4 Cell Communication and Cell Cycle

Describe the ways that cells can communicate with one another.

• Cells communicate with one another through direct contact with other cells or from a distance via chemical signaling.

Explain how cells communicate with one another over short and long distances.

• Cells communicate over short distances by using local regulators that target cells in the vicinity of the signal-emitting cell.
• Signals released by one cell type can travel long distances to target cells of another type.

Describe the components of a signal transduction pathway.

• Signal transduction pathways link signal receptions with cellular responses.
• Many signal transduction pathways include protein modifications and involve phosphorylation cascades.

Describe the role of components of a signal transduction pathway in producing a cellular response.

• Signaling begins with the recognition of a chemical messenger—a ligand—by a receptor protein in a target cell.
  • i. The ligand-binding domain of a receptor recognizes a specific chemical messenger, which can be a peptide (protein) or a small molecule.
  • ii. G protein-coupled receptors are an example of a receptor protein in eukaryotes.
  • iii. Receptors may be located on the surface of a target cell or in the cytoplasm or nucleus of the target cell.
• Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals, resulting in the appropriate responses by the cell. Responses could include cell growth, secretion of molecules, or gene expression.
  • i. After the ligand binds, the intracellular domain of a receptor protein changes shape, initiating transduction of the signal.
  • ii. Enzymes and second messengers such as cyclic AMP (cAMP) relay and amplify the intracellular signal.
  • iii. Hormones are an example of a signaling messenger that can travel long distances in the bloodstream.
  • iv. The binding of ligands to ligand-gated channels can cause the channel to open or close.

Describe the different types of cellular responses elicited by a signal transduction pathway.

• Signal transduction may result in changes in gene expressions and cell function, which may alter phenotype or result in programmed cell death (apoptosis).

Explain how a change in the structure of any signaling molecule affects the activity of the signaling pathway.

• Changes in signal transduction pathways can alter cellular responses. Mutations in any domain of the receptor protein or in any component of the signaling pathway may affect the downstream components by altering the subsequent transduction of the signal.
• Chemicals that interact with any component of the signaling pathway may activate or inhibit the pathway.

Explain how positive and negative feedback helps maintain homeostasis.

• Organisms use feedback mechanisms to maintain their internal environments in response to internal and external changes.
  • i. Negative feedback mechanisms maintain homeostasis by reducing the initial stimulus to regulate physiological processes. If a system is perturbed or disrupted, negative feedback mechanisms return the system back to its target set point. These processes operate at the molecular, cellular, and organismal levels.
  • ii. Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved further away from the initial set point. Amplification occurs when the stimulus is further intensified, which, in turn, initiates an additional response that produces system change.

Describe the events that occur in the cell cycle.

• The cell cycle is a highly regulated series of events that controls the growth and reproduction of eukaryotic cells.
  • i. The cell cycle consists of sequential stages of interphase (G1, S, G2), mitosis, and cytokinesis.
  • ii. G1 phase: The cell is metabolically active, duplicating organelles and cytosolic components.
  • iii. S phase: DNA is in the form of chromatin and replicates to form two sister chromatids connected at a centromere.
  • iv. G2 phase: Protein synthesis occurs, ATP is produced in large quantities, and centrosomes replicate.
  • v. A cell can enter a stage (G0) in which it no longer divides, but it can reenter the cell cycle in response to appropriate cues.
  • vi. Nondividing cells may exit the cell cycle or be held at a particular stage in the cell cycle.

Explain how mitosis results in the transmission of chromosomes from one generation of cells to the next.

• Mitosis is a process that ensures the transfer of a complete genome from a parent cell to two genetically identical daughter cells in eukaryotes.
  • i. Mitosis plays a role in growth, tissue repair, and asexual reproduction.
  • ii. Mitosis occurs in sequential steps (prophase, metaphase, anaphase, telophase) and alternates with interphase in the cell cycle.
  • iii. Prophase: Sister chromatids condense, mitotic spindle begins to form, and centrosomes move to opposite poles of the cell.
  • iv. Metaphase: Spindle fibers align chromosomes along the equator of the cell.
  • v. Anaphase: Paired sister chromatids separate as spindle fibers pull chromatids toward poles.
  • vi. Telophase: Mitotic spindle breaks down, a new nuclear envelope develops, and then the cytoplasm divides.
  • vii. Cytokinesis: A cleavage furrow forms in animal cells or a cell plate forms in plant cells, resulting in two new daughter cells. 90 Cell Communication and Cell Cycle UNIT 4

Describe the role of checkpoints in regulating the cell cycle.

• A number of internal controls or checkpoints regulate progression through the cell cycle.
• Interactions between cyclins and cyclindependent kinases control the cell cycle.
  • Exclusion: Knowledge of specific cyclin-CdK pairs or growth factors is beyond the scope of the AP Exam.

Describe the effects of disruptions to the cell cycle on the cell or organism.

• Disruptions to the cell cycle may result in cancer or apoptosis (programmed cell death).