GENERAL BIOLOGY

SCHOOL OF MLTFACULTY OF HEALTH SCIENCE

PREPARED BY:MANEGA

HDL 121GENE REGULATION

GENE REGULATION

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Learning Outcomes

After completing this lecture, students will be able to:

(a) Define gene regulation & operon

(b) Describe lac operon – operator, promoter & terminator

(c) Explain tryptophan operon

Topics

© 2010 Cosmopoint

GENE REGULATION

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Topic Outlines

1.1. Operon

1.2. Lac Operon

1.3 Tryptophan Operon

© 2010 Cosmopoint

GENE REGULATION

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Definition

Gene regulation: process that cells use to turn the information on genes into gene products

Operon: a functioning unit of key nucleotide sequences of DNA including an operator, a common promoter, & one or more structural genes, which is controlled as a unit to produce messenger RNA (mRNA), in the process of transcription by an RNA polymerase

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1.1. Operon

GENE REGULATION

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Introduction

How are genes regulated? They are turned off if there is no need for the enzymes they code for or turned on when the environment changes and the enzymes are once again needed

Example E. coli in an environment without lactose does not produce the enzymes for lactose digestion. When lactose is present the enzymes for digestion are produced.

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1.1. Operon

GENE REGULATION

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Why is this off/on switch important? Energy is not wasted. It would be similar to having all the electrical appliances in your house on at once. Which of course would be very wasteful. Also unnecessary materials would lead to sluggish functioning.

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1.1. Operon

GENE REGULATION

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How are the genes for a particular metabolic pathway turned on or off? On the prokaryote chromosome a combination of genes and regulatory DNA sequences known as the operon accomplishes this.

An example of an operon discovered by Jacob and Monod in E. coli The lac operon which is off if no lactose is present but can be induced to turn on in the presence of lactose.

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1.1. Operon

GENE REGULATION

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Lactose Operon

Francois Jacob & Jacques Monod demonstrated how some genes in the E. coli are regulated at the biochemical level.

The E. coli contains a set of genes that encodes for three proteins that the bacteria use to obtain energy from the sugar lactose.

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1.2 Lac Operon

GENE REGULATION

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Repressor/Regulator Gene - Produces a repressor protein that fits in the operator to turn the operon off

Promoter - RNA polymerase attaches here to begin transcription of the genes

Operator - The active repressor fits in notches to block RNA polymerase and turn off transcription.

Structural Genes - Metabolic pathway genes with code for enzymes to digest lactose

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1.2 Lac Operon

GENE REGULATION

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1.2 Lac Operon

GENE REGULATION

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1.2 Lac Operon

GENE REGULATION

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1.2 Lac Operon

GENE REGULATION

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1.2 Lac Operon

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1.2 Lac Operon

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1.2 Lac Operon

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Three enzymes produced

β-galactosidase: catalyst the hydrolysis process of lactose into glucose & galactose

Lactose permease: absorption of lactose by bacteria

Transacetylase: function not clear

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1.2 Lac Operon

GENE REGULATION

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1.2 Lac Operon

GENE REGULATION

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1.2 Lac Operon

GENE REGULATION

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Trp Operon

Trp operon – gene activity is repressed when a tryptophan is added unlike Lac operon which is induced when lactose is added

5 structural genes (trp A, trp B, trp C, trp D & trp E)The presence of tryptophan in the cell shut down the

operonWhen Trp is present, it binds to a site on the Trp

repressor & enables the Trp repressor to bind to the operator

When Trp is not present, the repressor leaves its operator, & transcription of the 5 structural genes begins

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1.3 Trypthon Operon

GENE REGULATION

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1.3 Trypthon Operon

GENE REGULATION

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1.3 Trypthon Operon

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Enzyme: Tryptophan synthethase

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1.3 Trypthon Operon

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Repression

The repressor for the trp operon is produced upstream by the trp R gene, which is continually expressed

When tryptophan is present, it binds to the tryptophan repressor tetramers, & causes a change in conformation, which allows the repressor to bind the operator, which prevents RNA polymerase from binding or transcribing the operon, so tryptophan is not produced

When tryptophan is not present, the repressor cannot bind the operator, so transcription can occur. Therefore, this is called negative feedback mechanism

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1.3 Trypthon Lac Operon