Campbell Biology · Chapter 12

The Cell Cycle

pp. 228–245 · 3 sections

Every cell comes from an earlier cell, and this chapter shows how one cell becomes two with the same DNA. It covers how DNA is packed into chromosomes, the stages a cell passes through as it grows, copies its DNA and splits, and the molecular checks that decide when division happens. That's Topics 4.5 and 4.6 of the AP course, and the last part explains how cancer starts when those checks fail.

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Chapter quiz (36 questions)Flashcards (43 terms)

12.1 Chromosomes and identical copies

pp. 229–230

On the AP exam? Yes

Topic 4.5 expects you to count chromosomes and sister chromatids and to know that mitosis makes identical cells for growth, repair and asexual reproduction; gametes and meiosis come back in Topic 5.1. You won't need any species' chromosome count.

In the course: Topic 4.5 Cell Cycle, Topic 5.1 Meiosis (notes, videos and more questions)

Key points

Cell division has three big jobs: it's how a single-celled organism reproduces, how one fertilized egg grows into a many-celled body, and how that body keeps repairing damage and swapping in new cells for ones that die.
A genome is all the DNA a cell carries. In eukaryotes it's split among several linear chromosomes, and each chromosome is one long DNA molecule wrapped up with proteins. That DNA-plus-protein material is called chromatin.
For most of a cell's life its chromatin is loose and spread out, so genes can be read. As division nears, it packs down into compact chromosomes you could see with a light microscope, which can be moved without tangling or snapping.
In S phase each chromosome is copied. The two identical copies, called sister chromatids, are held together by cohesin proteins, most tightly at the centromere. Count chromosomes by counting centromeres: a copied chromosome is still one chromosome.
When the sister chromatids split apart in mitosis, each one is counted as a separate chromosome from then on. The two nuclei that form get identical chromosome sets, and cytokinesis then divides everything else.
Body (somatic) cells carry two sets of chromosomes, one from each parent; in humans that adds up to 46. Eggs and sperm carry just one set because they come from meiosis, a different kind of division (Topic 5.1). When an egg and sperm join, the zygote is back to two sets, and every later round of mitosis passes on that same count.

Key terms (14)

cell division: How one cell becomes two. It lets single cells reproduce and lets bigger organisms grow and heal.
genome: All of a cell's genetic information: every bit of its DNA, across every chromosome.
chromosome: One long DNA molecule plus the proteins that package it. Eukaryotic cells have several straight (linear) ones; most bacteria have a single circular one.
chromatin: The DNA-and-protein material chromosomes are made of. It's loose between divisions and tightly coiled during them.
gene: A stretch of DNA that holds the instructions for one product, usually a protein or an RNA, and so helps shape a trait.
somatic cell: A body cell that isn't a sex cell, such as a skin, muscle or root cell. In humans it holds 46 chromosomes in two sets.
gamete: A sex cell, either an egg or a sperm. It holds only one set of chromosomes, and two gametes join at fertilization.
sister chromatids: The two matching copies of a chromosome made in S phase. They stay stuck together until anaphase.
cohesin: A ring of proteins that keeps sister chromatids together after DNA is copied. When an enzyme cuts it, the chromatids can come apart.
centromere: The pinched-in spot where sister chromatids are held most tightly. Each chromosome has exactly one, so counting centromeres counts chromosomes.
mitosis: Splitting of the nucleus so that each new nucleus gets a complete set of chromosomes identical to the original.
cytokinesis: Splitting of the cytoplasm into two separate cells, which usually happens right as mitosis ends.
zygote: The single cell made when an egg and a sperm fuse. Rounds of mitosis turn it into a whole body.
meiosis: A two-step division that makes gametes with half the chromosome number. Unlike mitosis, its products are not identical.

Check yourself: 12.1 Chromosomes and identical copies

4 questions on 12.1 Chromosomes and identical copies. Pick an answer to see if you got it, and why.

Question 1 of 4

A student lists four terms for describing a human cell's DNA. Which pairing of term and description is correct?

Question 2 of 4

A skin cell from a horse has 64 chromosomes in G₁. The cell then completes S phase. Just before mitosis begins, how many chromosomes and how many chromatids does it contain?

Question 3 of 4

A gardener snips a stem from a pothos vine and sets it in a glass of water. Over several weeks, new roots grow from the cutting by cell division. Compared with cells in the original stem, the new root cells most likely

Question 4 of 4

Tomato body cells contain 24 chromosomes, in two sets of 12. Which cell from a tomato plant would normally contain 12 chromosomes?

0 of 4 answered

12.2 Interphase, mitosis and cytokinesis

pp. 230–238

On the AP exam? Yes

This is Topic 4.5: know G₁, S and G₂, the phases of mitosis and cytokinesis in plants and animals. The AP course uses four phases and folds prometaphase into prophase. Motor-protein details, bacterial binary fission and how mitosis evolved are background.

In the course: Topic 4.5 Cell Cycle, Topic 6.1 DNA and RNA Structure, Topic 7.7 Common Ancestry (notes, videos and more questions)

Key points

The cell cycle covers a cell's whole life, from the division that made it to the division that ends it. Interphase fills most of that time while the cell grows and prepares; the M phase (mitosis plus cytokinesis) is usually brief.
Interphase has three parts. In G₁ the cell grows and does its everyday job, in S it copies its DNA, and in G₂ it keeps growing and gets set to divide. The cell makes proteins and organelles throughout interphase, but DNA replication is limited to S. When some cell types cycle faster than others, the difference is mostly in how long they spend in G₁.
The AP course uses four phases of mitosis. Prophase: chromosomes condense, the spindle starts to form and the nuclear envelope breaks up (some books split this last part off as prometaphase). Metaphase: chromosomes line up across the middle. Anaphase: sister chromatids separate and head to opposite ends. Telophase: new nuclear envelopes form and the chromosomes loosen.
The spindle is made of microtubules. In animal cells they grow out from two centrosomes, each holding a pair of centrioles, yet plant cells build spindles with no centrioles at all. A kinetochore on each sister chromatid grabs microtubules from one pole, so each chromosome is held under tension from both sides until it sits at the cell's midline.
Anaphase starts when an enzyme cuts cohesin. Kinetochore microtubules then shorten, hauling each chromosome centromere-first toward a pole. Meanwhile, microtubules from the two poles that overlap in the middle slide past each other, which lengthens the whole cell.
Cytokinesis depends on the cell. An animal cell tightens a ring of actin and myosin just under its membrane, which pinches it in two along a cleavage furrow. A plant cell can't pinch because of its wall, so Golgi vesicles collect in the middle and fuse into a cell plate that becomes a new wall.
Bacteria divide by binary fission, with no mitosis. They copy their single circular chromosome starting from one origin, move the two copies apart and pinch in two. A few bacterial division proteins are cousins of tubulin and actin, which hints that mitosis grew out of an older, simpler way of dividing.

Key terms (15)

cell cycle: The full run of growth, DNA copying and division that takes a cell from its own formation to splitting into two.
interphase: The long stretch between divisions (G₁, S and G₂) when the cell grows, works and copies its DNA. It isn't a resting stage.
G₁ phase: The first gap after a division. The cell grows, builds proteins and organelles and does its normal job. Its length varies a lot between cell types.
S phase: The synthesis stage, when the cell copies its DNA so every chromosome ends up with a sister chromatid.
G₂ phase: The second gap, after DNA copying. The cell keeps growing and makes what it needs for mitosis.
prophase: The first phase of mitosis: chromosomes condense, the spindle starts forming and, by the end, the nuclear envelope breaks up.
metaphase: The phase when every chromosome sits along the cell's middle, its two kinetochores tied to opposite poles.
anaphase: The phase when cohesin is cut, sister chromatids split, and each new chromosome travels toward one end of the cell.
telophase: The last phase of mitosis: a nuclear envelope forms around each group of chromosomes, which begin to unwind.
mitotic spindle: A football-shaped set of microtubules that attaches to chromosomes and moves them during mitosis.
centrosome: The area near an animal cell's nucleus that microtubules grow out from. It's copied before mitosis, and the two copies become the spindle's poles.
kinetochore: A patch of proteins on each chromatid's centromere that grips spindle microtubules.
cleavage furrow: The groove that appears around a dividing animal cell as a ring of actin and myosin tightens and pinches it in two.
cell plate: A disk of fused Golgi vesicles that grows across a dividing plant cell and turns into the new wall between the two cells.
binary fission: How a bacterium divides: it copies its circular chromosome, moves the copies apart and splits in two, without mitosis.

Check yourself: 12.2 Interphase, mitosis and cytokinesis

4 questions on 12.2 Interphase, mitosis and cytokinesis. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers give a culture of dividing cells radioactive thymidine, a building block used only to make DNA, for just 15 minutes. They then wash it away and immediately check which cells took it up. Which cells will be labeled?

Question 2 of 4

Under a microscope, a dividing animal cell shows two V-shaped groups of chromosomes moving toward opposite ends, centromeres first. Which event directly triggered the start of this stage?

Question 3 of 4

A student films a dividing cell and writes down four observations, out of order: 1. A nuclear envelope forms around each of two clusters of chromosomes. 2. Chromosomes become visible as compact, X-shaped structures. 3. All the chromosomes sit in a single plane across the middle of the cell. 4. Two identical sets of chromosomes head for opposite sides of the cell. Which order matches the sequence of mitosis?

Question 4 of 4

Researchers give dividing tobacco cells a fluorescent tag that is built only into newly made cell wall material. In a cell that has just finished separating its chromosomes, where would the glow most likely appear first?

0 of 4 answered

12.3 Controlling the cycle, and how cancer escapes

pp. 238–243

On the AP exam? Yes

Topic 4.6 covers checkpoints, cyclins and CDKs, cancer and apoptosis, and G₀ is in Topic 4.5. You won't need MPF by name, specific cyclin-CDK pairs, named growth factors or particular cancer drugs.

In the course: Topic 4.6 Regulation of Cell Cycle, Topic 4.5 Cell Cycle, Topic 4.3 Signal Transduction Pathways, Topic 6.7 Mutations (notes, videos and more questions)

Key points

Different cells divide at very different rates. Cells lining your stomach are replaced every few days, many lymphocytes (a type of white blood cell) wait until an infection calls for them, and most mature neurons never divide. These differences come from molecular controls.
The cycle isn't just a chain where finishing one step automatically starts the next. Chemical signals in the cytoplasm push a nucleus into S phase or mitosis, and a dedicated set of control molecules runs the whole cycle.
The cycle pauses at checkpoints. The G₁ checkpoint looks at the cell's size, nutrients, growth signals and DNA; without a go-ahead it leaves the cycle for G₀, a non-dividing state it may or may not come back from. At G₂ it checks that the DNA is fully and correctly copied, and the M checkpoint holds off anaphase until every chromosome is attached to both poles.
Two kinds of proteins drive the cycle: cyclins and the cyclin-dependent kinases (CDKs) they switch on. CDK levels stay fairly steady, but a CDK only works while bound to a cyclin, and cyclin levels rise and fall as cyclins are made and then destroyed. An active cyclin-CDK phosphorylates target proteins that launch the next stage.
One cyclin-CDK pair, called MPF, pushes cells past the G₂ checkpoint into mitosis, and breaking down its cyclin during anaphase lets the cell finish mitosis. You only need the general cyclin-CDK idea for the AP exam.
Outside signals matter too. Most animal cells divide only when growth factors reach their receptors, stop dividing once they're packed against neighbors (density-dependent inhibition), and divide only while attached to a surface (anchorage dependence).
Cancer cells ignore these controls, usually because of mutations in genes for signaling or checkpoint proteins. They divide without growth factors, pile up, can keep dividing indefinitely and often dodge apoptosis. A benign tumor stays in one place; a malignant one invades nearby tissue and can spread through blood and lymph (metastasis).

Key terms (14)

cell cycle control system: The network of proteins inside a cell, such as cyclins and CDKs, that sets the pace of the cycle and decides when the cell may move on.
checkpoint: A stage in the cycle where the cell confirms that things are ready. If they aren't, the cycle pauses until they are.
G₀ phase: A non-dividing state outside the cycle. Some cells stay there for life; others can return to the cycle when they get the right signals.
cyclin: A protein whose amount rises and falls through the cycle. It switches on its CDK partner.
cyclin-dependent kinase (CDK): An enzyme that adds phosphate groups to target proteins, but only while a cyclin is bound to it.
MPF: A cyclin-CDK complex that pushes a cell from G₂ into mitosis. Its name stood for maturation-promoting factor.
growth factor: A signaling protein, made by one cell, that tells target cells carrying its receptor to start dividing.
density-dependent inhibition: The way normal cells halt division once neighbors surround them on all sides. Also called contact inhibition.
anchorage dependence: The need of most animal cells to be stuck to a surface, such as the matrix around them, before they will divide.
transformation: When a normal cell picks up changes that make it a cancer cell, free of the usual limits on division.
benign tumor: A mass of abnormal cells that hasn't invaded nearby tissue or spread. Surgery can often take all of it out.
malignant tumor: A tumor whose cells push into surrounding tissue and can seed new tumors elsewhere. This is what doctors call cancer.
metastasis: When cancer cells leave the original tumor, travel in the blood or lymph, and start new tumors in other organs.
apoptosis: Programmed cell death: a cell's orderly self-destruct, used to get rid of damaged or unneeded cells.

Check yourself: 12.3 Controlling the cycle, and how cancer escapes

4 questions on 12.3 Controlling the cycle, and how cancer escapes. Pick an answer to see if you got it, and why.

Question 1 of 4

Question 2 of 4

A drug that blocks protein synthesis is added to cells in early G₂, before the cyclin that triggers mitosis has built up. Which outcome is most likely?

Question 3 of 4

Two cell lines are treated with a low dose of a compound that slows spindle assembly, so some kinetochores take much longer than usual to attach. Line 1 has a normal M checkpoint. Line 2 has a mutation that disables it. Which result is most likely?

Question 4 of 4

Many lymphocytes, a type of white blood cell, go months without dividing. When one meets the particular virus it recognizes, signals push it to divide many times within a few days. Before the infection, these lymphocytes are best described as

0 of 4 answered