Nucleic acids are organic macromolecules composed of a limited variety of monomers (nucleotides) linked together into polymer strands (DNA, RNA) with characteristic stability (DNA more stable; RNA less stable).
The monomeric unit of nucleic acid is a nucleotide, which in turn is composed of three parts: a sugar ring, a heterocyclic base, and a phosphate group. A corresponding nucleoside is structurally similar with a sugar ring and heterocyclic base but lacks a phosphate group.
The sugar ring can be either a ribose (found in RNA) or 2′-deoxyribose (found in DNA).
In the structure of a nucleotide, the sugar subunit is situated as a hub, linked on one side to the phosphate group and, on another side, to the base. This arrangement lends itself to the polymer construction of nucleic acids by the formation of phosphodiester bonds that connect the sugar of one nucleotide to the phosphate group of the next nucleotide in the strand. Following these sugar-phosphate linkages down the length of the nucleic acid polymer gives the impression of a backbone with a variety of bases, each extending from its sugar link.
The five common nucleotides found in DNA and RNA are divided in to purines (double ring structure) and pyrimidines (single ring structure).
|Base||Ring structure||Found in DNA||Found in RNA|
The Watson-Crick model of the structure of DNA elucidated a double stranded composition with the two strands wound into a double helix. In addition to the helical formation, each strand runs antiparallel (its nucleotides oriented in the opposite direction of its partner strand), with the sugar-phosphate backbone running along the outside and bases projected into the center of the helix where they hold the formation by hydrogen bonding to the bases projected inward from the other strand.
The hydrogen bonding between bases on each strand of a double stranded molecule of DNA is arranged with specificity between certain base pairs with each pair composed of a purine and a pyrimidine.
|Purine||Pyrimidine||Number of hydrogen bonds|
The structure of nucleic acids as polymers with unique sequences of bases (by way of their nucleotide residues) gives way to a high fidelity means of transmitting genetic information by reading and replicating the base sequence for a strand of DNA. This process is performed in DNA replication, whereby each strand of the double-stranded DNA molecule is introduced to a new partner strand by matching new nucleotides with the correct base pairing, and in transcription, where a new molecule of RNA is created by linking nucleotides that pair with the sequence of bases on a template strand of DNA.
The double helix of double-stranded DNA is stabilized by the hydrogen bonding between base pairs along the length of the molecule. Disruption of the hydrogen bonds, such as in the case of high temperature, can cause the unwinding of the two strands (denaturation), which can then also be brought back together when proper conditions return (reannealing).
A single strand of DNA will readily bind to another single strand of DNA in the process of hybridization where there is a significant amount of base pair matching between their sequences (in conditions conducive to its hydrogen bonds).
Learn more about planning and tracking your MCAT prep!
MCAT.me Tour →