What you need to know
- 3.14mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. produces genetically identical daughter cells.
- 3.15Observing mitotic stages in a root tip squash.
- 3.16Calculating mitotic indices.
By the end, you should be able to…
- Describe chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. behaviour in mitosis.
- Distinguish mitosis from cytokinesisDivision of the cytoplasm to form two cells, after nuclear division..
- Calculate a mitotic index and interpret its limits.
Build your understanding
Prophase and metaphase
In prophaseThe first stage of mitosis: chromosomes condense and the nuclear envelope breaks down., chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. condense and become visible as paired sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere.. The nucleolusA dense region in the nucleus where rRNA is made and ribosomes are assembled. disappears, the nuclear envelopeThe double membrane around the nucleus. Its pores let mRNA leave. breaks down as division progresses, and a spindle forms. Plant cells can form a spindle without animal-type centrioles.
At metaphaseThe stage of mitosis when chromosomes line up at the equator, attached to spindle fibres by their centromeres., chromosomes align individually at the equator. Spindle fibres attach at the centromere region, connecting sister chromatids to opposite poles. homologous chromosomesA pair of chromosomes, one from each parent, with the same genes at the same loci but possibly different alleles. do not pair into bivalents in mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent..
Anaphase and telophase
At anaphaseThe stage of mitosis when centromeres divide and sister chromatids are pulled to opposite poles., sister chromatids separate at their centromeres and move to opposite poles as daughter chromosomes. Each pole receives one copy of each chromosome. This equal separation follows the accurate copying of DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. in interphaseThe stage of the cell cycle between divisions, when the cell grows and its DNA replicates..
During telophaseThe last stage of mitosis: chromosomes reach the poles and a nuclear envelope forms around each set. the chromosomes reach the poles and decondense; nuclear envelopes reform and the spindle disassembles. cytokinesisDivision of the cytoplasm to form two cells, after nuclear division. partitions the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles.. Animal cells develop a cleavage furrow; plant cells form a cell plate that contributes to the new separating wall.
Recognise the evidence in a preparation
A diffuse nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. with no distinct condensed chromosomes usually indicates interphase. Condensing chromosomes suggest prophase; an equatorial arrangement suggests metaphase; two separating chromosome groups suggest anaphase; two reforming nuclei suggest telophase.
Stages are a continuous process. A tilted cell, overlapping chromosomes or a transitional appearance may make classification uncertain. A single fixed image records a moment, not movement. Use several visible features rather than identifying a stage from the shape of the cell alone.
Mitotic index
Mitotic index = number of cells in mitosis / total number of cells counted. Include prophase, metaphase, anaphase and telophase in the numerator; include interphase in the denominator. Multiply the fraction by 100 only if a percentage is requested.
In an illustrative example, 18 prophase, 9 metaphase, 6 anaphase, 7 telophase and 160 interphase cells give 40/200 = 0.20, or 20%. A higher index means more cells were observed in mitosis, not necessarily that individual cells complete division faster. A delay in mitosis can also raise the index.
NOT: mitosis does not halve the chromosome number. Cytokinesis is not a chromosome stage.
- Starts after replication, so each chromosome has two chromatids
- Daughter nuclei are genetically identical, mutation aside
- Used in growth, repair and asexual reproduction
- The cytoplasm divides by cytokinesis
Two traps this diagram blocks
- Do not say homologous pairs separate in mitosis. That is meiosis I.
- Do not say the daughter cells are haploid.
From the diagram alone
Question
Describe what happens to sister chromatids during mitosis and state the chromosome number of each daughter nucleus. [3]
Show a model answer
Sister chromatids separate in anaphase and move to opposite poles. Each daughter nucleus therefore receives one copy of each chromosome. The chromosome number stays diploid, the same as the parent nucleus.
Make your reasoning visible
Say what separates
Sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere. separate in mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent.. homologous chromosomesA pair of chromosomes, one from each parent, with the same genes at the same loci but possibly different alleles. separate in meiosis I.
Explain identical outcomes
Connect DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. replication with equal distribution of sister chromatids; “it divides” is insufficient.
Use the full denominator
Divide by all counted cells, including interphaseThe stage of the cell cycle between divisions, when the cell grows and its DNA replicates., rather than by the mitotic cells only.
Avoid this misconception
The claim: Mitosis produces four haploid cells.
The correction: Normal mitosis produces two nuclei retaining the starting chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. number; cytokinesisDivision of the cytoplasm to form two cells, after nuclear division. normally makes two cells.
How does one cell become two identical cells?
Before a cell divides it copies its DNA. Mitosis then shares the copies out exactly, so each new nucleus gets a full set of chromosomes.
What the model shows
A model cell with four chromosomes going through interphase, the four stages of mitosis and cytokinesis, with a graph of how much DNA the cell holds.
What to look for
When the DNA doubles but the chromosome number does not. Which parts separate in anaphase. Whether the two daughter cells end up with the same chromosomes as each other and the parent.
What it tells us
Mitosis and the cell cycle produce genetically identical daughter cells, which is what growth, repair and asexual reproduction need.
Be careful what you claim
This is a simplified model cell, not a real organism. Real chromosomes are uncoiled in interphase, and interphase lasts much longer than mitosis.
Try it in the model: press Next stage once and watch every chromosome become two sister chromatids. Then read the DNA content in the graph.
Mitosis and the cell cycle
Follow one cell through interphase, the four stages of mitosis and cytokinesis. Watch the DNA double and then be shared out, so both daughter cells get the same chromosomes.
What you see
- One model cell with 2n = 4: a long and a short pair. Pink chromosomes came from the mother and blue from the father; the long pair carries alleles A/a and B/b.
- The track along the top shows the cell cycle. The graph shows the DNA content of one cell, with chromosomes per cell underneath.
- The panel on the right shows which chromosomes are in the parent cell and, at the end, in each daughter.
How to use it
- Step through with Next stage and Back, press Play, or tap a stage on the track to jump to it.
- Switch between an animal cell (centrioles, cleavage furrow) and a plant cell (no centrioles, cell plate). Try 2n = 6 or 8.
What it shows
- DNA replication in S phase doubles the DNA (2C to 4C) but not the chromosome number (3.14).
- Sister chromatids, not homologues, separate in anaphase, so both daughter cells receive the same set.
- Mitosis produces genetically identical daughter cells for growth, repair and asexual reproduction.
Limits of the model
- Chromosomes are drawn condensed in interphase so you can count them; really they are uncoiled chromatin.
- The cell is a teaching model, not a named species; real cells have many more chromosomes.
- Stage lengths are not to scale: interphase takes up most of a real cell cycle.
Guided tour · step 1 of 11
- The cell starts in G1 of interphase.
- In Stages, just below these steps, press Next stage to go through S phase.
In S phase every DNA molecule is replicated, so each chromosome becomes two identical sister chromatids joined at a centromere. The DNA doubles from 2C to 4C, but the chromosome number stays 2n.
Stages
Interphase is not a resting stage. In G1 the cell grows and makes proteins and organelles. Each chromosome is one long DNA molecule, uncoiled (drawn condensed here so you can count them).
Questions
Final check: order the stages
These four drawings are one model cell, not a root-tip photograph. Put them in the order a nucleus passes through mitosis. Interphase, including DNA replication, comes before this list. Cytokinesis is not one of the four stages.
- 1
Each chromosome sits alone at the equator. Fibres attach at the centromere, with sister chromatids facing opposite poles.
- 2
The chromosomes are at the poles and beginning to decondense. Nuclear envelopes are reforming and the spindle is breaking down.
- 3
Chromosomes have condensed into sister chromatids. The nuclear envelope is breaking down and a spindle is forming.
- 4
Sister chromatids have separated and are moving to opposite poles as daughter chromosomes.
Drag a drawing, or use Earlier and Later. Then check.
A teaching model with two chromosomes so the movements stay readable. It is not a count for a named species. Plant cells form a spindle without centrioles. A real squash is a fixed moment, so neighbouring cells can be in different stages.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A cell shows two groups of condensed daughter chromosomes moving apart. Identify the stage and justify it.
Show suggested answer
Anaphase: sister chromatids have separated and are moving towards opposite poles.
Practice question
Retrieval and explanation · suggested answer
36 of 240 cells are in mitosis. Calculate the index as a fraction and a percentage.
Show suggested answer
36/240 = 0.15; 15%.
Practice question
Retrieval and explanation · suggested answer
Why does a root-tip squash not show the same cell progressing through all stages?
Show suggested answer
Fixation preserves cells at particular points; different cells supply snapshots of different stages.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A substance prevents spindle-mediated separation. The mitotic index rises. Has growth necessarily accelerated?
Show a hint
Consider how long cells remain in the counted stages.
Explore the reasoning
No. Cells can accumulate in mitosis because they cannot complete it. A snapshot index does not independently measure production of daughter cells.
Recall this lesson
Lesson recall questions
Question 1 of 5: List the four stages of mitosis in order (after interphase).
Mitosis and the cell cycle
Practice question
Retrieval and explanation · suggested answer
List the four stages of mitosis in order (after interphase).
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
From cells to a research question
3B · MitosisScientists studying cell-cycle regulation need to distinguish entry into division from successful completion. A count of dividing cells is one observation, not a complete explanation. A mitotic index…Specification 3.16Read & use the scienceClose article
Why this matters
Scientists studying cell-cycle regulation need to distinguish entry into division from successful completion. A count of dividing cells is one observation, not a complete explanation. A mitotic index is the number of cells visibly in mitosis divided by the total cells counted. It is a snapshot of a population, so its interpretation depends on how long cells spend in each stage as well as how many enter division. Researchers may also track labelled cells through time. This distinction matters when testing a drug: more cells with condensed chromosomes could mean division is frequent, or that the cells are stuck before completion. A root-tip sample and a tumour sample are different biological settings, but the sampling caution is shared.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.16 be able to calculate mitotic indices
Use the science
What extra observation could distinguish faster cycling from arrest?
Explore a possible response
Follow cells over time and count completed divisions, using suitable controls alongside the snapshot counts.
Explore the source
- Nobel Prize: the 2001 Physiology or Medicine awardNobel Prize Outreach
Two ways of counting growth
3B · MitosisA mitotic index counts cells visibly in mitosis. In cancer pathology, a Ki-67 score uses a marker associated with cell proliferation, giving a related but different measurement. The…Specification 3.16 · 3.15Read & use the scienceClose article
Why this matters
A mitotic index counts cells visibly in mitosis. In cancer pathology, a Ki-67 score uses a marker associated with cell proliferation, giving a related but different measurement. The National Cancer Institute describes its clinical use. Neither number is a direct clock for every cell: a snapshot also depends on how long cells spend in the counted state and which area was sampled. Students can calculate a mitotic index from a root-tip image and then ask why a clinician might use another marker. This comparison strengthens data interpretation without changing the required Unit 2 calculation.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.16 be able to calculate mitotic indices
- 3.15 CORE PRACTICAL 6: prepare and stain a root tip squash to observe the stages of mitosis
Use the science
Two samples have the same mitotic index but different Ki-67 scores. Must they have identical cell-cycle behaviour?
Explore a possible response
No. The measures count different cell states, and sampling or time spent in each state may differ. Further observations are needed.
Explore the source
- Ki-67 proliferation indexNational Cancer Institute