Reproduction is one of the things that all living things do.
All animals and plants will die and are programmed to reproduce, to organise replacements for themselves
This is a very primitive method of reproduction in a species. It allows one animal, or plant to produce copies of itself, without the need for one male and one female sex cells (gametes) to combine.
Species that reproduce asexually have not changed for millions of years, because there is no mixing of genes at every new generation, no survival of the fittest, no natural selection of the 'best' genes. Bacteria and fungi reproduce using asexual reproduction.
There are other species that use both asexual reproduction and sexual reproduction; strawberry plants, buttercups and hammerhead sharks all use both techniques. Buttercups and strawberry plants send out 'runners', above ground roots, which develop into new plants where they touch the ground. The new plants are genetically identical to the parent plant. In some species of sharks and turkeys the female resorts to asexual reproduction when no male is available.
This is the means by which natural selection occurs. The animals most suited to their environment survive longest and are most successful at reproduction, so their genes become more common in the total population.
Whenever sexual reproduction occurs, whether it is in plants or anim
als, two sex cells called gametes must join together to form a single cell that then develops into a new organism.
Sex cells, gametes, have only half the number of chromosomes that a normal cell has. When the two gametes combine, the new cell will have the whole complement of chromosomes, a totally unique combination of genes and the potential to develop into a new plant or animal.
Flowers are the plants' sex organs. Most flowers produce both male and female sex cells, though there are some plant species, like courgettes, which have two types of flower, one male and one female.
Pollen grains are the male gametes, the female ones are called ovules or eggs. Pollen grains are very small so are transported by insects or wind to other flowers, where they land on the sticky stigma in the middle of the flower. The pollen grains 'dissolve away' a tube down the stigma and style and eventually end up in the ovary, where they join with ovules in a
process called fertilisation. The fertilised egg cell develops into a seed, which is then dispersed by the plant. Plant reproduction is very inefficient in some ways producing massive numbers of seeds because only a small percentage will survive and grow into new plants.
Sexual reproduction in animals needs both male and female animals. The female produces gametes called eggs in her ovaries. The male produces gametes called sperm. When the two gametes combine the fertilised egg, called a zygote, develops into a new animal.
Reproduction in Fish
In most fish the male and female swim close to each other, releasing male and female gametes in close proximity to each other to increase the likelihood of their meeting and joining. The fertilised eggs develop in the water.
Reproduction in Birds and Reptiles
In birds and reptiles the female gamete is fertilised inside the female. In birds most males have no phallus so the male places his genital opening next to that of the female and sperm are transferred to the female. They are stored and used to fertilise each egg cell as it is produced, before the shell forms.
Reptilian reproduction is very similar to that of birds. Males of most reptiles have a penis that is used to place the sperm deeper inside the female. The fertilised eggs are then laid, in a similar way to birds. Reptiles' eggs have a leathery 'shell' different from birds' hard shell.
Reproduction in Mammals
Mammalian reproduction is very complex because the blood supply of foetus and mother need to be kept separate.
The female's ovaries produce egg cells. The eggs travel down the fallopian tubes, egg tubes, towards the uterus, womb.
The male's sperm, produced in the testes, are placed in the female's vagina, using his penis, which becomes rigid to enable penetration of the vagina to take place.
The sperm travel up the vagina and uterus to the fallopian tubes, where they join with the egg cells to form fertilised eggs, zygotes. The zygotes implant themselves in the wall of the uterus.
A placenta develops that enables the mother's blood supply to be kept separate from that of the developing embryo. The embryo's blood vessels and the mother's blood vessels are very close together in the placenta. This allows oxygen and nutrients from the mother's blood supply to cross to that of the embryo. Waste carbon dioxide and other waste products from the embryo's blood supply cross the placenta in the opposite direction to the mother's blood supply. The embryo's waste products are excreted by the mother's body, in the same way as her own.
The time the embryo spends in the womb is called the gestation period of that animal. Mice have a gestation period of 19 days. A female elephant, at the other extreme, is pregnant for 22 months.
Mammalian reproduction is different to other classes in that female mammals produce milk for the new-born animal in mammary glands. The babies' digestive system has not developed fully at birth and the milk provides an easily digested source of nutrients
Reproduction in Humans
Humans are mammals and reproduce in this way. In humans the female's ovaries start to produce egg cells between the ages of 9 and 16. The male's testes start to produce sperm from about 13 or 14 years of age.
Human females' ovaries usually produce a single egg about every 28 days. The walls of the uterus become thick and rich with blood vessels, ready to receive the zygote if an egg is fertilised. If the egg is not fertilised the uterus lining breaks down and passes down the vagina, causing bleeding. This bleeding is called a period or menstruation.
After the period the lining builds up again ready for the next occasion that a fertilised egg may implant itself.
Human reproduction is usually a deliberate act of love, rather than the reproduction by instinct of other animals.
Artificial insemination is where sperm are injected into the vagina, using a syringe. This is widely used in cattle farming, to fertilise the cow's egg without the risk of her being harmed by the weight of the bull as he copulates with her, from behind.
There are different forms of assisted reproduction in humans. Test-tube babies are the result of human sperm and egg cells being put together in the laboratory. Fertilised eggs that result are implanted in the woman's uterus and she has a normal pregnancy in every other respect.