Cell Division

Inside the nucleus of every body cell sit your chromosomes — long, coiled threads made of DNA wrapped around proteins (called histones). When the cell is dividing, the chromosomes condense into the familiar 'X' shape often shown in textbooks. At other times they exist as much thinner, looser threads.

Each chromosome carries hundreds to thousands of genes. A gene is a short section of DNA that codes for a particular protein (e.g. for eye colour, for an enzyme, for haemoglobin).

Order of size (smallest to biggest):

1. Nucleotide (one rung of DNA ladder) → bases A, T, C, G
2. Gene (sequence of bases coding for one protein)
3. DNA molecule (full chromosome strand, very long)
4. Chromosome (DNA + protein, coiled)
5. Nucleus (membrane-bound organelle containing all chromosomes)
6. Cell

Genome = the entire DNA of an organism — all the chromosomes combined.

In humans, every body cell (except gametes) contains 23 pairs of chromosomes = 46 total. We call this diploid (2n).

The pairs are homologous — each pair has one chromosome from your mother (in the egg) and one from your father (in the sperm). Both members of a pair carry the same genes in the same order, but may carry different versions (alleles).

Pair 23 = sex chromosomes:

• Females: XX
• Males: XY

The first 22 pairs are called autosomes (you don't need this term for AQA but useful to know).

Gametes (sperm and egg) contain only ONE chromosome from each pair — 23 chromosomes total. We call this haploid (n).

At fertilisation, a sperm (23) fuses with an egg (23) → zygote (46 = 23 pairs). The diploid number is restored.

Why does this matter? It's the basis of inheritance — you get half your chromosomes from each parent, so half your genes. Differences between siblings come from which combinations of the parents' chromosomes ended up in the gametes (more on this in 4.6 Meiosis).

Stage 1 — Growth and DNA replication (the largest stage; ~90% of the cycle):

• The cell grows in size.
• The number of organelles (mitochondria, ribosomes) increases.
• The DNA is replicated — each chromosome is copied to form two identical strands (sister chromatids) joined at a centromere. The chromosome now looks like an 'X'.

Stage 2 — Mitosis (nuclear division):

• Chromosomes line up at the centre (equator) of the cell.
• Spindle fibres attach to centromeres and pull the two strands of each chromosome apart, to opposite poles (ends) of the cell.
• The nuclear membrane reforms around each set → two nuclei.

Stage 3 — Cytokinesis (cytoplasm division):

• The cell membrane pinches inward and divides the cytoplasm in two.
• In plant cells, a new cell wall forms across the middle.
• Result: two genetically identical daughter cells, each with the same number of chromosomes as the parent.

Mitosis is used for:

1. Growth — multicellular organisms grow by increasing cell number (and size).
2. Repair — replacing damaged or dead cells (e.g. skin healing after a cut).
3. Asexual reproduction — single-celled organisms (some bacteria, yeast budding) and some plants (runners, tubers) produce offspring identical to themselves.
4. Embryonic development — the fertilised egg (zygote) divides repeatedly by mitosis to build the embryo.

Why are daughter cells identical?

• DNA is replicated BEFORE mitosis, so each chromosome becomes two identical copies.
• During mitosis, one copy goes to each daughter cell.
• Result: both daughters have the same DNA, same genes, same chromosome number as the parent.

Contrast with meiosis (covered in 4.6 Inheritance): meiosis produces gametes that are genetically DIFFERENT from the parent and have HALF the chromosome number.

A stem cell has two key properties:

1. It can divide repeatedly by mitosis.
2. It can differentiate into one or more types of specialised cell.

Once a stem cell has differentiated, it usually loses both abilities — it becomes a specialised, non-dividing cell.

Three types of stem cell:

Type	Source	Can become …
Embryonic stem cells	Very early embryo (4–5 days old)	ANY type of body cell (most flexible)
Adult stem cells	Specific places in body (bone marrow, gut lining)	A LIMITED range — e.g. bone marrow stem cells → different blood cells
Plant meristem stem cells	Root and shoot tips	ANY type of plant cell, throughout plant's life

Why are embryonic stem cells so useful for research? Because they can become any cell type, they could potentially be used to replace damaged cells in conditions like:

• Type 1 diabetes (replace insulin-producing pancreas cells)
• Paralysis (regrow damaged nerve cells)
• Heart disease (replace damaged heart muscle)

Therapeutic cloning is one possible technique: scientists take a nucleus from a patient's body cell and put it into an egg with its nucleus removed. The resulting embryo's stem cells have the patient's genes — so any tissue grown from them won't be rejected by the patient's immune system.

Potential benefits:

• Treat / cure currently untreatable conditions (diabetes, paralysis, Parkinson's, blindness).
• Avoid donor-organ shortages — grow tissues to order.
• Avoid immune rejection (if therapeutic cloning is used).

Risks:

• Cultured stem cells could be contaminated with viruses that infect the patient.
• Stem cells transplanted into a patient could divide uncontrollably and form a tumour (cancer risk).
• Long-term effects in humans still being studied — therapies are mostly experimental.

Ethical issues (the most commonly examined point):

• Embryonic stem cell research destroys the embryo — some religious and secular groups consider an embryo a potential human life and oppose its destruction.
• Where do the embryos come from? Often from leftover IVF embryos that would otherwise be discarded.
• Adult stem cells avoid this ethical issue but are less flexible.

Plant uses (less controversial):

• Take cuttings from meristem tissue → grow many identical, disease-resistant plants quickly.
• Preserve rare or endangered plant species.
• Agricultural uses — e.g. cloning crop plants that have a particularly good yield.

Evaluation pattern. AQA exam questions often ask 'evaluate the use of stem cells' — give 1 benefit, 1 risk, 1 ethical issue, and a balanced conclusion. Four marks; one for each.