Control, Genomes and Environment

Cellular Control – The lac operon

Starter

• Cystic fibrosis (CF) in humans is caused by mutations of a gene coding for transmembrane protein (CFTR) which acts as an ion pump. A large number of different mutations of the gene have been found. Explain what is meant by a gene mutation. (2 marks)

• a change in the genetic material; unpredictable / AW;extra detail; e.g. addition / substitution / deletion / frame shift / small part of chromosome / may code for different protein /may code for no protein

Learning Outcomes

• Explain genetic control of protein production in a prokaryote using the lac operon

Control of protein synthesis and activation

• Protein synthesis and activation is tightly controlled - ensuring body development, cellular processes and reactions all take place at appropriate times and rates.

• In eukaryotes:• Protein synthesis can be controlled at the

transcriptional and translational levels.

• Genes that aren’t transcribed are ‘switched off’• Genes that are transcribed, but not translated are

‘silenced’

Genetic control in prokaryotes• Differential gene expression due to environment,

eg. nutrient availability.

• Involves ‘operons’ - a section of DNA containing several genes:– Eg Structural genes (protein products), control

elements (P,O)– regulatory gene is not strictly part of the operon, but

its product has an important role.

Regulatory gene Control sites Structural genes

A

Complete the table

Part of operonLetter I P O Z Y

Function name

Regulatory Gene

Structural gene

Product/detail of function

Repressor protein

b-galactosidase

Lactose• Bacteria produce enzymes to metabolise certain

nutrients only when they are present in their environment.

• E.coli normally respires glucose, but can also use lactose as its source of carbon.

• The essential enzyme to allow the metabolism of lactose is b-galactosidase.

• If lactose is not present, there will be very few b-galactosidase molecules in the cell.

• The presence of lactose in the environment causes a large increase in the production of b-galactosidase

Lactose not present

I- Regulator gene – produces a repressor protein which binds to operator (O)P- Promoter region – binding site for RNA polymerase. Repressor at O prevents it binding.Z is b-galactosidase, Y is lactase permease. A is transacetylaseZ, Y, A are not transcribed when R is at O

A

I- Regulator gene – produces a repressor protein which is prevented from binding to O by lactose binding and changing the shape of the repressor.P- Promoter region – binding site for RNA polymerase.Z, Y, A are transcribed when RNA polymerase binds at P

Lactose present

A

Plenary

Component Function

A. structural genes 1. produces repressor protein

B. regulator gene 2. binds to repressor

C. promoter 3. codes for lac enzymes

D. operator 4. binds to RNA polymerase

A – 3B – 1C – 4D - 2

Activators

• Proteins which activate genes.

• Activators and repressors are transcription factors.

• Eg: cAMP can bind to protein kinase A and cause the protein to change shape, thus activating the protein.

Plenary

• The bacterium Escherichia coli (E. coli) uses glucose as a respiratory substrate. In the absence of glucose, E. coli can use lactose. The use of a different substrate is determined by the interaction between genes and the environment. Describe how this happens in this example.

• (5 marks)

Answer

• 1. ref to operon;2 normally repressor substance bound to operator;3 prevents RNA polymerase binding (at promoter) / prevents transcription;4 lactose binds to repressor;5 changes shape of protein molecule;6 unable to bind (to operator);7 RNA polymerase binds (at promoter) / transcription occurs / genes switched on;8 AVP; e.g. production of lactose permease / production of beta-galactosidase;

Homework

• Gene control in prokaryotes sheet

• Q p103 green book• Q4 p149 purple book

Learning Outcomes

• Explain genetic control of protein production in a prokaryote using the lac operon