Before mitosis begins, the cell has undergone S phase and G phases as it is a part of the cell cycle. The 'S' phases lead to the formation of duplicate chromosomes (2x+2x) required for chromosomal division. While the 'G' phases provide cell growth and size required for the process. Also, the cell cycle proteins help to check for any errors in chromosomes and prevent the cycle from going into mitosis. This is a protective mechanism that prevents abnormal cell formation.