Mitosis and the cell cycleEdexcel International A Level Biology: Revision notes
Section 1
The cell cycle
The cell cycle is the ordered series of events in a cell between one division and the next. It has two main parts.
Interphase is the longest part. In G1 the cell grows and makes proteins and organelles; in S phase the DNA is replicated, so each chromosome now has two identical chromatids; in G2 the cell prepares for division.
Mitosis (nuclear division) is followed by cytokinesis (division of the cytoplasm). Because interphase is so much longer, most cells in a tissue sample are in interphase.
Interphase is not a resting phase. The cell is busy growing and replicating its DNA.
Section 2
The stages of mitosis
Mitosis produces two genetically identical daughter nuclei.
- Prophase: chromosomes condense and become visible as two chromatids joined at a centromere; the nuclear envelope breaks down; the spindle forms.
- Metaphase: chromosomes line up at the equator, attached to spindle fibres by their centromeres.
- Anaphase: centromeres divide and spindle fibres shorten, pulling the chromatids to opposite poles.
- Telophase: chromatids reach the poles and uncoil; nuclear envelopes reform.
In cytokinesis the cytoplasm divides. In animal cells the membrane pinches in; in plant cells a cell plate forms and becomes a new cell wall.
Section 3
The role of mitosis: growth and asexual reproduction
Growth: multicellular organisms grow by increasing cell number. Mitosis makes new cells that are genetically identical to the parent cell, so a tissue keeps the same genetic information. Mitosis also replaces damaged and worn-out cells.
Asexual reproduction: one parent produces offspring by mitosis alone, with no gametes or fertilisation. The offspring are clones, genetically identical to the parent (apart from mutations). Examples are strawberry runners and budding in yeast. This is quick and needs only one parent, but there is no genetic variation.
Section 4
Calculating the mitotic index
The mitotic index shows what proportion of cells in a tissue are dividing.
Mitotic index = number of cells in mitosis ÷ total number of cells observed
Worked example: 320 cells counted, 48 in mitosis. Mitotic index = 48 ÷ 320 = 0.15 (15%).
A high index shows rapid cell division, as in a root tip meristem or a tumour. A low index means few cells are dividing. Count many fields of view so that the sample is large enough to be reliable.
Divide by ALL the cells counted, not just the interphase ones.
Section 5
Core Practical 6: root tip squash
Aim: to observe mitosis and calculate a mitotic index.
- Cut about 1–2 mm from a growing root tip, where the meristem is.
- Warm it in dilute hydrochloric acid (about 1 mol dm⁻³, 55–60 °C, for a few minutes) to separate and soften the cells.
- Place on a slide and add a stain such as acetic orcein or toluidine blue, which binds to the chromosomes.
- Lower a coverslip and squash it to spread the cells into a thin layer.
- View under the light microscope, find cells in each stage and count cells in several fields of view.
Safety: acid and stain are hazardous, so wear eye protection and take care when warming.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Mitosis and the cell cycle
- A student prepares a squash from the tip of an onion root and views it under a light microscope. She sees many cells in different stages of the cell cycle.The student cut only the first 1–2 mm from the root tip rather than a section from the middle of the root. Explain why.2 marks
- A student counted the cells in several fields of view of a stained root tip squash. In total she counted 320 cells, of which 48 were in mitosis and the remainder were in interphase.Explain why most of the cells counted were in interphase.2 marks
- A student investigates the effect of a chemical on cell division in garlic root tips. She grows one set of roots in water (root tip A) and another set in a solution of the chemical (root tip B), then examines a root tip squash of each with a light microscope. In root tip A she counted 450 cells, of which 63 were in mitosis. In root tip B she counted 380 cells, of which 19 were in mitosis.Describe how the student should prepare a root tip squash so that chromosomes can be seen.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).