The Nuts and Bolts of Fertility

Quickfire science on sperm and eggs.
21 March 2017


How do you make a baby? Tom Crawford and Georgia Mills have been finding out; here’s the “nuts and bolts” about the cells that make reproduction possible…

Tom - Men produce sperm non-stop throughout their life. It takes about two months per sperm cell from beginning to end.

Georgia - Women are very different. They start making eggs nine weeks after fertilisation and by the time they’re born they have all of their eggs already in their ovaries that they will ever produce.

Tom - Males make loads of sperm - half a teaspoon contains about 200 million. Each sperm has a lifetime of between 40 and 70 days.

Georgia - In comparison, females only release about 500 eggs during their entire reproductive life.

Tom - Sperm are tiny. The head is 1/2000th of a centimetre, but if you lined up all 200 million head to tail, they would extend over six miles.

Georgia - Eggs, on the other hand, are one of the largest cells in the human body at about 0.2 millimetres in size. They can just about be seen with the human eye.

Tom - Sperm can survive for about two days inside a woman and swim towards an egg by following chemical signals that they sniff out.

Georgia - Eggs ensure that only one sperm can enter and fertilise them by transforming their surface coat into an impenetrable outer shell immediately after fertilisation.

Tom - Both eggs and sperm have only half of the DNA that’s found in all other cells. Sperm have either an X or a Y chromosome.

Georgia - Eggs can only have an X chromosome so, if they meet a Y carrying sperm a male baby is formed, and an X bearing sperm results in a female. So sperm, in other words Dad, determines the sex of a future baby.


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