DNA - recipe for life

To bake a cake you need a recipe, which lists the ingredients needed and how to mix them together. Your DNA sequence is your recipe book. It contains all the information needed to make you – and every living organism has its own DNA instruction book called its genome.

However, your DNA doesn’t just have everything in one big list. Like a cookery book, it is broken down into individual recipes – which we call genes. Each gene has the information to make one small component, such as an enzyme. It also has words to spell out each bit of the recipe.

Here we’ll introduce you to some of the basics of genetics.

 

What is DNA?

The recipe for you is carried by an amazing molecule called Deoxyribonucleic acid, or DNA for short. DNA is an astonishingly long molecule that looks like a twisted ladder – it’s often called the double helix.

It’s composed mostly of 4 basic molecules called nucleotides, which are normally known by the first letter of their name.

The 4 nucleotides are:

Adenine (A)        Cytosine (C)       Guanine (G)       Thymine (T)

Pairs of nucleotides form the rungs of the ladder, with one on each side of the helix and are known as base pairs. They like to pair up in a particular way. Adenosine pairs with Thymine and Guanine pairs with Cytosine.

What does DNA look like?

If you were to extract the DNA from all the cells in your body you’d get a gunky white substance that doesn’t look like much at all. But that’s because you’ve mushed up all the cells to get at it.

Normally DNA is found in a compartment in the cell called the nucleus.

It is neatly bundled into X-shaped structures called chromosomes, which are visible under a microscope.

Chromosomes are X-shaped bundles of DNA

In human cells, except for eggs and sperm, there are 46 chromosomes. These are divided into 23 pairs of different chromosomes. Eggs and sperm have a single copy of each of these 23 chromosomes. A particular gene is nearly always found in the same place on the same chromosome in each person.

If you unwind the each of the chromosomes you get down to a single thread, which is the DNA.

There are around 20,000-25,000 genes in the human genome made up of around 3 billion base pairs. Not every base pair is part of a gene though!

 Some bits of DNA are regulatory sequences – they help control whether a gene is used in a particular cell or not. After all, it's no good having the product of every gene made in every cell or else all the cells would be the same! The regulatory sequences determine whether a gene is on or off, or how much to make of each protein. Apart from the regulatory sequences there’s lots of DNA that does things we just don’t know about yet

Alleles

We don't all have the same forms of each gene - if we did we'd all look the same. Instead we have different alleles.  This is what we call different versions of each gene.

For example we all have genes that colour our eyes.  You might have blue eyes or brown, whilst mine are grey. We have different alleles of each eye colour gene.

You inherit the different alleles from each parent, meaning you have two for every gene in your genome.

 

 

 

 

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