Cell division, mitosis and stem cells

All multicellular organisms grow, repair tissue and reproduce asexually using mitosis — a type of nuclear division that produces two genetically identical daughter cells.

The cell cycle

The cell cycle has three main stages:

1. Interphase (longest) — the cell grows, doubles its sub-cellular structures (mitochondria, ribosomes) and replicates its DNA so each chromosome becomes two identical sister chromatids joined at the centromere.
2. Mitosis — the chromosomes line up at the equator, sister chromatids are pulled to opposite poles by spindle fibres, and a new nucleus forms at each pole.
3. Cytokinesis — the cytoplasm and cell membrane split, giving two genetically identical daughter cells.

Why mitosis matters

• Growth — multicellular organisms increase in cell number from a single zygote.
• Repair — replacing damaged or worn-out cells (skin, gut lining).
• Asexual reproduction — many plants, fungi and some animals produce identical offspring this way.

Stem cells

Stem cells are undifferentiated cells that can divide repeatedly and become specialised.

Type	Source	Potency
Embryonic	Early embryo	Can become any cell type (pluripotent)
Adult	Bone marrow, etc.	Limited range (e.g. blood cells only)
Plant meristem	Root and shoot tips	Can become any plant cell

Therapeutic uses

• Treating leukaemia by transplanting bone marrow stem cells.
• Research into spinal cord repair, type 1 diabetes (insulin-producing cells) and macular degeneration.
• Therapeutic cloning — making stem cells genetically matched to a patient to avoid rejection.

Ethical issues

Embryonic stem cells require destroying an embryo, which raises ethical objections; adult stem cells avoid this but are less versatile.

WJEC exam tip

A common 6-mark question asks you to describe the cell cycle. Always mention DNA replication BEFORE mitosis — students lose marks by jumping straight into "chromosomes line up". Use the order: grow, copy DNA, mitosis, cytokinesis.