What is a chromosome?
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A chromosome is:
- A DNA molecule folded in a particular way
- Always part of a pair
Human beings have 23 different pairs, making a total of 46 chromosomes. Of each of these 23 pairs of chromosomes, one comes from the mother and the other from the father. One pair is known as the sex chromosomes. A woman's sex chromosomes consist of two X chromosomes, one from her mother and one from her father, while a man has one X chromosome from his mother and one Y chromosome from his father.
What is a protein?
A protein consists of a number of amino acids. These are the structural building blocks of our bodies.
If we want to make a protein containing 100 amino acids, we need a chunk of DNA molecule that codes for these 100 amino acids in the correct order.
Since three bases are needed on a DNA molecule to code for each amino acid, we need a chunk of DNA with 300 bases in exactly the right order to get our protein.
This chunk of DNA is known as the gene for that protein.
Genes and proteins
We have just learned that a gene is a chunk of DNA that contains the code for a particular protein. This means that the genes determine which proteins a cell can produce.
All cells in an organism contain exactly the same DNA. So all cells should only be able to make exactly the same proteins. But this is only how it works in theory - things are different in practice.
Although all cells contain the same genes, different cells make use of different genes.
Some genes are active while others are inactive. During the countless cell divisions that take place as we develop from fertilised egg to fully developed organism, those genes not needed for a cell's special functions are inactive.
What are the differences between cells?
In a liver cell, only some of the genes will be active. The active genes are those that code for the production of specific liver proteins.
In a nerve cell, other genes are active and so different proteins are produced. This means that liver cells and nerve cells are very different - and so able to fulfil very different functions.
It is the active genes in each individual cell that determine whether it is a liver cell or a nerve cell. This means that when a liver cell divides it will always become two liver cells. A liver cell will never turn into a muscle cell or a skin cell.
Which genes are active in a cell determines which proteins the cell can produce.
Each cell uses thousands of different proteins. These include:
- Enzymes
- Receptors
- Transport proteins
- Structural proteins
Some of these proteins are used in vast amounts while others are needed only in small amounts. This means that some genes need to be more active than others. The cell needs to be able to regulate the activity of the genes that can be activated.