KS3 / KS4 Biology: Human fertilisation

This is the story of your creation. It began with a sperm, the smallest cell in the human body, fusing with the biggest cell… an egg.

Every day, the human male produces hundreds of millions of DNA torpedoes, otherwise known as sperm. Each ejaculation can contain 250 million of them. That's enough, at least in theory, to cover every inch of Manhattan with people. Fortunately, the human female has other ideas. For her, it's quality, not quantity. Her ovaries usually produce just one egg every month.

The process of reproduction is so complex, it's a wonder it happens at all. Most of the time, the sperm die in a pool, trapped inside the vagina. Because the entrance into the womb, through the cervix, is out of reach, blocked shut. For the sperm to have a chance of getting in, timing is everything. There are just a few days in any month when the woman's body offers them an opportunity. Hormones soften the blockage at the entrance to the cervix. Transforming it from a barrier into a life line.

But the sperm have to be strong to make the 15cm journey. It's a long way for the smallest cell in the body. The straighter and faster they can swim, the better chance they have of making it. The deformed, lazy, and the dead are left behind. And, of course, the more fit sperm you have, the greater the chance of success. If 250 million sperm began the journey, only 1%, that's two and a half million, will make it through to the cervix. As they swim out of the vagina, they're just at the beginning of a long and perilous journey.

The woman's body is about to launch an attack. They have entered the cervix - a labyrinth of dead ends, perfect for an ambush. The sperm have triggered the body's defence system. White blood cells have recognised the sperm as a foreign invader and, just as they would if they were fighting an infection, they've been mobilised to kill. They attack the sperm as they swim through the cervix and into the uterus. By the time the surviving sperm reach the fallopian tube, where they are safe from the white blood cells, there could be as few as 20 left. They will be 20 of the very best on offer.

Here, scientists have recently discovered that the woman's body has come up with an amazing trick. She takes control of the sperm by holding them, one by one, on the walls of her fallopian tube. She then powers them down. They're alive, safe, but fast asleep. The woman now has up to five days to release the precious egg growing inside her ovary. As soon as it's ripe, the egg is released. And it's wafted into the opening of the fallopian tube.

Once the egg is ready and waiting, it's time to wake up the sleeping sperm… Sending out a powerful chemical beacon, the egg guides the sperm in. It's the precursor of every new life. The sperm - some male, some female - compete to reach the egg. But there can be only one winner. The competing sperm break off the surrounding cloud of cells. Until one finally pushes through the soft shell underneath. The egg is now in danger. If a second sperm gets in, the egg will die. It must protect itself - and quickly. Under the shell, tiny granules detonate in a chain reaction. Firing out chemicals, hardening the shell, making the egg impenetrable. Fertilised, the egg is now safe.

This was how we all began. You truly are an amazing creation.