Revision notes · Cell biology
Cell division
Chromosomes4.1.2.1
Definition: A chromosome is a long, coiled molecule of DNA found in the nucleus, carrying many genes.
Chromosomes carry the genetic instructions cells need to develop and function.
- •Chromosomes usually exist in pairs in body cells.
- •A human body cell has 23 pairs of chromosomes (46 in total).
- •A gamete (sex cell) contains only one chromosome from each pair (23 in total) — half the normal number.
- •Each chromosome carries many genes, each coding for a particular protein.
⚠️ Common mistake: Writing '23 chromosomes' for a human body cell instead of 23 pairs (46 chromosomes) — 23 single chromosomes is the gamete number.
🧠 Remember: Body cells: 23 pairs (46). Gametes: 23 singles (half).
Mitosis and the cell cycle4.1.2.2
Definition: Mitosis is the stage of the cell cycle in which the nucleus divides to produce two genetically identical daughter cells.
The cell cycle is the sequence of growth and division that produces new cells.
- •Before division, the cell grows, increases its number of sub-cellular structures (e.g. ribosomes, mitochondria), and duplicates its DNA.
- •During mitosis, one complete set of chromosomes is pulled to each end of the cell as the nucleus divides.
- •The cytoplasm and cell membrane then divide, producing two genetically identical cells.
- •Mitosis is used for growth, and to repair or replace damaged cells.
⚠️ Common mistake: Thinking DNA is copied during mitosis itself — replication happens during the growth stage of the cycle, before mitosis begins.
🧠 Remember: Mitosis = identical copies, for growth and repair.
Stem cells4.1.2.3
Definition: A stem cell is an undifferentiated cell that can divide to produce more stem cells, or differentiate into a specialised cell type.
Stem cells are found in early embryos, in some adult tissues, and in plant meristems.
| Source | Can become... |
|---|---|
| Embryonic (human embryo) | Any cell type in the body |
| Adult (e.g. bone marrow) | A limited range of related cells |
| Meristem (plant roots/shoots) | Any plant cell type, throughout the plant's life |
- •Stem cells can be used to treat some diseases, e.g. via therapeutic cloning, so replacement cells are not rejected by the patient's immune system.
- •Risks and objections include transferring viral infection during transplant, and ethical or religious objections to using human embryos.
⚠️ Common mistake: Claiming adult stem cells can become 'any' cell type — only embryonic stem cells (and plant meristem cells) have that full potential.
🧠 Remember: Embryonic = unlimited. Adult = limited. Meristem = unlimited, but plants only.
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