The genome is an organism's entire hereditary makeup. The genome can be thought of as a book(see link below) and in the case of humans, this book would have 23 chapters (chromosomes) which in turn contain our genes, which is made of DNA, which is made of nucleotides, which contains base pairs, etc. A mutation is a change in the genomic sequence of an organism. Mutations can be spontaneous and occur suddenly due to radiation, chemicals, and errors not corrected during DNA replication. Some mutations are hereditary. Mutations can have no effect, a slight effect or cause disease or death of an organism. Genes code for particular proteins. When the gene has a mutation, there will be a change in the protein due to the fact that the protein may be unable to function properly. Most mutations are harmful, however, some can be neutral or maybe even benefical or may increase the amount of genetic variation of the gene pool of a species. Examples of mutations are point mutations--where a single nucleotide is exchanged for another, due to DNA replication error, or exposure to mutagenic agents like chemicals. Insertions are one or more extra nucleotides added into the DNA, which can alter the gene product depending on where they were added. Deletions are removal of one or more nucleotides resulting in the reading frame of the gene being altered. Sometimes chromosomes are translocated resulting in an exchange of genes between non-homologous chromosomes. These are just a few examples of gene mutations. Since most mutations are harmful, it affects the fitness of an organism by decreasing it. If it is advantageous, it may increase the fitness of the organism. If it is neutral, it won't harm or benefit the organism, but may be added to the gene pool at a steady rate. This is called the molecular clock. The genome of various organisms has changed over time at a steady pace due to the fact that mutations have occurred over time.