Disjunction is a process which sees homologous chromosomes, that are lined up along the equatorial plate, gradually drawn towards opposite poles of a cell, by structures known as spindle fibres. The process of disjunction occurs in both mitosis and meiosis cell division and is associated with the anaphase stage. If two sister chromatids do not separate correctly during the process, Nondisjunction occurs. This then leads to an incorrect number of chromosomes, in one of the two dividing cells, and this is known as Aneuploidy. This can lead to disorders such as a trisomy 21 (Downs syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau's Syndrome).