biology

The Stages of Mitosis

Mitosis is the division of one nucleus into two genetically identical nuclei. The main stages are prophase, metaphase, anaphase and telophase, followed by cytokinesis, which divides the cytoplasm and produces two daughter cells.

Visual note, a 2:58 episode with transcript, a mind map, 12 flashcards and a 5 question quiz.

The Stages of Mitosis at a glance

Listen: The Stages of Mitosis

Two hosts talking it through, 2:58. The full transcript is below, so you can read along or skip the audio.

Transcript

AnnaWhen we talk about mitosis, what exactly is dividing?

MarcoOne nucleus divides into two genetically identical nuclei. Then cytokinesis divides the cytoplasm, producing two daughter cells. The usual order is prophase, metaphase, anaphase, telophase, and then cytokinesis.

AnnaSo where does prophase begin?

MarcoThe DNA copied before mitosis coils and condenses into visible chromosomes. Each chromosome has two identical sister chromatids joined at a centromere. The nucleolus disappears, the spindle begins forming, and centrosomes move toward opposite poles.

AnnaWhat would we notice under a microscope?

MarcoDark, shortened, threadlike or rodlike chromosomes become visible, but they are scattered rather than lined up across the middle. Early on, the nuclear envelope may still be partly visible. Some descriptions call its later breakdown prometaphase.

AnnaThen what changes in metaphase?

MarcoSpindle fibres attach to kinetochores at the centromeres. They position the duplicated chromosomes along the equator, or middle, of the cell. The sister chromatids are still joined. That is important: they do not separate until anaphase.

AnnaIs the microscope clue simply a line of chromosomes?

MarcoExactly. Metaphase shows a clear line across the centre, often like small, thick rods or duplicated chromosomes arranged in a plate. The nuclear envelope is no longer present.

AnnaWhat happens when the cell enters anaphase?

MarcoThe centromeres divide, and the sister chromatids separate. Each chromatid is now an individual daughter chromosome. Spindle fibres shorten and pull the daughter chromosomes toward opposite poles.

AnnaAnd how can we recognise that movement?

MarcoThere are two groups moving away from each other. The chromosomes may look like V shapes or other bent shapes, because their centromeres lead the way. They are not yet enclosed in new nuclei.

AnnaWhat makes telophase different?

MarcoThe chromosomes reach opposite poles and begin to uncoil into less distinct chromatin. A new nuclear envelope forms around each group, nucleoli reappear, and the spindle breaks down. Under a microscope, two groups sit at opposite ends, and two nuclei may be forming.

AnnaDoes the cell already split at that point?

MarcoNot completely. Cytokinesis divides the cytoplasm. In an animal cell, a contractile ring creates a cleavage furrow that pinches inward. In a plant cell, vesicles gather in the centre and form a cell plate, which develops into a new cell wall.

AnnaSo how do we avoid confusing cytokinesis with anaphase or telophase?

MarcoLook for the nuclei or chromosome groups. In cytokinesis, the nuclei have already reformed, and the cell is physically separating. In anaphase, chromosomes are still moving apart. The key sequence is condensing, lining up, separating, forming two nuclei, and then splitting the cytoplasm. Interphase comes before mitosis, but it is not one of its stages.

The notes

Prophase

During prophase, the DNA that was copied before mitosis coils and condenses into visible chromosomes. Each chromosome consists of two identical sister chromatids joined at a centromere. The nucleolus disappears, the spindle begins to form, and the centrosomes move towards opposite poles of the cell.

Under a microscope, prophase cells have dark, shortened, threadlike or rodlike chromosomes that are becoming visible, but the chromosomes are not arranged across the middle of the cell. The nuclear envelope may still be partly visible at early prophase. In some descriptions, the later breakdown of the nuclear envelope is called prometaphase rather than prophase.

Metaphase

During metaphase, spindle fibres attach to structures at the centromeres called kinetochores. The spindle fibres position the duplicated chromosomes along the equator, or middle, of the cell. The sister chromatids are still joined together at this stage.

Under a microscope, metaphase is recognised by the chromosomes forming a clear line across the centre of the cell. They often look like small, thick rods or duplicated chromosomes arranged in a plate. The nuclear envelope is no longer present.

Anaphase

During anaphase, the centromeres divide and the sister chromatids separate. Each chromatid is now considered an individual daughter chromosome. Spindle fibres shorten and pull the daughter chromosomes towards opposite poles of the cell.

Under a microscope, anaphase cells show two groups of chromosomes moving away from each other. The chromosomes may look like V shapes or other bent shapes because their centromeres lead as they are pulled towards the poles. The groups are not yet enclosed in new nuclei.

Telophase

During telophase, the chromosomes reach opposite poles and begin to uncoil, becoming less distinct as chromatin. A new nuclear envelope forms around each group of chromosomes, and nucleoli reappear. The spindle breaks down.

Under a microscope, telophase is recognised by two groups of chromosomes at opposite ends of the cell. These groups may look less dark and less sharply defined than the chromosomes in metaphase or anaphase. Two new nuclei can often be seen forming, and the cell may begin to pinch in the middle.

Cytokinesis

Cytokinesis is the division of the cytoplasm. In animal cells, a contractile ring causes a cleavage furrow to form, which deepens until the cell splits into two daughter cells. In plant cells, vesicles collect at the centre and form a cell plate, which develops into a new cell wall between the daughter cells.

Under a microscope, cytokinesis in an animal cell appears as a narrowing or pinching at the cell surface. In a plant cell, it appears as a developing partition across the middle. The nuclei have already reformed, so cytokinesis can be distinguished from anaphase by the presence of two nuclei or two chromosome groups in the new daughter cells.

Identifying the stages in order

The stages can be identified by following the position and appearance of the chromosomes. In prophase, chromosomes condense but are scattered. In metaphase, duplicated chromosomes line up at the equator. In anaphase, sister chromatids have separated into two moving groups. In telophase, two nuclei form and the chromosomes uncoil. Cytokinesis then completes the physical separation of the cells.

Microscope images are not always perfectly clear, and cells may not show every stage equally well. The key distinction is whether chromosomes are condensing, lined up, separating, or enclosed in two new nuclei. Interphase is not part of mitosis, although it is often shown in diagrams because the cell grows and copies its DNA before mitosis begins.

What to remember

In prophase, copied DNA condenses into visible chromosomes, each made of two sister chromatids.
In metaphase, duplicated chromosomes line up across the equator of the cell.
In anaphase, centromeres divide and sister chromatids become separate daughter chromosomes.
In telophase, chromosomes reach opposite poles, uncoil and become enclosed in two new nuclei.
Cytokinesis divides the cytoplasm and produces two daughter cells.
Animal cells form a cleavage furrow, while plant cells form a cell plate.
The stage order is prophase, metaphase, anaphase, telophase, then cytokinesis.