• Cells perform various reactions to detect and adapt to extracellular chemicals and physical stimuli.
  • In multicellular organisms, many signaling molecules are exchanged between cells.
  • Signals are detected by receptor proteins located on the cell surface. Some receptors are located in the cell.
  • Signaling molecules capable of binding to receptors are efficiently detected.
  • Receptors change their structure when bound with signaling molecules and transduce signals into the cell.
  • Intracellular signal transduction takes place by the four main methods of phosphorylation, G proteins, second messengers such as cAMP and Ca2+, and protein degradation.
  • There are four main receptor types: enzyme-linked receptors with kinase or other enzymes, G protein-coupled receptors, channel receptors and transcription factor receptors.
  • In the short term, transduced signals change the functions of proteins, thereby altering the structure and functions of the cell. In the long term, the signals change gene transcription, thereby affecting cell growth and differentiation as well as various other functions.
  • One cell replicates its genetic information and divides into two cells.
  • There are two types of cell division: symmetric cell division (by which two cells with the same characteristics are produced) and asymmetric cell division (by which two cells with different characteristics are produced).
  • Cell division progresses in a cyclic reaction known as the cell cycle, consisting of the G1, S, G2 and M phases.
  • The cell cycle of many cells in multicellular organisms is arrested in the G1 phase.
  • Growth signals cause the cell cycle to resume.
  • The cell cycle is regulated by the continuous activation and inactivation of the cyclin-CDK complex, centering around the cyclic expression of cyclins.
  • Cells progress to the next stage after verification that the previous stage is complete (checkpoints).
  • Cancer cells start to grow independently after the mutation of genes that encode proteins involved in the signal transduction mechanism of cell growth into cancer genes, resulting in the loss of the functions of protein that suppress cell division (tumor suppressor genes).
  • Cells have signal transduction pathways that induce their own death as an active process (apoptosis).

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