The human genome
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Chromosomes in cells
Picture 2. Each human cell has a nucleus that contains the 23 pairs of chromosomes.
The nucleus of a human cell contains around between 30 000 and 40 000 genes. These are carried on DNA which winds itself up inside chromosomes. Whenever a cell divides, the chromosomes in the new cell carry a copy of the original chromosomes.
Where are the chromosomes?
We find chromosomes inside the nucleus of nearly all cells. The nuclei of human cells have 23 pairs of chromosomes. The information they carry has a strong influence over our personal characteristics.
Picture 3. The 23 pairs of human chromosomes seen under a microscope. Take a closer look at chromsome 2.
Picture 4. A human chromosome. The DNA is tightly wound up inside it.
What's inside a chromosome
Each chromosome contains a single, long piece of DNA (deoxyribonucleic acid). This is a coiled, double stranded molecule.

A chromosome is about 0.004 mm long and contains a single piece of DNA that is about 4 cm long. This is about 10 000 times longer than the chromosome. So it has to twist and coil to fit inside.

Where are the genes?
About 3% of the DNA in a chromosome represents genes. The genes are what determine a person's physical characterisitcs. Each chromosome carries a couple of thousand genes and many of these are common to all human beings. So, despite obvious differences between people, 99.9% of your DNA is identical to anyone else's. It is the remaining 0.1% that influences the differences between us, such as height, hair colour and susceptibility to a particular disease.

Environmental factors, such as lifestyle (e.g. smoking and nutrition) also influence the way we look and our susceptibility to disease.

Question 2
a) Chromosomes are found in the nucleus of almost all cells. Can you name a human cell which does not contain chromosomes?

b) All of our DNA is involved in coding for our characteristics - true or false?