The final stage of meiosis in the ovum doesn’t take place until the sperm starts to penetrate the surface cell membrane. Then it is mitosis all the way to produce the billions of cells needed to make a new individual – until the first meiotic divisions take place to form the oocytes in the ovaries if the fetus is a girl.
It takes many sperm to fertilise an egg, although only one will penetrate the ovum. The enzymes from many acrosomes are needed to digest away the protective zona pellucida. When the head of a single sperm touches the membrane of the ovum it triggers a number of changes
Sperm entering the ovum
- The chromosomes of the secondary oocyte move into their second meiotic division to give a haploid ovum nucleus
- Ion channels in the membrane change so the inside of the ovum becomes positive with respect to the outside instead of negative. The charge difference temporarily blocks the entry of any more sperm
- A fertilisation membrane forms around the membrane, blocking the entry of any more sperm and allowing the ion balance to return to normal.
- The head of the sperm moves into the ovum leaving the tail outside
- It absorbs water and swells, releasing the chromosomes. They combine with the chromosomes of the ovum at the end of their second meiotic division to form a diploid zygote.
- The zygote undergoes mitosis to form an embryo, a fetus and eventually a baby.
The moment of fertilisation (Photo credit: cropbot. Licensed under the Public Domain Mark 1.0)
Early human embryo undergoing mitosis (Photo credit: Ekem. Licensed under the Public Domain Mark 1.0)
Produce an infographic on meiosis and mitosis in human beings – find out as many useful and interesting facts as possible about length of divisions, number of divisions, number of cells made etc and display as effectively as possible.