Transcript 4. Lac Operon

Name the four different types of gene mutation.
Identify the ones that can potentially be more harmful and give a reason for
why.
Point mutations: these are only minor mutations that will affect the organism
only slightly or not at all
Substitution
Inversion
Frameshift mutations: these lead to major changes as large portions of DNA
are misread
Insertion
Deletion
Homework
 LP 18 due in today
 No homework = detention 
 New homework – Due Thursday 28th November
25 November 2015
Today’s Title: The lac operon
Aims:
 (h) explain genetic control of
protein production in a prokaryote
using the lac operon;
Key phrases/words
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RWCM: write notes to describe how
protein synthesis can be controlled
Regulator gene
Lactose permease
Structural gene
Promoter region
Operator gene
Repressor protein
Operon
Control sites
ß-galactosidase
How is protein synthesis
controlled?
What you need to know
 Explain the genetic control of protein production –
using the lac operon as an example
 Key terms you need to know.......
 Operon – length of DNA made from structural genes
and control sites
 Structural genes – code for protein
 Control sites – operator and promoter region of the
DNA
The lac operon
 We know that.......
• mRNA – codes for a particular protein
• The code on the mRNA is complementary to the base
sequence on the DNA template strand
• Therefore that code on the mRNA is a copy of the base
sequence on the DNA coding strand
 Proteins are specified by mRNA
Background information
 Bacteria can synthesise different enzymes (proteins)
depending on what food substrate they are growing on
 E.Coli can synthesise over 3000 different enzymes
 Enzymes involved in basic cell functions are
synthesised at a constant rate
 Inducible enzymes are synthesised as and when they
are needed
What is enzyme induction?
 Discuss what you think this means and why it is
important – you have 2 minutes
 Share your thoughts with the class.
Background information
 E.Coli can adapt to its environment by producing enzymes
to metabolise certain nutrients..........
 but only when those particular nutrients are present
 E.Coli normally respires using glucose but can also respire
using lactose (sugar found in milk/disaccharide)
 Needs to synthesise ß-galactosidase & lactose permease
Background information
 ß-galactosidase – hydrolyses lactose into glucose &
galactose
 lactose permease – transport protein that becomes
embedded in the e.coli membrane – helps transport more
lactose into the cell
 When placed in a lactose substrate, e.coli increases the
synthesis of these two proteins by 1000x
 Lactose triggers the enzymes production – inducer
molecule
Task 1
 Make flash cards of the following words with definitions on the back.
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Regulator gene
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Lactose permease
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Structural gene
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Promoter region
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Operator gene
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Repressor protein
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Operon
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Control sites
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ß-galactosidase
Learning objectives
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explain genetic control of protein
production in a prokaryote using the
lac operon;
Mini Plenary
 Match the components with the functions
Component
Function
A. Structural Gene
1. Produces repressor protein
B. Regulatory gene
2. Binds to repressor
C. Promoter
3. Codes for lac enzymes
D. Operator
4. Binds to RNA polymerase
The lac operon
 Lac system genes (in the bacterial DNA) form the operon –
consists of structural genes and control genes.
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Z - ß-galactosidase
Y - lactose permease
O – operator region – switches Z & Y on and off
P – Binding site for RNA polymerase for transcription of Z & Y
If lactose is absent
• Regulator gene is expressed and produces repressor protein
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Repressor protein binds to the operator region (P & O)
Partially covers the promoter region (P)
RNA polymerase can’t bind – Z & Y genes can’t be translated
Z & Y are switched off
If lactose is present
• Inducer molecule (lactose) binds to the repressor protein
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Repressor protein dissociates from the operator region
Promoter is now unblocked
RNA polymerase can now bind promoter region
Z &Y can now be transcribed – mRNA produced
ß-galactosidase /lactose permease can now be synthesised
The lac operon
• As a result of the 2 enzymes being made, e.coli can now.....
• Take up lactose from its environment because lactose
permease acts a transport protein.
• Using ß-galactosidase, lactose (disaccharide) can
hydrolysed into glucose & galactose
• E.coli can use these sugars for respiration
• E.coli is gaining energy from the lactose
Mini Plenary
 What are the functions of:
• Repressor protein
• RNA polymerase
• Regulatory gene
 http://www.youtube.com/watch?gl=GB&v=oBwtxdI1zvk
Task 2
• Get into groups of 3 or more
• Use the large A2 paper as a story board
• Each person is to cut out the parts of the lac operating
system on the handout provided
• Arrange the cut out parts to show the story of how the
lac operon works in both the absence and presence of
lactose