Gene structure and regulation of gene expression

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Gene Structure and Regulation of Gene Expression

What is meant by a genome?

A genome is the complete set of genes or DNA is an organism

Why do you not use all your genome at the same time?

Cells do not express all of the genes in their genome at the same time as it is not energy or time efficient. Instead, specific genes get switched on at specific times when they are required.

What is meant by gene expression?

Gene expression is the process that leads to the transformation of the information stored in a gene to a functional product (usually a protein or RNA molecule)

The expression of genes (whether they are switched on or off) seems to depend on the cell type, the stage of development and the conditions within or around the cell

How is DNA structure different in eukaryotic and prokaryotic cells?

Compare the roll of structural genes and regulatory genes in gene expression

Regulatory gene: a gene that codes for transcription factors (which in turn control gene expression at the transcription stage)

Structural gene: A gene that codes for protein and RNA molecules that are not involved in gene regulation (e.g. enzymes)

How is gene expression controlled in prokaryotes?

What is an operon? Promoter, operator, structural genes

OPERON:

a segment of DNA containing adjacent genes including structural genes, an operator region and a promoter region.

Therefore means it is a functional unit of transcription and genetic regulation

Are found in many prokaryotic organisms

PROMOTER: Upstream region of a gene (a specific DNA sequence) to which RNA polymerase attaches, initiating transcription

OPERATOR: The segment of DNA that is the binding site of the transcription factor

STRUCTURAL GENES: A gene that codes for protein and RNA molecules that are not involved in gene regulation (e.g. enzymes)

How does a regulator gene and its repressor protein control gene expression?

Regulatory gene: gene found upstream which codes for a protein that controls the expression of the structural protein (i.e repressor protein (OFF SWITCH)

Repressor Protein: protein which binds to the operator region of the operon and blocks RNA polymerase from binding

RNA polymerase binds to which part of the operon?

The PROMOTER is the region of the operon that RNA polymerase binds to

Know the Lac operon example - what happens in bacteria when lactose is present and when lactose is absent

Experiments were carried out on the bacterium E. Coli, which inhabits the human gut and consumes sugars as part of its diet

If ONLY glucose is present, E.coli uses it for cellular respiration

If ONLY lactose is present, E.coli produces enzymes to change the lactose into glucose that can then be used in cellular respiration

If BOTH glucose and lactose are present, E.coli turns off the lactose metabolism in favor of glucose metabolism (as it is easier and uses less energy) - the enzymes needed to change lactose are no longer made and the gene is switched off

IF NO LACTOSE is present for these bacteria:

the repressor protein binds to the operator, covering part of the promoter

This means that RNA polymerase cannot bind to the promoter and transcription of the gene is blocked

Thus no enzymes for lactose metabolism are manufactured

IF LACTOSE is present for these bacteria:

The lactose binds to the repressor protein

This alters the repressor proteins shape so that it cannot bind to the operator

The three genes are able to be transcribed and the enzyme are made

Lactose can be converted into glucose for cellular respiration

How is gene expression controlled in eukaryotic cells? Transcription factors: Enhancers and Activators

The control of gene expression in eukaryotes is similar but MORE COMPLEX than that in prokaryotes

Transcription is regulated by proteins (activators) that bind to specific sites on the DNA molecule, hence turning it on

Enhancers are regions found in eukaryotic DNA that act as binding sites for some activator proteins. They seem to act by increasing the number of RNA polymerase molecules transcribing the associated gene

Gene expression ca be activated in eukaryotic cells by chemical modification - adding a methyl or an acetyl group to histone proteins

Attaching a methyl or a acetyl group (METHYLATION) to a histone makes it loosen its grip on the DNA that is wound around it.

This makes the gene regions available for transcription. The genes are switched on

When the methyl or acetyl groups were missing on the histone, the DNA was inactive. The genes were switched off