Cell division

The nucleus of a cell contains chromosomes, and each chromosome is made of a DNA molecule. Each one carries a large number of genes.

In body cells the chromosomes are normally found in pairs — a human body cell has 23 pairs, 46 in total. Keeping the three words straight is what most marks here turn on: DNA is the molecule, a chromosome is one long DNA molecule, and a gene is a section of that molecule.

Cells divide in a series of stages called the cell cycle. During the cycle the genetic material is doubled and then divided into two identical cells. There are three overall stages to describe:

1. Growth. Before a cell can divide it grows and increases the number of its sub-cellular structures, such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome.

2. Mitosis. One set of chromosomes is pulled to each end of the cell, and the nucleus divides.

3. Cytokinesis. Finally the cytoplasm and cell membranes divide, forming two identical cells.

Cell division by mitosis is important in the growth and development of multicellular organisms, and you should be able to recognise where it is happening in a situation you are given. Mitosis is at work whenever an organism grows, whenever damaged tissue is repaired or worn-out cells are replaced, and in asexual reproduction, where the offspring are genetically identical to the parent.

A model helps fix the order. Think of the cell cycle as copying a document before dividing an office in two: first you photocopy every page, so there are two complete sets (DNA replication, in the growth stage); then you carry one full set to each end of the room (mitosis); then you build a wall down the middle (cytokinesis). Both offices end up with an identical set of documents.

Like every analogy it breaks down — chromosomes are not paper, and nothing is sorting them deliberately — but it holds the three stages in the right order, which is what the question asks for.

One thing you do not need: the specification says you need the three overall stages of the cell cycle but not the different phases of the mitosis stage. Prophase, metaphase, anaphase and telophase are not required at GCSE, so time spent memorising them is time taken from something that is examined.

A stem cell is an undifferentiated cell of an organism which can give rise to many more cells of the same type, and from which certain other cells can arise by differentiation.

Where they come from decides what they can become.

The specification asks for their function in three places, so learn them as three jobs rather than one definition. In an embryo, stem cells divide and differentiate to produce every tissue of the developing organism. In adult animals they replace cells that are damaged or worn out — bone marrow stem cells make the replacement blood cells your body needs every day. In the meristems of a plant they let the plant keep growing and making new tissue for its whole life, which is why a plant can grow new roots and shoots when an animal cannot regrow a limb.

Treatment with stem cells may be able to help conditions such as diabetes and paralysis. In therapeutic cloning, an embryo is produced with the same genes as the patient. Stem cells from that embryo are not rejected by the patient's body, so they may be used for medical treatment.

There are drawbacks. The use of stem cells carries potential risks, such as the transfer of viral infection, and some people have ethical or religious objections to using embryos. You are not expected to know the techniques themselves — only the uses, the risks and the objections.

In plants, stem cells from meristems can be used to produce clones of plants quickly and economically. Rare species can be cloned to protect them from extinction, and crop plants with a useful feature such as disease resistance can be cloned to give farmers large numbers of identical plants.

Questions on stem cells usually ask you to evaluate, not to list — to weigh the practical risks and benefits against the social and ethical issues and then say what you think. Hold three groups in your head.

Benefits: treatment may help conditions such as diabetes and paralysis, and cells from therapeutic cloning are not rejected, because the embryo was produced with the same genes as the patient.

Practical risks: stem cells could carry a viral infection to the patient, and many treatments are still unproven.

Social and ethical issues: producing the stem cells destroys an embryo. Some people object on religious or ethical grounds, holding that an embryo has the status of a person; others argue that an embryo at this stage is a small cluster of cells and that relieving serious suffering matters more. There is also the question of cost, and who would get the treatment.

A full-mark answer gives both sides and then reaches a judgement. Stopping at "some people disagree" is where most marks are lost.