| The 23 pairs of chromosomes hold between 20,000 and 25,000 genes. This sequence of genes and what they do is called the Human Genome. Researchers have now mapped the Human Genome so they know the sequence of bases for a 'typical' person. They are now looking for differences between individuals which may lead to a person developing a particular disease. You can find out more in The Human Genome resource.
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Nearly all cells
of the human body have a nucleus
that contains 23 pairs of chromosomes.
These can be seen in the photograph below. Roll over the picture
to take a closer look at chromosome pair number 1. |
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| Picture 8. Human chromosomes that have been prepared for examination under a microscope. |
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| Picture 9. A diagram of DNA showing the base pairs. The DNA molecule is a complex structure.. |
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| What is a chromosome? |
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A chromosome is a long strand of DNA. The smallest chromosomes contain a strand that is about 2cm long. The largest ones have about 8 cm of DNA.
The DNA is tightly packaged into the chromosomes to allow it to fit into the nucleus.
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| What is DNA? |
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DNA is the molecule that carries our genes. It is made up of a sequence of four bases:
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C).
Each strand of DNA is made from billions of bases. The arrangement of the bases determines the effect of a gene. They are the genetic alphabet that make the instructions to control the cell. It takes about a million bases to make a single gene and each strand carries between one and two thousand genes.
But there are also sections of DNA between the genes that do not appear to have any function. Some researchers suggest that these redundant sections are involved in the regulation and repair of the DNA.
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| What are genes? |
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Genes determine things like eye colour, hair colour, tongue rolling and the enzymes made by a cell. Human cells contain about 40,000 different genes. Each one is made from millions of bases, so only a short section is shown here.
A gene is a section of DNA that holds the instructions to make one protein.
Genes also control the cell's growth. If this control goes wrong then the cell can form a cancer.
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| Genes, cell division and cancer |
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Each of our 40,000 genes is a section of DNA that control the cell's activity. This includes directing the production of proteins that:
- make up the structure of the cell
- become hormones like insulin
- are enzymes which carry out chemical reactions in the cytoplasm.
Certain genes play a vital role in the control of cell division. These genes tell the cell when to copy and divide. If this control goes wrong then the cell may start to grow uncontrollably. A lump of cells can begin to form and in some cases it produces a malignant tumour, or cancer.
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