A mutation is a spontaneous random sudden change that occurs in the genetic make up of an organisms, my be in the DNA or in the gene. In this case a mutation that knocked out proofreading function of DNA polymerase would result in a higher than normal rate of DNA synthesis error. Without the proofreading function, incorrect or the wrong nucleotides would be inserted about once every 100,000 bases instead of once every 10,000,000 bases which is the normal rate.
