Biology

Reproduction

In sexual reproduction, the parent organism produces sex cells called gametes. Eggs and Sperm are examples of gametes. When two of these gametes join and their nuclei fuse together, this is called fertilization. The new cell which is formed by fertilization is called a zygote. The zygote divides again and again to eventually form the organism. The 46 chromosomes in an ordinary human cell are of 23 different kinds. There are two of each kind. The two chromosomes of one kind are called homologous chromosomes. A cell which has the full number of chromosomes is called a diploid cell. An egg or sperm, only has 23 chromosomes and it is called an haploid cell. Gametes are always haploid.

MeiosisGametes need to have only half as many chromosomes (23) as their parent cell (46), division by mitosis will not work. When gametes are being made, cells divide in a different way called meiosis.

MITOSIS VS MEIOSIS

The Female Reproductive SystemThe female gametes are called ovum/ova (egg/eggs) which is large and stationery when compared to sperm. Eggs are made in both ovaries which is the site for meiosis.

The Male Reproductive SystemThe male gametes are called spermatozoa or sperm which is small, mobile and large in number when compared to an egg. They are made in both testes which is the site for meiosis.

The Menstrual Cycle

If the egg is not fertilized, it is dead by the time it reaches the uterus. It does not sink into the spongy wall, but continues onwards, down through the vagina. As the spongy lining is not needed now, it gradually disintegrates. It too, is slowly lost through the vagina and this is called menstruation or period.

Initially, a follicle develops inside an ovary. The developing follicle secretes a hormone called oestrogen. The oestrogen makes the lining of the uterus grow thick and spongy. When the follicle is fully developed, ovulation takes place. The follicle stops secreting oestrogen and it becomes a corpus lutem. The corpus luteum starts to secrete another hormone called progesterone. Progesterone keeps the uterus lining thick and spongy, and well supplied with blood, in case the egg is fertilised. If it is not fertilised, then the corpus luteum gradually disappears. As a result, progesterone is not secreted any more, and so the lining of the uterus breaks down and menstruation occurs. A new follicle starts to develop in the ovary and the cycle begins again.

However, if the egg is fertilised, the corpus luteum does not degenerate so quickly. It carries on secreting progesterone until the embryo sinks into the uterus wall and a placenta develops. Then the placenta secretes progesterone and carries on secreting it all through the pregnancy. The progesterone maintains the uterus lining, so that menstruation does not happen during the pregnancy.

Sexual Reproduction of a Flower

Pollination

Pollen must be carried from the anther to a stigma. For fertilisation to take place the male gametes must travel to the female gametes which is pollination. Sometimes, pollen is carried to the stigma of the same flower or to another flower on the same plant which is called self-pollination. If pollen is taken to a flower on a different plant of the same species, this is called cross-pollination. Flowers are normally pollinated by wind-pollination or insect-pollination.

Wind Pollination Insect Pollination-small, inconspicuous petals or none. - large, conspicuous petals-No scent or nectaries - Have nectaries and strongly scented-Anthers dangling outside flower - Anthers and stigmas inside for insect to brush past it-Stigma large and feathery outside- Pollen grains are smooth and light -Pollen grains are sticky or spiky.