Biology for Dan and Sophie: Part 2 - Evolution (I)

Evolution is a simple concept when explained well (which I hope I will be able to do!) but in order to fully understand it I need to explain a few things about cell biology. I will make this as brief and simple as possible because it’s not our focus at the moment (though if I am able to keep up these posts it will be at a later date) but it involves some complicated words and ideas which I can't really avoid so apologies in advance.

What is a cell?

If you are able to get your hands on a microscope get your mum or dad to cut a bit of onion and then peel off a bit of the thin membrane that you can find between each layer (this website shows the bit I mean) and have a look at it under a low magnification. You should be able to see something like this:

Image from http://serenalegeremicroscopy.weebly.com

The black lines are cell walls. They are a bit like your skin, protecting the inside of the cell. You can also see little blobs inside each cell. These are the nuclei and they are the part I want to talk about. There are other parts of the cell that are too small to be seen in this photo. The nucleus and these other parts are collectively called organelles (which just means 'small organ') and they do lots of important things that keep you, and every other organism, alive. We will hopefully get to talk about them later but for now let's look at the nucleus.

What is a nucleus?

The nucleus is where DNA is found. DNA is a very important molecule, as I discussed briefly last time. Most of the time the strands of DNA are all jumbled up but at certain times each strand comes together to form a neat structure called a chromosome.

Image from www.frontiers-in-genetics.org

Different organisms have different numbers of chromosomes but they always have two sets. For example, humans have 46 chromosomes, fruit flies have 8 and dogs have 78. When organisms want to have babies (a process called reproduction) they make special cells called gametes. Males have gametes called "sperm" while females have gametes called "eggs". Gametes only have one set of chromosomes so that when sperm and egg combine the result has two sets.

DNA is made up of smaller parts called 'genes'. Each gene is an instruction to make a protein, a type of chemical, and it is these proteins that make a cell (proteins are a bit like bricks, they are used to construct cells that are used to build organisms in the same way that bricks are used to construct walls that are used to make houses). The study of genes is called genetics. 

How do organisms reproduce?

To make gametes a cell must split in half, a process called meiosis. It is when a cell is preparing to undergo meiosis that the chromosomes are formed.

I'm going to use an example from baking to try and explain how this works. Make (or pretend to make) some sweet dough. The dough is going to be our nucleus. Add some currants or similar dried fruit to act as the chromosomes. Mix the dough so the 'chromosomes' are spread about the mixture fairly equally. When it's ready, knock it down and shape it into a ball. This is our nucleus at the start of meiosis. Now pinch the dough in the middle and squeeze so that you form two smaller balls. These new balls will have half the number of currants in them, just like the gametes will have half the number of chromosomes:

What does any of this have to do with evolution?

That's a very good question. To fully answer it I will have to explain what evolution is which I will do but for now you need to know that evolution requires something called "variation". Variation just means differences. You are different from you sister so you both have variation. These differences are due to differences in your genes and you have differences in your genes because of this process of meiosis. DNA must copy itself to make new cells and when it does this it can make small mistakes, which biologists call mutations. These mistakes mean that no gamete cell is identical which is why even though you are both made from gametes from your mum and dad, you look and act differently. Also, your mum and dad have different genes because they come from different families so they have a lot of differences. This means that when their gametes combine there's lots of ways they can be different, so even if your mum and dad gave you lots and lots and lots of sisters and brothers, they would all be different.

What is evolution?

Finally, the big question! It's got a fairly simple answer which will get more complex as we get into the details. . .

Evolution is just a big word that means ‘change’. If something evolves it changes. For example, Manchester United evolved from being a small local football club to become a massive club whose name is known around the world.

In biology, evolution means something a bit more specific than just ‘change’. Usually (but not always – I’ll try and remember to explain later) when a biologist says “evolution” what they really mean is “evolution by the process of natural selection”. This is a bit of mouthful and it’s why we just say “evolution” for short. But it’s important not to forget its just a shortened way of saying something longer.

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I think I'm going to leave it there for today. I know I haven't really answered the question but I think we've gone over a lot of really complicated stuff and I should give some time for you to understand this before we do anything more. I will explain evolution and natural selection in the next post, I promise! 

In preparation for next time you might want to find some sheets of coloured paper. You will need 2 sheets of one colour and 2 sheets of another colour. It's for a little experiment to try and help explain how evolution works. 

Summary

All organisms have DNA. In most organisms DNA is found in the nucleus of the cell. Most of the time DNA is tangled up and messy but just before meiosis it forms chromosomes. Most cells have 2 sets of chromosomes but after meiosis the resulting gamete cells only have 1 set so that when the male and female gametes combine the resulting cell has 2 sets, one from each parent. 

DNA is formed from genes. Different people have different genes and this gives a population variation. Variation is required for evolution. Evolution means 'change', but for biologists is short for "evolution by means of natural selection".

Next time . . .

Evolution and natural selection!

New words used:
cell
chromosome
DNA
evolution
gamete
gene
genetics
meiosis
population 
protein
variation