Animal cell division, animation. Cells divide and proliferate through the process of mitosis (nuclear division) and cytokinesis (cell division). At the start of mitosis cell organelles called centrosomes (yellow) are seen moving to opposite sides of the cell. The nuclear membrane breaks down. The cell's chromosomes, which are formed from DNA (deoxyribonucleic acid) have replicated and line up along the equator of the cell. Fibres, known as spindles, grow from the centrosomes and attach to the chromosomes to pull them apart. Nuclear membranes then reform around the two sets of chromosomes and the cell divides into two new cells, a process known as cytokinesis.