DNA - Deoxyribonucleic Acid

Elizabeth Dallman, Pharm.D. Candidate; Caitlin Munro, Pharm.D. Candidate; Emily Loudermilk, Pharm.D. Candidate.

What is DNA?

Deoxyribonucleic acid (DNA) is found in almost every cell of the body. DNA is made up of a sequence of four chemicals called nucleotide bases. These bases include:

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)

These 4 bases are the building blocks of our DNA and, ultimately, the human genome (a collection of a human’s genetic material stored as DNA). DNA builds genes. Genes, which contain information to build what are called proteins, are also the main components of the 23 different chromosomes, or threadlike structures found in the nucleus of a cell. There are two copies of each chromosome, one copy from each parent. Over 3 billion nucleotide bases make up the 23 chromosomes you receive from just one parent.

Based on DNA, we are all about 99.9% the same. Differences among individuals can be described as genetic variation. In some cases, a genetic variation can be the result of a change in nucleotide base sequence when one base replaces another. For instance, adenine (A) can take the place of guanine (G) to cause genetic variation.


Consider the following example:

A possible series of nucleotide bases may be-

TCC CAG CTG GAA TCC GGT GTC

We will refer to this strand as our “common” sequence for the rest of this discussion.

A variation of our “common” sequence may be-

TCC CAG CAG GAA TCC GGT GTC

Notice where A (adenine) replaced T (thymine) from the original strand?

Since one base replaced another, this is called a single nucleotide change or single nucleotide polymorphism (SNP, pronounced “snip”). This change may result in a difference between two compared individuals.

These pieces of DNA are then used to determine the sequence of amino acids, ultimately leading to making a protein.


What is an amino acid?

In the example above, note how the letters in the sequences are grouped in threes. This is to make the point that each group of three letters (i.e. three nucleotide bases, called a codon; pronounced "code on") gives the cell directions to make a specific chemical called an amino acid. There are 20 different amino acids that can be made.

For instance, based on the "common" sequence in the above example, the following amino acids are made:

TCC CAG CTG GAA TCC GGT GTC

TCC makes the amino acid Serine
CAG makes the amino acid Glycine
CTG makes the amino acid Leucine
GAA makes the amino acid Glutamic Acid
TCC makes the amino acid Serine
GGT makes the amino acid Glycine
GTC makes the amino acid Valine

Note: a given amino acid, like Glycine above, can be made by a number of different codons.

Therefore, the amino acid sequence made by this "common" nucleotide base sequence is:

Serine-Glycine-Leucine-Glutamic acid-Serine-Glycine-Valine

What is a protein?

As seen in both the example and the image above, when amino acids are sequenced together, they are called a protein. When discussing drugs, proteins can act as receptors, transporters, and enzymes. These three roles of a protein can be referred to as "drug targets".

Getting back to the nucleotide base sequence as shown above, the variation noted in red would result in the following:

TCC CAG CAG GAA TCC GGT GTC

TCC makes the amino acid Serine
CAG makes the amino acid Glycine
CAG makes the amino acid Glycine (changed from Leucine because A replaced T)
GAA makes the amino acid Glutamic acid
TCC makes the amino acid Serine
GGT makes the amino acid Glycine
GTC makes the amino acid Valine

Therefore, the amino acid sequence made by the "variant" nucleotide base sequence is:

serine-glycine-glycine-glutamic acid-serine-glycine-valine

Because this amino acid sequence is different, the protein formed is different and may have a different level of function. This may cause the drug targets (receptors, transporters, and enzymes) to work differently or not at all.

Sources of Information

Sadava D, Hillis DM, Heller HC, and Berenbaum MR. Life: The Science of Biology. 9th ed. Sunderland, MA: W. H. Freeman and Company; 2011.