Meiosis I and meiosis II

Understand the idea

Meiosis is eukaryotic cell division that makes haploid gametes. A diploid human cell has 46 chromosomes (23 pairs). After meiosis, egg or sperm has 23. Fertilisation returns the number to 46. Meiosis is two divisions after one DNA copy: meiosis I then meiosis II.

Meiosis I separates homologous pairs. In prophase I, homologues pair, form tetrads, and may swap DNA at chiasmata (crossing over). Metaphase I lines bivalents; anaphase I sends homologues to opposite poles. The two cells are already haploid for chromosome pairs, though each chromosome still has two chromatids.

Meiosis II is like a mitosis of those haploid cells: sister chromatids separate. Variety comes from crossing over in meiosis I and from random lining-up of maternal and paternal chromosomes. Humans have 23 pairs, so independent assortment alone allows over 8 million combinations.

What to remember

• Meiosis: one DNA copy, two divisions, haploid gametes.
• Meiosis I: homologues pair, cross over, then separate.
• Meiosis II: sister chromatids separate.

Recap

Meiosis exists so chromosome number does not double every generation, and so offspring are not copies. Pairing, crossing over, and a second split are the story.