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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BIO BC
Nucleic Acid Structure and Function

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Description

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).

Nucleotides and nucleosides

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.

SugarBasePhosphate
Nucleotide
Nucleoside

The sugar ring can be either a ribose (found in RNA) or 2′-deoxyribose (found in DNA).

Sugar phosphate backbone

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.

Pyrimidine, purine residues

The five common nucleotides found in DNA and RNA are divided in to purines (double ring structure) and pyrimidines (single ring structure).

BaseRing structureFound in DNAFound in RNA
Adeninepurine
Guaninepurine
Cytosinepyrimidine
Thyminepyrimidine
Uracilpyrimidine

Deoxyribonucleic acid (DNA): double helix, Watson-Crick model of DNA structure

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.

Base pairing specificity: A with T, G with C

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.

PurinePyrimidineNumber of hydrogen bonds
AdenineThymine2
GuanineCytosine3

Function in transmission of genetic information

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.

DNA denaturation, reannealing, hybridization

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).

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