Biomedical Engineering Reference
In-Depth Information
The simplified version of the Central Dogma, shown in
Figure 1-5
, in which DNA is duplicated through
replication, transcribed to RNA, which is in turn translated to protein, only hints at the complexity of
the information transfer process that is the driving force for bioinformatics. Consider that the archive
of an individual's genetic information or genome is encoded in DNA as a sequence of four different
nitrogenous bases on a sugar-phosphate backbone. This deoxyribonucleic acid can adopt a variety of
conformations, including the infamous right-handed double helix first described by Watson and Crick
in 1953. The sequence of four nitrogenous bases—some combination of Adenine (A), Thymine (T),
Cytosine (C), and Guanine (G)—in each strand of the double helix mirror each other in a predefined
manner; Adenine on one strand always binds with Thymine on the other, and Cytosine always binds
with Guanine.
In human cells, DNA is organized and compressed into 23 pairs of chromosomes, with one member
of each pair inherited from each parent. Most of this DNA—on the order of 98.5 percent—is
considered "junk," in that its function is unknown. The remainder of the DNA is in the genes—the
stretches of DNA involved in the transcription process.
Not only are there duplications in the remaining DNA, but there are additional non-coding nucleotide
sequences. Interrupting the sequences of base pairs that will be expressed (the exons), there are
interruptions in the sequence by segments that aren't expressed (the introns). Like the much larger
expanses of "junk DNA" in the chromosome, these smaller interruptions in the DNA have unknown
functions. Whether some of the non-coding DNA are remnants of provirus infections during hominoid
evolution or somehow involved in compacting the DNA is open to conjecture.
In the process of RNA synthesis within the cell nucleus, DNA is transcribed to single-stranded nuclear
RNA (nRNA), which is then processed to form mature messenger RNA (mRNA), as illustrated in
Figure 1-6
. Small nuclear RNA (snRNA) is involved in this maturation process, which includes excising
the introns from the mRNA strands and concatenating the remaining exons according to their original
order in the mRNA. As an information transport medium, RNA differs from DNA in that it's single-
stranded, much shorter, and the nitrogenous base Uracil (U) is substituted for Thymine.
Figure 1-6. Messenger RNA (mRNA) Synthesis. DNA is transcribed to
nuclear RNA (nRNA) this is in turn processed to mature mRNA in the
nucleus. Maturation involves discarding the junk nucleotide sequences
(introns) that interrupt the sequences that will eventually be involved in
translation (exons).
