What Is Mitosis? Cell Division Explained Simply

Before mitosis begins, the cell copies its DNA during an earlier part of the cell cycle. Each chromosome therefore consists of two identical sister chromatids joined together. Mitosis does not perform the DNA copying itself. Its job is to organise and separate those copied chromosomes accurately.

During early mitosis, chromosomes condense so they become shorter and easier to move. Structures called spindle fibres form and help organise chromosome movement. The nuclear envelope also changes as the cell prepares to separate the genetic material into two groups.

When you ask what mitosis is, keep the order clear: DNA replication comes first, then mitosis separates the copies. Mixing those two stages together makes the process seem more complicated than it is.

One of the clearest stages is metaphase, when chromosomes line up around the middle of the cell. Spindle fibres connect to chromosome regions so the sister chromatids can later move toward opposite ends. This arrangement helps each new nucleus receive one copy of every chromosome.

During anaphase, the sister chromatids separate and move apart. Once they are separated, each chromatid is considered an individual chromosome. The cell now has two matching groups of chromosomes moving toward opposite poles.

During telophase, new nuclear envelopes form around the separated chromosome sets and the chromosomes begin to become less condensed. Mitosis is then complete, although the whole cell may still need to divide its cytoplasm in a separate process called cytokinesis.

Mitosis supports several familiar biological processes:

• •Growth from a fertilised egg into a multicellular organism.
• •Replacement of worn-out cells in tissues such as skin.
• •Repair after some kinds of tissue damage.
• •Asexual reproduction in certain organisms.
• •Maintenance of the chromosome number in the daughter cells.

Mitosis is tightly controlled because errors can cause cells to receive the wrong chromosome number or to divide when they should not. Checkpoints in the cell cycle help the cell monitor whether DNA has been copied properly and whether chromosomes are attached correctly before separation. If serious damage is detected, the cycle can pause or the cell may be directed toward cell death. This control is part of understanding mitosis in a living organism: successful division depends not only on moving chromosomes, but also on deciding whether the cell is ready to divide at all.

Mitosis also differs from meiosis. Mitosis usually produces two daughter nuclei with the same chromosome number as the original cell, while meiosis involves two divisions and produces cells with half the chromosome number for sexual reproduction. Keeping those purposes separate prevents a common biology mix-up. That distinction also helps you choose the right diagram when answering cell division questions.

Mitosis is not just a textbook diagram. You can watch its stages in a classic school practical using the root tips of onions or garlic. The tips grow quickly, so many of their cells are dividing at any moment. A few millimetres of root tip are cut off and warmed briefly in dilute hydrochloric acid, which breaks down the material holding cells together. The tissue is then stained with a dye that colours chromosomes, squashed gently under a coverslip into a single layer of cells, and examined under a light microscope at high magnification.

In a good preparation you can find cells in each stage: a tidy line of chromosomes in metaphase, two separating groups in anaphase, and so on. Counting gives a useful measurement called the mitotic index, which is the number of cells visibly in mitosis divided by the total number of cells counted. Because most of the cell cycle is spent outside mitosis, the majority of cells will look like they are doing nothing. The proportion seen in each stage also gives a rough idea of how long that stage lasts, since a brief stage like anaphase will be caught less often than a longer one.

The chromosome movements of mitosis look much the same in plants and animals, but the machinery around them differs. In animal cells, the spindle is organised from two centrosomes, each containing a pair of small cylinders called centrioles, which move to opposite ends of the cell. Most plant cells have no centrioles at all, yet they still build a working spindle from protein fibres that gather around the nucleus. This is a useful reminder that living things can do the same job with quite different equipment.

The biggest difference comes during cytokinesis, the splitting of the cytoplasm. An animal cell pinches in two: a ring of contractile proteins, including actin and myosin, tightens around the middle and forms a cleavage furrow, rather like pulling the drawstring on a bag. A plant cell cannot do this because it is surrounded by a rigid cell wall. Instead, small vesicles carrying wall materials gather along the middle of the cell and fuse into a flat structure called the cell plate. It grows outwards until it meets the existing wall, becoming a new wall between the daughter cells.

Because mitosis depends on the spindle, some important cancer drugs work by sabotaging it. Spindle fibres are built from protein tubes called microtubules, which must constantly grow and shrink to pull chromatids into place and then apart. Vincristine and vinblastine, compounds first obtained from the Madagascar periwinkle, stop microtubules assembling properly. Paclitaxel, originally extracted from the bark of the Pacific yew tree, does the opposite: it locks microtubules so they cannot shrink. Either way the spindle cannot do its job, the cell stalls partway through mitosis, and many such cells eventually die.

These drugs affect any rapidly dividing cell, not only cancerous ones, which helps explain familiar side effects of some chemotherapy, such as hair loss and damage to the lining of the gut and to the bone marrow that makes blood cells. A related chemical, colchicine, has two other uses. Geneticists add it to cell cultures to trap many cells at the stage when chromosomes are most condensed, making them easier to photograph and count in a karyotype. Plant breeders use it to block division after DNA has been copied, producing cells with doubled chromosome sets, a technique used in developing some crop varieties.

Mitosis is the chromosome-separation stage of cell division, happening after DNA has already been copied. Chromosomes condense, line up, separate into matching groups, and become enclosed in two new nuclei. Keep mitosis separate from DNA replication and cytokinesis, and the cell cycle becomes much easier to follow.