The basic “building bricks” of the body are cells. There are over 200 different types, such as blood cells, nerve cells and muscle fibre cells. They vary greatly in size and shape, although most cells are far too small to see except through a powerful microscope. About 30 typical cells placed in a row would stretch only one millimetre. The whole body contains more than 50 million million cells. Most are in the blood and the brain.
INSIDE A CELL
A typical cell is a bag of jelly, or cytoplasm, containing even smaller parts called organelles. Mitochondria are small and sausage-shaped. They break up substances such as sugar (glucose) to release energy for use in the cell. The cell’s skin-like covering, the cell membrane, allows only certain substances to pass in and out. The cell’s factory for making various substances and products is called the endoplasmic reticulum. The biggest organelle is usually the nucleus, a dark lump near the cell’s centre. It contains genes in the form of DNA.
Genes contain all the information a living thing needs to develop, grow and maintain itself through life. A tiny worm has a few hundred genes. The human body has more than 100,000 genes. They determine whether you have dark hair, long legs, and a tendency to develop certain diseases, and so on.
Genes are encoded like an “instruction manual” in DNA. The twisted-ladder shape of DNA is called a double helix. The rungs or cross-links are made of chemical sub-units, known as nucleotides. There are four: A, T, G and C (adenine, thymine, guanine and cytosine). A always forms a cross-link with T, and G with C. The order of sub-units A, T, G and C along the DNA is the genetic code, containing information for the genes in chemical form. Every body cell has a full set of DNA including all genes. But each cell uses only a tiny part of the DNA for its own life processes.
HOW DNA MULTIPLIES
Cells do not live forever. Every minute the body makes about 3000 million new cells to replace those which naturally wear out and die. Also, reproduction involves making new cells. When a cell divides to make two new cells, its set of DNA is copied to make a duplicate set. Then each of the two new cells has a full DNA set with all the genes. DNA duplicates (copies) itself by breaking the cross-links that hold together its two strands. Each nucleotide sub-unit then joins to a new partner of its usual kind—A with T, and G with C. The row of new sub-units forms its own new strand. The result is two lengths of DNA, each identical to the other.
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