Science Buddies: "Ask an Expert"

I am preparing for my big end of year biology exam and I have come across an area that has me a bit confused. I am confused whether meiosis occurs before or after fertilisation. It is my understanding that meiosis is the prosses in which gametes are produced, for example, a sperm cell or an egg cell. However many of the resources I have come across online, talk about the diploid cell that undergoes meiosis, consisting of half parental chromosomes and half maternal chromosomes. I thought this combination occurred only after fertilisation? this one has me pretty stumped, any help would be greatly appreciated.

"Fertilization establishes the regular diploid number of chromosomes in the zygote. Meiosis occurs over two cycles of cell division, which sperm cells complete before fertilization. Meiosis in the egg cell stops during metaphase of the second cycle."

https://sciencing.com/happens-zygote-fo ... 20062.html

Hopefully this helped

Hi Huemaster, this is a confusing topic because the terms aren't specific.

This will be easier if we discuss it in terms of N, which I'm assuming you've seen. N refers to the number of complete chromosome sets existing in a cell. All of our non-reproductive cells are 2N, meaning one set of chromosomes from each parent.

So you are correct, meiosis starts with 1 original diploid cell (2N), which possesses 1N from that person's mom and 1N from their dad. The diploid replicates its DNA (4N) and then divides twice, making 4 gametes that are 1N each.

At fertilization, 2 gametes combine, one carrying 1N from mom, and one carrying 1N from dad, again making a 2N cell.

The reason the research you've been reading is confusing is because its focus is meiosis, and not fertilization. So the 'one set from each parent' being mentioned is pre-fertilization, talking about the makeup of the 2N cell that will eventually make gametes. This is the focus of a lot of research, because the second step of meiosis, (1 2N cell -> 2 1N gametes) is craaaazy complicated and amazing.

If you have a textbook I'd stick to learning its diagrams for your test.

Good luck!

Hi Huemaster,
LilGreenFrog has described it very nicely already and I hope things are making more sense now. I would like to add a comment so this is even simpler to understand: The half paternal chromosomes and half maternal chromosomes in a diploid cell undergoing meiosis come from the previous generation. If person A's cell (2N) is undergoing meiosis, recombination is happening between DNA they inherited from both their Mom and Dad so that the gametes produced (1N) inherit a bit of DNA from both sides of the family. This gamete will then fuse with another gamete during fertilization to create a 2N diploid cell with its own unique maternal and paternal DNA combinations. Hope this helps!