The DNA is the key basic component of gene, which carries the
genetic information that is transcribed onto ribonucleic acid and then
translated as the particular polypeptide. The basic structure of DNA molecule
was first described by Watson and Crick for which they were honored with the
Nobel Prize in Medicine. The DNA molecule is composed of two strands of
complementary nucleotides wound together in the form of a double helix. The
double helix has a diameter of 2 nm. Each full turn of the double helix
contains 10 nucleotide pairs and is 3.4 nm in length.
Each DNA strand has a backbone of deoxyribose (sugar) and phosphate
group residues arranged alternately. It has a sugar– phosphate backbone
substituted with purine and pyrimidine bases. It contains four nitrogenous
bases, two purines (adenine and guanine), and two pyrimidines (thymine and
The two complementary strands are held together by hydro-gen bonds
between the nitrogenous bases on the opposite strands. The hydrogen bonding
follows a specific binding manner in which hydrogen bonds are formed only
between guanine and cytosine and between adenine and thymine. Guanine and
cytosine form a complementary base pair and adenine and thymine form another
A molecule of DNA therefore contains as many
units of adenine as thymine and of guanine as cytosine. For example, when the
arrangement of bases along one strand is AGCTAG, the arrangement on the other
strand will be TCGATC. The ratio of adenine and thymine to guanine and cytosine
is constant for each species, but varies widely from one bacterial species to
During replication, the DNA molecule replicates, first by unwinding
at one end to form a fork and then by separation of strands at the other end.
Each strand then acts as a template for the synthesis of a complementary strand
with which it then forms a double helix.
It is a segment of DNA that carries codons specifying for a
particular polypeptide. A DNA molecule consists of a large num-ber of genes,
each of which contains hundreds of thousands of nucleotides. The DNA of a
bacterial chromosome is usually arranged in a circular form and when
straightened, it measures around 1000 μ. The length of DNA is usually expressed as
kilo-bases (1 kbp = 1000 base pairs, or bp).
Bacterial DNA measures usually 4000 kbp and the human genome about 3 million
Basically, the structure of RNA is similar to that of DNA except
for two major differences:
In DNA, the sugar is D-2-deoxyribose; in RNA, the sugar is D-ribose.
The RNA contains the nitrogenous base uracil instead of thymine
that is present in DNA.
On the basis of structure and function, the RNA can be
differentiated into three types:
Messenger RNA (mRNA),
Ribosomal RNA (rRNA), and
Transfer RNA (tRNA).
The RNA molecules range in size from the small
tRNAs (which contain fewer than 100 bases) to mRNAs (which may carry genetic
messages extending to several thousand bases). Bacterial ribo-somes contain
three kinds of rRNA with respective sizes of 120, 1540, and 2900 bases and a
number of proteins. Corresponding rRNA molecules in eukaryotic ribosomes are
A few RNA molecules have been shown to function as enzymes
(ribozymes). For example, the 23S RNA in the 50S ribosomal subunit catalyzes
the formation of the peptide bond during protein synthesis. Some small RNA
molecules (sRNA) function as regulators either (a) by binding near the 5' end of an mRNA, preventing ribosomes from
translating that message, or (b) by
base pairing directly with a strand of DNA near the promoter, preventing