The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 5

desmin filaments in muscle cells. Both ends of these coiled structures are capable of
extending. All three types of filaments act in consortium and mediate the intracellular
and cellular movement [1-6] .
1.2 Genes, Chromosomes, DNA, and RNA
Nearly all cells of an organism contain similar genetic material called its genome.
Scientists have defined the gene as that part of the chromosome that encodes the
necessary information for the synthesis of a functional biological product, which
could be either a protein or RNA. The units carrying genetic information are made
up of DNA. Besides DNA, the cells contain another type of nucleic acid, RNA,
which is produced by transcription of DNA, as discussed later in this chapter. The
nucleic acid molecule is made up of nucleotides, which in turn have three basic
units: a nitrogenous base, a pentose sugar, and a phosphate group. A phosphate-less
molecule is termed a nucleoside. The nitrogenous bases found in the nucleic acids
are of five different types—adenine (A), guanine (G), cytosine (C), thymine (T), and
uracil (U)—derived from purine and pyrimidine parent structures. DNA is known
to be made up of A, T, G, and C, while RNA has A, U, G, and C. The pentose sugar
is also different in the two nucleic acids. DNA contains deoxyribose, and RNA has
ribose present in its structure.
Apart from the regular purines and pyrimidines, other bases, usually in methylated
forms, may also be present. The nucleotides of DNA and RNA are linked to each
other by phosphodiester linkage, wherein the 5 phosphate group of one is attached
to the  hydroxyl end of the other. So the backbone structure has the sugar and phos-
phate group linked with the nitrogen base protruding from the backbone at regular
intervals ( Fig. 1.2 ). These phosphodiester linkages have the same orientation, giving
a specific  and 5 end to the nucleic acid and thus a specific polarity.
A short linear polymer of nucleotide is termed an oligonucleotide, whereas lon-
ger chains containing 50 or more nucleotides are called polynucleotides. The DNA
molecule is made up of two polynucleotide chains, or DNA strands, held together
by the hydrogen bonding that exists between their nitrogen bases. The nitrogen-base
pairing is very specific; A always pairs with T with the help of two hydrogen bonds,
and G pairs with C with the help of three hydrogen bonds. In essence, a purine
always pairs only with a pyrimidine. This specific or complementary base pairing
allows the two strands to be held together in a thermodynamically favorable antipar-
allel double helix. The sugar phosphate backbones twist around one another to give
shape to this double helical structure. The hydrophilic backbone of sugar and phos-
phate groups is on the outer side, whereas the hydrophobic nitrogen bases are on
the inner side; this creates a major and minor groove on the surface of the duplex.
Crystal analysis of DNA revealed that the bases stacked in the helix are . Å apart,
and it takes a complete turn for every 10 bases, in other words, at Å.
Erwin Chargaff and coworkers showed that the DNA in an organism does not
change because of external or internal factors like age, nutrition, and so forth. The