Division of the cytoplasm (cytokinesis) usually occurs, producing two new…

The idea

Cytokinesis is division of the cytoplasm, and it follows telophase rather than forming part of nuclear division itself. In an animal cell a ring of protein filaments just beneath the cell-surface membrane contracts, pinching the membrane inwards to form a cleavage furrow that deepens until the cell is separated into two. A plant cell cannot pinch inwards because of its rigid cellulose wall, so vesicles from the Golgi apparatus line up along the equator and fuse, forming a cell plate that grows outwards until it meets the existing wall and becomes the new cell-surface membranes and middle lamella. Organelles such as mitochondria are shared out roughly rather than exactly. The word usually in the specification matters: mitosis can occur without cytokinesis, and the result is one cell containing two or more nuclei.

What earns the marks

• Cytokinesis being division of the cytoplasm, after telophase, producing two separate cells.
• The animal cell mechanism: the cell-surface membrane constricts or pinches inwards to form a cleavage furrow.
• The plant cell mechanism: vesicles fuse along the equator to form a cell plate, because the cellulose wall cannot pinch inwards.
• Recognising that if cytokinesis is prevented the cell is left with two nuclei, appearing to be stopped at telophase.
• Identifying that the block is on cytoplasm or new membrane formation, not on nuclear division.

Common mistakes and how to put them right

• Listing cytokinesis as a fifth stage of mitosis instead of a separate division of the cytoplasm.

• Describing a cell plate forming in an animal cell, or a cleavage furrow in a plant cell.

• Saying a drug that blocks cytokinesis stops DNA replication or prevents mitosis, when nuclear division has already finished.

• Assuming organelles are divided in exactly equal numbers between the daughter cells.

• Omitting the observable evidence, two nuclei in one cell, when asked what a cytokinesis inhibitor would produce.

Exam tips

• When a stem says a substance stimulates cytokinesis, the predicted effect of inhibiting it is binucleate cells, not fewer mitoses.
• Use the cellulose cell wall as the reason for the difference between plant and animal cytokinesis; it is the explanation that carries the mark.
• Say cytoplasm divides, not the cell splits, so it is clear which part of division you mean.