Bases:
Nitrogenous Rings and Their Properties
A base in nucleic acids refers to a ring
structure containing nitrogen atoms that possess unshared pairs of electrons
capable of acquiring a proton. Bases can be classified into two types: purines
and pyrimidines. Purines, such as Adenine and Guanine, consist of a double-ring
structure, while pyrimidines, namely cytosine, thymine, and uracil, possess a
single-ring structure. When a base is connected to a sugar molecule, it is
referred to as a nucleoside. Further attachment of a phosphate group to a
nucleoside results in the formation of a nucleotide.
Nucleotides:
Sugar, Phosphate, and Nitrogen Base
A nitrogen base is linked to carbon number 1 of
a pentose sugar, while a phosphate group is attached to carbon number 5' of the
sugar. Additionally, a free hydroxyl (OH) group is connected to the 3' carbon
atom. Nucleotides are named based on the specific base attached to them, such as
Adenine nucleotide (Adenine deoxyribose phosphate). The bases are represented
by their initial letters: A, G, T, C, and U.
 |
| DNA nucleotide |
 |
| Polynucleotide |
Polynucleotide
Formation: Phosphodiester Bonds
In a DNA polymer, each nucleoside is connected
to its neighboring nucleosides by phosphate groups. These phosphate groups link
the 3' carbon of one sugar with the 5' carbon of the adjacent sugar, forming a
covalent bond. This connection occurs through a dehydration synthesis reaction,
which eliminates a water molecule. The resulting linkage is known as a
phosphodiester bond since the phosphate group forms two ester (P-O-C) bonds
with the sugars. Multiple nucleotides can join together, forming a lengthy
polynucleotide chain.
RNA vs.
DNA: Ribose vs. Deoxyribose
The structure of an RNA polynucleotide strand
is similar to DNA, except that ribose replaces deoxyribose as the sugar
component. Linear strands of DNA or RNA, regardless of their length, typically
possess a free 5' phosphate group at one end and a free 3' hydroxyl group at
the other. The direction in which a polynucleotide strand runs within a
molecule can be described by referencing the 3' and 5' carbons of the sugars.
Understanding the molecular components and
structure of nucleic acids is essential for unraveling the intricate mechanisms
underlying genetic information storage and transmission. The arrangement of
bases, nucleosides, and nucleotides within DNA and RNA contributes to the vast
diversity and complexity of life's genetic code, paving the way for further
scientific exploration and discoveries.
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