Unit 4: Cell Communication and Cell Cycle

21 terms. Say what each one means, then open the card to check.

• Ligand

  A signal molecule that binds a specific receptor protein. Binding changes the receptor's shape and starts the cell's response.

  Example. Insulin is the ligand for the insulin receptor on a muscle cell; the cell ignores signals its receptors do not fit.

• Signal transduction

  The chain of steps inside a cell that turns a signal bound at a receptor into a response, such as an active enzyme or a gene switched on.

  Example. A hormone at the surface of a liver cell leads, through several relay proteins, to an enzyme that releases glucose.

• G protein-coupled receptor

  A membrane receptor that, when its ligand binds, switches on a G protein inside the cell, which then activates an enzyme or a channel.

  Example. When epinephrine binds this kind of receptor on a liver cell, the G protein turns on the enzyme that makes cyclic AMP.

• Receptor tyrosine kinase

  A membrane receptor whose two halves pair up when the ligand binds and add phosphate groups to each other, which activates relay proteins inside the cell.

  Example. Many growth factors act through this kind of receptor, starting a kinase cascade that switches on genes for cell division.

• Intracellular receptor

  A receptor inside the cell, in the cytoplasm or nucleus, that binds small nonpolar signals able to cross the plasma membrane.

  Example. Cortisol, a steroid hormone, diffuses into a cell and binds its receptor there; the pair then turns on certain genes.

• Second messenger

  A small molecule or ion made or released inside the cell after a signal binds its receptor, which spreads the signal through the cytoplasm.

  Example. Cyclic AMP made after a hormone binds its receptor activates protein kinase A in the cytoplasm.

• Protein kinase

  An enzyme that adds a phosphate group from ATP to another protein, often switching that protein on. Phosphatases remove the phosphate.

  Example. In a phosphorylation cascade, kinase 1 phosphorylates kinase 2, which phosphorylates kinase 3, passing the signal along.

• Negative feedback

  Control in which the response reverses the change that triggered it, holding a variable near a set point.

  Example. When body temperature rises, sweating cools the skin, and sweating slows once the temperature falls back toward normal.

• Positive feedback

  Control in which the response adds to the change that triggered it, pushing a process further until it reaches an end point.

  Example. Platelets stuck to a torn blood vessel release chemicals that make more platelets stick, until the tear is plugged.

• Interphase

  The part of the cell cycle between divisions: G1 (growth and normal work), S (DNA copied) and G2 (more growth and preparing to divide).

  Example. A skin cell spends most of its cycle in interphase and only a small part of it dividing.

• Sister chromatids

  The two identical copies of a chromosome made in S phase, held together at the centromere until they separate in anaphase.

  Example. A human cell in G2 has 46 chromosomes, each made of two sister chromatids, for 92 chromatids in all.

• Mitosis

  Division of the nucleus into two nuclei with the same chromosomes as the parent, in prophase, metaphase, anaphase and telophase.

  Example. Cells at the edge of a healing cut divide by mitosis, so each new skin cell has the same DNA as the cell it came from.

• Cytokinesis

  Division of the cytoplasm after mitosis. An animal cell is pinched in two by a ring of actin; a plant cell builds a cell plate.

  Example. In a dividing root tip cell, vesicles gather at the middle of the cell and fuse into a cell plate that becomes a new wall.

• G0 phase

  A nondividing state outside the cell cycle. Some cells stay in it for life; others can return to the cycle when signaled.

  Example. Most mature nerve cells stay in G0, while liver cells in G0 can start dividing again after part of the liver is damaged.

• Checkpoint

  A control point in the cell cycle where the cell checks conditions, such as DNA damage or spindle attachment, before going on.

  Example. A cell with a chromosome not yet attached to the spindle waits at the checkpoint in metaphase instead of starting anaphase.

• Cyclin

  A protein whose level rises and falls through the cell cycle. It binds and activates a cyclin-dependent kinase.

  Example. A mitotic cyclin builds up through G2, peaks as mitosis starts, and is broken down around the start of anaphase.

• Cyclin-dependent kinase (CDK)

  A kinase that is active only when bound to a cyclin. The active pair phosphorylates proteins that move the cell past a checkpoint.

  Example. A CDK is present all through the cycle, but it pushes a cell into mitosis only once enough mitotic cyclin has bound it.

• Growth factor

  A signal protein released by one cell that binds receptors on other cells and stimulates them to divide.

  Example. At a wound, platelets release a growth factor that makes nearby connective tissue cells divide and fill the gap.

• Oncogene

  A mutated form of a gene that normally promotes division (a proto-oncogene), whose protein is now overactive or made in excess.

  Example. A mutation that leaves a relay protein in a growth factor pathway stuck on makes the cell divide with no growth factor.

• Tumor suppressor gene

  A gene whose protein stops or pauses division or triggers cell death. Mutations that disable it remove a brake on division.

  Example. The p53 gene is a tumor suppressor; cells with both copies disabled keep dividing even when their DNA is damaged.

• Apoptosis

  Programmed cell death: a cell breaks down its own contents in an orderly way, and neighboring cells clear away the pieces.

  Example. In a human embryo, the cells between the developing fingers die by apoptosis, separating the fingers.