S-Cool Revision Summary

S-Cool Revision Summary

Some differences, or variations, cover a whole range of things and are called continuous variations.

Others differences have only a very few possible options, these are called discontinuous variations.

Continuous variation

Some of these differences vary over a whole range, for example the height, skin colour and weight of people. We call this type continuous variation.

So if you lined up a hundred people you would find a whole range of heights - within sensible limits! People don't just come in set heights like shoe sizes.

Discontinuous variation

However there are other differences where there are only a few possible forms. For example, I can't roll my tongue lengthways. Can you?

Try it now in a mirror. Don't try it if there are any big, strong aggressive people about! The only options are that you can or cannot roll your tongue, you can't half roll your tongue.

Other examples of discontinuous variation are blood groups and eye colour in humans. An example that could apply to plants and animals would be resistance to a particular disease.

But how do all the various differences arise?

Where do they come from in the first place?

The answers have been found only during the last one hundred years. Differences between animals or plants come about through either genetic variation or environmental variation.

The genetic code is carried by an amazing molecule called Deoxyribonucleic acid, or DNA to its friends. DNA is an amazingly long and complicated molecule.

The DNA is found in the nucleus of all cells. It is formed into X-shaped bundles called chromosomes. In human cells, except for eggs and sperm, there are 46 chromosomes. These are divided into 23 pairs.

Each chromosome has the appearance of two knitted sausages tied together in the middle. The more scientific description would be that a chromosome is made up of two chromatids held together in the middle by a centromere. You choose which is easier to remember!

This DNA strand looks a bit like a ladder twisted into a double helix. The rungs of the ladder are made up of pairs of base molecules connected to each other.

It is the order of the bases (that form the rungs across it like on a ladder) that carry the actual genetic code.

To make things a bit easier for once, there are only 4 different types of bases.

Each is usually known by the first letter of its name:

Adenine (A),

Cytosine (C),

Guanine (G)

Thymine (T)

Even easier is the fact that the order the bases join up to form the 'rungs' is fixed.

Adenine and Thymine always join together, and Cytosine and Guanine always join.

The genetic code contained in our chromosomes is of no real use unless it can be used to make new cells.

The code is passed on to the new cells using either of two processes, mitosis or meiosis.


Mitosis is the process used during growth to make new cells within a plant or animal. It is also used during asexual reproduction, in which an individual can clone itself to produce identical offspring.

However our body often has to make new cells to replace damaged ones or as we grow. So human cells also go through mitosis in the same way as animal and plant cells but is for growth and repair.

The offspring cells have the same number of chromosomes as the parent cells, therefore they are diploid.


Meiosis is a slightly different process. It is used to create the gametes, these are the sperm or eggs, used in sexual reproduction. The offspring produced during sexual reproduction have characteristics, selected from those of the parents.

The main difference in meiosis as compared to mitosis is that the new cells have half the number of chromosomes as the diploid 'parent' cell. One chromosome comes from each homologous pair of chromosomes. So these offspring cells are haploid not diploid.