of Nucleic Acids
Having identified the
genetic material as the nucleic acid DNA (or RNA), we proceed to examine the
chemical structure of these molecules. Generally nucleic acids are a long chain
or polymer of repeating subunits called nucleotides. Each nucleotide subunit is
composed of three parts: a nitrogenous base, a five carbon sugar (pentose) and
a phosphate group.
There are two types of
nucleic acids depending on the type of pentose sugar. Those containing
deoxyribose sugar are called Deoxyribo Nucleic Acid (DNA) and those with
ribose sugar are known as Ribonucleic Acid (RNA). DNA is found in
the nucleus of eukaryotes and nucleoid of prokaryotes. The only
difference between these two sugars is that there is one oxygen atom less in
The bases are nitrogen
containing molecules having the chemical properties of a base (a substance that
accepts H+ ion or proton in solution). DNA and RNA both have four bases (two
purines and two pyrimidines) in their nucleotide chain. Two of the bases,
Adenine (A) and Guanine (G) have double carbon–nitrogen ring structures and are
called purines. The bases, Thymine (T), Cytosine (C) and Uracil (U) have single
ring structure and these are called pyrimidines. Thymine is unique for DNA,
while Uracil is unique for RNA.
It is derived from
phosphoric acid (H3PO4), has three active OH- groups of
which two are involved in strand formation. The phosphate functional group (PO4)
gives DNA and RNA the property of an acid (a substance that releases an H+ ion
or proton in solution) at physiological pH, hence the name nucleic acid.
The bonds that are formed from phosphates are esters. The oxygen atom of
the phosphate group is negatively charged after the formation of the
phosphodiester bonds. This negatively charged phosphate ensures the retention
of nucleic acid within the cell or nuclear membrane.
The nitrogenous base is chemically linked to one molecule of sugar (at the 1-carbon of the sugar) forming a nucleoside. When a phosphate group is attached to the 5' carbon of the same sugar, the nucleoside becomes a nucleotide. The nucleotides are joined (polymerized) by condensation reaction to form a polynucleotide chain. The hydroxyl group on the 3' carbon of a sugar of one nucleotide forms an ester with the phosphate of another nucleotide. The chemical bonds that link the sugar components of adjacent nucleotides are called phosphodiester bond (5'→3'), indicating the polarity of the strand.
The ends of the DNA or
RNA are distinct. The two ends are designated by the symbols 5' and 3'. The
symbol 5' refers to carbon in the sugar to which a phosphate (PO 4)
functional group is attached. The symbol 3' refers to carbon in the sugar to
which hydroxyl (OH) functional group is attached. In RNA, every nucleotide
residue has an additional –OH group at 2' position in the ribose.
Understanding the 5'→3' direction of a nucleic acid is critical for understanding the aspects of replication and transcription.
Based on the X - ray
diffraction analysis of Maurice Wilkins and Rosalind Franklin,
the double helix model for DNA was proposed by James Watson and
Francis Crick in 1953. The highlight was the base pairing between
the two strands of the polynucleotide chain. This proposition was based on the
observations of Erwin Chargaff that Adenine pairs with Thymine (A = T) with two
hydrogen bonds and Guanine pairs with Cytosine (G ≡ C) with three hydrogen
bonds. The ratios between Adenine with Thymine and Guanine with Cytosine are
constant and equal. The base pairing confers a unique property to the
polynucleotide chain. They are said to be complementary to each other, that is,
if the sequence of bases in one strand (template) is known, then the sequence
in the other strand can be predicted. The salient features of DNA structure has
already been dealt in class XI.