How can I understand the difference in ploidy between meiosis and mitosis?

Instead of trying to memorise the facts by heart, the differences between meiosis and mitosis become much easier to understand if we reason on what the purpose of each process is. The key is that: – MITOSIS creates clones of the mother cell. – MEIOSIS has the purpose of creating gametes for reproduction. If we remember this then we can reason that mitosis needs to produce diploid cells (like the mother cell), because they need to be ready to go and survive on their own. On the other hand, because the products of meiosis will need to fuse with another cell before they can have a complete genome, they must have half the genetic information – they must be haploid.Once we understand this important fact, all the other details will fall into place more easily and make a lot more sense. As one example, both processes start from a single cell, so the genetic material needs to "add up": because mitosis produces two diploid cells, meiosis will have to produce four haploid cells!