Lesson 5: Control of Gene Expression Flashcards
Gene Expression
The process of transcription and translation are collectively known as gene
expression.
Gene expression in eukaryotes is controlled by several processes:
Transcriptional regulation – prevent transcription
Post-transcriptional regulation – control of mRNA after it has been
produced
Translational regulation – prevent translation
Transcriptional Regulation 1
Transcription can be regulated by nucleosomes and by binding proteins.
Nucleosomes are the result of DNA packaging. Each nucleosomes
consists of DNA wound around 8 histones.
Nucleosomes can prevent gene expression by blocking the activators and
enzymes involved in transcription from binding to certain regions of the
DNA.
DNA in nucleosomes may be wrapped or unwrapped and does not need
to be completely released in order to be transcribed. This allows for DNA
site exposure.
Transcriptional Regulation 2
RNA polymerase requires the presence of transcription factors before transcription
can begin.
Many different transcription factors have been found and each one is able to
recognize and bind to a specific sequence of DNA. A specific combination of
factors is necessary to activate a gene.
Activators (a type of transcription factor) must bind to a section of DNA
known as the enhancer. The enhancer may be some distance from the
gene.
Other proteins must bind in order to bend the DNA so that the enhancer
region is brought close to the promoter region.
An initiation complex is created when this occurs and transcription can
begin.
Transcription factors are controlled by signals from other molecules such as
hormones.
Transcription factors may also be controlled by other molecules in the
environment of the cell .
Both the organism’s internal and external environment my influence
gene expression. Temperature, light, chemicals (including drugs) and
even the sex of the organism can alter gene expression.
Examples of Environmental Control
Drugs
Thalidomide in the 1950s was prescribed to pregnant women to treat
morning sickness. It had no effects on gene expression in adults bit
prevented gene expression during limb formation in the fetus.
Sex
Male pattern baldness is activated by the presence of both testosterone
and dihydrotestosterone
Temperature
The Himalayan rabbit which has a gene for pigmentation. Inactive above
35 degrees but very active between 15 and 25 degrees.
Chemical modification to the DNA itself can also regulate transcription.
The addition of a methyl group to cytosine does not alter the sequence
but in areas that are heavily methylated transcription does not occur.
Removal of the methyl groups reactivates the gene.
Post – Transcriptional Regulation
Post – transcriptional regulation involves modification of the mRNA.
Sequences of DNA that are transcribed but do not appear in the mature
mRNA are called introns. The introns are removed from the mRNA
sequence leaving only the exons which are joined together to create the
mature mRNA that will be translated.
This allows for a gene to code for more than one protein. It is thought
that the average human gene can code for three proteins.
Translational Regulation
There are many proteins involved in the start of polypeptide synthesis.
These proteins prevent translation from occurring. The mRNA will
eventually be recycled by enzymes.
