Chapter 7: How do Organisms Reproduce? (Biology)

DNA in the nucleus carries the information for making proteins, and hence for the body design and characteristics of the organism. When a cell reproduces, it makes a copy of its DNA so that each new cell gets the complete set of instructions. This ensures that the offspring has the same basic body design as the parent and can survive in the same habitat. Small copying changes also create the variation on which evolution depends.

It passes the information for body design and characteristics from parent to offspring, and its small errors create variation.

Conditions such as temperature or water level can change drastically. If all members of a species were identical, all might die. If there is variation, some individuals may be better suited to the new conditions and survive, so the species continues. For an individual, however, a particular variation may be useless or even harmful in its present surroundings.

Variation helps some members of a species survive changing conditions, so the species survives; for a single individual a variation may not be useful.

Binary fission gives two daughter cells; multiple fission gives many at the same time.

• A single plant (like Rhizopus) produces a very large number of spores.
• Spores are tiny and light, so they spread easily through air and water to new places.
• Each spore has a thick protective wall, so it can survive unfavourable conditions until it reaches a moist surface and germinates.

Spores are produced in large numbers, spread easily and survive harsh conditions because of their thick walls.

Complex organisms have many kinds of specialised cells organised into tissues, organs and organ systems that work together. Regeneration needs cells that can divide and then develop into all these different cell types in the right arrangement, which is not possible from a cut piece of such a body. So only simple organisms such as Hydra and Planaria can regenerate.

Their bodies are made of highly specialised tissues and organ systems, which cannot all be rebuilt from a single body part.

• Plants that do not produce viable seeds (such as banana, orange, rose and jasmine) can be grown.
• New plants bear flowers and fruits earlier than those grown from seed.
• All the new plants are genetically similar to the parent, so desirable characters (such as fruit quality) are kept.
• It is quick and easy.

It allows plants without viable seeds to be grown, gives earlier flowering and fruiting, and keeps the parent's desirable traits.

Reproduction makes new individuals with the same body design as the parent. The information for this design is in the DNA. So the cell must copy its DNA and give one copy to each new cell; without this, the new cells would not have the information to function and grow. (The cell also makes extra cellular apparatus to go with the DNA.)

The body design information is in DNA, so each new individual needs its own copy of it.

Pollination is the transfer of pollen to the stigma; fertilisation is the fusion of male and female gametes inside the ovule, which happens after it.

They add their secretions to the sperms. The fluid provides nourishment to the sperms and makes their transport easier by giving them a liquid medium (semen).

Their secretions nourish the sperms and make their movement easier.

• The breasts begin to develop and the nipples darken.
• Hair grows in the armpits and the genital area.
• Menstruation (periods) begins.
• The hips widen, and there is a growth spurt.
• The skin may become oily and pimples may appear.

Breast development, growth of hair in the armpits and genital area, the start of menstruation, and widening of the hips.

The embryo is attached to the wall of the uterus by a special tissue called the placenta. It is a disc with villi on the embryo's side, surrounded by blood spaces from the mother's side, which gives a large area. Glucose and oxygen pass from the mother's blood to the embryo through the placenta, and the embryo's wastes pass back to the mother's blood.

Through the placenta, which passes food and oxygen from the mother's blood to the embryo.

No. A copper-T is placed inside the uterus and only prevents pregnancy. It does not stop contact between body fluids, so infections such as gonorrhoea, syphilis and HIV can still be passed on. Only a barrier method, such as a condom, gives some protection against STDs.

No; it only prevents pregnancy. A barrier method such as a condom is needed to reduce the risk of STDs.

Amoeba and Leishmania reproduce by binary fission, Plasmodium by multiple fission.

(b) Yeast

The vas deferens carries sperms from the testis in males.

(c) Vas deferens

(d) pollen grains

• It combines DNA from two parents, so it produces much more variation in the offspring.
• Variation helps some individuals to survive changes in the environment, which ensures the survival of the species.
• Accumulated variations are the basis of evolution of new species.

It creates more variation, which helps the species survive changing conditions and drives evolution.

• It produces sperms (male germ cells).
• It secretes the hormone testosterone, which regulates the production of sperms and brings about the changes in boys at puberty (such as a deeper voice and facial hair).

It makes sperms and secretes the hormone testosterone.

Every month the ovary releases an egg, and the lining of the uterus becomes thick and spongy, with a rich blood supply, to receive and nourish a fertilised egg. If the egg is not fertilised, this lining is not needed. It breaks down slowly and comes out through the vagina as blood and mucus. This is menstruation; it happens about every 28 days and lasts 2 to 8 days.

When the egg is not fertilised, the thickened uterus lining that was prepared for it breaks down and is shed.

See the labelled diagram: sepal, petal, stamen (anther and filament), pistil (stigma, style and ovary with ovules), and receptacle.

• Barrier methods: condoms (for the penis) and similar coverings for the vagina stop sperms from reaching the egg; they also help protect against STDs.
• Hormonal methods: oral pills change the hormonal balance so that eggs are not released.
• Intrauterine devices: a loop or copper-T placed in the uterus prevents pregnancy.
• Surgical methods: the vas deferens is blocked in males (vasectomy), or the fallopian tubes in females (tubectomy), so that sperms or eggs cannot pass.

Barrier methods, oral pills, devices such as copper-T, and surgical methods (vasectomy, tubectomy).

• Unicellular organisms reproduce by simple cell division: the single cell divides into two (binary fission, e.g. Amoeba) or many (multiple fission, e.g. Plasmodium), or forms a bud (yeast).
• Multicellular organisms have complex bodies with specialised cells, so they cannot simply divide cell by cell. They use specialised methods: fragmentation and regeneration (in simple ones such as Spirogyra and Planaria), budding (Hydra), spores (Rhizopus), vegetative propagation in plants, and sexual reproduction with specialised reproductive organs and germ cells.

Unicellular organisms reproduce by cell division (fission, budding); multicellular organisms need specialised reproductive organs and methods, often sexual reproduction.

Members of a population die from age, disease and predators. Reproduction produces new individuals that replace those that die, keeping the population steady. Because DNA copying keeps the body design almost the same, the new individuals fit the same niche. The small variations also help some individuals survive changes, so the population continues over many generations.

It replaces individuals who die and keeps the body design suited to the habitat, so the population size stays stable.

• To prevent unwanted pregnancies and plan the size of the family and the gap between children.
• To protect the mother's health, since frequent pregnancies harm her body.
• To control population growth, so that resources and living standards are maintained.
• Barrier methods also help prevent the spread of sexually transmitted diseases.

To avoid unwanted pregnancies, protect the mother's health, control population, and (with barrier methods) prevent STDs.