DNA and Chromosomes
We all started off as a single cell: a fertilised egg. That cell divided again and again to form all the different cells, tissues and organs of our bodies. How did the cells 'know' how to do this? How does a nerve cell 'know' how to behave like a nerve cell and not like a white blood cell? Scientists have shown that DNA controls much of how a cell behaves; what it looks like and how it divides.
What is DNA?
DNA is shorthand for deoxyribonucleic acid. It is a very complicated molecule and it is in nearly every cell in our bodies. It is also in plant cells, fungal cells, bacteria and even some viruses. This molecule is very important to life on Earth.
The DNA molecule is made of two extremely long strands that twist around each other (called a double helix). The picture shows an extremely small bit of DNA with one of its strands coloured red and the other blue. Each strand is made of sugar molecules (to keep it in a single strand) and nucleotide bases (which join the two strands together).
There are four types of nucleotide bases in DNA. The order in which they appear on the strand is important. The order of bases on the strand contains a code - which controls the way the cell containing the DNA works.
What is a Chromosome?
A chromosome often refers to a long strand of DNA that has been wound round like a twisted piece of string. This happens so that the extra long bit of DNA will fit into the nucleus of the cell. The word chromosome actually means coloured body. It was called this because some chemical dyes made the chromosomes stand out under the microscope.
Many cells have more than one strand of DNA in their cells. These strands are wrapped up to form chromosomes. Different species have different numbers of chromosomes. If you find a cell and count its chromosomes you can find out what organism it came from:
| Species | Number |
|---|---|
| Fruit Fly | 8 |
| Earthworm | 36 |
| Human | 46 |
| Potato | 48 |
| Garden Snail | 54 |
| Kingfisher | 132 |
DNA and Proteins
Proteins are very important molecules in cells. They are can make very tough structures such as hair and nails. They make enzymes which control the cell chemistry. They also make the cell membrane that controls what can enter and leave the cell.
DNA makes the proteins. A special structure called a ribosome actually converts the DNA code to protein. The DNA can be thought of as a program that tells the cell how to work and how to build a copy of itself. All through the proteins.