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Biology Unit 4: Cell communication and cell cycle

Cells talk to each other with chemical signals, and they divide in a tightly controlled cycle. When control breaks down, cancer can result.

Covers: Signal transduction, feedback, mitosis, cell cycle checkpoints, cancer

Study guide

Cells communicate through signaling pathways with three stages. In reception, a signal molecule (ligand) binds a receptor, either on the cell surface or inside the cell for small nonpolar signals like steroid hormones. In transduction, the signal is relayed and amplified inside the cell, often through a cascade of protein modifications (such as phosphorylation by kinases) and second messengers like cyclic AMP. In the response, the cell changes, such as turning genes on, activating enzymes, or dividing. Mutations in signaling proteins can disrupt these pathways.

Feedback keeps systems stable. Negative feedback counteracts a change and maintains homeostasis, as in body temperature and blood sugar regulation. Positive feedback amplifies a change until a goal is reached, as in childbirth contractions and blood clotting.

The cell cycle includes interphase (G₁ growth, S phase DNA replication, G₂ preparation) and the mitotic phase. Mitosis divides the copied chromosomes: in prophase chromosomes condense, in metaphase they line up in the middle, in anaphase sister chromatids separate, and in telophase new nuclei form. Cytokinesis then divides the cytoplasm. Mitosis produces two genetically identical cells for growth, repair and asexual reproduction.

Checkpoints at G₁, G₂ and M check conditions like DNA damage and chromosome attachment before the cycle continues. Proteins called cyclins and cyclin-dependent kinases drive the cycle forward. Cancer results when mutations disable these controls, such as damaged tumor-suppressor genes or overactive proto-oncogenes, so cells divide uncontrollably.

Many cell-surface receptors are G protein–coupled receptors. When a ligand binds, the receptor activates a G protein by causing it to swap GDP for GTP. The active G protein then turns on an enzyme that makes a second messenger such as cAMP. If the G protein can't take up GTP, the signal stops at that step and there is no response. Blood glucose is controlled by two pancreatic hormones. Insulin lowers blood sugar by causing cells to take up glucose and the liver to store it as glycogen, and glucagon raises it by triggering glycogen breakdown. In type 2 diabetes, cells stop responding to insulin, so blood glucose stays high.

Spindle fibers (microtubules) grow from centrosomes and attach to each chromosome's kinetochore at the centromere. In anaphase the fibers shorten and pull chromatids to opposite poles. Anything that blocks spindle attachment or breakdown stops chromosomes from separating properly. Mutations in body (somatic) cells, such as UV damage to skin cells, can cause cancer but are not inherited. Only mutations in cells that make gametes (the germ line) pass to offspring. Healthy cells can also trigger apoptosis, programmed cell death, when their DNA is badly damaged.

Key ideas

• Signaling has three steps: reception, transduction, response.
• Negative feedback keeps conditions stable; positive feedback amplifies a change.
• The cell cycle: G1, S (DNA copied), G2, then mitosis (PMAT) and cytokinesis.
• Checkpoints stop the cycle if DNA is damaged or not ready.
• Cancer comes from mutations that disable checkpoints or overactivate growth signals.

Formulas and rules

Common mistakes

• Thinking DNA is copied during mitosis. It's copied in S phase.
• Confusing negative feedback (stabilizes) with positive (amplifies).

Words to know

Cell signaling

Reception, transduction, response.

S phase

The part of interphase when DNA is replicated.

Metaphase

The phase when chromosomes line up in the middle of the cell.

Positive feedback

A response that amplifies the change, like contractions during childbirth.

Cell cycle checkpoint

A control point that stops division until conditions are right.

Mitosis phases

Prophase, metaphase, anaphase, telophase (PMAT).

Reception

A signal molecule (ligand) binds a receptor.

Transduction

The signal is relayed and amplified inside the cell.

Second messenger

A small molecule like cyclic AMP that relays a signal.

Negative feedback

A response that counteracts a change and maintains homeostasis.

Interphase

G₁, S and G₂: growth and DNA replication before mitosis.

Anaphase

Sister chromatids separate to opposite poles.

Cytokinesis

Division of the cytoplasm.

Cyclins and CDKs

Proteins that drive the cell cycle forward.

Cancer

Uncontrolled division when mutations disable cell-cycle controls.

Tumor-suppressor gene

A gene that normally slows division; its loss can cause cancer.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.