Chromosome behaviour in mitosis and meiosis

1. Overview

Mitosis is the nuclear division that produces two daughter nuclei that are genetically identical to each other and to the parent nucleus. It follows DNA replication in the cell cycle, so each chromosome enters mitosis as two identical sister chromatids joined at the centromere.

During mitosis, four structures change in a fixed sequence:

• Chromosomes — condense, separate, then decondense.
• Nuclear envelope — breaks down, then re-forms.
• Spindle — assembles and pulls the chromatids apart.
• Cell surface membrane (or cell wall in plants) — divides the cytoplasm.

The stages, in order, are prophase, metaphase, anaphase and telophase, and you must be able to recognise each one in plant and animal cells from diagrams, photomicrographs and slides.

Key Definitions

• Chromatid: one of the two identical copies of a chromosome that are joined together at the centromere after DNA replication; each chromatid carries two telomeres.
• Centromere: the constricted region that holds two sister chromatids together and to which spindle microtubules attach during division.
• Spindle: a structure made of protein microtubules that forms across the cell and moves the chromosomes during mitosis.
• Nuclear envelope: the double membrane that surrounds the nucleus; it breaks down at the start of mitosis and re-forms at the end.
• Mitosis: nuclear division that produces two daughter nuclei genetically identical to each other and to the parent nucleus.
• Prophase: the first stage of mitosis in which chromosomes condense and become visible and the nuclear envelope breaks down.
• Metaphase: the stage in which chromosomes line up along the equator (middle) of the spindle.
• Anaphase: the stage in which the centromeres split and sister chromatids are pulled to opposite poles of the cell.
• Telophase: the final stage in which chromosomes reach the poles, decondense and a new nuclear envelope forms around each group.
• Mitotic index: the proportion of cells in a sample that are in mitosis, calculated as the number of cells in mitosis divided by the total number of cells counted, often expressed as a fraction or percentage.

Content

What the chromosomes look like at the start

Before mitosis begins, DNA replication in interphase copies every chromosome. As a result, each chromosome is made of two identical sister chromatids joined at a single centromere. Each chromatid is one complete DNA molecule wound around proteins, and because every chromatid has a telomere at each of its two ends, a chromosome built from two sister chromatids has four telomeres in total. This replicated, two-chromatid form is what condenses and becomes visible when mitosis starts.

Prophase

The chromosomes supercoil and condense, becoming shorter, thicker and visible under the light microscope as paired chromatids. The nuclear envelope breaks down (it is no longer visible by the end of prophase), and the nucleolus disappears. The spindle begins to form from protein microtubules that extend across the cell between the two poles. In animal cells the spindle grows from a pair of centrioles that move to opposite poles; most plant cells lack centrioles but still form a spindle.

Metaphase

The completed spindle stretches from pole to pole. Spindle microtubules attach to the centromere of each chromosome, and the chromosomes are moved so that they line up single file along the equator (middle) of the spindle. This central arrangement is the clearest feature for recognising metaphase.

Anaphase

The centromere of each chromosome splits, separating the two sister chromatids. The spindle microtubules shorten, pulling the now-separated chromatids to opposite poles of the cell, centromere first, so they take on a characteristic V-shape as they are dragged along. Anaphase is fast, and a cell caught in it shows two groups of chromatids moving apart. Because the two chromatids of each chromosome were identical copies, each pole receives an identical set.

Telophase

The chromatids (now called chromosomes again) reach the poles and decondense (uncoil), becoming long, thin and no longer individually visible. A new nuclear envelope forms around each group, the nucleolus reappears, and the spindle breaks down. The result is two genetically identical nuclei.

Cytokinesis: how the cytoplasm divides

Telophase is followed by division of the cytoplasm, called cytokinesis, which differs between cell types:

Feature	Animal cells	Plant cells
How the cell divides	The cell surface membrane pinches inwards from the outside	A new cell wall (cell plate) is built across the middle
Mechanism	A cleavage furrow constricts the cell until it splits in two	Vesicles gather at the equator and fuse, forming the wall from the centre outwards
Why they differ	No rigid wall, so the flexible membrane can pinch in	The rigid cell wall cannot pinch in, so a new wall must be laid down

Identifying the stages in micrographs, diagrams and slides

When you look at a slide or photomicrograph, use these visual clues:

• Interphase (not part of mitosis): an intact nucleus with no individual chromosomes visible, often with a clear nucleolus.
• Prophase: condensed, visible chromosomes scattered in the cell; nuclear envelope breaking down or absent.
• Metaphase: chromosomes lined up in a single row across the middle of the cell.
• Anaphase: two separate groups of chromatids being pulled towards opposite ends, often V-shaped.
• Telophase: two groups of chromosomes gathered at the poles, with new nuclei forming; in plants a cell plate may be visible across the middle.

Working out the order — and the timing — from a photograph: count the cells in each stage. A stage that lasts a long time (such as prophase) is seen in many cells, while a short stage (such as anaphase) is seen in only a few. Because the cells in a root tip divide at random times, the proportion of cells caught in a stage equals the proportion of the whole cell cycle spent in that stage. This lets you estimate how long a stage lasts, as the second worked example shows.

Mitotic index

The same cell counts give the mitotic index: the fraction of counted cells that are actually dividing. Add up the cells in all four mitotic stages (prophase, metaphase, anaphase and telophase) and divide by the total number of cells counted. For the 90-cell count in the second worked example below:

$$\text{mitotic index} = \frac{18+6+2+4}{90} = \frac{30}{90} = 0.33 \;(33\%)$$

A higher mitotic index means a larger proportion of cells are dividing, so it is used to compare rates of division between tissues, for example fast-growing tissue against more slowly dividing tissue.

Worked example

Exam-style question: A student examines a stained root-tip squash under the microscope and finds one cell in which the chromatids have just been separated and are being pulled towards the two ends of the cell. State the stage of mitosis shown, and describe the role of the spindle and the centromeres in producing this appearance. [3]

Model answer:

• The cell is in anaphase.
• The centromere of each chromosome has split, separating the two sister chromatids so that each can move independently.
• The spindle microtubules shorten/contract and pull the separated chromatids, centromere first, to opposite poles of the cell, giving the V-shaped, two-group appearance described.

Worked example

Exam-style question: A student counts 90 cells in a root-tip squash and records the stage of each cell: 60 in interphase, 18 in prophase, 6 in metaphase, 2 in anaphase and 4 in telophase. The complete cell cycle in these cells takes 24 hours. Estimate the time, in minutes, that one cell spends in metaphase. [3]

Model answer:

• Fraction of cells in metaphase $= \dfrac{6}{90} = 0.0667$.
• This fraction equals the fraction of the cycle spent in metaphase, so the time is $0.0667 \times 24 = 1.6$ hours.
• Converting to minutes: $1.6 \times 60 \approx 96$ minutes (about 96 min).

Key Equations

This is mainly a qualitative topic, but one relationship is used to estimate how long a stage lasts from a cell count:

$$\text{time in a stage} = \frac{\text{number of cells in that stage}}{\text{total number of cells counted}} \times \text{total cell-cycle time}$$

The proportion of cells caught in a stage is taken as the proportion of the whole cell cycle spent in that stage.

Common Mistakes to Avoid

• Miscounting telomeres on a replicated chromosome. A single chromatid has two telomeres (one at each end), so a chromosome made of two sister chromatids has four telomeres, not one or two. Decide whether the chromosome is replicated before you count.
• Writing "nuclear membrane" instead of "nuclear envelope". The structure around the nucleus is a double membrane, correctly called the nuclear envelope; use that exact term when describing prophase and telophase.
• Saying chromosomes (rather than chromatids) separate in anaphase. In anaphase the centromere splits and the sister chromatids are pulled apart; calling them chromosomes at this point loses marks.
• Putting metaphase chromosomes "in two rows" or "in pairs at the equator". In mitosis the chromosomes line up singly in one row along the equator — pairing at the equator describes meiosis, not mitosis.
• Adding shading or drawing individual cells on a plan (low-power) drawing. Plan drawings show only tissue boundaries, drawn with a sharp pencil in clear, continuous lines and no shading.
• Putting units inside the body of a results table. Place units only in the column headings, separated by a solidus (for example, "Number of cells / arbitrary units"), and keep the body of the table for numbers only.

Exam Tips

• Learn the four stages in the order P-M-A-T (Prophase, Metaphase, Anaphase, Telophase) and pair each with one unmistakable feature: envelope breaks down, line up at the equator, chromatids move to poles, nuclei re-form.
• In "describe the behaviour of..." questions, follow one structure at a time through the cycle (chromosomes, then nuclear envelope, then spindle, then membrane/wall) so you do not miss any.
• When identifying a stage from a micrograph, state the stage and then justify it with the visible feature you can see ("metaphase, because the chromosomes are aligned along the centre").
• Always say the two daughter nuclei are genetically identical to the parent and explain why — because sister chromatids are exact copies that are shared one to each pole.
• Be precise about cytokinesis: animal cells form a cleavage furrow by membrane pinching, plant cells build a new cell wall (cell plate); do not swap these.
• In timing calculations, show your fraction first, then multiply by the cycle time, and finish by converting to the units the question asks for (hours or minutes).
• Do not round halfway through a calculation. Keep the full (unrounded) value in your calculator while you work, and round only the final answer to a sensible precision, with units. For example, carry $6/90 = 0.0667$ unrounded through the multiplication and only then quote about 96 min, rather than rounding the fraction early and losing accuracy.