Knowing PMAT is one thing. Being handed a micrograph of root tip cells and asked which phase a particular cell is in is another. The good news is that each phase has one giveaway feature, and once you know what to look for it takes seconds.
📘 What you need to know
Most cells in a micrograph are in interphase, because that is where cells spend most of the cell cycle.
Interphase: chromatin is visible but individual chromosomes are not, so the nuclei look dark and solid.
Prophase: chromosomes visible as one group; the nuclear envelope is breaking down.
Metaphase: chromosomes lined up along the middle of the cell.
Anaphase: chromosomes moving apart towards opposite poles, often with a characteristic V shape.
Telophase: two separate groups of chromosomes, one at each pole, beginning to uncoil.
What to look for
Phase
The giveaway
Also look for
Interphase
No separate chromosomes visible; the nucleus looks dark and evenly filled
Most of the cells in any field of view will look like this
Prophase
Chromosomes visible, all in one group
The edge of the nucleus looks broken or fuzzy as the envelope goes
Metaphase
Chromosomes in a neat line across the middle of the cell
Nothing between the line and the poles
Anaphase
Two sets pulling apart, with V shapes pointing inwards
The V shape comes from being dragged centromere first
Telophase
Chromosomes in two groups at opposite poles
They look fuzzier as they uncoil; a new envelope is forming
Cytokinesis
A furrow pinching in (animal) or a cell plate across the middle (plant)
Two nuclei already present, one on each side
Notice how many cells are in interphase. In a real root tip that proportion is what the mitotic index measures.
The one that catches people out
Prophase and telophase can look surprisingly similar in a photomicrograph — in both, the chromosomes are visible and the nuclear envelope is somewhere between present and absent. Use this:
Count the groups. In prophase there is only one group of chromosomes in the cell. In telophase there are two, one at each pole. That single check settles it almost every time.
One group means the cell is on its way into division. Two groups means it is on its way out.
Mitotic index
Once you can identify phases, you can measure how actively a tissue is dividing. The mitotic index is the proportion of cells in a sample that are in mitosis:
Mitotic index
mitotic index = number of cells in mitosis ÷ total number of cells
Count every cell you can see, then count how many of those are in prophase, metaphase, anaphase or telophase. Cells in interphase are not in mitosis, so they go in the total but not in the top of the fraction. The answer is a decimal, or a percentage if you multiply by 100.
A high mitotic index means a lot of division is going on — normal in a root tip or a growing embryo, but a warning sign in a tissue sample that should be quiet.
Count carefully and count everything. The most common error is not miscounting the dividing cells but forgetting to include all the interphase ones in the total.
Worked examples
WORKED EXAMPLE
Calculate a mitotic index
In a field of view of a root tip there are 45 cells. Of these, 4 are in prophase, 2 in metaphase, 2 in anaphase and 1 in telophase. Calculate the mitotic index as a percentage.
Step 1: Add up the cells in mitosis4 + 2 + 2 + 1 = 9 cellsStep 2: Divide by the total number of cells9 ÷ 45 = 0.2Step 3: Convert to a percentage0.2 × 100 = 20Mitotic index = 20%The total is all 45 cells, not just the 9. Interphase cells count in the denominator.
WORKED EXAMPLE
Estimate how long a phase lasts
A population of cells has a cell cycle lasting 20 hours. The mitotic index is 0.2. Estimate how long the cells spend in mitosis.
Step 1: State the assumption
The proportion of cells seen in a phase is roughly the proportion of time spent in it.
Step 2: Multiply0.2 × 20 hours = 4 hoursAbout 4 hours in mitosisSo 16 of the 20 hours are interphase — which is why most cells you see are in interphase.
WORKED EXAMPLE
Justify an identification
A student says a cell in a micrograph is in anaphase. State two features they should point to as evidence.
Feature 1
There are two sets of chromosomes moving apart, away from the middle towards opposite poles.
Feature 2
The chromosomes show the characteristic V shape, because they are being pulled centromere first.
Two sets separating, V-shaped“State two features” wants observations from the image, not a description of what anaphase means.
💡 Exam tip
Describe what you can see, then name the phase. An identification with evidence scores; a bare guess often does not.
Use the one-group-or-two check to separate prophase from telophase.
Expect most cells to be in interphase. If you have labelled half the field as dividing, recheck.
For a mitotic index, show the fraction before you work it out. Method marks are available even if the arithmetic slips.
Watch the units of the answer: a decimal and a percentage are both acceptable, but say which one you have given.
Root tips and growing embryos are the usual sources of these images, because that is where division is happening fastest.
⚠ Common mix-up
Leaving interphase cells out of the total when calculating a mitotic index.
Counting interphase as a phase of mitosis. It is part of the cell cycle, not part of mitosis.
Calling every cell with visible chromosomes prophase. Check whether they are lined up, separating, or in two groups first.
Confusing metaphase with anaphase. Metaphase is a single tidy line; anaphase is two sets already moving apart.
Assuming a dark nucleus means the cell is dead or damaged. It usually just means interphase chromatin.
Giving a mitotic index greater than 1. If that happens, you have divided the wrong way round.
Up next: Meiosis — the division that halves the chromosome number, why a sexual life cycle needs it, and how it manufactures variation.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
