Cells and organisation, Year 10: mitosis and stem cells

mitosis; cytoplasm

HintOne is the name of the division of the nucleus; the other is the jelly-like part that fills the rest of the cell.

The whyThe cell cycle has three overall stages. First the cell grows and copies its DNA. Second, in mitosis, the copies are separated and the nucleus divides. Third, the rest of the cell splits in two. AQA does not ask for the named phases within mitosis.

It is copied, so there are two copies of each chromosome

HintTwo new nuclei are about to be made, and both need a full set of instructions.

The whyThe DNA replicates during the first stage of the cell cycle. Mitosis then pulls one copy of every chromosome to each end of the cell, so both new cells receive the complete set.

8

HintEach round doubles the number of cells.

The whyOne cell becomes 2, then 4, then 8: the number doubles each time, so after three rounds there are 2 × 2 × 2 cells. Repeated doubling is why a single fertilised egg can grow into an organism of billions of cells.

identical

HintEach one received a complete copy of the same set of chromosomes.

The whyBecause the DNA is copied exactly and one copy of each chromosome goes to each new cell, both cells carry the same genes as the cell they came from. That is what makes mitosis suitable for growth and for replacing damaged cells.

So that each of the two new cells has enough to work

HintWhatever is inside is about to be shared between a pair.

The whyDividing splits the contents of one cell between two. If the cell did not first increase its sub-cellular structures, each new cell would start with only half the usual number.

A stem cell

HintIt shares its name with a part of a plant.

The whyA stem cell has not yet become specialised. It can do two things: make more unspecialised cells like itself, and give rise to specialised cells. Most body cells can do neither.

embryos; bone marrow

HintOne is a very early stage of human development; the other is the soft tissue inside large bones.

The whyEmbryonic stem cells can become most types of human cell. Adult stem cells in the soft tissue inside bones are more limited: they form many types of cell, including blood cells.

Embryonic stem cells can become most types of cell; adult stem cells only some types

HintOne kind has a whole body still to build; the other mostly tops up one tissue.

The whyStem cells from an embryo can be made to differentiate into most types of human cell. Adult stem cells, such as those inside bones, can form many types of cell including blood cells, but not every type.

meristem

HintThe word begins with the same four letters as "merit".

The whyMeristem tissue stays unspecialised and keeps dividing. Its cells can become any kind of plant cell, for as long as the plant lives. This is why plants keep growing at their tips.

To divide and differentiate into all the different types of cell the body needs

HintThe embryo begins as a small ball in which everything looks alike.

The whyEvery specialised cell in the body, from nerve cells to muscle cells, comes from the unspecialised cells of the early embryo. They divide many times and then become specialised as the embryo develops.

They could be made to differentiate into new nerve cells to replace the damaged ones

HintAsk which kind of tissue this patient has lost, and what the treatment might grow.

The whyDamaged nerve cells in the spinal cord are not replaced naturally. The hope is that stem cells could be grown into the missing type of cell. In the same way, they might replace the insulin-producing cells that are lost in some people with diabetes.

The patient's body will not reject them

HintThink what the immune system does to a transplanted organ that does not match.

The whyThe body's immune system attacks cells it recognises as foreign, which is why transplants can be rejected. Cells with the patient's own genetic make-up are not seen as foreign. AQA does not ask how the technique is carried out.

Passing on a viral infection

HintIt is a type of pathogen that lives inside cells.

The whyIf the cells used are infected, the infection can be transferred to the patient along with them. Any treatment that puts living cells into the body carries this kind of risk, and it has to be weighed against the benefit.

The embryo is destroyed, and they regard it as a human life

HintThink about where these particular stem cells come from and what happens to that source.

The whyCollecting embryonic stem cells means the embryo cannot go on to develop. People disagree about the moral status of an early embryo, so they disagree about whether this is acceptable. Others point to the patients who might be helped. You are expected to weigh both sides.

To produce large numbers of identical plants quickly and cheaply

HintThink what a farmer gains if one plant with a useful feature can be multiplied.

The whyA plant with a special feature, such as disease resistance, can be copied many times over, and every copy has the feature. The same method can multiply a rare species to protect it from extinction.