BIOLOGY 101

CHAPTER 18: Gene Expression:

Turning genes on and off

18.1 Gene Regulation

BACTERIAL TRANSFORMATION:

• Bacteria have the ability to pick up DNA from their surroundings and transcribe it as if it was their own.

Gene Expression:Turning Genes On and Off

• When this occurs, it is referred to as Bacterial Transformation

• A bacterial transformation is an event in which a bacterial cell picks up foreign DNA, and subsequently, expresses a new trait as a result

18.1 Gene Regulation

BACTERIAL TRANSFORMATION:

• Bacteria have the ability to pick up DNA from their surroundings and transcribe it as if it was their own.

Gene Expression:Turning Genes On and Off

• Plasmids are small circular DNA molecules that many bacteria possess naturally. They often carry accessory genes

• Once a plasmid has entered a cell, it is treated as native DNA, being replicated and expressed along with host DNA

• Transformation can involve the transfer of a plasmid from one bacteria to another

Gene Expression:Turning Genes On and Off

• Plasmids are often genetically engineered to act as vectors to deliver genetic material to bacterial cells

• Bacterial cells are treated in such a way that they will readily take in plasmids and become ‘transformed’

The term ‘transformed’ came from Frederick Griffith in his experiments with mice and bacterial pathogens

18.1 Gene Regulation

BACTERIAL TRANSFORMATION:

• Bacteria have the ability to pick up DNA from their surroundings and transcribe it as if it was their own.

Gene Expression:Turning Genes On and Off

• Plasmids are often genetically engineered to act as vectors to deliver genetic material to bacterial cells

• Genetically engineered plasmids have ‘built-in’ sites to insert foreign genes, and often confer a selective trait, such as antibiotic resistance

18.1 Gene Regulation

BACTERIAL TRANSFORMATION:

• Bacteria have the ability to pick up DNA from their surroundings and transcribe it as if it was their own.

Gene Expression:Turning Genes On and Off

• Such ‘reporter genes’ may generate a color, fluoresce or confer resistance to an antibiotic

• Reporter genes somehow change the phenotype of transformed cells

• Antibiotic resistance is a convenient reporter gene type because it allows you a way to kill off all non-transformed cells

18.1 Gene Regulation

BACTERIAL TRANSFORMATION:

• It is important to include a gene on the plasmid that lets you know if bacteria have been transformed

18.1 Gene Regulation

pGLO Plasmid: Utilizing -lactamase to confer ampicillin resistance

• The pGLO plasmid has been genetically engineered to take advantage of a bacterial gene for ampicillin resistance

Gene Expression:Turning Genes On and Off

• Numerous antibiotics utilize a structure known as-lactam rings.

• A major pathway to antibiotic resistance is the destruction of -lactam rings with -lactamase

• pGLO continually transcribes -lactamase, making cells that have the plasmid resistant to ampicillin

o This acts as a selective agent to identify transformed cells

18.1 Gene Regulation

OPERONS:

• Gene regulation is often carried out at the level of transcription

Gene Expression:Turning Genes On and Off

• This generally involves transcription factors that control the binding of RNA Polymerase to the promoter region of a gene

• In bacteria, small groups of functionally related genes are sometimes under the control of a single promoter. A cluster of genes like this is called an operon.

18.1 Gene Regulation

The Arabinose Operon: Negative and Positive Regulation

• Glucose is a preferred energy source for bacteria, but they can also metabolize arabinose if glucose levels are low.

Gene Expression:Turning Genes On and Off

• The Arabinose Operon controls a group of genes involved in the metabolism of arabinose.

• Structural genes araBAD are under the control of a single promoter

• Another gene product, araC, is a regulatory protein that is involved in the positive and negative regulation of the operon

18.1 Gene Regulation

The Arabinose Operon: Negative and Positive Regulation

• Glucose is a preferred energy source for bacteria, but they can also metabolize arabinose if glucose levels are low.

Gene Expression:Turning Genes On and Off

• When arabinose is absent, the AraC protein repressestranscription of the Ara Operon by forming a DNA loop

18.1 Gene Regulation

The Arabinose Operon: Negative and Positive Regulation

• Glucose is a preferred energy source for bacteria, but they can also metabolize arabinose if glucose levels are low.

Gene Expression:Turning Genes On and Off

• When arabinose is absent, the araC protein repressestranscription of the Ara Operon by forming a DNA loop

• When arabinose is present, it binds to the araC protein, changing it into a positive regulator

o This initiates transcription of araBAD by RNA Polymerase

18.1 Gene Regulation

pGLO Plasmid: Utilizing the regulation of the arabinose operon

• The pGLO plasmid has been genetically engineered to take advantage of the arabinose operon regulatory pathway

Gene Expression:Turning Genes On and Off

• araBAD genes have been cut out and replaced by the Green Fluorescent Protein (GFP)

• If glucose levels are low and arabinose levels are high, the GFP protein will be transcribed instead of the araBAD genes