细胞通讯与细胞周期

A single molecule of adrenaline 肾上腺素 arriving at a liver cell can release millions of glucose 葡萄糖 molecules within seconds. A single faulty switch in the same kind of pathway can make a cell divide without stopping. This unit follows a signal from the outside of a cell to its response, and shows where the control can fail.

Cells coordinate by sending and receiving chemical signals 信号. In chemical signaling, a ligand 配体 binds a receptor 受体 on a target cell 靶细胞. The ligand may be released or held on a cell surface. Signals travel over different ranges: direct contact (cell junctions 细胞连接), local signaling (nearby cells, like neurotransmitters 神经递质), and long-distance signaling (hormones 激素 carried through the blood in animals; plants use other transport routes).

Signal transduction 信号转导 converts an outside signal into a cellular response in three stages: reception 接收 (ligand binds receptor), transduction 转导 (a relay of molecules inside the cell), and response 响应 (a change in the cell's activity, such as switching on a gene). Receptors are specific, so a cell only responds to signals it can receive.

In the transduction stage, the signal passes through a pathway 通路 – often a cascade 级联 of proteins that activate one another, frequently amplifying 放大 the signal so a few ligands trigger a large response. Second messengers 第二信使 (like cyclic AMP 环磷酸腺苷 or calcium ions) spread the signal quickly through the cell. A change in one step can alter the whole outcome.

Feedback 反馈 keeps systems balanced:

• Negative feedback 负反馈 counteracts a change to restore a set point 设定点 (like a thermostat 恒温器) – it maintains homeostasis 稳态.
• Positive feedback 正反馈 amplifies a change to push a process to completion (like childbirth contractions 宫缩 or blood clotting 血液凝固).

The cell cycle 细胞周期 is the life of a cell from one division to the next: interphase 间期 (G1 growth, S DNA replication DNA复制, G2 preparation) followed by M phase, including mitosis 有丝分裂 and cytokinesis 胞质分裂, which normally produce daughter cells 子细胞 with the same chromosome 染色体 number and very similar genetic information. Interphase takes most of the time; DNA is copied only in S phase.

The cycle is controlled at checkpoints 检查点 that verify conditions before proceeding (Is the DNA intact? Are chromosomes attached?). Internal signals (cyclins 周期蛋白 and their kinases 激酶) and external signals drive the cycle forward. When this control fails – for example, a mutation 突变 that ignores a checkpoint – cells divide uncontrollably, which underlies cancer 癌症.

Worked example. Signal amplification in a cascade: one hormone activates one receptor, which switches on about 100 relay proteins, and each of those makes about 1,000 second-messenger molecules — so a single signal produces roughly $100 \times 1000 = 10^{5}$ product molecules. This is why a hormone concentration as low as $10^{-9}\,\text{M}$ can trigger a large cellular response.

Worked example (predict the effect of a mutation). In a pathway, a ligand activates a G protein, which switches on an enzyme that makes a second messenger. A mutation leaves the G protein permanently active, even with no ligand. Predict the result: the enzyme keeps making second messenger and the response continues without the signal. The cell can no longer switch the response off. In growth-signal pathways, this kind of always-on mutation is a step toward cancer.

Worked example (timing the cell cycle). In an onion root tip, 200 cells are counted: 160 in interphase, 20 in prophase, 8 in metaphase, 6 in anaphase and 6 in telophase. The cycle takes about 24 hours.

• The mitotic index 有丝分裂指数 is the fraction of cells in mitosis: $\dfrac{20+8+6+6}{200}=20\%$.
• For this rough phase-duration estimate, assume an asynchronously dividing sample and use phase fractions as approximate time fractions; growth and sampling biases can affect the estimate. Interphase: $\dfrac{160}{200}\times24=19.2$ hours. Metaphase 中期: $\dfrac{8}{200}\times24\approx1.0$ hour.

DNA content through the cycle. A cell starts G1 with an amount $x$ of DNA. It doubles to $2x$ during S phase and stays at $2x$ through G2 and mitosis. At cytokinesis each daughter cell returns to $x$. A graph of DNA per cell against time is a step up in S, then a step down at division.

Answer each before reading the answers below.

1. Name the three stages of cell signalling, in order.
2. A drug blocks the receptor for a hormone. Predict the effect on the target cell's response, and explain.
3. In a root-tip count, 25 of 250 cells are in mitosis and the cycle lasts 20 hours. Find the mitotic index and the time spent in mitosis.
4. Classify each as positive or negative feedback: (a) body temperature falls and shivering begins; (b) contractions during childbirth release oxytocin, which strengthens the contractions.
5. A student says: "A cell in G2 has the same amount of DNA as a cell in G1." Correct the student.

1. Reception, transduction, response.
2. The response is reduced or absent. The hormone cannot bind, so the signal never starts the transduction pathway, even though the hormone is present.
3. Mitotic index $=\dfrac{25}{250}=10\%$; time in mitosis $\approx0.10\times20=2$ hours.
4. (a) Negative feedback: shivering raises the temperature back toward the set point. (b) Positive feedback: the change is amplified until the baby is born.
5. DNA is copied in S phase, between G1 and G2, so a G2 cell has twice the DNA of a G1 cell. The amount halves again only when the cell divides.

• In signal transduction name the three stages: reception → transduction → response.
• Negative feedback reverses a change to keep conditions steady (homeostasis); positive feedback amplifies a change to completion (childbirth, clotting).
• Order the cell cycle: interphase (grow, copy DNA in S phase) then mitosis → two identical daughter cells.
• The DNA is copied once, in S phase, so each daughter gets a full copy.
• Failed checkpoints allow uncontrolled division → cancer.