GCSE Biology: Reproduction

Justina: There are about 250 babies born every minute around the world.

Georgina: And every single one of them is genetically unique.

Justina: Formed through an incredible process that starts at the cellular level.

Georgina: And that’s what we’re going to be looking at today: reproduction.

Justina: There are two main forms of reproduction: asexual and sexual.Asexual reproduction is generally quicker as only one parent is needed.It uses the process of mitosis to create identical copies of the parent cell.Bacteria, most fungi and some plants use asexual reproduction to create offspring.

Georgina: But humans and many other animals go through a process called sexual reproduction.Sexual reproduction is a slower process, requiring two parents, that usually involves the joining of two reproductive cells, or gametes, to create genetically unique offspring.In humans, the two gametes are the sperm cell from the male parent and the ovum, or egg, from the female parent.

Both the male and female reproductive systems produce hormones which play an important role in sexual reproduction.The testes release testosterone, which causes male secondary sexual characteristics to develop at puberty, such as the enlarging of the testes so they can produce sperm cells.The ovaries release oestrogen, which causes female secondary sexual characteristics to develop at puberty, such as the start of the menstrual cycle, which controls the release of eggs by the ovary into the oviduct, ready for fertilisation.But how does the human reproductive system actually work?The male gamete, the sperm cells, are produced in the testes.They travel past the prostate gland, which secretes fluid to carry the sperm down the urethra, where it can be delivered via the penis, into the female reproductive system during sexual intercourse.Sperm cells are highly adapted.They have a tail called a flagellum, that helps them swim through the female reproductive system.They also have many mitochondria that release energy for swimming, and they contain a haploid nucleus which contains half the number of chromosomes required to develop a human embryo:

Georgina: The sperm is delivered through the vagina into the uterus, where it swims up into the oviduct.The female reproductive system produces eggs, or ova, in the ovaries.The ovum then travels to the oviduct.It’s in the oviducts that a sperm cell meets and joins with an ovum in a process called fertilisation.Each egg, or ovum, like the sperm cell, contains a haploid nucleus with Georgina:3 chromosomes.the sperm and ovum join in the oviduct to form a diploid zygote, which now contains the 46 chromosomes required for a human embryo to develop.This zygote is the very first cell of a new embryo.As the zygote travels towards the uterus, it divides by mitosis to form a ball of cells, known as an embryo.It’s implanted in the uterus wall, where it begins to receive nutrients and grow into a foetus.There are a variety of ways to prevent fertilisation from occurring and this is known as contraception.Barrier methods such as condoms, prevent sperm from entering the vagina.Hormonal contraceptives, such as contraceptive pills, prevent eggs from being matured and released by the ovaries.And there are surgical contraceptives such as a vasectomy, that cuts the sperm tubes to prevent sperm from entering the penis.

Georgina: But if fertilisation has happened, after a 40 week gestation period in the uterus, the fully formed foetus passes through the cervix and vagina, and out into the world.

Justina: And a baby is born.

Georgina: Exactly.