Lecture: The Cell Cycle & Mitosis
Lecture script · about 15–20 minutes spoken · Week 9
Opening
Think about the last time you scraped your knee. Over the next several days, new skin crept in from the edges and closed the gap. Every one of those new skin cells came from an older cell that split in two. Nothing arrived from outside. Your knee healed by cell division, done many times over.
You did the same thing on a far bigger scale. You started life as one cell, and every cell in your body traces back to it through rounds of division. A nursing student needs this to understand healing and some cancer treatments. A gardener rooting a houseplant cutting is watching it happen. Someone counting cells under a microscope in a lab is measuring it.
Here's where we are. In Week 3 you met DNA, the long molecule that stores a cell's instructions. In Week 4 you toured the cell and met the nucleus, which holds that DNA, and the cell wall around a plant cell. In Week 8 you picked up a habit: for any process, say the stages in order and name where each one happens. In this lecture I'll put that habit to work on a cell splitting in two. We'll walk through the cell cycle, the chromosome, the four phases of mitosis, how the cell finishes the split, what mitosis is for, what keeps it in check, and finally how to put a number on it. Next week, in Week 10, you meet meiosis, the other kind of division.
1. The cell cycle is a loop
Picture a bakery. Most of the shift goes to getting ready: mixing dough, letting it rise, shaping loaves. The part a customer sees, bread coming out of the oven, takes 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.
Let me take the loop one piece at a time. 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. It has three parts, always in the same order. First comes G1, spoken "G one". The G1 phase is the first part of interphase, in which the cell grows and carries out its normal work. Then comes S. 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. Then comes G2. The G2 phase is the last part of interphase, in which the cell makes its final preparations for division.
After interphase, the cell divides. The M phase is the dividing part of the cell cycle: mitosis, the division of the nucleus, plus cytokinesis, the division of the cytoplasm. Each new cell then starts its own loop back at G1. So the order is G1, then S, then G2, then M, and around again.
Pause and try this. A classmate says, "DNA gets copied during mitosis." Where does the copying really happen?
Think back to the three parts of interphase.
Here's the answer. The DNA is copied in the S phase of interphase, before mitosis ever starts. Mitosis divides the copies; it doesn't make them.
Here's a line to carry with you: grow, copy, prepare, divide.
2. One X is one chromosome
Imagine you photocopy a long recipe and staple the copy to the original at one corner. You still have one recipe. It's now in two identical copies, held together at one spot. That's what a chromosome looks like after the S phase.
A 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's a single thread. After copying, it has two identical halves. Sister chromatids are the two identical copies of a chromosome made in the S phase, joined at the centromere until they separate in anaphase. And the place they're held together has its own name. The centromere is the pinched region where two sister chromatids are held together, giving a duplicated chromosome its X shape.
So when you picture that familiar X, here's how to read it in words. The X is one chromosome. Its left pair of arms is one chromatid, and its right pair of arms is the sister chromatid. The pinch in the middle, where they meet, is the centromere.
Let me count it out for a human body cell. A human body cell has 46 chromosomes. After the S phase, it still has 46 chromosomes, because each X counts as one. Each of those is made of 2 sister chromatids, so that's 46 times 2, which is 92 chromatids. Later, when the sisters are pulled apart, each one counts as a chromosome on its own. Split those 92 evenly between two new cells, and each gets 46. That's the same number the parent cell started with.
Pause and try this. Right after the S phase, a human body cell has 92 chromatids. How many chromosomes does it have?
Count the centromeres, not the arms.
Here's the answer. It has 46 chromosomes. Each X-shaped chromosome is made of two sister chromatids, so 46 chromosomes hold 92 chromatids between them.
3. Mitosis, in order
Picture moving day in a busy apartment. First you pack everything into tight boxes. Then you line the boxes up by the door. Then you split them into two equal loads. Then each load is unpacked in a new home. Mitosis makes those 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. 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. That's the packing. The long, loose chromosomes coil up into short, thick rods. Some courses treat prometaphase as a separate phase; in this course, it's 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. That middle line is the metaphase plate: the imaginary plane across the middle of a dividing cell where the chromosomes line up in metaphase. That's the boxes at the door.
Anaphase is the third phase of mitosis, in which the sister chromatids separate and move apart to opposite poles of the cell. That's the split into two loads. And 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. That's the unpacking.
So the order is prophase, metaphase, anaphase, telophase. Pack, line up, pull apart, unpack. A trick for the middle two: metaphase sounds like middle, and anaphase sounds like apart.
Pause and try this. Under a microscope you see a cell whose chromosomes sit in one neat row across its middle. Which phase is it in?
Ask yourself: condensing, lined up, splitting, or re-forming?
Here's the answer. It's in metaphase. Chromosomes lined up single file across the middle is the clue for metaphase, the second phase.
4. Finishing the split
At the end of telophase, the cell has two nuclei. But it's still one cell. Think of 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 is different, because it has a cell wall, the support layer you met in Week 4. 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.
So the whole M phase runs prophase, metaphase, anaphase, telophase, and then cytokinesis. Cytokinesis isn't a phase of mitosis. Mitosis divides the nucleus. Cytokinesis divides the cytoplasm. Together they make the M phase.
Pause and try this. A cell at the tip of an onion root has finished telophase. How will it split its cytoplasm?
Think about what an onion cell has that a skin cell doesn't.
Here's the answer. It builds a cell plate. An onion root cell is a plant cell with a cell wall, so it builds a new wall down the middle instead of pinching in at a cleavage furrow.
5. What mitosis is for
Picture a houseplant 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 was built entirely by mitosis.
Mitosis has four everyday jobs. It builds a growing body. It repairs, like your scraped knee. It replaces worn-out cells, like the ones lining your gut. And it makes new organisms by 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.
Every time, the result is the same. Mitosis makes 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. A human body cell with 46 chromosomes makes two daughter cells with 46 each.
Next week you meet meiosis, and here's the contrast to hold now. Mitosis makes two identical cells. Meiosis makes four unique ones. Those two words sound almost the same, so give yourself a test. Mitosis keeps things the same: two cells, identical to the parent, with the same chromosome number. If someone describes four cells, or a halved chromosome number, they're describing meiosis.
Pause and try this. A study guide says, "Mitosis makes four cells, each with half the chromosome number." What would you correct?
Run the test: does mitosis keep things the same?
Here's the answer. Mitosis makes two cells, not four, and each keeps the full chromosome number. Four cells with half the number describes meiosis, which comes next week.
6. The brakes, and what happens without them
Picture a car on a steep hill. It needs an accelerator to move and brakes to stop. Jam the accelerator down, or lose the brakes, 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.
Two kinds of genes work 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 most famous 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 stops working, a damaged cell can keep on dividing. It's mutated in roughly half of human cancers. Hold that figure in words, as roughly half, not 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. So more division isn't always good.
Here's a connection to medicine. Many cancer drugs work by hitting cells that are dividing. A tumor's cells divide constantly, so they're hit. But the cells that build a hair, in the hair follicles, also divide fast. The drug can't tell them apart, which is why hair loss can follow treatment.
Pause and try this. Which kind of gene works like a brake on division, and which like an accelerator?
Think of the car on the hill.
Here's the answer. A tumor-suppressor gene, such as the one for p53, is a brake. A proto-oncogene pushes the cycle forward, and when it mutates into an oncogene it's like an accelerator stuck down.
7. Putting a number on division
Picture a crowded hallway between classes. If you counted 100 people and 20 were running, you could say one in five was in a hurry. Biologists count dividing cells the same way.
The mitotic index is the percent of counted cells that are in mitosis: cells in mitosis divided by total cells counted, times 100.
Let's work it on the onion root tip example from your lab. These counts are illustrative, made up for this course, not real data. Out of 100 cells, 80 are in interphase, 9 in prophase, 4 in metaphase, 3 in anaphase, and 4 in telophase. First, check the total. 80 plus 9 plus 4 plus 3 plus 4 is 100, so every cell was counted once. Next, add only the cells in mitosis: 9 plus 4 plus 3 plus 4 is 20. Interphase cells aren't dividing, so they stay out of that sum. Now divide by the total. 20 divided by 100 is 0.20, and 0.20 times 100 is 20%. That's the mitotic index.
What does 20% mean? 100 divided by 20 is 5, so about one cell in five is dividing. That's a high rate, and it fits a root tip that's growing.
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 in that stage. On a 24-hour cycle, interphase takes about 0.80 times 24, which is 19.2 hours. Mitosis takes about 0.20 times 24, which is 4.8 hours. And 19.2 plus 4.8 is 24, the whole cycle.
Two slips to avoid. Don't divide by the wrong total: 20 divided by 80, the interphase count, gives 25%, and that's wrong. And don't report 80% as the index. That's the share of cells not dividing.
Pause and try this. A lab partner divides the 20 dividing cells by the 80 interphase cells. What went wrong, and what's the right index?
Ask what belongs underneath the division.
Here's the answer. The bottom of the division must be all the cells counted, which is 100, not the 80 in interphase. 20 divided by 100, times 100, gives the correct index of 20%.
Closing
Let me pull it together, in the order we took it. The cell cycle is a loop: G1, S, G2, then M, and most of it is interphase. DNA is copied in the S phase, before mitosis. One X is one chromosome made of two sister chromatids, joined at the centromere. Mitosis runs prophase, metaphase, anaphase, telophase: pack, line up, pull apart, unpack. Cytokinesis finishes the job, with a cleavage furrow in an animal cell and a cell plate in a plant cell. Mitosis builds, repairs, replaces, and reproduces, and it always makes two identical cells. Checkpoints and tumor suppressors like p53 are the brakes, and cancer is division without them. And the mitotic index puts a number on division: cells in mitosis over all cells counted, times 100.
Next in this module, read the Readings & Resources page, then work through the lecture tutorial, the practice set, and the quiz, and then the discussion and the assignment. Next week, in Week 10, you meet meiosis and sexual reproduction, the division that makes four unique cells.
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