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The cell nucleus contains DNA, the information for making proteins, which decide the body design. During reproduction, cells make copies of their DNA by biochemical reactions, along with additional cellular apparatus, so the new cell gets the same body design. The copying is not perfectly accurate, which gives variations.
Single-celled organisms divide into two or more new cells.
The body breaks into small pieces, and each piece grows into a new individual, for example Spirogyra, a filamentous alga. It works only for simple, undifferentiated organisms.
If an organism is cut or broken into pieces, each piece may grow into a complete organism, for example Hydra and Planaria. Specialised cells divide to form masses of cells (blastema) which differentiate into different tissues. Regeneration is not the same as reproduction: most organisms cannot reproduce by cutting.
A bud, a small outgrowth, develops on the parent, grows, and detaches to become a new individual. Examples: Hydra and yeast.
New plants are obtained from parts such as roots, stems and leaves.
Many simple organisms like Rhizopus (bread mould) have blob-like structures on hyphae called sporangia, which contain spores. Spores are covered by a thick protective wall and can survive unfavourable conditions. When they land on a suitable moist surface they germinate and grow into new individuals.
New plants are grown from the cells or tissue of the plant in a nutrient medium. A small part of the plant is placed in a medium to form a mass of cells called callus, which is then moved to hormone-containing media to form roots and shoots. It is used for ornamental plants and for disease-free plants.
The reproductive parts of a flowering plant are the flowers. A flower has four whorls:
Flowers with only stamens or only pistil are unisexual (papaya, watermelon); flowers having both are bisexual (Hibiscus, mustard).
The transfer of pollen from the anther to the stigma is pollination. It may be self-pollination (to the stigma of the same flower or another flower of the same plant) or cross-pollination (to a flower of another plant), by wind, water or animals such as insects.
After the pollen lands on a suitable stigma, a pollen tube grows through the style to the ovule in the ovary. The male gamete moves through the tube and fuses with the egg cell. This fusion of gametes is fertilisation and forms the zygote.
Each month the uterus prepares itself to receive a fertilised egg, by developing a thick, soft lining rich in blood vessels. If the egg is not fertilised, the lining is not needed. It breaks down and comes out through the vagina as blood and mucus. This is menstruation, which lasts for about 2 to 8 days, and occurs roughly every month (a 28-day cycle).
When a sperm fertilises the egg, the zygote forms. It divides to form an embryo, which gets implanted in the thick lining of the uterus, and develops into a foetus. The embryo gets nourishment from the mother's blood through a special tissue called the placenta, a disc embedded in the uterine wall with many villi that give a large surface area. Oxygen and nutrients pass from the mother to the embryo, and wastes pass the other way. The development takes about nine months (gestation), after which the child is born by the contractions of the uterus.
Diseases spread by sexual contact: bacterial (gonorrhoea, syphilis) and viral (warts, HIV-AIDS). The use of a condom prevents the transmission of many of them; Copper-T, pills and surgical methods do not protect from STDs.
Female foeticide (the killing of female foetuses) is a serious social issue that lowers the sex ratio; sex determination before birth is illegal in India.
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The DNA in the cell nucleus has the information for making proteins, which decide the body design. By making a copy of its DNA, a cell passes the same blueprint to the daughter cells so the offspring have the same body design. The copying has small errors, which give the variations that are useful for evolution.
Variations help some individuals to survive when the environment changes (for example a rise in temperature kills most bacteria but a few heat-resistant variants survive), ensuring the survival of the species. An individual with a variation may, however, be less suited and die, so the variation is not necessarily useful for it.
In binary fission, the parent cell divides into two daughter cells (Amoeba). In multiple fission, the parent cell divides into many daughter cells at the same time (Plasmodium).
Spores are protected by a thick wall, so they can survive unfavourable conditions such as heat and dryness and germinate when moisture and conditions are suitable. They are light and easily spread, and the organism produces many of them.
Complex organisms have highly specialised cells organised into tissues, organs and systems. Regeneration needs undifferentiated cells that can form all the tissues, which most complex organisms do not have in large numbers. Their bodies are too organised to be rebuilt from a piece.
It is used for plants that have lost the capacity to produce seeds (banana, orange, rose, jasmine). The plants grown bear flowers and fruits earlier than those from seeds, and are genetically identical to the parent, preserving the desirable qualities.
It makes sure that the new individual gets the same genetic information (body design) as the parent, and allows the species to keep its characteristics; the small variations also give diversity.
Pollination is the transfer of pollen grains from the anther to the stigma of a flower. Fertilisation is the fusion of the male gamete (carried by the pollen tube) with the female gamete (egg) in the ovule, forming the zygote. Pollination happens first, outside the ovule; fertilisation happens in the ovule.
Their secretions form the fluid part of semen. They make the transport of the sperms easier and provide nourishment to them.
Breasts develop and the nipple skin darkens, hair grows in the armpits and genital area, the hips widen, the skin may become oily with pimples, and menstruation begins (the ovaries start releasing eggs).
Through the placenta, a disc-like tissue embedded in the uterine wall. The villi of the placenta bring the blood of the embryo close to the mother's blood, and the nutrients and oxygen pass from the mother to the embryo, and wastes from the embryo to the mother.
No. Copper-T is placed in the uterus to prevent the implantation of the embryo, and so prevents pregnancy, but it does not stop the transmission of sexually transmitted diseases. Only barrier methods like condoms offer protection.
Answer: (b) Yeast
Answer: (c) Vas deferens (a part of the male system)
Answer: (d) pollen grains
Sexual reproduction combines the DNA of two parents, so the offspring show greater variation. This variation helps the species to adapt to changes in the environment and gives raw material for evolution. Asexual reproduction gives nearly identical offspring, which would all be affected by the same change.
The testes make sperms (the male gametes) and secrete the hormone testosterone, which controls the formation of sperms and brings about the changes at puberty.
Every month the uterus develops a thick lining rich in blood vessels to receive a fertilised egg. If the egg is not fertilised, the lining is not needed. It breaks down and is passed out through the vagina with blood and mucus; this is menstruation.
See the diagram in the notes: sepals, petals, stamen (anther and filament), and pistil (stigma, style, ovary with ovules).
Physical barriers (condoms, diaphragm, cervical cap); chemical methods (oral pills, which change the hormonal balance); the intrauterine device (Copper-T or loop); and surgical methods (vasectomy in males, tubectomy in females).
Unicellular organisms reproduce mostly by fission (binary or multiple) or budding, as the cell divides and the whole organism is the cell. Multicellular organisms of simple structure use fragmentation, regeneration, budding or spores, while complex ones depend on specialised organs for sexual reproduction (flowers in plants, reproductive systems in animals), or on vegetative propagation.
Reproduction produces new individuals to replace those that die, so the number of individuals of a species is maintained in a population. It also passes on the body design (DNA) and gives variation that helps the species survive changes.
To avoid unwanted pregnancies, to space the births of children, to control the population, to protect the health of the mother, and (condoms) to protect from sexually transmitted diseases.