In plant cells, the daughter nuclei are separated from each other by the formation of a cell wall. In animal cells, the cytoplasm invaginates to form two daughter cells. A human body cell contains 23 pairs of chromosomes. A human sperm nucleus has 23 single chromosomes, one from each pair in a testis cell. Similarly, an egg contains one chromosome from each pair in an ovarian cell.
Fertilisation is a key feature of sexual reproduction.
Here, the nuclei of the sperm and the egg join to form a new nucleus, called the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg. Before a cell divides, the chromosomes are copied exactly. The DNA of each chromosome is replicated to form two chromatids. The chromosomes migrate to the equator of the cell where they pair to form bivalents.