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A human being begins life as a single fertilised cell, yet an adult body contains tens of trillions of cells. Every one of those cells came from that first one by a process of cell division. Throughout your life, cells keep dividing — to make you grow, to repair damaged tissue and to replace worn-out cells. This lesson opens Topic B2 (Scaling up) of OCR Gateway Science A by looking at how a cell copies itself exactly through the cell cycle, of which mitosis is one stage. Getting this secure now sets up everything that follows in B2: differentiation, stem cells and the whole problem of supplying a body that is made of so many cells.
By the end of this lesson you should be able to describe the stages of the cell cycle (including interphase and mitosis), explain that mitosis produces two genetically identical diploid daughter cells, state why cell division by mitosis is important, and carry out a simple calculation about the cell cycle.
A cell cannot simply grow larger and larger forever. As you will see later in this topic, a bigger cell has a smaller surface area compared with its volume, so it becomes harder to supply with the substances it needs. Instead, organisms grow and maintain themselves by making more cells, each one a faithful copy of its parent. 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 original parent cell. Every new cell carries an exact copy of the same DNA. Keep that idea central and the detail of the cell cycle falls into place.
Exam Tip: Examiners often ask why an organism needs mitosis. The mark-worthy answers are growth, repair/replacement of cells, and asexual reproduction — and the products are always genetically identical. Do not confuse this with the cell division that makes sex cells (gametes), which produces different cells and is not required here.
The instructions for building a cell are carried by DNA, which is packaged into structures called chromosomes. In humans, body cells contain 46 chromosomes, arranged as 23 pairs. A cell that has its chromosomes in pairs like this is described as diploid. Because mitosis must give each new cell a complete set of 46 chromosomes, the cell has to copy all of its DNA before it divides — that copying is the first big job of the cell cycle.
When a chromosome has been copied, it temporarily exists as two identical halves joined together. Each half is called a chromatid, and the two copies are called sister chromatids because they are exact duplicates. During mitosis these sister chromatids are pulled apart so that each new cell gets one copy. This is the mechanism that guarantees the two daughter cells are 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. The number does not halve in mitosis.
Cell division is not a single event but part of a repeating sequence called the cell cycle. Most of the cycle is taken up not by division itself but by the cell growing and preparing. The cycle has two main parts: a long interphase, then a shorter period of division (mitosis followed by 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 before division. Three things happen:
A cell would not divide successfully without interphase: it needs to be big enough, have enough organelles, and — most importantly — have a complete second copy of its DNA, so that each daughter cell receives a full set.
Mitosis is the stage in which the nucleus divides. In simple terms:
Because the two chromatids of each chromosome were identical copies, the two new nuclei contain 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.
Notice in the diagram that each daughter cell ends up with one red and one blue chromosome — exactly the same combination as the parent. That is what "genetically identical" looks like.
OCR expects you to handle simple data about the cell cycle, including working out what percentage of the cycle a stage takes, or how many cells are produced after several divisions. These are reliable sources of marks.
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 already in hours, so no conversion needed):
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 the 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 each round the number is multiplied by 2:
1→2→4→8→16
You can also calculate this 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 growth is by doubling (×2 each 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 is worth being clear about why mitosis produces genetically identical cells, because examiners reward the explanation, not just the statement. The key is the order of events. First, during interphase, the entire DNA is copied, so each chromosome becomes two identical sister chromatids. Then, during mitosis, these identical chromatids are separated, with one going to each new cell. Because each daughter cell receives one copy of every chromosome — and those copies were made by the precise base-pairing rule of DNA replication — the two cells end up with exactly 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 concentrated in 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 replaced continually, because their cells are worn away or rubbed off and must be renewed. Bone marrow produces 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 this control is lost, and cells begin to divide uncontrollably. A mass of these 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 please. This is also why one of the risks of using stem cells (which you meet 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 marks 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 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 distinguishes interphase preparation from nuclear division, uses precise terms (sister chromatids, cytokinesis, diploid), and finishes by stating the products exactly — identical and diploid. This is the 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 Science A GCSE Biology (J247), Topic B2 Scaling up. Refer to the official OCR specification document for the exact wording.