The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 7

and the part of the gene that is translated is called an exon. In fact, it is claimed
that only 1.5% of the total human genome consists of exon. Also the presence of
transposable elements is very common—short, repeated sequences of DNA that keep
moving in the genome; these are also called molecular parasites. About % of the
genome consists of simple sequence repeats that are a few base pair long sequences
known as “satellite” bands. There are specific areas in chromosomes that, by vir-
tue of their unique sequences or base composition, help the chromosomes in par-
ticular functions; for example, the centromere is the site where the chromosome
attaches to spindle fibers with the aid of proteins. These sequences are usually rich in
AT pairs. The sequences, called telomeres, help to stabilize the chromosome. The
sequence-specific changes in chromosomes become very crucial in certain diseases
and can be used for other applications, hence should be characterized. For example,
the simple sequence repeats of the chromosome are individual specific because of
unequal crossing over during meiosis. The difference can be traced by Southern blot-
ting, polymerase chain reaction and forms the basis for DNA fingerprinting [-11] .
RNA
The second major type of nucleic acid present in cells is RNA, of which there are
three types: ribosomal RNA (rRNA), messenger RNA (mRNA), and transfer RNA
(tRNA). All three have specific intermediary functionality to ensure that the genetic
message encoded in the DNA is expressed. Because DNA is present mostly in the
nuclear region in the cell, whereas the site for protein synthesis is in cytoplasm, a
mechanism for transfer of the genetic message is essential. RNA is the molecule that
fills this lacuna. This is evident because RNA is present both in the nuclear region
and in the cytoplasm, and increased protein synthesis is seen with increased levels
of cellular RNA. Single-stranded RNA results from the transcription of DNA, which
also assumes a right-handed helical conformation. Weak base stacking interactions
stabilize its structure, which is unique like proteins and unlike DNA. Among the
three types of RNA, rRNA is associated with ribosomes, the machinery for protein
synthesis. mRNA are messenger molecules that transfer the genetic information from
DNA to the ribosomes to allow protein synthesis. mRNA serves as the template that
codes for the amino acid chain (polypeptide). If mRNA codes for one polypeptide, it
is referred to as monocistronic, and it is called polycistronic when several polypep-
tides are encoded in its message. Most of the mRNA is monocistronic, and its length
varies with the polypeptides it codes for. Apart from the coding region, there are
additional bases present that may serve to control protein synthesis. tRNA are mol-
ecules that transfer the information of mRNA into the amino acid chain by serving
as adaptors. tRNA carries a specific amino acid on its end and attaches with the
mRNA to transfer the amino acid to the growing amino acid chain [12,1] .
Properties of Nucleic Acid
Isolated DNA solutions exhibit a marked decrease in viscosity on heating. The ther-
mal change causes it to denature, or melt down, because of the breaking of hydrogen