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Lecture 21

BIO 1140 Lecture Notes - Lecture 21: Lac Operon


Department
Biology
Course Code
BIO 1140
Professor
Doug Johnson
Lecture
21

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Cell Bio Lecture 21
Monday March 24, 2014
Regulation:
Why is understanding regulation the key to understanding so much of life?
- Changes in environment or changes in nutrients
- Changes in make up of cell is response to signals
- In a multicellular organism, cellular differentiation
- Conserve energy
Gene regulation refers to the regulation of activity and may occur at any level. While the
main control is at the level of transcription addition controls are at the posttranscriptional,
translation and posttranslational levels. At the level of transcription, “trans-acting”
proteins interact with “cis-acting” DNA sequences
- In terms of the lac operon, it’s how one small molecule regulates the lac operon
Trans-acting – The protein
Cis-acting –
Regulation of Gene Expression in Prokaryotes
Start with a simple, but powerful model. Prokaryotic gene expression reflects life history.
In many cases rapid, reversible responses to the environment are observed.
- Typically RNA polymerase binds to a DNA sequence 5 to the gene called the
promoter. Within the promoter may be the consensus sequence 5’-TATAAT-3
called a TATA box
- Repressor proteins binding to other regulatory DNA sequences may prevent the
gene from being expressed
- Activator proteins binding to other regulatory DNA sequences may turn on
expression of the gene
Repressors and activators may regulate the same gene
- In prokaryotes, many genes are organized into clusters (transcription units) that
are implicated in a single function
- At a smaller scale many genes are organized into operons (one or more operons
may be found within a transcription unit)
- So, an operon is 2 or more genes that are regulated together and are also regulated
by a single promoter and made into ONE RNA. So the RNA that is made contains
many genes/possible protein coding regions/open reading frames
- The operon itself can be considered as a unit of transcription with several genes
controlled by a single promoter. In effect an operon is a cluster of genes and DNA
sequences involved in their regulation. RNA polymerase binds at the promoter
and transcribes all the genes in the operon into one mRNA (called polycistronic
because it contains several cistrons – an older definition of a gene used in
genetics)
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