Biology · Simulation and virtual lab Free

Mitosis and meiosis: chromosomes, stages and a root tip count

Watch chromosomes move through mitosis and meiosis, then count the cells in an onion root tip and calculate the mitotic index.

Best for S3 · Unit 5: Mitosis and meiosis
Best for S5 · Unit 4: Cell and nuclear division
Also useful S3 · Unit 13: Genetics and genetic crosses (gametes and variation)
Also useful S1 · Unit 4: Magnifying instruments and biological drawings (looking at a root tip squash)

▶ Start the simulation 🧪 Open the virtual lab

You will learn to: Describe and order the stages of mitosis and meiosis.
Compare chromosome numbers and DNA content at each stage.
Calculate the mitotic index of a root tip.
You will measure: Number of cells, chromosomes per cell and DNA per cell; counts of each stage; mitotic index (%).
Time needed: About 30–40 min
Equipment: Nothing extra. Real version: onion root tips, 1 M HCl, acetocarmine stain, slides, cover slips, microscope.
Price: Free: no payment, no account.

Simulation

Every day your body makes millions of new cells to grow and to repair wounds, and every child is different from its brothers and sisters. Mitosis and meiosis explain both.

Change and observe

Virtual lab

Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.

Aim

To describe the stages of mitosis and meiosis, to compare the number of chromosomes and the amount of DNA at each stage, and to calculate the mitotic index of an onion root tip.

What to do

Choose Mitosis. Use Next stage to go through all the stages. At each stage read the number of chromosomes and the DNA per cell, and tap Record this stage.
Choose Meiosis and do the same for all nine stages. Look for crossing over in prophase I.
Tap New cell and go to metaphase I again. Did the pairs line up the same way? What does this mean for the gametes?
Compare the two lines on the graph of DNA per cell.
Choose Onion root tip. Tap at least 30 cells, one by one, and note the stage of each. Tap Record my count.
Count two more fields of view (tap New field of view) and calculate the average mitotic index.

Results

📏 Read the instruments carefully

Instrument	Smallest reading	Uncertainty
Microscope at ×400	1 cell	count at least 100 cells for a reliable index
Chromosome count	1 chromosome	count centromeres, not chromatids

Write every reading with its unit, and with the same number of decimal places as the instrument. Use the 🔍 Magnifier on the picture to read small scales at eye level.

⚠️ Avoid these mistakes

Counting each chromatid as a chromosome: count the centromeres.
Counting the same cell twice or skipping cells: count in rows.
Saying meiosis gives identical cells: crossing over and independent assortment make them different.

✍️ Conclusion

Write your own conclusion first: what do your results and graph show, and which law or principle do they support?

Check your conclusion

Mitosis produces two genetically identical diploid cells for growth and repair. Meiosis produces four genetically different haploid cells (gametes): the chromosome number is halved, and crossing over and independent assortment create variation. In a root tip most cells are in interphase, the longest stage.

Questions

State two differences between mitosis and meiosis.

Mitosis gives two cells, meiosis four; mitosis keeps the chromosome number (2n), meiosis halves it (n); mitosis gives identical cells, meiosis gives genetically different cells.

What happens to the DNA during interphase?

It replicates, so each chromosome becomes two identical sister chromatids joined at the centromere.

What is a bivalent and when does it form?

A pair of homologous chromosomes lying side by side; it forms in prophase I of meiosis.

Explain how crossing over increases variation.

Chromatids of homologous chromosomes exchange pieces at chiasmata, so new combinations of alleles appear on the chromosomes of the gametes.

Why is the root tip used to observe mitosis?

The root tip (meristem) grows quickly, so many of its cells are dividing by mitosis.

Most cells in the root tip are in interphase. Why?

Interphase is the longest stage of the cell cycle, so at any moment most cells are in it.

For teachers: expected results and how the model works

The model cell has 2n = 4 (one long and one short pair; red = maternal, blue = paternal). DNA units: 2 in G1, 4 after replication; mitosis ends at 2 per cell; meiosis 4 → 2 → 1. Chromosome counts are counted by centromeres (anaphase shows the temporary 8). The root tip field has about 78 % interphase, 10 % prophase, 5 % metaphase, 3 % anaphase and 4 % telophase, chosen at random each field.

Quick check 3 questions · scored

Choose one answer for each question. You see at once if it is right, why, and your score at the end.

1. A cell with 2n = 8 divides by meiosis. How many chromosomes are in each gamete?

2. In which stage do homologous chromosomes pair and cross over?

3. 20 of 250 counted cells are dividing. Mitotic index?

Exam practice Exam set

National-exam style questions on this simulation (13 marks). Try each one on paper first, then open the marking scheme.

A cell with 46 chromosomes divides. State the number of chromosomes in each daughter cell after (a) mitosis, (b) meiosis. (2 marks)

Marking scheme: (a) 46 (1); (b) 23 (1).

In a root tip squash, 300 cells were counted; 15 were in prophase, 6 in metaphase, 4 in anaphase and 5 in telophase. Calculate the mitotic index. (3 marks)

Marking scheme: Cells in mitosis = 15 + 6 + 4 + 5 = 30 (1); mitotic index = 30 ÷ 300 (1) × 100 = 10 % (1).

Describe what happens to the chromosomes during anaphase of mitosis and anaphase I of meiosis. (4 marks)

Marking scheme: Mitosis: centromeres divide (1) and sister chromatids are pulled to opposite poles (1). Anaphase I: homologous chromosomes separate (1); chromatids stay joined at the centromere (1).

Give two ways in which meiosis produces genetic variation. (2 marks)

Marking scheme: Crossing over between homologous chromosomes in prophase I (1); independent (random) assortment of pairs in metaphase I (1).

State two roles of mitosis in living organisms. (2 marks)

Marking scheme: Growth (1); repair and replacement of tissues / asexual reproduction (1).

For class and homework

Teacher resources

📋 Free lesson plan: S3 · Unit 5: Mitosis and meiosis
📋 Free lesson plan: S5 · Unit 4: Cell and nuclear division
Worksheet and marking scheme: use the print buttons below.
Learners can download their results table with ⬇️ Download my results (CSV) under the table, and open it in Excel or Google Sheets.

The worksheet has the steps, an empty results table, a graph grid and all the questions with answer lines. The marking scheme has the expected results and every answer with its marks.

Donat Sciences and MathsBiology · Virtual lab worksheet

School: ........................................ Name: ................................ Class: ........ Date: ............

Mitosis and meiosis: chromosomes, stages and a root tip count

Aim: To describe the stages of mitosis and meiosis, to compare the number of chromosomes and the amount of DNA at each stage, and to calculate the mitotic index of an onion root tip.

Apparatus (real laboratory): onion root tips, 1 M HCl, acetocarmine stain, slides, cover slips, microscope.

What to do (with the simulation on www.donatstem.co.rw)

Choose Mitosis. Use Next stage to go through all the stages. At each stage read the number of chromosomes and the DNA per cell, and tap Record this stage.
Choose Meiosis and do the same for all nine stages. Look for crossing over in prophase I.
Tap New cell and go to metaphase I again. Did the pairs line up the same way? What does this mean for the gametes?
Compare the two lines on the graph of DNA per cell.
Choose Onion root tip. Tap at least 30 cells, one by one, and note the stage of each. Tap Record my count.
Count two more fields of view (tap New field of view) and calculate the average mitotic index.

Results

#	Process	Stage	Cells	Chromosomes per cell	DNA per cell
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

Graph

Plot the DNA per cell (vertical axis) against the stage number for mitosis and for meiosis on the same axes.

Questions

State two differences between mitosis and meiosis. [2]
What happens to the DNA during interphase? [2]
What is a bivalent and when does it form? [2]
Explain how crossing over increases variation. [2]
Why is the root tip used to observe mitosis? [2]
Most cells in the root tip are in interphase. Why? [2]
A cell with 46 chromosomes divides. State the number of chromosomes in each daughter cell after (a) mitosis, (b) meiosis. [2]
In a root tip squash, 300 cells were counted; 15 were in prophase, 6 in metaphase, 4 in anaphase and 5 in telophase. Calculate the mitotic index. [3]
Describe what happens to the chromosomes during anaphase of mitosis and anaphase I of meiosis. [4]
Give two ways in which meiosis produces genetic variation. [2]
State two roles of mitosis in living organisms. [2]

Donat Sciences and MathsBiology · Virtual lab worksheet

School: ........................................ Name: ................................ Class: ........ Date: ............

Marking scheme: Mitosis and meiosis: chromosomes, stages and a root tip count

Expected results

Answers

State two differences between mitosis and meiosis. [2]
Mitosis gives two cells, meiosis four; mitosis keeps the chromosome number (2n), meiosis halves it (n); mitosis gives identical cells, meiosis gives genetically different cells.
What happens to the DNA during interphase? [2]
It replicates, so each chromosome becomes two identical sister chromatids joined at the centromere.
What is a bivalent and when does it form? [2]
A pair of homologous chromosomes lying side by side; it forms in prophase I of meiosis.
Explain how crossing over increases variation. [2]
Chromatids of homologous chromosomes exchange pieces at chiasmata, so new combinations of alleles appear on the chromosomes of the gametes.
Why is the root tip used to observe mitosis? [2]
The root tip (meristem) grows quickly, so many of its cells are dividing by mitosis.
Most cells in the root tip are in interphase. Why? [2]
Interphase is the longest stage of the cell cycle, so at any moment most cells are in it.
A cell with 46 chromosomes divides. State the number of chromosomes in each daughter cell after (a) mitosis, (b) meiosis. [2]
(a) 46 (1); (b) 23 (1).
In a root tip squash, 300 cells were counted; 15 were in prophase, 6 in metaphase, 4 in anaphase and 5 in telophase. Calculate the mitotic index. [3]
Cells in mitosis = 15 + 6 + 4 + 5 = 30 (1); mitotic index = 30 ÷ 300 (1) × 100 = 10 % (1).
Describe what happens to the chromosomes during anaphase of mitosis and anaphase I of meiosis. [4]
Mitosis: centromeres divide (1) and sister chromatids are pulled to opposite poles (1). Anaphase I: homologous chromosomes separate (1); chromatids stay joined at the centromere (1).
Give two ways in which meiosis produces genetic variation. [2]
Crossing over between homologous chromosomes in prophase I (1); independent (random) assortment of pairs in metaphase I (1).
State two roles of mitosis in living organisms. [2]
Growth (1); repair and replacement of tissues / asexual reproduction (1).

Keep learning

S3 biology laboratory activitiesUnit 5 activities S5 biology laboratory activitiesCell and nuclear division S3 biology quizMitosis and meiosis All biology simulationsBack to the list

This simulation is an original model made for Rwandan learners. Real experiments can give slightly different numbers: a simulation never replaces handling real apparatus when your school has it.