Class notes
Class notes Biology
- Course
- Biology
- Institution
- 12th Grade
Gene Expression explains how genes are turned on and off indicating how genes are expressed.
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GENE EXPRESSIONRegulation of Protein Synthesis
- Cells need to be able to respond to their environment by changing the types and amount
of protein it produces through gene expression (Central Dogma).
- Some proteins are only necessary for certain moments in the cell cycle. Helicase is only
needed during DNA replication.
- Anabolic (building) processes require energy and resources. Cells can reduce energy
costs greatly by not synthesizing proteins they require until they actually need them.
There are four levels of control within the cell:
1. Transcriptional
2. Post-transcriptional
3. Translational
4. Post-translational
Transcriptional Control
Transcription begins when RNA polymerase binds to the DNA at the promoter region. To help
RNA polymerase bind to the DNA, regulator proteins called transcription factors need to bind
onto the promoter region first. Transcription factors can be negative or positive. The presence
and concentration of these transcription factors determine IF and HOW QUICKLY a gene is
transcribed into mRNA. This method of control is found in eukaryotes.
Transcriptional Control - Operons
One transcribed region could have more than one gene in it. It’s like having one switch that
controls multiple light bulbs. In this way, one regulatory protein can control many genes.
This is called an OPERON. This method of control is found mostly in prokaryotes but can
happen in some eukaryotic cells as well. The first operon (lactose operon) was discovered in E.
coli.
In this operon…
- The control region is make of a promoter and an operator
- The coding region contains genes for three different enzymes that work together to
break down lactose
Negative regulation - binding of a regulatory protein (“repressor”) to the operator to prevent
transcription
Positive regulation - binding of a regulatory protein (“activator”) to an area close to the promoter
region to stimulate transcription
Post-Transcriptional Control
In eukaryotes, the mRNA molecule formed in the nucleus must be modified before it can
become the mRNA transcript in the cytoplasm. If the 3’ poly-A tail and 5’ cap are not added,
translation can be blocked in two simple ways:
- If it lacks the poly-A tail, then the mRNA strand will not be able to leave the nucleus.
- If it also lacks a cap, nucleases (enzymes) will break it up, back into its individual
nucleotides.
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