Download Regulatory genes

Bacterial Genetics
Prokaryotic Cell
• Circular (and
naked) double
stranded DNA
• Bacteria have
very short
generation
spans (ex. E.coli
divides every 20
minutes)
Plasmid – small circular DNA found in some
bacteria
– Contain a few accessory genes (e.g. antibiotic
resistance)
– NOT required for bacterial cell reproduction or
survival
Side Note
Bacterial Cell Reproduction
• Binary fission – asexual (production of offspring
from one parent) process for prokaryotic cell
division
• Each fission results in two daughter cells each
with 1 copy of the original chromosome
Why bother?
• Bacteria rapidly reproduce which leads to
increase in genetic diversity (due to
mutations that may or may not confer
advantages to offspring)
– 9 million mutations per day per human host
– Beneficial mutations allow bacteria to continue
to survive in host and contribute to evolution of
bacterial populations
Mechanisms for gene transfer:
• Mutations cannot account for all bacterial
diversity
• Bacteria have three mechanisms for
exchanging genetic information between
themselves:
– 1. Transformation
– 2. Transduction
– 3. Conjugation
• These exchanges create new strains of
bacteria
1. Transformation
• Bacterium takes
up DNA from
environment
2. Transduction
• Viruses (bacteriophage) transfer genes
between bacteria
3. Conjugation
• Genes are
directly
transferred from
one bacterium to
another via
“mating bridge”
Transposable Elements
• In addition to the transfer of genetic
material (DNA) b/n bacterial cells, DNA of
a single bacterium can undergo
recombination via transposable elements
• Essentially, DNA “jumps” from one part of
chromosome to another
Regulation of gene expression
• Individual bacterium have to deal with
fluctuations in host environment so they
have developed levels to control gene
expression
• 2 types of genes in bacteria:
– Regulatory genes – produce proteins that
control gene expression
– Structural genes – produce proteins that do
something (e.g. break down lactose)
2 Levels for Metabolic Control
1. Use allosteric control (e.g. feedback
inhibition)
– Ex: Negative feedback to stop production of
tryptophan after thanksgiving dinner
2. Regulate expression of genes using an
operon system
Operons
• Prokaryotic genes are grouped into functional
units called operons
3 parts to an operon
1. Operator – controls access of RNA
polymerase to the promoter
2. Promoter – where RNA polymerase
attaches to begin transcription of genes
3. Genes – code for expression of proteins
related to one particular function (e.g.
breaking down galactosidase)
Regulatory gene
• Located away from operon itself
• Produce repressor protein that is able to
bind to operator and prevent RNA
polymerase from attaching to promoter
2 types of operons
1. Repressible – normally on but can be
inhibited (i.e. IS REPRESSIBLE)
– Usually anabolic (building essential molecules)
– Repressible protein (coded for by the regulatory
gene) is inactive
– If the essential molecule is present, it binds to the
repressible protein, activates it, and this
activated protein binds to the operator and turns
operon OFF (blocks RNA polymerase thus
REPRESSED!)
2. Inducible – normally off but can be turned on
(i.e. IS INDUCIBLE)
– Usually catabolic (breaking down food for
energy)
– Repressible protein is active
– Inducer (small molecule) binds to & inactivates
the repressible protein, preventing it from binding
to the operator. Now RNA polymerase
transcribes the genes (INDUCED!)
Lac Operon (No lactose present)
You drink milk (contains lactose) and…
Videos
• Lac Operon (inducible)
• Trp Operon (repressible)