Unit 6 (Gene Expression)

Gene expression

• Prokaryotes and eukaryotes must be able to regulate which genes are expressed at any given time

• Genes can be turned on or off based on environmental and internal cues

• On/off refers to whether or not transcription will take place

• Allows for cell specialization

Bacterial gene expression

• Operons: A group of genes that can be turned on or off

• Have three parts:

  • Promoter: where RNA polymerase can attach

  • operator: the on/off switch

  • genes: codes for related enzymes in pathway

• Can be repressible or inducible

  • Repressible (on to off): transcription is usually on, but can be repressed (stopped)

  • Inducible (off to on): transcription is usually off, but can be induced (started)

Regulatory gene

Produces a repressor protein that binds to the operator to block RNA polymerase from transcribing the gene

• Always expressed, but at low levels

• Binding of a repressor to an operator is reversible

Allosteric Regulation

• Activator: substrate binds to allosteric site and stabilizes the shape of the enzyme so that the active sites remain open

• Inhibitor: substrate binds to allosteric site and stabilizes the enzyme shape so that the active sites are closed (inactive form)

Repressible Operons

• Example: the trp operon in bacteria controls the synthesis of tryptophan

• Since it’s repressible, transcription is active

  • It can be switched off by the trp repressor

  • Allosteric enzyme that is only active when tryptophan binds to it

• When too much tryptophan builds up in bacteria, it is more likely to bind to the repressor turning it active, which will then temporarily shut off transcription for tryptophan

Inducible Operons

• Example: the lac operon controls synthesis of lactase, an enzyme that digests lactose (milk sugar)

• Since it is inducible, transcription is off

  • A lac repressor is bound to the operator (allosterically)

• The inducer for the lac repressor is allolactose

  • When present it will bind to the loc repressor and turn the lac repressor off (allosterically inactive)

  • The genes can now be transcribed

Eukaryotic Gene Expression

• The phenotype of a cell or organism is determined by a combination of genes that are expressed and the levels that they are expressed

  • Differences between cell types is known as differential gene expression

Chromatin Structure

• If DNA is tightly wound it is less accessible for transcription

• How can it be modified?

  • Histone acetylation adds acetyl groups to histones, which loosens the DNA

• DNA methylation adds methyl groups to DNA, which causes the chromatin to condense

Epigenetic inhertience

• Chromatin modifications do not alter the nucleotide sequence of the DNA, but they can be heritable to future generations

  • Modifications can be reversed, unlike mutations

  • Explains why one identical twin may inherit a disease while the other does not

Transcription Initiation

• Once chromatin modifications allow the DNA to be more accessible, specific transcription factors bind to control elements

  • Sections of non coding DNA that serve as binding sites

• Gene expression can be increased or decreased by binding of activators or repressors to control elements

Translation Initiation

• Translation can be activated or repressed by initiation factors

• MicroRNAs and small interfering RNAs can bind to mRNA and degrade it or block translation 

RNA processing

Alternative splicing of pre-mRNA

Eukaryotic Development

• During embryonic development, cell division and cell differentiation occurs

  • Cells become specialized in their structure and function

  • Morphogenesis: the physical process that gives an organism its shape

• How do cells differentiate during early development?

  • Cytoplasmic determinants: substances in specific regions of egg that cells in that area to assume specific properties during embrogenesis

  • Induction: cell to cell signals that can cause a change in gene expression

• Both cytoplasmic determinants and induction influence pattern formation

  • A “body plan” for the organism

  • Homeotic genes map out the body structures

• As cells differentiate, apoptosis plays a critical role

  • Apoptosis: programmed cell death 

    • Allows structures to take their form

      • Example: if apoptosis did not occur during the development of human hands and feet we would be born with webbed fingers and toes