Introduction to Molecular and Cell Biology, Biol. 220
Lecture 17: Genes and Regulation
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Introduction to Molecular and Cell Biology, Biol. 220 Lecture 17: Genes and Regulation |
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A gene is the entire nucleic acid sequence that is necessary for synthesis of a functional protein or RNA. This includes the includes both the coding region and the regulatory domains.
An operon is a transcriptional unit. In prokaryotes, these are often polycistronic while in eukaryotes they are monocistronic.
A cistron is the genetic unit that encodes a single polypeptide.
Gene--------------------------------------------------------------------->
regulatory domain, Operon------------------------------------------->
Untranslated, cistron, (cistron ...) -------UT->
Prokaryotic Genes (Fig. 9.1a)
Often polycistronic.
Control regions often short and adjacent to 5' end of operon.
Eukaryotic Genes (Fig. 9.1b)
Monocistronic.
Can have multiple control regions operating adjacent or distant, both upstream and downstream of operon.
Splicing and polyadenylation can generate alternative forms of mRNA. (Fig. 9.2)
Fig. 9.2 Alternative splicing of mRNA.
Gene Expression
Expression of genes is according to the following scheme:
DNA
|
| (transcription, splicing, modification)
V
RNA
|
| (translation)
V
Protein
|
| (folding, modification, localization)
V
Functional Protein
How many genes are there in a human?
Would you expect all genes to be expressed continuously?
How much energy is required to make a protein?
Since it is energitically expensive to make RNA and proteins, expression of such products only when needed helps conserves a cell's resources.
The lactose operon is a good example of gene regulation:
Lactose is a disaccharide that is made up of one glucose and one galactose molecule.
lactose = glucose-galactose
The lactose operon codes for three proteins that are responsible for the transport of lactose into the cell and the hydrolysis of lactose into glucose and galactose.
| Lactose | Glucose | Lactose operon transcription |
| none | none | none |
| none | abundant | none |
| abundant | abundant | very little transcription |
| abundant | none | very active transcription |
Fig. 10.2 Negative control
of the lactose operon.
The location on the DNA where RNA polymerase binds is called the promoter site.
The locations on the DNA where proteins bind and regulate transcription are called an operator sites.
Proteins binding at operator sites that facilitate transcription are called positive regulators.
Proteins binding at operator sites that prevent transcription are called negative regulators.
Molecules that bind to protein regulators and facilitate transcription are called Inducers.
Molecules that bind to protein regulators and prevent transcription are called Repressors.
Transcription composed of 4 steps:
Most of the regulation of gene expression takes place during binding of the RNA polymerase at the promoter.
| Created 2004 by CA Rinehart• email CA Rinehart | Index • CourseInfo LogIn • Syllabus • References • Other Resources |
