Gene Expression

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•  Gene expression is regulated by proteins that bind to specific base sequences in DNA

Transcriptional activity is regulated by two groups of proteins that mediate binding of RNA polymerase to the promoter

Transcription factors form a complex with RNA polymerase at the promoter

  • RNA polymerase cannot initiate transcription without these factors and hence their levels regulate gene expression

Regulatory proteins bind to DNA sequences outside of the promoter and interact with the transcription factors

  • Activator proteins bind to enhancer sites and increase the rate of transcription (by mediating complex formation)
  • Repressor proteins bind to silencer sequences and decrease the rate of transcription (by preventing complex formation)

The presence of certain transcription factors or regulatory proteins may be tissue-specific

  • Additionally, chemical signals (e.g. hormones) can moderate protein levels and hence mediate a change in gene expression

Differential Gene Expression

gene expression

Control Elements

The DNA sequences that regulatory proteins bind to are called control elements

  • Some control elements are located close to the promoter (proximal elements) while others are more distant (distal elements)
  • Regulatory proteins typically bind to distal control elements, whereas transcription factors usually bind to proximal elements
  • Most genes have multiple control elements and hence gene expression is a tightly controlled and coordinated process

Enhancers and Silencers

enhancer silencer

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•  The environment of a cell and of an organism has an impact on gene expression

Changes in the external or internal environment can result in changes to gene expression patterns

  • Chemical signals within the cell can trigger changes in levels of regulatory proteins or transcription factors in response to stimuli
  • This allows gene expression to change in response to alterations in intracellular and extracellular conditions

There are a number of examples of organisms changing their gene expression patterns in response to environmental changes:

  • Hydrangeas change colour depending on the pH of the soil (acidic soil = blue flower ; alkaline soil = pink flower)
  • The Himalayan rabbit produces a different fur pigment depending on the temperature (>35ºC = white fur ; <30ºC = black fur)
  • Humans produce different amounts of melanin (skin pigment) depending on light exposure
  • Certain species of fish, reptile and amphibian can even change gender in response to social cues (e.g. mate availability)

The Role of Soil pH in the Determination of Hydrangea Flower Colour