7. How do Organisms Reproduce?

DNA in the nucleus carries information for making proteins. These proteins decide the body design and features of an organism. In reproduction, DNA copying passes this information from parents to offspring. So the offspring have a body design similar to the parents.

Variations help some individuals of a species to survive when the environment changes. For example, some bacteria that can bear heat survive a heat wave. So the species does not get wiped out.

But a variation may not be useful to every individual in its present surroundings. So variation is beneficial to the species, but not necessarily to the individual.

• Spores are produced in large numbers.
• They are covered by thick walls. So they are protected till they find moist conditions to grow.
• Spores are light. So they spread easily to new places.

Complex organisms have a very complex body design. Their bodies are made of organs and organ systems with specialised cells. A cut piece of the body cannot make all these organs again. So they cannot give rise to new individuals through regeneration.

• Plants like banana, orange, rose and jasmine have lost the ability to produce seeds. They can be grown only by vegetative propagation.
• Plants grown this way bear flowers and fruits earlier than plants grown from seeds.
• All the new plants are genetically similar to the parent plant. So good qualities are kept.

DNA carries the information for the body design of an organism. During reproduction, a copy of DNA must pass to the new cell. Only then can the new organism have the same body design as the parent. So DNA copying is essential.

They add their secretions to the sperms. These secretions give a fluid medium in which sperms can move easily. They also provide nutrition to the sperms.

• Breasts start to develop.
• Hair grows in the armpits and genital area.
• Menstruation (periods) starts.
• Skin becomes oily and pimples may appear.

The embryo gets nourishment from the mother’s blood through a special tissue called the placenta. The placenta has villi on the embryo’s side, which give a large surface area. Glucose and oxygen pass from the mother’s blood to the embryo through it. Wastes from the embryo are removed through the placenta.

No. Copper-T is placed in the uterus. It only prevents pregnancy. It does not stop contact of body fluids. So it does not protect from sexually transmitted diseases.

Answer: (b) Yeast.

Answer: (c) Vas deferens

Vas deferens is a part of the male reproductive system.

Answer: (d) pollen grains.

• Sexual reproduction involves two parents. So it produces more variations.
• Variations help the species to survive in changing environments.
• Variations also help in the evolution of new species.

• Testes produce the male germ cells, called sperms.
• Testes secrete the hormone testosterone. It controls the production of sperms and brings about changes in boys at puberty.

Every month, the uterus prepares itself to receive a fertilised egg. Its lining becomes thick and spongy and gets a rich blood supply.

If the egg is not fertilised, this lining is not needed. So it breaks and comes out through the vagina as blood and mucus. This is called menstruation. It occurs roughly every month and lasts for about two to eight days.

• Barrier methods: Condoms and similar coverings stop the sperm from reaching the egg.
• Hormonal methods: Oral pills change the hormonal balance so that eggs are not released.
• Intra-uterine devices: Copper-T or loop is placed in the uterus to prevent pregnancy.
• Surgical methods: The vas deferens in males or the fallopian tube in females is blocked.

Unicellular organisms have a simple body. They reproduce by fission, budding or multiple fission. The single cell divides to form new individuals.

Multicellular organisms have a complex body with specialised cells. So they use methods like fragmentation, regeneration, spore formation, vegetative propagation and sexual reproduction. Complex organisms reproduce sexually using special reproductive organs and germ cells.

Reproduction produces new individuals of the same kind. It replaces the individuals that die. So the population size of the species is maintained. Small variations in reproduction help the species to survive in changing conditions. In this way, reproduction gives stability to populations.

• To prevent unwanted pregnancies.
• To keep a gap between births. This keeps the mother and child healthy.
• To control the population size.
• Some methods like condoms also prevent sexually transmitted diseases.