Genes and inheritance
Genes control the growth, repair, and functions of cells in the body. Genes are made of DNA (deoxyribonucleic acid), which is found in the nucleus of cells as structures called chromosomes. DNA provides cells with instructions for the development and growth of the body's organs and structures, through building proteins and making molecules that control cellular processes. Genes are the means by which physical and some mental characteristics are passed on to children.
There are about five billion cells in the adult human body, and all of them, except red blood cells, have a set of genes made of DNA. This chemical has a double helix shape and is made of two strands of molecules joined together in the centre by a series of nucleotide bases. The DNA is coiled into structures called chromosomes that are stored in the nucleus of cells.
Human genes are arranged on 22 pairs of matching chromosomes, plus two sex chromosomes. One chromosome in each pair is inherited from each parent. Therefore, body cells contain two copies of genes, with the genes for the same characteristic carried on the matching chromosomes in a pair. Egg and sperm cells, called sex cells, have 23 single chromosomes so that a paired set of genes is created when a sperm cell fertilizes an egg.
There are about 35,000 pairs of genes in a human body cell. These genes, which vary in size, provide the cell with the information that enables the cell to make proteins. The order of nucleotide bases along the DNA provides this information. Each body cell contains the same genes, but each tissue or organ needs to make different proteins. For this reason, many genes in an individual body cell are permanently switched off, and a system exists to turn on genes only when they are required. Both of the genes in a matching pair can be identical. However, some matching genes occur in slightly different forms called alleles. Some genes may have two to several hundred different alleles. These different forms account for differences between individuals, such as shade of hair colour or length of nose. Differences in just 0.1 per cent of our DNA create all our unique features.
Most of the differences that occur between genes do not affect function. For example, blue eyes work as well as brown ones. However, some genetic differences have important effects. For example, inherited diseases, such as cystic fibrosis and sickle-cell disease, are caused by different forms of abnormal genes. Genes also play a part in disorders such as coronary artery disease and some common cancers, such as colorectal and breast cancer.
Every time a cell divides into two during growth or repair, its genes are duplicated so that each new cell has a full set. When sperm and egg cells are made, the chromosomes pair and exchange genetic material between each other before the cell divides. This ensures that when an egg and a sperm fuse, the new child is different both from its parents and from its siblings.
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