Every organism, including human beings, has a genome containing all the biological information required to build and maintain a living example of that organism (NCBI, 2004). The biological information contained in the genome is encoded is deoxyribonucleic acid (DNA), and is divided into discrete units known as genes. The term "gene" was coined in 1909 by Danish botanist Wilhelm Johanssen for the heredity unit found on a chromosome. A single gene consists of a unique sequence of DNA that provides the instructions for making a functional product - a protein. Genes instruct each cell type to make discrete sets of proteins at just the right time, and this specificity leads to unique organisms being formed. Genes also code for proteins that attach to the genome at the appropriate positions and switch on a series of reactions called gene expression.
Not all genes are present in the nucleus. Cell organelles known as mitochondria possess their own genetic material which differs from nuclear genetic material in that it is all maternally derived: nuclear DNA is inherited half from the father and half from the mother (NCBI, 2004). This is because although there are mitochondria in the tail of sperm, the tail breaks off when the sperm enters the egg during fertilization, so all the mitochondria of the fertilized egg come from the maternal egg cytoplasm. Also, there is no recombination in mitochondrial DNA so there is no shuffling of genes from one generation to another as there is in nuclear genes.
Genes make up about one percent of the total DNA in the human genome (NCBI, 2004). The coding portions of the genes are called exons, and they are interrupted by sequences called introns. A eukaryotic gene does not code for a particular protein in one continuous stretch of DNA: both exons and introns are transcribed into mRNA, and the primary mRNA transcript is edited before it is transported to the ribosome. This process removes th...