Prophase I
- Homologous chromosomes pair and crossing over, or recombination, occurs--a step unique to meiosis.
- Chromosomes form structures called synapsis.
- The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent.
- At this stage, non-sister chromatids may cross-over at points called chiasmata.
- The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads."
- During this stage, individual chromosomes begin to condense into long strands within the nucleus.
- However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
- The chromosomes in the leptotene stage show a specific arrangement where the telomeres are oriented towards the nuclear membrane.
- Hence, this stage is called "bouquet stage".