Cell Division: Mitosis and Meiosis
Cell Division Glossary
Chromosome - A molecule of DNA wrapped around histones that becomes visible during prophase of cell division.
Chromatid - A replicated chromosome: each strand of the 'X' is a chromatid.
Diploid - Cells that have two copies of each chromosome in their nuclei.
Haploid - Cells that have one copy of each chromosome in their nuclei.
Homologous Chromosomes - Chromosomes that have the same genes in the same places. For each homologous pair, one comes from the mother, the other from the father.
What are the Stages of Mitosis and Meiosis?
All living things are made of cells - this is one of the key parts of Cell Theory. In order to survive, these cells must be able to make more cells. Cells achieve this using Cell Division - perhaps one of the more complicated topics I teach my A Level Biology students.
Cell division is the process by which biological cells multiply. There are three major types of cell division:
- Mitosis - used by Eukaryotic organisms to grow or reproduce asexually;
- Meiosis - used by Eukaryotic organisms to create sex cells (gametes);
- Binary Fission - used by Prokaryotic organisms to reproduce.
Despite their differences, remember that all three types of cell division begin with DNA replication - the act of doubling the amount of DNA in the cell.
Teaching Tip! The stages of cell division are complex and subtle. When examined on them, try to use labelled diagrams and tables to summarise the key information. Also ensure that all key terms are used appropriately, as this can save you time and energy.
The Stages of MitosisClick thumbnail to view full-size
Mitosis is the process of cell division that forms two genetically identical nuclei from on parent cell nucleus. It is used for:
- Asexual reproduction (e.g. Paramecium)
- Growth (increasing cell number)
- Repair and Maintenance (replace damaged cells with identical replacements)
Although we traditionally break down mitosis into a series of stages and sub-stages, it is actually a continuous process. In the micrographs opposite, you can see that mitosis is not necessarily synchronised and looks much messier than the clean, idealised textbook diagrams!
Prophase - The chromosomes supercoil and become visible under a light microscope. The chromosomes assume their classic 'X' shape - two sister chromatids joined in the middle at the centromere. Other key events are:
- Nuclear Envelope breaks down;
- Centriole divides in two, travels to opposite poles of the cell to form the spindle.
Metaphase - An easy stage to identify, Metaphase is characterised by the chromosomes lining up, single file, along the middle (the equator) of the cell. At this point, each chromosome becomes attached to the spindle at its' centromere.
Anaphase - Another easily recognisable stage! Anaphase sees the chromosomes split at the centromere, separating the sister chromatids:
- The sister chromatids are pulled apart to opposite poles of the cell
- At this point, each chromatid becomes an individual chromosome - identical to the original parent chromosome
- Spindle fibres shorten, pulling each chromatid by the centromere - this causes the chromatids to look like Vs
Telophase - a simple stage to recognise - you will see two nuclei starting to form in early telophase; in late telophase you will no longer be able to see the chromosomes, just two complete nuclei at opposite ends of the cell.
Interphase, Prophase, Metaphase, Anaphase and Telophase
DNA is replicated, Cell builds up energy reseves and grows.
DNA Packaged - the chromosomes shorten and thicken
Chromosomes line up in the middle of the cell
Chromatids break apart at the centromere and move to opposite poles
Two nuclei formed after nuclear envelopes reform around each group of chromosomes
Summary of Mitosis
Do you think it is important for High School Students to learn about cell division?
The Stages of Meiosis
Human body cells have 46 chromosomes. These are arranged in pairs, with one copy of each chromosome from Mum, and the other from Dad. If your sperm and eggs were made using mitosis, when these two cells fused at fertilisation, the egg would have 96 chromosomes. Definitely not human!
Meiosis is the process of cell division that halves the chromosome number and makes gametes (human gametes contain 23 chromosomes). This ensures that at fertilisation the number of chromosomes found in normal body cells - the diploid number - is restored.
Meiosis involves two divisions of the nucleus:
- The first division generates most of the variation
- The second division halves the chromosome number.
Meiosis is a very technical process that is most easily described in diagrams and tables (see above and below).
What are Homologous Chromosomes?
In the photo to the right, you can see all the chromosomes from a human cell. In the inset box, you see all of the chromosomes paired up according to size - this is called a karyotype. Each pair is known as a homologous pair. In each pair, one chromosome comes from Mum and the other comes from Dad.
Homologous chromosomes are:
- The same size;
- The same shape;
- Have the centromere in the same place;
- Have the same genes.
The X and Y chromosomes have only one small region that is homologous, but are still grouped together as they determine the sex of an individual.
What is Crossing Over?
Crossing over (and recombination) is where homologous chromosomes exchange genetic material and create 'recombinant chromosomes.'
During Prophase I, homologous chromosomes line up with each other and swap small amounts of their DNA from similar regions. Without recombination and crossing over, all alleles on a single chromosome would be inherited together. This system creates new gene combinations which increases our capacity (as a species) to respond to environmental change.
Chromosomes condense, Crossing over occurs
Homologous chromosomes pair up and align in middle of cell
Homologous chromosomes pulled apart
Nuclear Envelope reforms
Cell splits into two
Centrioles divide and move to opposite poles
Chromosomes attach to spindle fibres and line up along equator
Sister chromatids break apart at centromere and migrate to opposite poles
Nuclei reform, Chromosomes uncoil
Summary of Meiosis
Cell Division Resources
Comparison of Mitosis and Meiosis
Number of daughter cells made
Are the daughter cells identical
Number of nuclear divisions?
Haploid or Diploid?
Where does this occur?
Do homologous chromosomes pair up?
Growth and Asexual Reproduction
To make sex cells (gametes)
Crash Course in Meiosis
The above video analyses meiosis in great detail. It also has links to videos on related topics. This is a fantastic revision resource that all of my students are required to watch.
Cell Division Links
- Cell Division Animations
Excellent animations showing the stages of mitosis, meiosis and many more key biological processes. (Select using the drop-down menu in the top left corner)
- Meiosis Tutorial
A series of illustrations showing the stages of meiosis. Complete with knowledge check. Suitable for Degree level students (similar pages exist for mitosis)
- Mitosis: An Interactive Animation
Interactive animation showing the stages of animal cell mitosis.
© 2013 Rhys Baker