Mitosis and the cell cycle

Graze your knee, and new skin closes over it within a week or so. Where did all those new skin cells come from? From skin cells nearby, each splitting into two. But a split shares everything out, so each new cell should get half the instructions in the nucleus. Split again and it is quarters, then eighths. Yet not one of your skin cells is running on half a set. That graze comes back once the cell's three stages are laid out.

A cell keeps its instructions in the nucleus, on chromosomes. A chromosome is a long strand of DNA carrying genes, the instructions that run the cell. In a body cell, the chromosomes come in pairs. A human body cell has forty-six chromosomes, in twenty-three pairs. A body cell makes two new cells by going through the cell cycle. It is called a cycle because each new cell can go round again. The cycle starts from one ordinary body cell, like a skin cell at the edge of that graze. No egg, no sperm and no second parent come into it anywhere. Suppose that skin cell simply split down the middle, with its forty-six chromosomes as they are. If it split right now, how many chromosomes would each new cell get? Twenty-three, half a set each. Split those cells again and twenty-three cannot be shared out evenly at all.

The cell cycle has three stages, and stage one comes before anything splits. First, the cell grows. It makes more of its sub-cellular structures, the parts inside it, such as ribosomes and mitochondria. That gives enough of everything for two whole cells, not two half-empty ones. Then the DNA replicates, which means it is copied. Every chromosome is copied, so the cell now holds two copies of each one. Count them: forty-six chromosomes, each one doubled. Nothing has been pulled apart yet, but the cell holds two full sets. That solves the halving problem. Because the copying comes first, the split has two full sets to share out, and each new cell gets one whole set. A student describes stage two like this: the chromosomes are copied, and then the nucleus divides. What has the student got wrong about stage two? The copying is in the wrong stage. It belongs in stage one, so that by stage two there are already two sets to share out. One examiner's report on a Higher paper records the same slip. Two copies of each chromosome forming "was often included in stage two, rather than stage one." It adds: "This error, or if stage numbers were omitted, limited the response to a maximum of four marks." The fix is a habit: put the copying under stage one, and number every stage you describe.

Stage two is mitosis. One copy of every chromosome is pulled to each end of the cell. Then the nucleus divides, so each end now has a complete set of chromosomes inside its own nucleus. Stage three is the split. The cytoplasm and the cell membrane divide, and one cell becomes two. Each new cell got one copy of every chromosome, and the copies were exact. So the two cells are genetically identical. They carry the same genes as each other, and as the parent cell. A fruit fly body cell has eight chromosomes, in four pairs. At the end of stage one, how many copies of chromosomes are in the cell? Sixteen. Every one has been copied, so there are two copies of each. Now, once the whole cycle is complete, how many chromosomes does each new cell have? Eight, the same number as the parent, still in four pairs. A cell with its chromosomes in pairs like this is called diploid. A student says each new fruit fly cell gets four. What has that student done wrong with the number? Halved it, as if the cell split without copying first. Mitosis never makes a cell with half a set. Halving happens in a different division, meiosis, much later in the course. The cycle ends there, with two genetically identical cells. Becoming specialised for a job happens to them afterwards, and that is the video called Cell differentiation and how organisms grow.

A property of the new cells is what they are like. A use of mitosis is what the body does with it, such as growth, repair or replacing worn-out cells. One use needs care. Mitosis makes genetically identical cells. Asexual reproduction, with just one parent, makes genetically identical offspring. It works because one parent's cells divide by mitosis, so the offspring are genetically identical. Reproduction itself has its own video later in the course. Six answers have come in to a question asking for two properties of the cells a healing bone makes by mitosis. Which answers are properties, which are uses, and which one is wrong? Properties: genetically identical, diploid, and two of them. Uses: repair and growth, true but not what was asked. Haploid is wrong. Now the reason: why is haploid wrong for a cell made by mitosis? Haploid means just one chromosome from each pair, half the usual number. Mitosis keeps the full number, so every new cell is diploid. One examiner's report records that "Most candidates scored highly on this question recognising that cells produced by mitosis had the properties of being genetically identical or diploid." The same report names two ways marks went. "No credit was given to the uses of these cells in growth, repair or asexual reproduction." And "Candidates lost marks where they gave contradictory answers to one of the marking points such as haploid cells or genetically different cells." The fix: check whether the question wants a property or a use, and never add a word that contradicts a right answer.

Inside stage two, mitosis runs through its events in a fixed order, and some courses give each one a name. These five drawings of one dividing cell are out of order. In what order do these five drawings of a dividing cell belong? In order: first, the chromosomes become visible and the membrane round the nucleus breaks down. Then they line up across the middle. Then the copies are pulled to opposite ends. Then a new nucleus forms at each end, and last, the cell membrane pinches the cell in two. If your course names the phases, those five are prophase, metaphase, anaphase, telophase and cytokinesis. Fibres called spindle fibres do the pulling. The growing and copying before all five is called interphase.

Here is a cell-cycle diagram for a skin cell at that graze. It shows three stages, and stage two is already labelled mitosis. Can you describe what happens to the cell in each stage before I show you? Stage one: the cell grows, makes more sub-cellular structures such as ribosomes and mitochondria, and its DNA replicates, so there are two copies of each chromosome. Stage two: one set of chromosomes is pulled to each end of the cell, and the nucleus divides. Writing that mitosis happens earns nothing here, because the diagram already says it. Stage three: the cytoplasm and the cell membrane divide, forming two genetically identical cells. One examiner's report on a Foundation paper says: "Many students said mitosis happens, but as this was written on the cell cycle diagram it did not gain credit." It goes on: "For Stage 3 many students only referred to two daughter cells forming, when reference to two identical cells was required." The fix: describe the events instead of naming the stage, and finish on two genetically identical cells, never just two cells.

Three questions to close, and the first uses an animal this video never mentioned. A dog body cell has seventy-eight chromosomes. After mitosis, does each new cell have more, fewer or the same? The same. The copying comes before the split, so the number never halves. Here's another: in which stage are the chromosomes copied, and why that one? Stage one, before mitosis begins, so that there are two full sets ready to share out. Now a different one: is repairing tissue a property of the new cells, or a use? A use. A property is what the cells are like: genetically identical, diploid, and two of them. And that graze on your knee: every new skin cell over it carries a full set of forty-six, because the chromosomes were copied before every split.

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