Cell division and stem cells — AQA GCSE Biology revision

The cell cycle

• The cell grows, copies its DNA and doubles its organelles.
• Mitosis: one set of chromosomes goes to each end and the nucleus divides.
• The cytoplasm and membrane split, giving two genetically identical daughter cells.

Stem cells

• Stem cells are undifferentiated — they can become other cell types.
• Embryonic stem cells can become almost any cell; adult stem cells (e.g. bone marrow) are more limited.
• Plant meristems make stem cells for the whole life of the plant — used for cloning rare plants.
• Therapeutic cloning gives cells with the patient's genes, so they are less likely to be rejected.

Key terms

Mitosis

Division producing two genetically identical cells.

Differentiation

A cell becoming specialised for a particular job.

Meristem

Plant tissue where stem cells divide.

Rejection

When the immune system attacks transplanted cells.

Meiosis

Cell division producing four genetically different gametes with half the chromosome number.

Chromosome

A thread of DNA carrying genetic information, found in the nucleus.

Common exam traps

• Saying mitosis produces different cells — that is meiosis.
• Only listing ethical objections and giving no benefit in evaluate questions.
• Thinking mitosis makes genetically different cells — it makes identical clones.

Practice questions with answers

1. 1. Mitosis produces daughter cells that are:

   • • Genetically identical
   • • Genetically different
   • • Always gametes
   • • Always stem cells

   Answer: Genetically identical

   DNA is copied first, so each daughter cell gets the same genes.

2. 2. Why are stem cells useful in medicine?

   Answer: They can differentiate into other cell types to replace damaged cells, e.g. in diabetes or paralysis.

   Give a use as well as the definition.

3. 3. Give one risk of using stem cells in treatment.

   Answer: Possible transfer of viral infection, or the cells may not be accepted by the patient.

   Any sensible scientific risk gains the mark.

4. 4. How many chromosomes does a human gamete have?

   • • 46
   • • 23
   • • 92
   • • 12

   Answer: 23

   Gametes have half the number of chromosomes (23) so fertilisation restores 46.

5. 5. State one reason mitosis occurs in the human body.

   Answer: For growth or to repair damaged tissue (or asexual reproduction in some organisms).

   Mitosis produces identical cells needed to replace or add cells.

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