Biomedical Engineering Reference
In-Depth Information
Transcription
Translation
D
R
N
P
Reverse
transcription
DNA
replication
RNA
replication
Figure 3.1
Schematic representation of the so-called central dogma of molecular biology. DNA replication is
essential to the transmission of genetic information from one generation to the next in most life forms (i.e. in
living forms whose genomes are DNA based). RNA replication is essential to the transmission of genetic infor-
mation in the context of a small number of viruses whose genomes are RNA based. Transcription describes the
copying of selected DNA sequences into RNA, and translation describes the conversion of the genetic informa-
tion inherent in mRNA into a polypeptide of defi ned amino acid sequence. The process of reverse transcription
is a central feature of certain viruses (retroviruses) containing an RNA-based genome which, as part of their
life cycle, infect eukaryotic cells and convert their RNA-based genomes into a DNA-based one (see Box 14.1)
3.2 Nucleic acids: function and structure
Nucleic acids represent a prominent category of biomolecule present in living cells. The term
incorporates both DNA and RNA. DNA represents the repository of genetic information (the
genome) of most life forms. RNA replaces DNA as the repository of genetic information in some
viruses. In most life forms, however, RNA plays a role in mediating the conversion of genetic in-
formation stored in specifi c DNA sequences (genes) into polypeptides. There are three subcatego-
ries of RNA, each playing a different role in the conversion of gene sequences into the amino acid
sequence of polypeptides. Messenger RNA (mRNA) carries the genetic coding information from
the gene to the ribosome, where the polypeptide is actually synthesized. Ribosomal RNA (rRNA),
along with a number of proteins, forms the ribosome itself, and transfer RNA (tRNA) functions as
an adaptor molecule, transferring a specifi c amino acid to a growing polypeptide chain on the ri-
bosomal site of polypeptide synthesis. Therefore, nucleic acids, between them all, mediate the fl ow
of genetic information via the processes of replication, transcription and translation as outlined in
what has become known as the central dogma of molecular biology (Figure 3.1).
Structurally, nucleic acids are polymers in which the basic recurring monomer is a nucleotide
(i.e. nucleic acids are polynucleotides). Nucleotides themselves consist of three components: a
phosphate group, a pentose (fi ve-carbon sugar) and a nitrogenous-containing cyclic structure
known as a base (Figure 3.2). The nucleotide sugar associated with RNA is ribose, whereas that
found in DNA is deoxyribose (Figure 3.3). In total, fi ve different bases are found in nucleic acids.
They are categorized as either purines (adenine and guanine, or A and G, found in both RNA and
DNA) or pyrimidines (cytosine, thymine and uracil, or C, T and U). Cytosine is found in both
RNA and DNA, whereas thymine is unique to DNA and uracil is unique to RNA (Figure 3.4).
The DNA or RNA polymer consists of a chain of nucleotides of specifi c base sequence, linked
via phosphodiester bonds (Figure 3.5). RNA is a single-stranded polynucleotide, although RNA
molecules tend to adopt higher order three-dimensional shapes. DNA, on the other hand, is a
double-stranded molecule (Figure 3.6) that assumes a double helical structure. The two polynu-
cleotide strands face each other in an antiparallel manner (Figure 3.6), with the hydrophilic sugar
and phosphate residues facing outwards, towards the surrounding aqueous-based environment,
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