Biology Core- Variation

Once upon a time there was a pop song with the line 'Everyone is beautiful in their own way'.

A nice thought I'm sure you'll agree. I guess it means that we're all different and have a beauty in the way we're put together.

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?

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.

Genetic variation

We all know that we get our features and characteristics from our parents through their genes.

Even with the same parents, brothers and sisters can be very different. For one thing they might be a different sex! Each one of us receives a unique combination of genes.

An interesting exception to this is where you get identical twins who have developed from the same fertilised egg. Both will be the same sex and share identical genes.

Identical twins are similar but can still have differences. One might end up being much stronger than the other, this due to environmental variation.

Environmental variation

This is where the environment that you grow up in or live in has an effecton you. For example if you had an identical twin who was brought up from childhood in a much poorer environment than you, where they might not get fed as much as you.

What effect might the environment have on their development?

You might find that when you met up later they were perhaps lighter in weight than you, perhaps shorter in height, and they might have performed less well at school.

To easily and clearly see the effects of environmental variation look at plants, as they are often susceptible to environmental effects.

For example: Imagine two plants grown in the same soil but one is in the shade of a large tree. The plant that is in the shade will not grow as large as the one that is not shaded.

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.

Where is the genetic code?

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!

What is the genetic code?

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:

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.

• Adenine (A),
• Cytosine (C),
• Guanine (G),
• Thymine (T).

What DNA does

But so what?

Nice molecule, but what does it actually do? Each group of 3 bases on one side of the DNA carries the genetic code for one of the 20 different amino acid molecules.

Once the whole code for one gene is read the cell can make the specific protein. Each gene codes for one complete protein. Many of these proteins are actually enzymes.

So the chromosomes in each cell contain every gene needed to create a new human cell or whole body! Or a plant or animal!

Amazing!! It's like carrying around quite a stack of filing cabinets each stuffed full of sheets with instructions on how to build a new you! And that's in each and every cell.

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

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.

Humans don't make clones of themselves naturally like plants can.

For example: strawberry plants send out runners.

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

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. The following list summarises the slightly more complicated events of meiosis

1. Meiosis starts with all the chromosomes lining up in their (homologous) pairs.
2. One chromosome from each pair enters a new nucleus so that 2 daughter cells are fomed.
3. The 2 daughter cells now have half the number of chromosomes each compared to the 'parent' cell. They are haploid.
4. The chromatids that make up each chromosome are pulled apart by fibres.
5. Each single chromatid joins the others in another new nucleus. These are the gamete cells. These cells are haploid too.
6. The chromatids in the gamete cells are replicated to re-create the X-shaped chromosome. The gametes are haploid.

Finally then you end up with 4 gamete cells from each parent cell that splits up. These gamete cells, whether eggs or sperm, can then go on to fertilisation.

The beauty of meiosis is that it mixes up all the chromosomes. No gamete is ever the same as the next as the chromosomes get shuffle about so much.

So when an egg and sperm 'get it together' and you pop up 9 months later you are unique. Different, but definitely unique!