Mitosis

Where it happens

Mitosis happens in body (somatic) cells during growth and repair. It follows interphase, when the cell grows and copies its DNA.

In animals, mitosis is most active in tissues that are constantly renewed: the skin epidermis, the lining of the alimentary canal, and the red bone marrow that produces blood cells. In plants, mitosis is concentrated in the meristems, the apical meristem at the root tip and shoot tip, and the cambium that widens the stem.

Mitosis is one phase of the cell cycle, which consists of interphase (G1, S and G2), the mitotic phase (nuclear division) and cytokinesis (division of the cytoplasm). Interphase takes up most of the cycle, and the S phase is when DNA is replicated.

Inputs and outputs

• Input: one diploid parent cell that has passed the checkpoints of interphase and is large enough to divide.
• Input: a replicated set of chromosomes, each made of two identical sister chromatids joined at the centromere.
• Input: energy from respiration to assemble spindle fibres from microtubules and to move the chromosomes.
• Output: two daughter cells, each with a complete diploid set of chromosomes identical to the parent cell.
• Output: two new nuclear membranes, re-formed around each set of chromosomes in telophase.
• Output: in plant cells, a new cell plate that grows into a cell wall between the daughter cells; in animal cells, a cleavage furrow that pinches the cell in two.

The steps

1. Interphase: the cell grows, makes new organelles and replicates its DNA, so each chromosome becomes two identical sister chromatids joined at the centromere.
2. Prophase: the chromatin condenses into visible chromosomes, the nucleolus disappears, the nuclear membrane breaks down, and in animal cells the centrioles move to opposite poles and spindle fibres begin to form.
3. Metaphase: spindle fibres attach to the centromere of each chromosome and the chromosomes line up in a single row along the equator (metaphase plate) of the cell.
4. Anaphase: the centromeres divide, and the spindle fibres shorten to pull the sister chromatids apart to opposite poles. Each chromatid is now a separate chromosome.
5. Telophase: the chromosomes reach the poles and uncoil, a nuclear membrane re-forms around each set, the nucleolus reappears and the spindle breaks down.
6. Cytokinesis: the cytoplasm divides. In animal cells the membrane pinches inward as a cleavage furrow; in plant cells a cell plate forms at the equator and becomes a new cell wall.
7. The two daughter cells enter interphase, and the cycle can repeat.

Why it matters and how it is controlled

Mitosis lets a single fertilised egg grow into a whole organism and lets the body replace and repair cells throughout life. Because the daughter cells are identical, they carry the same genetic information.

The cell cycle is controlled at checkpoints. Before the cell commits to S phase, it checks that it is large enough and that its DNA is undamaged; before mitosis, it checks that DNA replication is complete; and during metaphase, it checks that every chromosome is attached to the spindle.

Mitosis is also the basis of asexual reproduction in unicellular organisms such as Amoeba (binary fission) and in plants that reproduce vegetatively, so the offspring are clones of the parent. When checkpoint control fails, cells divide without restraint and form a tumour; a tumour whose cells can spread to other tissues is cancer.

Understanding mitosis is also the foundation for cloning and tissue culture, in which cells are stimulated to divide outside the organism.

How it is examined

You may be asked to identify a stage from a diagram, to compare mitosis with meiosis, or to state the purpose and outcome of mitosis.

The most frequent format is a set of unlabelled micrographs or drawings, which you must arrange in order and name. Look for these clues: chromosomes visible but scattered means prophase, a single row at the equator means metaphase, a V-shaped set of chromatids moving to each pole means anaphase, and two clusters with a re-forming nuclear membrane or a cleavage furrow means telophase.

You may also be asked to calculate how long a stage lasts, given the percentage of cells in that stage in a sample and the total length of the cycle, or to state two differences between cytokinesis in plant and animal cells.

Common misconceptions

Worked exam-style question

Question. A student examined a stained root tip of an onion under a microscope and counted 200 cells. Of these, 160 were in interphase, 20 were in prophase, 8 in metaphase, 6 in anaphase and 6 in telophase.

(a) Explain why a root tip is a suitable tissue for observing mitosis. (b) State one observable feature that identifies a cell in metaphase.

(c) The complete cell cycle in this tissue takes 20 hours. Calculate the time a cell spends in prophase.

(d) Explain why the daughter cells produced are genetically identical to the parent cell.

Model answer. (a) The root tip contains the apical meristem, whose cells are actively dividing by mitosis for growth, so a high proportion of cells are in a division stage. (b) The chromosomes are lined up in a single row at the equator of the cell, attached to spindle fibres.

(c) Fraction of cells in prophase = 20 ÷ 200 = 0.1; time in prophase = 0.1 × 20 hours = 2 hours. (d) During interphase, each chromosome is replicated into two identical sister chromatids.

In anaphase the sister chromatids are separated to opposite poles, so each daughter cell receives one complete copy of every chromosome and therefore the same genetic information as the parent.

Source:SRC-DSKP-EN

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