DNA, also known as deoxyribonucleic acid, is a fundamental
molecule found in all living things. It serves as the basis for
heredity, specifying which traits are passed on from parents to
children through the generations. It also contains instructions
for our body cells to perform their specific functions.
In humans, most of the DNA is in the form of tightly coiled strands
called chromosomes, found inside the cell nucleus. There are 46
chromosomes in a human cell. If you unwind each chromosome and place
them end-to-end, you will have a long, double-stranded helix that
is about 3 meters in length—all from one microscopic human
DNA helix looks like a twisted ladder. The two sides are composed
of the four bases: adenine (A), thymine (T), guanine (G), and cytosine
(C), and the rungs of the ladder represent hydrogen bonds that connect
specific pairs of these molecules together: A–T and G–C.
The arrangement of these molecules, called the DNA
sequence, spell out the instructions for our physical characteristics
and body functions. These instructions are found in units called
The specific pairing of DNA molecules presents a simple mechanism
for their replication. Replication occurs whenever a cell divides
in two during growth and development.
During replication, the helix unwinds and an enzyme
separates the two strands. Another enzyme, DNA polymerase, adds
the four molecules A, T, C, and G to each strand based on the strands
sequences: A is added to a T on the strand, and C is paired with
The products of replication are two sets of double-stranded
DNA molecules that have exactly the same sequence as the original.
During cell division, each cell receives one set of DNA. In this
way, all the cells of the body have the same DNA molecules.
In the laboratory, scientists use an artificial
method for replication to examine specific portions of DNA. To learn
more, please visit the Polymerase Chain Reaction page.