REGULATORY GENES

ASWATHY ANAND A2010-106

• Regulatory gene is a gene involved in controlling the expression of one or more other genes.

• The regulation of gene expression plays a central role in development from a zygote to a multicellular organism.

TYPES OF GENES

REGULATION IN PROKARYOTES

CONTRL SEQUENCES• Stretches of DNA that cordinates gene

expression. OPERON• is a functioning unit of genomic DNA

containing a cluster of genes under the control of a single regulatory signal or promoter

PROMOTER• It’s a control sequence• Transcription starts here. OPERATOR• DNA sequence between promoter and

enzyme genes.• Act as an on or off switch for the genes.

OPERON

• These are group of genes transcribed at the same time.

• The first operon to be described was the lac operon in E. coli.

• The 1965 Nobel Prize in Physiology and Medicine was awarded to François Jacob, André Michel Lwoff and Jacques Monod for their discovery.

An operon is made up of 3 basic DNA components:

• Promoter – a nucleotide sequence that enables a gene to be transcribed.

• The promoter is recognized by RNA polymerase, which then initiates transcription.

• CONTROL ELEMENTS o Operator – a segment of DNA that a regulator

binds to. It is a segment between the promoter and the genes of the operon.

o Repressor -The protein physically obstructs the RNA polymerase from transcribing the genes.

• Structural genes – the genes that are co-regulated by the operon.

4 conditions are possible1. When glucose is present and lactose is absent the E.

coli does not produce β-galactosidase.

2. When glucose is present and lactose is present the E. coli does not produce β-galactosidase.

3. When glucose is absent and lactose is absent the E. coli does not produce β-galactosidase.

4. When glucose is absent and lactose is present the E. coli does produce β-galactosidase

LACTOSE ABSENT

• A repressor protein is continuously synthesized. It sits on a sequence of DNA just in front of the lac operon, the Operator site

• The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing

LACTOSE PRESENT

• A small amount of sugar allolactose is formed within the bacterial cell. This fit on the repressor protein at another active site.

• This causes a conformational change in the repressor protein. So cant bind to the operator and thus transcription takes place.

GLUCOSE AND LACTOSE PRESENT

• When both lactose and glucose is present RNA pol cant sit in the promoter site and thus fal;l off.

GLUCOSE ABSENT LACTOSE PRESENT

• Another protein is needed which is an activator.

• This stabilizes RNA pol.• Activator work only when glucose is present.

EUKARYOTIC GENE REGULATION

• Eukaryotic genes are controlled individually and each gene has specific control sequences preceding the transcription start site.

• DNA packaging• Transcription• RNA processing• mRNA Export• mRNA modification • mRNA degradation• Translation• Protein modification• Protein transport• Protein degradation

• When DNA is tightly packed around the histones, there is no way for replication to occur.

• Packaging is normally done by Histone Deacetylase Complexes (HDAC).

• Acetylation is associated with gene activation.• Histone methylation is associated with gene

inactivation.

EUKARYOTIC CONTROL MECHANISMS

• Involve regulatory proteins.• Regulate transcription.

– Transcriptional regulation in eukaryote is complex.

• It involves many proteins collectively called Transcription Factors.

– Transcription factors bind to DNA sequences called Enhancers.

– Repressor proteins inhibit transcription • By binding to DNA sequences called Silencers.

• Cis-acting element DNA sequences close to a gene that are required for gene expression.

• Insulator It blocks enhancers

• Trans-acting factor Proteins that bind to the cis-acting elements to control gene expression.

TRANSLATIONAL REGULATION

• Blocking the attachment of mRNA with the ribosome.

• Regulation of protein processing.• Protein modification.