Transcript Gene structure in prokaryotes

Gene structure in prokaryotes
• *In prokaryotic cells such as
bacteria, genes are usually found
grouped together in operons.
• *The operon is a cluster of related
genes, often coding for enzymes in a
metabolic pathway, which are under the
control of a single promotor regulatory
region.
*One example is the Lac operon,
which coded for the enzymes
responsible for lactose
catabolism. *Within the operon
there are three genes that code for
proteins (structural proteins) and an
upstream control region including
the promotor and a regulatory site
called The operator
*Laying outside the operon is
the repressor gene, which
codes for a protein (Lac
repressor) that binds to the
operator site and is responsible
for the suppression of the
operon by blocking the binding
of RNA polymerase.
*The system of grouped genes in
prokaryotic cells means that the
transcribed mRNA may contain
information for more than one protein.
Such a molecule is known as
a polycistronic mRNA.
The synthesis of these mRNA is
regulated in a cordance with the needs of
the cell at any one time thus enables the
cell to adapt quickly to changing
environmental conditions. Fig 6
Gene structure in eukaryotes
*Gene structure and function in eukaryoties is
more complex than in prokaryotes.
*Eukaryotic genes contain extra pieces of DNA
that did appear in the mRNA.
*These sequences are known as intervening
sequences or introns, with the sequences
that will make up the mRNA being called exons
*Within the nucleus the introns are
spliced out of the primary transcript
and a cap at the -5 terminus and a
tail of adenine residues at the -3
terminus, were added.
*The end product of RNA processing
is a fully functional mRNA that is
ready for export to the cytoplasm for
translation ( Fig 7)