AP Biology Unit 4 Notes: How Cells Divide and How Division Is Controlled

Cell cycle

Ordered series of events in which a cell grows, replicates its DNA, and divides to produce two genetically identical daughter cells (typical body cells).

Interphase

Portion of the cell cycle (G1, S, G2) when the cell grows, duplicates contents (including DNA), and prepares for division; not a “resting” stage.

G1 phase

First gap phase of interphase; cell grows, performs normal functions, makes RNA/proteins, and decides whether conditions are favorable to divide.

G0 phase

Nondividing state entered from G1; may be temporary (can re-enter the cycle) or long-term (e.g., many mature neurons remain here).

S phase

Synthesis phase of interphase in which DNA is replicated, producing duplicated chromosomes made of two identical sister chromatids; DNA amount doubles.

Sister chromatids

Two identical copies of a chromosome produced during S phase, held together until they separate during anaphase of mitosis.

Kinetochore

Protein complex assembled on the centromere region of a chromosome; spindle microtubules attach here (not directly to DNA).

G2 phase

Second gap phase of interphase; additional growth and final preparation for mitosis, including synthesis of mitotic proteins and checking that DNA replication is complete/undamaged.

Mitosis

Nuclear division that distributes duplicated chromosomes into two nuclei so each daughter nucleus receives a complete, identical set of chromosomes.

Cytokinesis

Division of the cytoplasm to form two separate daughter cells (animals use a cleavage furrow; plants form a cell plate).

Prophase (and prometaphase)

Chromatin condenses into visible chromosomes, spindle begins forming, and centrosomes move apart; nuclear envelope breaks down and microtubules attach to kinetochores.

Metaphase

Stage of mitosis in which chromosomes align at the cell’s equator (metaphase plate), reflecting balanced attachment to opposite spindle poles.

Anaphase

Stage of mitosis when sister chromatids separate and are pulled to opposite poles; once separated, each chromatid is considered an individual chromosome.

Telophase

Stage of mitosis in which chromosomes arrive at poles and begin decondensing, and new nuclear envelopes form around each set, restoring two nuclei.

Mitotic spindle

Microtubule-based structure that moves and segregates chromosomes by attaching to kinetochores and pulling chromatids toward opposite poles.

G1 checkpoint (restriction point)

Checkpoint that asks if the cell is big enough, has sufficient nutrients, has growth signals, and has undamaged DNA; passing commits the cell to DNA replication.

G2 checkpoint

Checkpoint that verifies DNA replication is complete and DNA is not damaged before the cell proceeds into mitosis.

M checkpoint (spindle checkpoint)

Checkpoint (often at metaphase) that ensures all chromosomes are properly attached to the spindle via kinetochores and under correct tension before anaphase begins.

Cyclin

Regulatory protein whose concentration rises and falls during the cell cycle; activates CDKs to help drive progression through specific stages/checkpoints.

Cyclin-dependent kinase (CDK)

Protein kinase present more consistently in cells that becomes active when bound to a cyclin; phosphorylates target proteins to push the cell into the next phase.

Growth factor

Extracellular signaling molecule (often a protein) that stimulates cell division by promoting progression through the cell cycle (especially past G1).

Anchorage dependence

Requirement (common in many animal cells) that a cell be attached to a surface/ECM to divide; loss of this control is associated with cancerous behavior.

Proto-oncogene

Normal gene that promotes cell cycle progression under appropriate conditions; mutations that increase its activity/expression can convert it into an oncogene that drives excessive division.

Tumor suppressor gene

Gene that normally slows the cell cycle, repairs DNA damage, or triggers apoptosis when damage is severe (e.g., p53, Rb); loss of function can allow uncontrolled division.

Apoptosis

Programmed, orderly cell death used to eliminate cells that are too damaged to repair, helping prevent propagation of harmful mutations.