How do Organisms Reproduce?

Reproduction is not needed to keep the individual alive. It carries the species forward. The plan of the body is in the DNA. Copying makes two DNA molecules, and each copy is joined by new cell equipment. Only then can both cells stay alive.

No bio-chemical copy is fully reliable. So the offspring are like the parent, but not exact. A variation that spoils the cell too badly kills that cell. The variations that survive are the basis of evolution later. If the niche changes, that small variation can save the species.

Section 7.2 is about modes of reproduction used by a single organism. The bud of yeast was the first mode. The second is bread mould. The book calls this Activity 7.2.

The threads of Rhizopus are hyphae. They are not the reproductive organs. The small round body on a stick is a sporangium. Spores sit inside it. A thick wall protects them until they meet a moist surface and can grow into a new Rhizopus.

In a unicellular organism, division of the cell itself makes a new organism. Amoeba splits into two equal halves. The plane of the split can be any plane.

Leishmania (kala-azar) has a whip-like part at one end. So its binary fission has a fixed direction. The malaria parasite Plasmodium splits into many daughter cells at one time. That is multiple fission.

Cells of a complex organism make tissues and organs, and the organs sit in definite places. Such a body cannot be cut cell by cell into a new organism. The body of Spirogyra is simple enough that broken pieces grow into new filaments. That is fragmentation.

A cut piece of Hydra or Planaria can become a whole organism. That is regeneration. Special cells multiply and, in the order of development, become many kinds of cells. Even so, this is not the ordinary method of reproduction, because organisms normally make offspring without being cut.

For reproduction, Hydra puts out a bud by cell division at one fixed place. The bud becomes a tiny Hydra and separates. The idea is like budding in yeast, but on a multicellular body.

In vegetative propagation a root, stem or leaf becomes a new plant in suitable conditions. Layering and grafting are used for sugarcane, roses and grapes. Such plants can flower and fruit earlier than plants grown from seed.

Banana, orange, rose and jasmine have lost the ability to make seeds, yet they are grown by the vegetative method. The offspring stay like the parent. Buds along the leaf margin of Bryophyllum fall on the soil and become new plants.

On the stem of a money-plant the place where a leaf comes out is a node. A bud sits at the node. The bare part between two leaves does not make a new plant.

In tissue culture, tissue or cells are taken from a growing tip and placed in an artificial medium. They divide fast and make a callus. In a medium with hormones, small plantlets form, and these are placed in soil. Many disease-free plants can be made from one parent. The method is common for ornamental plants.

Sexual reproduction joins variation from two organisms. If DNA doubled in every generation, control would break down. So germ-cells are made by meiosis with half the DNA. The male gamete can move. The female gamete holds stored food.

In a flower the sepals and petals are helper parts. The stamen is the male part and makes pollen. The carpel is the female part: stigma, style and ovary. The ovary holds ovules and the egg. Papaya and watermelon may be unisexual. Hibiscus and mustard are bisexual.

The arrival of pollen on the stigma is pollination. If it happens in the same flower it is self-pollination; if it comes from another flower it is cross-pollination. Wind, water or animals carry pollen. A pollen tube grows through the style to the ovary. Fertilisation makes a zygote. The ovule becomes a seed and the ovary a fruit.

At puberty the reproductive tissues mature, as the general growth of the body begins to slow. Both boys and girls may grow new hair, oily skin and pimples. In girls the breasts develop and menstruation begins. In boys facial hair appears and the voice changes. These changes are slow and are not completed at the same age in every person.

The testes lie outside the abdomen in the scrotum, because sperm formation needs a temperature lower than body temperature. The testes also make testosterone. Sperms travel through the vas deferens. The prostate and the seminal vesicles add a fluid that gives nutrition and helps transport. The urethra is a common passage for both sperms and urine.

Eggs are made in the ovaries. Each month one egg enters an oviduct. Fertilisation happens in the oviduct. The zygote divides into an embryo and settles in the wall of the uterus. The uterus opens into the vagina through the cervix.

The placenta is a disc set in the wall of the uterus. Villi lie on the embryo side and blood spaces on the mother side. Glucose and oxygen pass from the mother to the embryo. Waste of the embryo passes into the mother blood. Pregnancy lasts about nine months.

If the egg is not fertilised it lives for about a day. The thick lining of the uterus is no longer needed. It leaves through the vagina as blood and mucus. This is menstruation. It happens about every month and usually lasts two to eight days.

Ways to avoid pregnancy: a mechanical barrier such as a condom stops sperm from reaching the egg and also lowers the risk of many infections to some extent. Pills change the hormone balance so that an egg is not released, but side-effects can occur. A loop or a copper-T placed in the uterus avoids pregnancy, not infection. If the vas deferens or the fallopian tube is closed by surgery, fertilisation does not occur. Surgery done poorly can itself cause infection. The law stops sex-selective removal of a female foetus after a test before birth.