Chap 8

Return to Essential Concepts

  • A typical eukaryotic cell expressed only a fraction of its genes, and the distinct types of cells in multicellular organisms arise because different types of genes are expressed as cells differentiate.
  • Although all of the steps involved in expressing a gene can in principle be regulated, for most genes in the initiation of transcription is the most important point of control.
  • The transcription of individual genes is switched on and off in cells by transcription regulators. These act by binding to short stretches of DNA called regulatory DNA sequences.
  • Although each transcription regulator has unique features, most bind to DNA using one of a small number of structural motifs. The precise amino acid sequence that is folded into the DNA-binding motif determines the particular DNA sequence that is recognized.
  • In bacteria, transcription regulators usually bind to regulatory DNA sequences close to where RNA polymerase binds. They can either activate or repress transcription of the gene. In eukaryotes, regulatory DNA sequences are often separated from the promoter by many thousands of nucleotide pairs.
  • Eukaryotic transcription regulators act in two fundamental ways: (1) they can directly affect the assembly process of RNA polymerase and the general transcription factors at the promoter, and (2) they can locally modify the chromatin structure of promoter regions.
  • In eukaryotes, the expression of a gene is generally controlled by a combination of transcription regulators.
  • In multicellular plants and animals, the production of a different transcription regulators in different cell types ensures the expression of only those genes appropriate to the particular type of cell.
  • Cells in multicellular organisms have mechanisms that enable their progeny to 'remember' what type of cell they should be.
  • A single transcription regulator, if expressed in the appropriate precursor cell, can trigger the formation of a specialized cell type or even an entire organ.
  • Cells can also regulate gene expression by controlling events that occur after transcription has begun. Many of these mechanisms rely on RNA molecules that can influence their own transcription or translation.
  • MicroRNAs (miRNAs) control gene expression by base-pairing with specific mRNAs and regulating their stability and their translation.
  • Cells have a defense mechanism for destroying 'foreign' double-stranded RNAs, many of which are produced by viruses. Scientists can take advantage of this mechanism, called RNA interference, to inactivate genes of interest by simply injecting cells with double-stranded RNAs that target the mRNAs produced by those genes.

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