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Every one of us started out as a single fertilised cell, yet the body you carry around today is built from tens of trillions of cells. All of them trace back to that first cell through repeated cell division. Dividing does not stop once you are born, either: your cells keep on dividing so that you can grow, so that damaged tissue can heal, and so that worn-out cells can be replaced. This lesson opens Topic B2 (Scaling up) of your OCR Gateway Combined Science course by looking at how a cell makes an exact copy of itself through the cell cycle, of which mitosis is just one part. Nail this down now and everything else in B2 — differentiation, stem cells, and the whole challenge of supplying a body made of so many cells — will make much more sense.
By the end of this lesson you should be able to describe the stages of the cell cycle (interphase and mitosis), explain that mitosis produces two genetically identical diploid daughter cells, state why cell division by mitosis matters, and carry out a simple calculation about the cell cycle.
This lesson builds AO1 (understanding of the cell cycle and what mitosis produces) and AO2 (applying that knowledge in the cell-cycle calculation).
A cell cannot just keep swelling to a larger and larger size forever. As you will discover later in this topic, a bigger cell ends up with a smaller surface area compared with its volume, which makes it harder and harder to supply with the substances it needs. Instead, living things grow and keep themselves in good repair by producing more cells, each a faithful copy of the one before it. Cell division by mitosis matters for three reasons:
The single most important rule to hold onto is this: mitosis produces two cells that are genetically identical to each other and to the parent cell they came from. Each new cell carries an exact copy of the same DNA. Keep that idea at the centre and the detail of the cell cycle slots neatly into place.
Exam Tip: Examiners often ask why an organism needs mitosis. The answers that score are growth, repair/replacement of cells, and asexual reproduction — and the products are always genetically identical. Do not muddle this with the type of cell division that makes sex cells (gametes), which produces cells that are different and is not required in B2.
The instructions for building and running a cell are stored in its DNA, which is packaged into structures called chromosomes. Human body cells contain 46 chromosomes, arranged as 23 pairs. A cell that keeps its chromosomes in matching pairs like this is described as diploid. Because mitosis has to give each new cell a complete set of 46 chromosomes, the cell must first copy all of its DNA before it divides — and that copying is the first big job of the cell cycle.
Once a chromosome has been copied, it exists for a while as two identical halves joined together. Each half is a chromatid, and because the two copies are exact duplicates they are called sister chromatids. During mitosis these sister chromatids are pulled apart so that each new cell receives one copy. This is the mechanism that guarantees the two daughter cells end up genetically identical.
Exam Tip: "Diploid" simply means the chromosomes are in pairs (in humans, 23 pairs = 46 chromosomes). Mitosis keeps the chromosome number the same: a diploid parent cell gives two diploid daughter cells, each with 46 chromosomes. A common misconception is that the number halves during mitosis — it does not; only the division that makes gametes halves the number.
Cell division is not a single sudden event but one part of a repeating sequence called the cell cycle. Most of the cycle is taken up not by the division itself but by the cell growing and getting ready. The cycle has two main parts: a long stage called interphase, followed by a shorter period of division (mitosis, then cytokinesis).
flowchart TD
A["Interphase<br/>cell grows; organelles duplicate;<br/>DNA replicates"] --> B["Mitosis<br/>chromosomes line up; sister chromatids<br/>pulled to opposite poles; nucleus divides"]
B --> C["Cytokinesis<br/>cytoplasm and cell membrane divide"]
C --> D["Two genetically identical<br/>diploid daughter cells"]
D --> A
Interphase is the longest stage of the cell cycle and is the period of preparation that happens before the cell divides. Three things go on:
Without interphase a cell could not divide successfully: it needs to be big enough, to have enough organelles, and — above all — to have a complete second copy of its DNA so that each daughter cell ends up with a full set.
Mitosis is the stage in which the nucleus divides. Put simply:
Because the two chromatids of each chromosome were identical copies, the two new nuclei hold exactly the same genetic information.
Finally, in cytokinesis, the cytoplasm and cell membrane divide, splitting the one cell into two separate cells. (In a plant cell, a new cell wall also forms between the two daughter cells.) The result is two genetically identical diploid daughter cells, each with a full set of 46 chromosomes and its own share of organelles.
Look carefully at the diagram: each daughter cell ends up with one red and one blue chromosome — exactly the same combination the parent had. That is what "genetically identical" looks like in practice.
OCR expects you to handle simple data about the cell cycle, such as working out what percentage of the cycle a stage takes up, or how many cells you get after several divisions. These are dependable sources of marks in the combined-science papers.
A cell cycle lasts 24 hours in total. The cell spends 1 hour in mitosis and the rest in interphase and cytokinesis. What percentage of the cycle is spent in mitosis?
Step 1 — write the percentage as a fraction of the whole cycle:
percentage=total cycle timetime in mitosis×100
Step 2 — substitute the values (both are already in hours, so there is no conversion to do):
percentage=24 h1 h×100=4.17%
Answer: about 4.2% of the cell cycle is spent in mitosis. This fits the general rule that a cell spends most of its cycle in interphase — growing and copying its DNA — and only a small fraction actually dividing.
Common error: dividing the wrong way round, or forgetting to multiply by 100. Always put the part over the whole, then multiply by 100 to turn the fraction into a percentage.
A single cell divides by mitosis. Each daughter cell then divides again, and so on. How many cells are there after 4 rounds of division?
Each division doubles the number of cells, so after every round the number is multiplied by 2:
1→2→4→8→16
You can also work this out directly as 24=16.
Answer: 16 cells after 4 divisions.
Common error: multiplying 1×4=4 instead of doubling each time. Because each cell splits into two, the numbers grow by doubling (×2 per round), not by adding.
Exam Tip: If a question gives you a time for the whole cycle and a time for one stage, it usually wants a percentage: whole cyclestage time×100. If it gives a starting number of cells and a number of divisions, the number of cells doubles each division.
It pays to be clear about why mitosis produces genetically identical cells, because examiners reward the explanation, not just the statement. The secret is the order of events. First, during interphase, the whole of the DNA is copied, so each chromosome becomes two identical sister chromatids. Then, during mitosis, those identical chromatids are separated, one going to each new cell. Because each daughter cell receives one copy of every chromosome — and those copies were made by the exact base-pairing rule of DNA replication — the two cells end up with precisely the same genes as each other and as the parent. This is completely different from the division that makes sperm and egg cells, which deliberately produces cells that are different and have half the number of chromosomes. For B2 you only need mitosis, where the watchwords are identical and diploid (same number).
Mitosis is going on in your body all the time, but it is busiest in the places where new cells are constantly needed. Growing tissues in a developing organism divide rapidly to add cells. The lining of the gut and the skin are renewed continually, because their cells are worn away or rubbed off and have to be replaced. Bone marrow turns out a steady stream of new blood cells. In plants, mitosis is concentrated at the growing tips of roots and shoots. In all these places the cell cycle turns over again and again, each round producing two identical cells.
Normally the cell cycle is carefully controlled, so that cells divide only when new cells are needed. Occasionally that control breaks down and cells begin to divide uncontrollably. A lump made of such cells is called a tumour, and this uncontrolled division is the basis of cancer. You do not need the detail of cancer for B2, but it is a useful reminder that mitosis is normally a tightly regulated process — cells do not simply divide whenever they like. It is also why one of the risks of using stem cells (met in a later lesson) is that, because they divide rapidly, they could divide uncontrollably and form a tumour.
Exam Tip: If a question asks where rapid cell division by mitosis takes place, good answers include growing tissues, the skin, the lining of the gut, bone marrow (blood cells) and, in plants, the tips of roots and shoots. Linking "uncontrolled mitosis" to a tumour is a useful extra point.
Question (6 marks): Describe what happens during the cell cycle, including the stages of interphase and mitosis, and state the type of cells produced.
Mid-band response: "In the cell cycle the cell grows and then divides. The DNA is copied. In mitosis the chromosomes are pulled apart and the cell splits into two. This makes two new cells that are the same."
Examiner-style commentary: This earns credit for cell growth, DNA being copied, chromosomes being pulled apart, and identical daughter cells. It is held back by vagueness — it does not name interphase, does not mention organelles duplicating, and does not say the cells are diploid. To climb a band, name the stages and add detail about sister chromatids.
Stronger response: "During interphase the cell grows, the number of organelles like mitochondria increases, and the DNA replicates so each chromosome becomes two copies. In mitosis the chromosomes line up in the middle and the copies are pulled to opposite ends of the cell, and the nucleus divides. Then the cytoplasm divides. This produces two genetically identical cells."
Examiner-style commentary: A clear, well-organised answer that names interphase and mitosis, includes organelle duplication and DNA replication, and describes the chromosomes separating. To reach the top band, use the term sister chromatids, name cytokinesis, and state that the daughter cells are diploid.
Top-band response: "The cell cycle begins with a long interphase, in which the cell grows, increases its number of sub-cellular structures such as mitochondria and ribosomes, and replicates its DNA so that each chromosome is copied to form two identical sister chromatids. In mitosis, the copied chromosomes line up across the centre of the cell and the sister chromatids are pulled to opposite poles, after which the nucleus divides into two. Finally, in cytokinesis, the cytoplasm and cell membrane divide, producing two genetically identical diploid daughter cells, each with a complete set of chromosomes (46 in a human cell)."
Examiner-style commentary: Full marks. The answer names every stage in order, correctly separates interphase preparation from nuclear division, uses precise terms (sister chromatids, cytokinesis, diploid), and finishes by stating the products exactly — identical and diploid. A model of a well-sequenced description.
| Misconception | The correct idea |
|---|---|
| Mitosis halves the number of chromosomes | Mitosis keeps the number the same — two diploid cells, each with 46 chromosomes |
| The daughter cells are slightly different | The daughter cells are genetically identical to each other and to the parent |
| Most of the cell cycle is spent dividing | Most of the cycle is interphase (growth and DNA copying); division is a small fraction |
| DNA is copied during mitosis | DNA is copied during interphase, before mitosis begins |
| "Chromatid" and "chromosome" mean the same thing | A copied chromosome is made of two sister chromatids; they separate during mitosis |
| Mitosis makes sperm and egg cells | Mitosis makes ordinary body cells; gametes are made by a different division (not required in B2) |
This content is aligned with OCR Gateway Combined Science A (J250), Topic B2 Scaling up. Refer to the official OCR specification for exact wording.