What exactly is the GENOME?

The genome is all of the DNA inside the nucleus of each cell.  In humans, that is a lot of DNA - in fact if you stretched it all out in a line it would measure 2 metres long!  All cells have their own copy of this DNA.

That gives the cell a huge problem - how can it fit so much DNA into its tiny (around 6 micrometres diameter) nucleus?

Chromosomes are the answer - inside the nucleus, our DNA occurs in 46 pieces, each of which is wrapped around special proteins called HISTONES to pack it tightly.  These 46 chromosomes can be seen using a light microscope when the cell divides.

What do chromosomes do?

You could think of them as the way DNA is packed (like 46 double-suitcases for going on holiday!) to make sure that no bits are lost when the cell is about to split itself into two new cells during cell division. 

Normally, during the life of the cell, the DNA is not packed as chromosomes, but is spread out inside the nucleus (luckily the DNA is very thin, so it can all fit!), where it gives the instructions to the cell for making proteins.

How does the DNA give instructions to the cell?

Genes are sequences of DNA which are copied into RNA molecules, which then leave the nucleus to go to the cytoplasm so that proteins, including enzymes that control all the reactions in the cell, can be made.

You can watch the video on transcription and translation to find out more.

How many genes do humans have?

Humans have approximately 25,000 genes - about the same number as a mouse!  But our genes have some clever ways of being a bit more complex than those found in a mouse!

Yet surprisingly, only a small amount of all the DNA in our nucleus is made of genes, about 3%.

Why is the human genome project important?

Learning more about our genes will help us to understand which genes are most important to keep us healthy, and maybe in the future we can make medicines to help people whose genes are not working properly.