Biomedical Engineering Reference
In-Depth Information
recombinase and a short specific DNA sequence are the key necessities apart from
several other auxiliary proteins and enzymes. One recombinase enzyme binds
at each of the two sites for recombination, which may be on the same or different
DNA strands. Then the DNA strands are cleaved and in some cases rejoined to form
Holliday intermediates at both the sites. In some systems, both DNA strands are cut
and rejoined simultaneously without the formation of Holliday intermediates.
Transposition is another type of recombination process. This occurs mostly to
allow for movement of mobile genetic elements or transposons. Mobile genetic ele-
ments have accumulated over ages by mutations and make up a sizeable part of the
genome. These elements usually code for the proteins or enzymes that aid in their
movement around the genome. The movement can be of a cut-and-paste type, where
the whole piece of DNA is cut from one chromosome and inserted as such in the
other. Movement can also be replicative, where a new copy of transposon is inserted
in the target DNA and the original remains with the donor DNA. Similar to DNA-
only transposons, as in the case of prokaryotes, eukaryotes have DNA-only trans-
posons as well as retrotransposons. The latter elements use RNA as an intermediate
molecule. The DNA mobile element is first transcribed and the RNA so formed is
acted upon by reverse transcriptase to yield a DNA replica element that is inserted
into the opposite DNA strand [2-0] .
1.3 Transcription
The information is carried in genes as DNA, but to transduce this information into
functional biomolecules requires the dual process of transcription and translation.
The genetic information is copied in the form of RNA by transcription and trans-
lated to produce proteins. As with DNA replication, the DNA double strand must
separate to allow for growth of the new polynucleotide chain by base pairing. The
genes that carry the information to code for proteins are copied to RNA, and such
RNA copies are termed mRNA molecules. The four-base language of DNA is con-
verted to the four-base language of RNA by transcription. The DNA strand acts as
a template strand for polynucleotide chain formation catalyzed by RNA polymerase
( Fig. 1.6 ). The RNA polymerase identifies and associates with a region in the DNA
called the promoter region. Transcription factors aid RNA polymerase in this pro-
moter site recognition. The site of transcription initiation is called 1 and the direc-
tion in which it is transcribed is downstream and the opposite direction is upstream.
RNA polymerase melts around 1 base pairs of DNA to create a transcription bub-
ble. Transcription progresses by the joining of ribonucleotides by a phosphodiester
bond formation at the  end of the growing chain. About half of the bases are paired
to ribonucleotides in the transcription bubble as the elongation complex comprising
RNA polymerase, template DNA, and the growing RNA moves ahead. In the final
stages, specific sequences in the template DNA direct RNA polymerase to terminate
transcription, release the RNA transcribed, and dissociate from the template DNA
strand, ready to transcribe again.
The transcription in prokaryotes differs from the eukaryotic process. The genes
in prokaryotes have a common metabolic goal, and such a group of genes is called
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