In order for the cell to make a protein it uses transcription factors which are molecules that bind to particular sites in the nucleus. This then starts the process of copying a gene to RNA. Once the gene is copied to RNA it is taken outside the nucleus and ribosomes use the RNA information to create the protein. Within the RNA is an instruction as to how many copies of this protein to build. This number is reduced each time a new protein is created. When the number reaches zero the RNA is destroyed. 

DNA can become mutated causing either deletions, additions or translocations of DNA. When this occurs the proteins that the cell creates have the wrong amino acids within. This changes their function which means that they no longer perform the tasks that they are designed to do. 

These mutations can cause increased expression of a particular genes, or the creation of incorrect responses to cell communication. They can even cause cells to communicate with itself causing excessive growth. 

Genes that cause a cell to divide are called proto-oncogenes but genes that have mutated and are no longer functioning properly are called oncogenes. There are another group of genes which inhibit growth and are called tumor suppressor genes. When these genes are mutated then tumors can no longer be effectively suppressed. 
