It is important to remember that a genome (all the DNA of an organism) is more than just its genes. The genome includes vast amounts of DNA outside of genes, and this too is subject to change over time. In fact, non-gene portions usually change at a faster rate than genes, because many of these changes have little or no effect on the organism's survival.
Evolution of genes and genomes includes sequence changes to existing genes, gene duplication, recombination of gene segments, and the varied actions of transposable elements as they move through the genome.
Point Mutations in Existing Genes
Genes are long strings of four nucleotides (abbreviated A, T, C, and G) whose order dictates the order of amino acids in proteins (or nucleotides in RNA). A point mutation is a change in a single nucleotide position at aparticular point in a gene. Point mutations can be changes that convert one type of nucleotide to another (a C to a T, for instance), or cause the deletion or addition of a single nucleotide. While the rate of mutation is slow, over long periods (millions of years) most possible sequence changes will have occurred several times in a population, and so natural selection is likely to have acted on most genes in almost every modern population.
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