Week 9 · Chapter

Chapter 9 — The Cell Cycle & Mitosis

Introduction to Biology Free sample edition

The module's primary reading · ~35–45 minutes · Objective 5 (describe the eukaryotic cell cycle and the ordered phases of mitosis, compute a mitotic index, contrast mitosis with meiosis, and explain how crossing over, independent assortment, and fertilization generate genetic variation, including the 2ⁿ count), with Objective 3 (compare prokaryotic and eukaryotic cells, relate each eukaryotic organelle to its function, explain how substances cross the plasma membrane by passive and active transport (including osmosis and tonicity), and use the surface-area-to-volume ratio to explain why cells stay small) carried forward · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Your body makes new cells every day, and this week you learn how one cell becomes two. You will meet the cell cycle (the repeating loop of a cell's life: a long stretch of growing and copying, then a short burst of dividing). You will learn mitosis (the orderly way a cell splits its nucleus so each new cell gets an identical set of instructions), step by step and in order. You will see what keeps division in check and what happens in cancer (cell division that has lost its brakes). And you will put a number on how fast a tissue divides by counting cells under a microscope.

Why you already care about this

Picture a scraped knee. Over the next several days, new skin creeps in from the edges and closes the gap. Every one of those new skin cells came from an older cell that split in two. Nothing arrived from outside; the repair was cell division, done many times over.

You did the same thing on a much larger scale. You started life as one cell, and every cell in your body today traces back to it through rounds of division. The onion on a kitchen counter does it too: at the tip of each root, cells divide so the root can push deeper into the soil. In this week's lab, a count of 100 cells from an onion root tip finds 20 of them caught in the act of dividing. A human body cell carries 46 chromosomes, and after it divides, each of the two new cells carries 46 as well. And on a 24-hour cycle, the counted cells suggest that dividing takes about 4.8 hours while the rest of the day goes to growing and copying.

Division is also where things can go badly wrong. A cell that divides when it should not is the start of a tumor. So this week is about a process that heals your knee, builds your body, and, when it breaks, causes one of the most studied diseases in medicine.

By the end of this chapter you will be able to:

describe the cell cycle as a loop, naming the three parts of interphase and the M phase in order;
tell a chromosome from a chromatid, and count both before and after a cell copies its DNA;
name the four phases of mitosis in order and identify a phase from a description of a cell;
contrast how an animal cell and a plant cell split their cytoplasm;
state what mitosis is for and what it produces;
explain checkpoints and describe cancer as division without the brakes, naming proto-oncogenes, tumor-suppressor genes, and p53;
compute a mitotic index from a cell count, read it, and turn it into hours.

Carry this line with you: copy in S, split in M; one X is one chromosome, not two.

Where we are

Already covered: Week 1 taught careful observation and the controlled experiment, Week 3 introduced DNA as the cell's instruction molecule, and Week 4 toured the cell, including the nucleus and the plant cell wall. Week 8 added the order-and-location habit: name the stages of a process in order and the place each one runs.
This week: the cell cycle as a loop, the chromosome and its sister chromatids, mitosis in order, cytokinesis, the purpose of mitosis, checkpoints and cancer, and the mitotic index.
How they connect: mitosis divides the nucleus that Week 4 described and hands each new cell a copy of the DNA from Week 3. It is a process with an order and a place, the kind Week 8's habit was built for.
Where we're going next: Week 10 introduces meiosis, the other kind of nuclear division, which makes four unique cells instead of two identical ones.

Week 4 gave you the nucleus as the organelle that holds the DNA and directs the cell's work, and it gave plant cells a cell wall for support and shape. Week 3 gave you DNA as a nucleic acid, a long chain of nucleotides that stores the cell's instructions. Week 8 gave you a habit: for any process, say the stages in order and name where each one happens. This week you run that habit on the most visible process in all of biology, a cell splitting in two. The order matters here more than anywhere else so far, because a cell that does the steps in the wrong order ends up with the wrong set of instructions.

1. Read the cell cycle as a loop

Think about a bakery that runs the same loop every day. Most of the shift goes to getting ready: buying flour, mixing dough, letting it rise. The part a customer sees, loaves coming out of the oven, takes up only a small slice of the day. A cell's life runs the same way.

The cell cycle is the repeating loop of a cell's life: interphase (G1, S, and G2), when the cell grows and copies its DNA, followed by the M phase, when it divides into two cells.

The loop has two big parts.

Interphase is the long, non-dividing part of the cell cycle, made of G1, S, and G2; most of a cell's life is spent here.
The M phase is the dividing part of the cell cycle: mitosis, the division of the nucleus, plus cytokinesis, the division of the cytoplasm.

Interphase itself has three parts, always in this order.

The G1 phase is the first part of interphase, in which the cell grows and carries out its normal work.
The S phase is the part of interphase in which the cell copies its DNA, so that afterward each chromosome is two identical sister chromatids.
The G2 phase is the last part of interphase, in which the cell makes its final preparations for division.

Then the cell enters the M phase and divides. Each of the two new cells starts its own loop at G1. Written as an ordered chain, the loop is:

G1 → S → G2 → M

The word "interphase" can sound like a rest period, but it is the busiest stretch of a cell's life. The cell is growing, doing its everyday work, and copying every bit of its DNA.

Worked example — placing events on the loop. You place each event in the part of the cycle where it happens.

A cell copies its DNA. The S phase, the middle part of interphase.
A new cell, fresh from division, grows larger. The G1 phase, the first part of interphase.
A cell with its DNA already copied makes its final preparations. The G2 phase, the last part of interphase.
The nucleus divides and then the cell pinches in two. The M phase: mitosis, then cytokinesis.

In words: grow, copy, prepare, divide, and then the loop starts again in each new cell.

Misconception check. Students often use "mitosis" and "the cell cycle" as if they meant the same thing. The slip is tempting because mitosis is the dramatic part, the part every diagram shows. But mitosis is only the division of the nucleus inside the M phase. The cell cycle is the whole loop, and most of it is interphase. A second slip is to say DNA is copied during mitosis. It is copied in the S phase, before mitosis ever starts.

Memory hook: grow, copy, prepare, divide.

2. Picture a chromosome: one X, two sister chromatids

Imagine you photocopy a long recipe and staple the copy to the original at one corner. You still have one recipe, now in two identical copies held together. That is what a chromosome looks like after the S phase.

Here is a chromosome described as a labeled figure.

Before the S phase. Each chromosome is one long DNA molecule with its packing proteins; after the S phase it is made of two sister chromatids, and it still counts as one chromosome. Before copying, it is a single thread.
After the S phase. The DNA has been copied. Sister chromatids are the two identical copies of a chromosome made in the S phase, joined at the centromere until they separate in anaphase.
The pinch point. The centromere is the pinched region where two sister chromatids are held together, giving a duplicated chromosome its X shape.

So the familiar X shape is one chromosome made of two identical sister chromatids. The two arms on the left side of the X belong to one chromatid, and the two arms on the right side belong to its sister, joined at the centromere in the middle.

Later in mitosis, the two sister chromatids are pulled apart. From that moment on, each separated chromatid counts as a chromosome in its own right. That is how one cell can hand an identical, complete set to each of two new cells.

Worked example — counting chromosomes and chromatids in a human cell. A human body cell has 46 chromosomes. You count through one round of copying and division.

Before the S phase. 46 chromosomes, each a single DNA molecule.
After the S phase. Still 46 chromosomes, because each X counts as one. Each is made of 2 sister chromatids, so 46 × 2 = 92 chromatids.
In anaphase. The sister chromatids separate, and each one now counts as a chromosome.
In each new cell. 92 ÷ 2 = 46 chromosomes, the same number the parent cell started with.

In words: copying doubles the chromatids, not the chromosomes, and division hands each new cell the full 46.

Misconception check. Students often count an X-shaped chromosome as two chromosomes, because it has two obvious halves. The cure is to count centromeres: one centromere, one chromosome. The X is one chromosome made of two sister chromatids. The count of chromosomes changes only when the sisters separate in anaphase and each chromatid becomes a chromosome again.

Memory hook: one X is one chromosome, not two.

3. Run mitosis in order: prophase to telophase

Picture a moving day in a busy apartment. First you pack everything into tight boxes. Then you line the boxes up at the door. Then you split them into two equal loads. Then each load is unpacked in a new home. Mitosis follows the same four moves, in the same order, every time.

Mitosis is the division of a cell's nucleus into two nuclei with identical sets of chromosomes, run in the order prophase, metaphase, anaphase, telophase; it is one part of the M phase, not the whole cell cycle.

Two structures do the moving.

The nuclear envelope is the double membrane around the nucleus; it breaks down in prophase and two new envelopes form in telophase.
The spindle is the set of protein fibers that forms in prophase and pulls the sister chromatids to opposite poles of the cell in anaphase.

Now the four phases, in order.

Prophase is the first phase of mitosis: the chromosomes condense, the nuclear envelope breaks down, and the spindle forms; this course counts prometaphase as its end. The long, loose chromosomes coil up into short, thick rods you can see under a microscope. Some courses count prometaphase, the moment the spindle fibers reach the chromosomes, as a separate phase. This course teaches it as the end of prophase.
Metaphase is the second phase of mitosis, in which the chromosomes line up single file across the middle of the cell. The metaphase plate is the imaginary plane across the middle of a dividing cell where the chromosomes line up in metaphase.
Anaphase is the third phase of mitosis, in which the sister chromatids separate and move apart to opposite poles of the cell. This is the moment each chromatid becomes a chromosome.
Telophase is the fourth phase of mitosis, in which two new nuclear envelopes form, one around each set of chromosomes, and the chromosomes loosen again.

prophase → metaphase → anaphase → telophase

Worked example — naming a phase from what you see. You look at four cells under a microscope and name each phase from one clue.

Condensing? The chromosomes are thick rods and there is no nuclear envelope. That is prophase.
Lined up? The chromosomes sit in one row across the middle. That is metaphase.
Splitting apart? Two groups of chromosomes are moving toward opposite ends. That is anaphase.
Re-forming? Two new nuclei are taking shape, one at each end. That is telophase.

In words: condense, line up, pull apart, re-form. Those four clues name the four phases, and in that order they also tell you which cell is furthest along.

Misconception check. Students often put the phases out of order, most often by swapping metaphase and anaphase. The names are new, and both begin with a sound that is easy to blur. The cure is to tie each phase to its clue: metaphase starts like middle, where chromosomes line up, and anaphase starts like apart, where sisters separate. Prophase comes first because the chromosomes must be packed before they can be moved, and telophase comes last because the nuclei re-form only after the sets are separated.

Memory hook: pack, line up, pull apart, unpack.

4. Finish the split: cytokinesis

At the end of telophase the cell has two nuclei, but it is still one cell. Picture a long balloon twisted in the middle until it becomes two small balloons. Something has to divide the rest of the cell too.

Cytokinesis is the division of the cytoplasm that follows mitosis and ends the M phase, splitting one cell into two daughter cells.

Animal cells and plant cells do this in two different ways.

An animal cell pinches in two. The cleavage furrow is the groove that pinches an animal cell in two during cytokinesis, like a drawstring pulled tight.
A plant cell has a cell wall, the support layer Week 4 described. It builds a wall down its middle instead. The cell plate is the new wall a plant cell builds down its middle during cytokinesis, separating the two daughter cells.

The full order of the M phase is now one ordered chain:

prophase → metaphase → anaphase → telophase → cytokinesis

Worked example — which kind of cell? You decide how each cell finishes dividing.

A skin cell closing a scraped knee. An animal cell, so it pinches in at a cleavage furrow.
A cell at the tip of an onion root. A plant cell with a cell wall, so it builds a cell plate.
A cell lining your stomach. An animal cell, so a cleavage furrow.

In words: an animal cell pinches; a plant cell builds a wall between.

Misconception check. Students often think cytokinesis is one of the phases of mitosis. It is tempting because it comes right after telophase and finishes the job. But mitosis divides the nucleus, and cytokinesis divides the cytoplasm. Together they make up the M phase. A second slip is to give a plant cell a cleavage furrow. A plant cell, with its cell wall, builds a cell plate.

Memory hook: animals pinch; plants build a plate.

5. Ask what mitosis is for: two identical cells

Think of a pothos or spider plant on a bookshelf. You snip a stem, set it in a glass of water, and in time it grows roots and becomes a whole new plant. That new plant is built entirely by mitosis, and it is genetically the same as the plant you cut it from.

Mitosis has four everyday jobs.

Growth. A body grows by adding cells; you went from one cell to a whole person this way.
Repair. A scraped knee or a broken bone is mended with new cells.
Replacement. Cells that wear out, such as the cells lining your gut, are replaced.
Asexual reproduction. Asexual reproduction is making a new organism from one parent by cell division alone, so the offspring is genetically identical to the parent, as when a plant cutting grows into a new plant.

Every time, the result is the same: two identical diploid daughter cells. Diploid means the cell keeps the full two sets of chromosomes it started with. A daughter cell is each of the two cells produced when a cell divides; after mitosis the two daughter cells are identical to each other and to the parent cell.

Worked example — what does mitosis produce? You follow one human body cell through one full cycle.

Start. One body cell with 46 chromosomes.
After the S phase. 46 chromosomes, each made of 2 sister chromatids, so 92 chromatids.
After mitosis and cytokinesis. Two daughter cells.
In each daughter cell. 46 chromosomes, identical to the parent's set.

In words: one cell in, two identical cells out, and the chromosome number does not change.

There is a second kind of nuclear division, and you meet it next week. Meiosis makes the egg and sperm cells, and it is the contrast to hold now: mitosis makes two identical cells, and meiosis makes four unique ones.

Misconception check. Students often say mitosis makes four cells, or makes gametes, or halves the chromosome number. Every one of those is true of meiosis, not mitosis, and the two words sound almost the same. The cure is a single test: mitosis keeps things the same. Two cells, identical to the parent, with the same chromosome number. If a description involves four cells or a halved number, it is describing meiosis.

Memory hook: mitosis makes two of the same.

Pause and reflect. Choose one of these two prompts and think it through, or jot a few sentences in your notes.

(a) Think of a time your body repaired itself, such as a cut, a scrape, or a sunburn that peeled. Describe what mitosis was doing during the repair, using the words growth, repair, or replacement.
(b) Think of a plant you have seen grown from a cutting, a runner, or a bulb. Explain why the new plant is identical to the old one, using the words mitosis and asexual reproduction.

6. Meet the brakes: checkpoints and cancer

Picture a car on a steep hill. It needs an accelerator to move and brakes to stop. Take away the brakes, or jam the accelerator down, and the car runs out of control. Cell division works the same way.

A checkpoint is a point in the cell cycle where the cell checks that conditions are right, such as whether its DNA is damaged, before it moves on to the next stage. Checkpoints are the cycle's quality control. A cell with a problem is held back until the problem is fixed.

Two kinds of genes act like the pedals.

A proto-oncogene is a normal gene that pushes the cell cycle forward; when mutated it can become an oncogene.
An oncogene is a mutated proto-oncogene that pushes the cell cycle forward too hard, like an accelerator stuck down.
A tumor-suppressor gene is a normal gene whose protein acts as a brake on the cell cycle; when a mutation stops it from working, the brake fails.

The best-known brake has a short name. p53 is the best-known tumor-suppressor protein; it detects damaged DNA and either pauses the cell cycle for repair or triggers apoptosis. Apoptosis is programmed cell death, in which a damaged or unneeded cell dismantles itself in an orderly way. When p53 is lost, a damaged cell can keep dividing. p53 is mutated in roughly half of human cancers, a figure you should know in words rather than as an exact number.

Put those pieces together and you have the disease. Cancer is a disease of cell division that has escaped control, in which mutations have piled up until cells divide without the normal brakes. Usually it takes several mutations, not one, for a cell to lose control.

Worked example — the chemotherapy connection. Many cancer drugs work by hitting cells that are dividing. You reason through what that means.

What the drug targets. Cells in the act of dividing.
Which cells divide fast in a tumor. The tumor cells, which have lost their brakes.
Which healthy cells also divide fast. The cells that build a hair, in the hair follicles, among others.
The result. The drug hits the tumor and the hair follicles alike, which is why hair loss can follow treatment.

In words: a drug aimed at division cannot tell a tumor cell from a healthy cell that is also dividing.

Misconception check. Students often assume more cell division is always good, since division builds and repairs the body. It is tempting because the first half of this chapter was all about the good that division does. But division is useful only when it is controlled. Uncontrolled division is cancer. The brakes, the checkpoints, the tumor-suppressor genes, and p53, matter as much as the accelerator.

Honesty note: this section is an overview. The proteins that run each checkpoint, and the full chemistry of how p53 reads damage, belong to later courses; here you need the idea of an accelerator, a brake, and what happens when either fails.

Memory hook: an accelerator stuck down, or brakes that fail.

7. Count the dividing cells: the mitotic index

Picture a crowded school hallway between classes. If you counted 100 students and found 20 running, you could say one student in five was in a hurry. Biologists count dividing cells the same way, to put a number on how fast a tissue divides.

The mitotic index is the percent of counted cells that are in mitosis: (cells in mitosis ÷ total cells counted) × 100.

The cells come from the onion root tip example, the dataset your lab uses this week. The counts below are an illustrative dataset constructed for this course, not real data. Your own count will give its own numbers.

Cells counted in each stage out of 100 onion root tip cells (illustrative, constructed for this course, not real data)
Stage	Cells counted
Interphase	80
Prophase	9
Metaphase	4
Anaphase	3
Telophase	4

Worked example — the onion root tip mitotic index. You compute and read the index with every step shown.

Question. How fast is the tissue at the tip of an onion root dividing?
Check the total. 80 + 9 + 4 + 3 + 4 = 100 cells, so you counted every cell once.
Cells in mitosis. Add the four mitosis phases: 9 + 4 + 3 + 4 = 20 cells. Interphase cells are not dividing, so you leave them out of this sum.
Mitotic index. (20 ÷ 100) × 100 = 0.20 × 100 = 20%.
Read the index. 100 ÷ 20 = 5, so about one cell in five is dividing, a high rate that fits a growing root tip.
Interphase share. (80 ÷ 100) × 100 = 80%. This is the share of cells not dividing, so 80% is never the mitotic index.
Biggest phase. Prophase holds 9 of the 20 dividing cells, the most of any mitosis phase.

In words: one cell in five is dividing at this root tip. A root tip is a growing tissue, so a high index is what you would expect, and a gardener can see the result as a root that keeps pushing into the soil.

From shares to hours

The count also tells you about time. If a cell spends most of its life in interphase, then at any moment most of the cells you catch will be in interphase. So the share of cells in a stage estimates the share of time a cell spends in that stage.

Worked example — the 24-hour read. You turn the shares into hours on a 24-hour cycle.

Interphase. Interphase ≈ 0.80 × 24 = 19.2 hours.
Mitosis. Mitosis ≈ 0.20 × 24 = 4.8 hours.
Check the hours. 19.2 + 4.8 = 24 hours, the whole cycle.

In words: on this model, a root-tip cell spends about 19.2 hours growing and copying versus about 4.8 hours dividing (19.2 ÷ 4.8 = 4, so four times as long). That is the loop from Section 1 put into numbers.

In practice (your tools). A free spreadsheet does this arithmetic for you once the counts are in. Type the five stage names in column A and the five counts in cells B2 to B6. In cell B7, total them with =SUM(B2:B6). In cell B8, add the four mitosis phases with =SUM(B3:B6). In cell B9, compute the index with =B8/B7*100. Then check the spreadsheet against your own hand arithmetic: B7 should equal the number of cells you counted.

Caveat: one count of 100 cells from one root is a small sample. Your own count will land near, not on, these numbers, and the spread of the class data shows how much one count can vary. The gradable facts are the formula, the arithmetic, and the read, never one cell count.

Misconception check — two slips. Students often divide by the wrong total. Dividing the 20 dividing cells by the 80 interphase cells gives 20 ÷ 80 = 0.25, or 25%, which is wrong; the denominator is always the total cells counted, 100. Students also report the interphase share as the index. The 80% is the share of cells not dividing. The mitotic index counts the dividing cells, so here it is 20%.

Memory hook: dividing over everyone, times 100.

Key terms

Cell cycle — The repeating loop of a cell's life: interphase (G1, S, and G2), when the cell grows and copies its DNA, followed by the M phase, when it divides into two cells. (first used: 1. Read the cell cycle as a loop)
Interphase — The long, non-dividing part of the cell cycle, made of G1, S, and G2; most of a cell's life is spent here. (first used: 1. Read the cell cycle as a loop)
M phase — The dividing part of the cell cycle: mitosis, the division of the nucleus, plus cytokinesis, the division of the cytoplasm. (first used: 1. Read the cell cycle as a loop)
G1 phase — The first part of interphase, in which the cell grows and carries out its normal work. (first used: 1. Read the cell cycle as a loop)
S phase — The part of interphase in which the cell copies its DNA, so that afterward each chromosome is two identical sister chromatids. (first used: 1. Read the cell cycle as a loop)
G2 phase — The last part of interphase, in which the cell makes its final preparations for division. (first used: 1. Read the cell cycle as a loop)
Chromosome — One long DNA molecule with its packing proteins; after the S phase it is made of two sister chromatids, and it still counts as one chromosome. (first used: 2. Picture a chromosome: one X, two sister chromatids)
Sister chromatids — The two identical copies of a chromosome made in the S phase, joined at the centromere until they separate in anaphase. (first used: 2. Picture a chromosome: one X, two sister chromatids)
Centromere — The pinched region where two sister chromatids are held together, giving a duplicated chromosome its X shape. (first used: 2. Picture a chromosome: one X, two sister chromatids)
Mitosis — The division of a cell's nucleus into two nuclei with identical sets of chromosomes, run in the order prophase, metaphase, anaphase, telophase; it is one part of the M phase, not the whole cell cycle. (first used: 3. Run mitosis in order: prophase to telophase)
Nuclear envelope — The double membrane around the nucleus; it breaks down in prophase and two new envelopes form in telophase. (first used: 3. Run mitosis in order: prophase to telophase)
Spindle — The set of protein fibers that forms in prophase and pulls the sister chromatids to opposite poles of the cell in anaphase. (first used: 3. Run mitosis in order: prophase to telophase)
Prophase — The first phase of mitosis: the chromosomes condense, the nuclear envelope breaks down, and the spindle forms; this course counts prometaphase as its end. (first used: 3. Run mitosis in order: prophase to telophase)
Metaphase — The second phase of mitosis, in which the chromosomes line up single file across the middle of the cell. (first used: 3. Run mitosis in order: prophase to telophase)
Metaphase plate — The imaginary plane across the middle of a dividing cell where the chromosomes line up in metaphase. (first used: 3. Run mitosis in order: prophase to telophase)
Anaphase — The third phase of mitosis, in which the sister chromatids separate and move apart to opposite poles of the cell. (first used: 3. Run mitosis in order: prophase to telophase)
Telophase — The fourth phase of mitosis, in which two new nuclear envelopes form, one around each set of chromosomes, and the chromosomes loosen again. (first used: 3. Run mitosis in order: prophase to telophase)
Cytokinesis — The division of the cytoplasm that follows mitosis and ends the M phase, splitting one cell into two daughter cells. (first used: 4. Finish the split: cytokinesis)
Cleavage furrow — The groove that pinches an animal cell in two during cytokinesis, like a drawstring pulled tight. (first used: 4. Finish the split: cytokinesis)
Cell plate — The new wall a plant cell builds down its middle during cytokinesis, separating the two daughter cells. (first used: 4. Finish the split: cytokinesis)
Asexual reproduction — Making a new organism from one parent by cell division alone, so the offspring is genetically identical to the parent, as when a plant cutting grows into a new plant. (first used: 5. Ask what mitosis is for: two identical cells)
Daughter cell — Each of the two cells produced when a cell divides; after mitosis the two daughter cells are identical to each other and to the parent cell. (first used: 5. Ask what mitosis is for: two identical cells)
Checkpoint — A point in the cell cycle where the cell checks that conditions are right, such as whether its DNA is damaged, before it moves on to the next stage. (first used: 6. Meet the brakes: checkpoints and cancer)
Proto-oncogene — A normal gene that pushes the cell cycle forward; when mutated it can become an oncogene. (first used: 6. Meet the brakes: checkpoints and cancer)
Oncogene — A mutated proto-oncogene that pushes the cell cycle forward too hard, like an accelerator stuck down. (first used: 6. Meet the brakes: checkpoints and cancer)
Tumor-suppressor gene — A normal gene whose protein acts as a brake on the cell cycle; when a mutation stops it from working, the brake fails. (first used: 6. Meet the brakes: checkpoints and cancer)
p53 — The best-known tumor-suppressor protein; it detects damaged DNA and either pauses the cell cycle for repair or triggers apoptosis. (first used: 6. Meet the brakes: checkpoints and cancer)
Apoptosis — Programmed cell death, in which a damaged or unneeded cell dismantles itself in an orderly way. (first used: 6. Meet the brakes: checkpoints and cancer)
Cancer — A disease of cell division that has escaped control, in which mutations have piled up until cells divide without the normal brakes. (first used: 6. Meet the brakes: checkpoints and cancer)
Mitotic index — The percent of counted cells that are in mitosis: (cells in mitosis ÷ total cells counted) × 100. (first used: 7. Count the dividing cells: the mitotic index)

Summary

The cell cycle is a loop: G1 → S → G2 → M, and most of it is interphase.
DNA is copied in the S phase, before mitosis, never during it.
One X-shaped chromosome is one chromosome made of two sister chromatids.
Mitosis runs prophase, metaphase, anaphase, telophase, in that order.
Cytokinesis follows: an animal cell pinches at a cleavage furrow; a plant cell builds a cell plate.
Mitosis serves growth, repair, replacement, and asexual reproduction.
Mitosis makes two identical diploid cells; meiosis, next week, makes four unique ones.
Checkpoints are quality control; cancer is division without the brakes.
Proto-oncogenes push the cycle forward; tumor-suppressor genes such as p53 brake it.
Mitotic index = cells in mitosis over all cells counted, times 100.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

In an illustrative count from a garlic root tip, you find 170 cells in interphase, 14 in prophase, 6 in metaphase, 4 in anaphase, and 6 in telophase. Find the total, the number of cells in mitosis, and the mitotic index, and turn the mitosis share into hours on a 24-hour cycle.
Four cells under a microscope show these clues: (a) two new nuclei forming at opposite ends; (b) thick, condensed chromosomes and no nuclear envelope; (c) chromosomes in one row across the middle; (d) two groups of chromosomes moving apart. Name each phase, then put the four cells in order.
An illustrative plant species has 12 chromosomes in each body cell. How many chromosomes and how many chromatids does one of its cells have after the S phase, and how many chromosomes will each daughter cell have after mitosis?
A classmate in a study group says, "A drug that stops cells from dividing would only harm the tumor, because healthy cells are fine." Explain what is wrong with that claim.

Answers (worked reasoning — read after attempting)

Total: 170 + 14 + 6 + 4 + 6 = 200 cells. Cells in mitosis: 14 + 6 + 4 + 6 = 30. Mitotic index: (30 ÷ 200) × 100 = 0.15 × 100 = 15%. Mitosis on a 24-hour cycle: 0.15 × 24 = 3.6 hours. The denominator is the full 200, not the 170 interphase cells; dividing 30 by 170 would be the wrong-total slip.
(a) Telophase, since nuclei are re-forming. (b) Prophase, since the chromosomes are condensed and the envelope is gone. (c) Metaphase, since the chromosomes are lined up in the middle. (d) Anaphase, since the sets are moving apart. In order: (b) prophase, (c) metaphase, (d) anaphase, (a) telophase.
After the S phase the cell still has 12 chromosomes, because each X counts as one. Each is made of 2 sister chromatids, so 12 × 2 = 24 chromatids. In anaphase the sisters separate, so each daughter cell gets 24 ÷ 2 = 12 chromosomes, the same number as the parent cell.
A drug that hits dividing cells cannot tell a tumor cell from a healthy cell that is also dividing. Healthy tissues that divide quickly, such as hair follicles, are hit too, which is why hair loss can follow some cancer treatments. The tumor is targeted because it divides without brakes, but it is not the only tissue dividing.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz; next week, meiosis shows how a different division makes four unique cells.

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