Mitosis and the cell cycle — Edexcel International AS Level Biology
The cell cycle and mitosis, Core Practical 6 on root tip squashes, and calculating the mitotic index.
Revision notes
3 short notes, in the order of the specification. Each one in short:
The cell cycle is the series of events from one cell division to the next. It includes interphase, in which DNA is replicated, then mitosis, then cytokinesis. Mitosis produces two daughter cells that are genetically identical to the parent cell, which is needed for growth, repair and asexual reproduction.
Core Practical 6: root tip squash
PracticalIn Core Practical 6, a root tip is warmed in hydrochloric acid, stained with a chromosome stain such as acetic orcein and squashed into a thin layer on a slide. Under a light microscope, cells from the growing region can be seen in different stages of mitosis. The root tip is used because it contains meristem cells that are dividing.
The mitotic index is the proportion of cells in a sample that are in mitosis. It is calculated by dividing the number of cells showing visible chromosomes in a stage of mitosis by the total number of cells observed. A high mitotic index shows that many cells are dividing, as in a meristem or a rapidly growing tumour.
Exam questions
8 exam-style questions (23 marks), each with its mark scheme.
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12 cards: flip them, mark what you knew, and practise the rest.
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The whole of cell structure, reproduction and development on one page, so you can see where this subtopic fits.
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Quick check questions
In which phase of the cell cycle is DNA replicated?
Show answer
The S phase of interphase.
What happens to the chromosomes in metaphase?
Show answer
They line up at the equator of the cell, attached to spindle fibres by their centromeres.
Why are daughter cells from mitosis genetically identical?
Show answer
DNA is replicated exactly in interphase and each daughter cell receives one copy of every chromosome.
Why is the root tip used for studying mitosis?
Show answer
It contains meristem cells, which are dividing rapidly.
How is the mitotic index calculated?
Show answer
Number of cells in mitosis divided by the total number of cells observed.
Frequently asked questions
What happens in each stage of mitosis?
In prophase the chromosomes condense and the nuclear envelope breaks down. In metaphase they line up on the equator. In anaphase the centromeres divide and sister chromatids are pulled to opposite poles. In telophase nuclear envelopes re-form and the chromosomes uncoil.
Why is mitosis important?
Mitosis produces two genetically identical daughter cells, so it is how multicellular organisms grow and replace damaged or worn-out cells. It is also the basis of asexual reproduction, in which offspring are genetic clones of one parent.
How do you make a root tip squash?
Warm a 2 to 3 mm root tip in 1 M hydrochloric acid at about 60 °C, rinse, stain with acetic orcein and squash it under a coverslip to make a single layer of cells. Then view it under a light microscope to find cells in each stage of mitosis.
How do you calculate the mitotic index?
Count the cells in mitosis (visible chromosomes) and the total number of cells in the field of view, then divide the number in mitosis by the total. Multiply by 100 for a percentage. For example, 40 dividing cells out of 250 gives 0.16, or 16%.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026