Download Lecture #9 Introduction to gene regulation and operons in Prokaryotes.

BIOLOGY 207 - Dr. McDermid
Lecture #9 Introduction to gene regulation and operons in Prokaryotes.
Readings for lectures #4 and #5:
Griffiths et al, 7th Edition: Ch. 11 pp 335-344, Ch 7 pp210-219 (skim only, beginning and end)
Problems for lectures #4 and #5:
Griffiths et al, 7th Edition: Ch. 11 Tier 1: #1,2,3,4,8 Tier 2: #9,14,15
Concepts:
How are prokaryotic genes regulated?
1. In prokaryotes, operons are clusters of genes
2. The lac operon - a classic prokaryote operon
3. Transcriptional regulation involves the binding of proteins to the DNA.
Central Dogma DNA ------> RNA -------> Protein
1) Transcription 2) Translation Gene Regulation
- frequently at the transcription step
Status:
Gene on Gene off Gene Regulation involves the activity of proteins
Transcription Translation Prokaryote Gene Regulation: Operons
Lac operon: Figure 11-1
The I gene product is a repressor protein
which acts by binding to the operator DNA
sequence thereby preventing the RNA
polymerase (which binds to the promoter-P
sequence) from transcribing a polycistronic
messager RNA of the cistrons Z, Y, A. (cistron = gene)
-Z-Y- NOTE – Griffiths mistake on pg 336, mix up definitions of Z and Y
-A-I- lac repressor I has 2 recognition sites
Lecture #4
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Example of negative control
Operon POZYA
Transcription of mRNA- polycistronic
- polycistronic mRNA
Note: not all prokaryotic genes are in operons
Genetics of the lac operon
Jacob and Monod in the 1950's and 1960's
Inducers - specific substrates that trigger the appearance of specific enzymes
beta-galactosidase could be induced with several types of beta-galactosides (Figure 11-3
IPTG).
The enzyme is not normally present in absence of an inducer.
Constitutive mutants of the I locus
Constitutive mutant
Genetically mapped close to, but distinct from, the ZYA genes
These mutants defined the I locus.
+
I cells
I- mutants
Table 11-2 & Figure 11-5
For further study, need
"heterozygous diploid strains"
F-factor - Ch. 7 pp 210-219
Fertility factor - Fig. 7-14
F-factor is an "episome" - capable of being either a free plasmid or a sequence integrated into
bacterial chromosome
If F factor is present then the strain is an F+ strain
F factor can contain some E.coli genes therefore making the cell a
partial diploid.
lac- host with F factor with lac+ = lac-/F lac+
From this one can examine lac mutants in partial diploid strains
Determine if mutations are cis or trans acting.
cis trans -
Lecture #4
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trans-acting locus
A gene able to affect the activity of other genes on a different DNA molecule usually through a
diffusible product (e.g. protein)
I+ is dominant over I- in the trans - position
Figure 11-6 Table 11-3
I+ Z -Y + / F I-Z + Y -
Z inducible; Y is inducible
IS is dominant to both I+ and I- in trans
Figure 11-7 Table 11-3
IS Z + Y + is uninducible.
Repressor protein: I gene product
I- mutation
Normal
Inducer
DNA
Inducer
Is mutation
XXX
XXXXXX
Summary of I mutant classes:
Dominant in trans therefore it is a diffusible product
Lecture #4
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DNA