Well there's Water, Earth, Fire, and Air. Not quite sure how this would be a school question. But here it is.
Two identical copies formed by the replication of a single chromosomes
Answer:
Prophase 1
Explanation:
Prophase I :It can be divided into five stages:
1)Leplotene: In this stage, chromosome becomes visible and shorten as a single structure.
2)Zygotene: Homologous / sister chromosome pair up. This process is known as synapse. Synapse is the process of attachment of homologous /sister chromosome. One pair comes from the male parent while the other comes from the female parent.
3)Pachytene: Bivalents shorten.
4)Diplotene:Point of attachment of non-sister chromatids. The two chromosomes are seen to be joined at several points along their length. These points are known as Chiasmata.(Chiasmata, a cross). The paternal gene A, B, C may swap for the maternal gene a, b, c for instance, leading to a new gene combination in the resulting chromatids. This is referred to as *CROSSING OVER*.
5)Diakenesis: Homologous centromere move apart.
So, crossing over occurs in *Prophase 1.
