At the core of each cell are molecules that store the blueprint of
life, deoxyribonucleic acid (DNA). DNA is composed of four types of
molecules known as nucleic acids or nucleotides. The four nucleotides
are: adenine (A), guanine (G), cytosine (C), and thymine (T). These
molecules are further classified into two families. Adenine (A) and
guanine (G) are known as purine and cytosine (C) and thymine (T) are
known as pyrimidine. During DNA synthesis, nucleotides are converted
into nucleic acids to so that they can be linked to form strands of
DNA. The assembled strand of DNA takes on the structure of a double
The chemical structures of the four nucleotides are planar due to
the delocalized electrons in the five- and six-membered rings, each
having a thickness of 3.4 angstroms. When the nucleotides form the
double helix structure, A-T and G-C are joined together by a hydrogen
bond to form a base pair. The base pairs are then joined together by
sugar bonds to form the helix. X-ray data shows that there are 10 base
pairs per turn of the helix.
The helical model of DNA also explains the theory of genetic
replication. James Watson once described it as the "pretty molecule"
because the method of replication is so self evident in this
structure. During replication, the hydrogen bonds between nucleotides
break and allow each single strand of DNA to serve as a template for
replication of the other half. The two identical copies of newly
synthesized of DNA are then distributed to two new daughter
cells. Because during each cycle of replication half of the old DNA is
preserved, DNA replication is said to be semi-conservative.
Although DNA contains the genetic blueprint of life, it requires
the assistance of ribonucleic acid (RNA) to be functional. RNA also
consists of strands of nucleic acids joined together by sugar-phosphate
bonds. Unlike DNA, RNA substitutes the nucleotide thymine (T) with
uracil (U) and exists as single strands. After DNA is converted into
strands of RNA, the messenger RNA (mRNA) is sent to the ribosome to
direct the synthesis of proteins.
Picture above: James Watson and Francis
Crick with DNA Model, 1953.
|Historical Fact: The structure of DNA was
not resolved until the early 1950's when James Watson and
Francis Crick assembled all of the previously known
information about DNA to construct a model of DNA. Their
result was published as a letter to the prominent
scientific journal Nature and was later recognized by the
The two strands of DNA are held together by hydrogen bonds between adjacent complementary nucleotides.