CDS Biology · Cell Biology
The Nucleus and Cell Division
What the nucleus holds and which cells have none, where RNA is found in a cell, how mitosis differs from meiosis, and how cancer starts and spreads.
Why this matters
Six CDS questions from 2016 (II) to 2026 (II). Two ask which cells have no nucleus (red blood cells, and platelets too in 2016 (II)), and the 2026 (II) paper added two on mitosis and cancer.
Concept 1 of 2: The nucleus, and the cells that have none
Definition
The nucleus:
- Bounded by a double membrane (the nuclear envelope) with pores. In a plant cell the chloroplasts also have their own double membrane, separate from the nucleus.
- Holds chromatin, which condenses into chromosomes (DNA with proteins) when the cell divides.
- The nucleolus, a dense body inside, makes ribosomal RNA for the ribosomes.
- RNA is made in the nucleus and works in the cytoplasm, so it is found in the nucleus, cytoplasm and ribosomes, and also in mitochondria and chloroplasts, which make their own.
Cells without a nucleus: mature red blood cells of mammals, and platelets, which are only fragments of a large cell. Muscle cells, white blood cells and plant root hair cells all have nuclei.
| Cell or part | Nucleus |
|---|---|
| Mature red blood cell (mammal) | None: it is lost as the cell maturesQ CDS 2016 (II): red blood cells and platelets are the two without a nucleus; root hair cells and monocytes have one. |
| Platelet | None: it is a cell fragment |
| White blood cell, muscle cell | Present |
| Plant cell | Present, in its own double membrane, as each chloroplast isQ CDS 2022 (II): in plant cells the nucleus and the chloroplasts are each surrounded by their own membrane. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 1 · Cell Biology · The Nucleus and Cell Division
Platelets have no nucleus either
RNA is not only in the nucleus
Concept 2 of 2: Mitosis, meiosis and cancer
Definition
Two kinds of division:
- Mitosis (equational division): two daughter cells with the same number of chromosomes as the parent. Used for growth, repair and replacing cells, and for asexual reproduction.
- Meiosis (reduction division): four daughter cells with half the chromosomes (haploid). Happens only in the cells that form gametes.
Cancer:
- Cells divide without control and form a tumour.
- Caused when normal genes become oncogenes, by radiation, chemicals such as those in tobacco smoke, or some viruses.
- Metastasis: cancer cells leave the tumour and spread through the blood and lymph to new sites.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells | Two | Four |
| Chromosome number | Same as the parent | Half of the parent (haploid)Q CDS 2026 (II): 'the cells become haploid' is the one statement that is not true of mitosis. |
| Where it happens | Body cells everywhere | Cells that form eggs and sperm |
| Used for | Growth, repair, asexual reproduction | Making gametes |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 2 · Cell Biology · The Nucleus and Cell Division
Mitosis never halves the number
Cancer spreads by blood and lymph
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
The nucleus, and the cells that have none4 rows
| Cell or part | Nucleus |
|---|---|
| Mature red blood cell (mammal) | None: it is lost as the cell maturesQ CDS 2016 (II): red blood cells and platelets are the two without a nucleus; root hair cells and monocytes have one. |
| Platelet | None: it is a cell fragment |
| White blood cell, muscle cell | Present |
| Plant cell | Present, in its own double membrane, as each chloroplast isQ CDS 2022 (II): in plant cells the nucleus and the chloroplasts are each surrounded by their own membrane. |
Mitosis, meiosis and cancer4 rows
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells | Two | Four |
| Chromosome number | Same as the parent | Half of the parent (haploid)Q CDS 2026 (II): 'the cells become haploid' is the one statement that is not true of mitosis. |
| Where it happens | Body cells everywhere | Cells that form eggs and sperm |
| Used for | Growth, repair, asexual reproduction | Making gametes |
Watch out for (4)
- Platelets have no nucleus either→ The nucleus, and the cells that have none
- RNA is not only in the nucleus→ The nucleus, and the cells that have none
- Mitosis never halves the number→ Mitosis, meiosis and cancer
- Cancer spreads by blood and lymph→ Mitosis, meiosis and cancer
Test yourself on Cell Biology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.