Download Control of gene expression in prokaryotes and eukaryotes

Control of Gene expression
Gene expression is transcription of DNA to make RNA and then using the RNA to make proteins. This
process can’t be left on indefinitely. The turning on and off of genes is critical to the development of an
organism and the organism functioning properly throughout its life.
Eukaryotic control
Pretranscriptional control:
Molecules called transcription factors bind to the promoter DNA before a gene. This allows the
RNA polymerase to bind to the promoter where it starts the process of transcription. Without the
transcription factors being present, no transcription or less transcription will occur. Transcription factors are
often made as a result of some change in the body or cell to start transcription of this gene.
Posttranscriptional control:
After the RNA is made via transcription it is called the primary transcript or premRNA. In
Eukaryotic cells this is in the nucleus and is not functional. The primary transcript is altered in three ways
before it is called functional mRNA.
1. Sections of the primary transcript are cut out (introns: intervening sequences) and the remaining
pieces (exons; expressing sequences) are the recombined. This combination of exons can be
arranged in different ways to produce different proteins. IF exon A, exon B, exon C is the order
one polypeptide is formed, but if the order is C, A, B another polypeptide might be formed.
This allows one gene to actually produce different polypeptides.
2. At this point a poly A tail, or string of A nucleotides are added to the 3’ sugar end of the RNA.
This allows the mRNA to thread the needle through the nuclear pore.
3. Finally, a GTP cap is added to the 5’ phosphate end of the RNA. A GTP is just like ATP but it
has the guanine (G) nitrogen base instead of the adenine (A) nitrogen base with a sugar and
three ATPs. This allows the functional mRNA to attach to the ribosome.
Prokaryotic control mechanisms
Bacteria sometimes have multiple genes under the control of one promoter. This series of genes is called
an operon. There are different mechanisms for controlling operons, but one is called the inducible operon.
Three different polypeptides are needed to make lactase in bacteria. The lac operon has all three of the
genes needed to make this lactase molecule. The control of this lac operon involves the following
molecules:
1. inducer: lactose sugar is the inducer. It binds to the repressor and removes it.
2. repressor: this protein binds to the operator portion of DNA next to the promoter and this stops
RNA polymerase from being able to bind to the promoter.
3. operator; the section of DNA near the promoter that is before the gene and is where the
repressor can bind.
4. promoter: the section of DNA where the RNA polymerase binds to start transcription.
If there is lactose sugar, it binds to the repressor and removes it. Once it is removed, the RNA polymerase
can bind to the promoter and make the needed polypeptides to assemble lactase. When no lactose sugar is
present the repressor stops the transcription process by stopping the RNA polymerase from binding to the
promoter.