We are currently working through the specification for an IGCSE in Biology. The following is copied straight from that specification and is all the information we need to know about meiosis:
Understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes.
Know that in human cells the diploid number of chromosomes is 46 and the haploid number is 23
The specification mentions the terms diploid and haploid. Ploidy refers to the number of chromosomes in a cell. Cells with two sets of chromosomes are refered to as diploid cells and cells with only one set of chromosomes are called haploid cells.
Meiosis is a form of cell division which forms gamete cells, however in its case the daughter cells are not identical either to each other or to their parent cells. Instead of containing the parental diploid number of chromosomes, the four daughter cells contain a haploid (or half) number of chromosomes. It is a specialised form of cell division which occurs in all sexually reproducing Eukaryotic cells including animal, plants and fungi.
Before I actually told them anything, I asked them to think about what they already knew about cell division (ie mitosis) and try to work out how one parent cell might replicate to make four daughter cells, each with half the amount of genetic material compared with the parent cell. They were all flummoxed. I gave them a hint about how mitosis might be used with some changes to create haploid daughter cells from a diploid parent. T13 figured it out immediately, followed quickly by the girls. They all realised that in order to obtain four daughter cells a parent cell would need to go through mitosis twice, and in order to create daughter cells with half the chromosomal material one stage would need to be left out of the second mitotic division. They agreed that there would be no interphase stage during the second division. Bingo!
To help consolidate this new found knowledge of meiosis we all watched the following video:
The most important thing for the children to understand was that meiosis describes the event which allows our bodies to create gametes. Human gametes are otherwise known as sperm and egg. A diploid human cell contains 46 paired chromosomes, whereas the haploid cell contains only 23. Gametes are a haploid cell. The sperm contains 23 of the father’s chromosomes whereas the egg contains 23 of the mother’s chromosomes. Thus when they join together during the process of fertilisation they with form a zygote cell which will contain a complete complement of paired chromosomes – 23 from the father and 23 from the mother. We would be covering genetic variation and mutations during our next lesson. For now though, I wanted to focus on how the process of meiosis is able to form four haploid cells from one diploid cell.
With this in mind I asked the children to do a similar exercise which they had done for mitosis. I gave them plates (cells), labels and jelly beans (chromosomes) and asked them to produce a simple model showing what happens to a cell during meiosis:
This model really was a very simple representation of meiosis, and wasn’t quite as accurate as the mitosis one we had made the day before.
You all know that we love any kind of hands on activity so I set about creating a tray full of the makings of a more complete meiosis model – a meiosis stages print out, labels and playdough in different colours:
Meiosis is essential a replica of mitosis which happens twice, although the second meiosis (known as Meiosis II) does not include the interphase stage of mitosis as no more chromosomal material is required for the formation of the haploid cells. You might remember that interphase is the stage of mitosis which allows the cell to replicate its genetic material. As only half the amount is required to form the haploid cell no more genetic material is required:
With sheet in hand the children worked as a team to create all the stages of meiosis from playdough:
If I had known it would take almost two hours to complete I possibly wouldn’t have started. I mean surely there are better things to do with our time than play with play dough? That said there was lots of chat around that table as we talked through some teen issues which were obviously bothering them. There was also lots of fun, as well as heaps of laughter. And if they learnt nothing, it was still probably worth it for the relationship building banter.
Having now spent time learning about mitosis and meiosis I asked the children to compare the two. I had printed off a venn diagram for them to fill in as they went along. This was great preparation for the short essay I wanted them to write comparing the two cell divisions, under exam conditions, which they will be doing tonight.
Our next genetics lessons will be focusing on genetic variation and how it is achieved.
Please see the science and maths page for all our lessons from the IGCSE Biology (scroll down to the bottom of the page)