
molecules occur in several types (to be covered in later plates) and translate the hereditary code into specific proteins that
determine how the cell (or a large organism made of large numbers of cells) will function under various conditions. There
is also some experimental evidence to suggest that RNA may play a role in memory.