Control of gene expression

Key words

  • Exon

    • Coding part of DNA

  • Intron

    • Non coding region of DNA

  • Operon

    • A group of genes that function as a single transcription unit

  • Transcription factor

    • Protein or short non coding RNA that can combine with a specific site on a length of DNA and inhibit or activate transcription of the gene

  • Acetylation

    • Addition of acetyl or phosphate groups

  • Methylation

    • Addition of methyl groups

Gene types

  • Housekeeping genes

    • Genes required constantly for essential metabolic pathways

    • Respiration enzymes

  • Tissue specific genes

    • Genes required only by specific cells at specific times

    • Protein based hormone for growth

Regulating genes

  1. Transcriptional control

    • When genes are turned on or off

  2. Post-transcriptional

    • mRNA can be modified (the editing of primary mRNA and the removal of introns to produce mature mRNA) which regulates translation & the types of proteins produced

  3. Translational control

    • mRNA can be modified (translation can be stopped or started)

  4. Post-translational

    • Proteins can be modified after synthesis which changes their functions

Transcriptional control

  • transcription ON switch

    • Acetylation

    • Reduces positive charge on histones

    • Attraction to DNA decreases

    • DNA uncoils

  • Transcription OFF switch

    • Methylation

    • Makes histones hydrophobic

    • Binds DNA tighter

Prokaryotic control: The Lac Operon

  • the lac operon is a section of DNA in the bacterial genome containing a number of genes

  • No lactose present

    • When lactose is absent the repressor binds to the operator, blocking transcription

    • The cell saves energy by not producing unnecessary enzymes

  • lactose present

    • Structural genes expressed as gene products are needed

    • Lactose causes repressor protein to undergo a conformational change

Post transcriptional control

  • Splicing

    • Removal of non coding introns and joining of coding exons

  • stabilisation

    • 5‘ cap and 3‘ tail delay degradation in the cytoplasm

  • RNA editing

    • Base sequences can be changed to create different proteins from the same gene

Translational control

  • degradation

    • Shortening the mRNA poly-A tail reduces its shelf life and stability

  • Inhibitory proteins

    • Binding of specific proteins to mRNA prevents it from attaching to ribosomes

      • Blocks translation

  • Initiation factors

    • Specialised proteins that aid in the binding of mRNA to ribosomes

      • Their activation is often required to start the process

Post translational control (insulin case study)

  • Preproinsulin

    • Folding and shorteining

  • Proinsulin

    • Addition of non protein groups

      • Sugars

      • Lipids

      • Phosphates

  • Active insulin

    • Activation by protein kinases

      • Phosphorylation

Fine tuning: up regulation by cAMP

  • the cell prefers glucose

  • It only speeds up lactose digestion when glucose is scarce

  • The cAMP-CRP complex acts as a positive regulator

    • Enhancing the transcription efficiency initiated by RNA polymerase in the absence of glucose