The ribosome links the amino acids together into a polypeptide chain and advances to the next group of three nucleotides, which in turn code for another specific protein. In this way, a long string of amino acids is assembled in a protein based on the original DNA genetic code. During copying, nucleotides can be added, deleted or mispaired. The DNA helix itself can be physically damaged by heat, toxins, UV light, or other environmental hazards, preventing copying of the strand or causing errors. These changes in the genetic code are called mutations.
Some mutations have little or no effect on the overall functioning of the cell.
Most are detrimental to the cell's ability to function, and a very rare few are helpful improvements to the cell's ability to survive.