Biomedical Engineering Reference
In-Depth Information
Figure 8.2
Coil configuration of a single polymer chain in solution.
1/2. Importantly, although for different physical reasons, the case of DNA double-
strand molecules falls into this last category. This arises because of the semiflexible
nature of this chain and it can then be shown that only unrealistically long chains
would be in good solvent [2]. The persistence length is the distance along the chain
above which the position of two monomers behaves independently. For DNA, this
length is 50 to 100 nm depending on the characteristics of the solution (pH, salin-
ity, etc.) (Figure 8.3). This picture of DNA molecules is meaningful only in a buffer
solution. In a cell's nucleus, DNA in the form of chromosomes is packed extremely
tightly by histones, a particular class of compaction proteins.
DNA microarrays that have been used for several decades exist in different ver-
sions. Generically, it consists of depositing thousands of spots of single-strand DNA
sequences on a solid substrate such as a glass slide and measuring the hybridization
efficiency with DNA or RNA single strands, by fluorescence or radioactive labeling.
When the target is itself DNA, these arrays can be used to identify genes such as
those implicated in some diseases. The quantification of the level of these genes can
be used for diagnostic [4, 5]. Compared to conventional techniques such as South-
ern blots that combine gel electrophoresis and hybridization for limited number of
DNA fragments, the analysis of the whole genome of an organism can be completed
in a single experiment.
8.1.1.2 RNA
Ribonucleic acid (RNA) molecules are similar to single-strand DNA from the point
of view of their sequence. One of the four bases is different, thymine being replaced
by uracyle. The phosphate backbone also presents some slight differences. The mes-
senger RNAs (mRNAs) are the molecules resulting from the transcription process.
They contain the same genetic information as DNA and come out of the nucleus to
be translated into proteins in the cytoplasm. There are other smaller RNA molecules
(transfer RNA : tRNA) that play a role in this translation process. The bases of
RNA are similar to the ones of the DNA associate themselves, but because of the
single-strand structure of RNA molecules, these interactions lead to partially folded
