NUCLEIC ACIDS are involved in the storage and transfer of genetic information
in all living organisms.
There are two types of nucleic acids in cells:
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Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Nucleic acids are so named because DNA was first discovered in the cell’s
nucleus (hence “nucleic”), but both DNA and RNA also occur in other parts of the
cell.
DNA is the genetic material that organisms inherit from their parents. When a cell
reproduces itself by dividing, its DNA molecules are copied and passed along
from one generation of cells to the next. Encoded in the structure of DNA is the
information that programs all of the cell’s activities. DNA acts as the cell’s
blueprint, containing all the information needed to create the different parts of the
cell. DNA, with the help of RNA, is how the cell makes its proteins.
A DNA molecule consists of two long chains made up of nucleotide subunits.
These long chains are called polynucleotides (“poly” means “many”). Hydrogen
bonds between the base portions of the nucleotides hold the two chains together.
Nucleotides are composed of a five-carbon sugar to which are attached one or
more phosphate groups and a nitrogen-containing base. In the case of the
nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate
group (hence the name deoxyribonucleic acid), and the base may be either
adenine (A), cytosine (C), guanine (G), or thymine (T).
Structure of a nucleotide
The nucleotides are covalently linked together in a chain through the sugars and
phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugarphosphate. Because only the base differs in each of the four types of subunits,
each polynucleotide chain in DNA is analogous to a necklace (the backbone)
strung with four types of beads (the four bases A, C, G, and T). These same
symbols (A, C, G, and T) are also commonly used to denote the four different
nucleotides—that is, the bases with their attached sugar and phosphate groups.
The three-dimensional structure of DNA—the double helix—arises from the
chemical and structural features of its two polynucleotide chains. Because these
two chains are held together by hydrogen bonding between the bases on the
different strands, all the bases are on the inside of the double helix, and the
sugar-phosphate backbones are on the outside.