Course 9.1Lesson 3 of 3

Cytokinesis and the work of mitosis

After the nucleus divides, the cytoplasm must divide too. Animal cells do it by a furrow, and plant cells by a cell plate. Then we see why a body needs mitosis: for growth, repair and a steady balance between the nucleus and the cytoplasm.

about 25 min1 simulation1 practice question1 Python program5 questions to check yourself

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Dividing the cell

Mitosis shares out the chromosomes. That is karyokinesis. The cell itself is still in one piece. It is divided by cytokinesis, and only then is cell division complete.

Two drawings side by side. On the left, an animal cell in cytokinesis: a furrow in the plasma membrane pinches in from the outside, with a ring of actin filaments, until the cell is cut in two. On the right, a plant cell with a cell wall: vesicles from the Golgi apparatus gather in the middle and fuse into a cell plate, which grows outward until it meets the side walls. A note says that the cell plate becomes the middle lamella between the two new cells.

Cytokinesis in an animal cell

An animal cell has no wall, so it simply pinches in two. A ring of actin filaments forms just inside the plasma membrane at the equator. It tightens, and a furrow appears in the plasma membrane. The furrow gets deeper and finally meets in the centre. The cytoplasm is cut in two.

Cytokinesis in a plant cell

A plant cell is held in a rigid wall, so it cannot pinch. It builds a new wall instead. Vesicles full of cell wall material gather in the centre of the cell. They fuse to form the cell plate. The plate grows outward until it meets the existing side walls. It becomes the middle lamella between the walls of the two new cells. Each daughter cell then lays down its own cell wall on its side of the plate.

In both kinds of cell, the organelles such as mitochondria and plastids are shared between the daughter cells.

When the cytoplasm does not divide

In some organisms, karyokinesis is not followed by cytokinesis. The cell then has many nuclei. This is a syncytium. The liquid endosperm of a coconut is an example. Many nuclei lie in one mass of cytoplasm until walls form later.

Why a body needs mitosis

Mitosis makes two cells that carry the same genes as the parent cell. This one fact explains its jobs.

Growth. A multicellular organism grows because the number of its cells grows. A fertilised egg becomes a body by mitosis.
Keeping the right balance. As a cell grows, the nucleus cannot keep up with the cytoplasm. Division restores the nucleo-cytoplasmic ratio.
Repair and replacement. The cells of the top layer of the skin, the lining of the gut and the blood cells are lost all the time. Mitosis replaces them.
Growth all through life in plants. Cells of the meristems divide for the whole life of a plant. The apical meristems make the plant longer, and the cambium makes it thicker.
Reproduction without gametes. A single celled organism, such as yeast or an amoeba, divides to make new individuals.

In animals, mitosis is seen mostly in diploid body cells. In plants it takes place in both haploid and diploid cells. The haploid gametophyte of a fern or a moss grows by mitosis. Some social insects are also an example: a drone honeybee is haploid, and its cells divide by mitosis.

You should see that the animal cell is cut from the outside, and that the plant cell is cut from the inside outward. Move the progress to 100 per cent. Both give two cells.

Try it in Python

The program counts the cells after a number of cycles and the number of divisions that took place.

Practise with new numbers

Common mistakes

Mixing up the two kinds of cytokinesis. The animal cell has a furrow from outside. The plant cell has a cell plate that grows out from the centre.
Thinking that karyokinesis and cytokinesis are the same. One divides the nucleus and the other divides the cytoplasm.
Saying that mitosis makes variation. The daughter cells are identical to the parent cell. Variation comes from meiosis and fertilisation.
Saying that the cell plate is made of the plasma membrane. It is made from the vesicles of the Golgi apparatus. It becomes the middle lamella.

Remember these

Cytokinesis divides the cytoplasm. Animal cells: furrow. Plant cells: cell plate that becomes the middle lamella.
A syncytium is a cell with many nuclei, for example the liquid endosperm of a coconut.
Mitosis gives growth, repair, a balanced nucleo-cytoplasmic ratio and, in single celled organisms, reproduction.
The daughter cells are identical to the parent cell.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

How does cytokinesis differ in an animal cell and in a plant cell?

What does the cell plate become?

What is a syncytium? Give one example.

Give three reasons why an organism needs mitosis.

A cell divides 4 times, and every cell divides each time. How many cells come from one starting cell?

References

NCERT (2023). Biology, Textbook for Class XI, chapter 10.
Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 10.2, The Cell Cycle.