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| 16S rRNA gene |
What is
the 16S rRNA Gene?
The 16S rRNA gene is a section of
prokaryotic DNA
found in all bacteria and archaea. This gene codes for an rRNA, and this
rRNA
in turn
makes up part of the ribosome. The first 'r'
in
rRNA stands for ribosomal. The ribosome is composed of two
subunits, the large subunit (LSU) and the small subunit (SSU).
These two
subunits sandwich the mRNA as it feeds through the ribosome for
translation. While there are also associated proteins helping
to
make up the functional units of the ribosome, in general, in bacteria,
the SSU is coded for by
the the 16S rRNA gene, and the LSU is coded for
by the 23S rRNA & 5S rRNA genes.
Why
is it Used?
The 16S rRNA gene is a commonly used
tool for
identifying bacteria for several reasons. First, traditional
characterization depended upon phenotypic traits like gram positive or
gram negative, bacillus or coccus, etc. Taxonomists today
consider analysis of an organism's DNA more reliable than
classification based solely on phenotypes. Secondly, researchers
may,
for a number of reasons, want to identify or classify only the bacteria
within a given environmental or medical sample. While there
is a
homologous gene in eukaryotes, the 18S rRNA gene, it is distinct,
thereby rendering the 16S rRNA gene a useful tool for extracting and
identifying
bacteria as separate from plant, animal, fungal, and protist DNA
within the
same sample. Thirdly,
the 16S rRNA gene is relatively short at 1.5 kb, making it faster and
cheaper to sequence than many other unique bacterial genes.
How is it Used?
Ribosomes (and correspondingly
the DNA that codes for them) have been mostly conserved over time,
meaning that their structure has changed very little over time due to
their important function, translating mRNA into proteins. But even
within this gene there are parts that have been conserved more than
others. This is due to the structure of the ribosome itself.
With the
way the ribosome folds, creating bonds with itself in some places
(conserved regions) while
other portions are looped and unbonded (hypervariable regions), the
degree to which any portion
of the gene is subject to mutations varies.
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