• Genes are not always active; their expression is regulated as appropriate.
  • Genes are roughly classified as housekeeping genes (essential for the survival and proliferation of cells) and those involved in differentiation (essential for the functioning of multicellular organisms).
  • Housekeeping genes are present in both prokaryotes and eukarytic multicellular organisms, whereas genes involved in differentiation are unique to eukaryotic multicellular organisms.
  • The housekeeping type consists of constitutive genes, which are continuously expressed, and regulated expression genes.
  • Regulated expression involves transcription factors (proteins) that recognize and bind to the sequence of the promoter region. Through this binding, transcription is regulated both positively and negatively.
  • Gene expression in prokaryotes is often caused by the presence/absence of mRNA synthesis, i.e., by transcriptional regulation.
  • In eukaryotes, posttranscriptional regulation occurs in addition to transcriptional regulation. This includes the modification of mRNA and the transportation of mRNA from the nucleus to the cytoplasm.
  • In addition to promoters, eukaryotes have gene expression regulation regions known as enhancers and silencers in their DNA. Gene expression is regulated when particular proteins (transcriptional regulator) bind to them.
  • In eukaryotes, DNA intertwines with histones (basic proteins) to form nucleosomes, which connect with other proteins to form chromatin.
  • Expression regulation by enhancers or silencers is sometimes accompanied by changes in chromatin structure (chromatin remodeling).
  • Genes that can be expressed are located in regions where chromatin is loosely packed (i.e., areas of euchromatin), whereas genes that are never expressed are often concentrated in regions where chromatin is tightly packed (i.e., areas of heterochromatin).

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