Class 10 Science · Chapter 7 NotesHow do Organisms Reproduce?

Reproduction is not needed to keep an individual organism alive, but it is essential for a species to continue. Organisms of the same species look similar because their body designs are similar, and these designs are controlled by information stored in DNA. During reproduction, a cell makes a copy of its DNA and also builds an additional cellular apparatus, so that each new cell gets its own DNA and cellular structure. No biochemical reaction is perfectly accurate, so the DNA copies are similar but not identical to the original. This inbuilt tendency to vary is the basis of evolution. Variation helps a species survive when the environment changes, because some individuals may be better suited to the new conditions.

• DNA (Deoxyribo Nucleic Acid) carries the information for inheritance of features.
• A basic event in reproduction is the creation of a DNA copy.
• Variation during reproduction is the basis for evolution.

In asexual reproduction, a new generation is created from a single individual. Unicellular organisms use fission. Many bacteria and protozoa simply split into two equal halves, a process called binary fission. In Amoeba, the split can occur in any plane, while in Leishmania, which has a whip-like structure at one end, binary fission occurs in a definite orientation. Plasmodium, the malarial parasite, divides into many daughter cells at once by multiple fission. Yeast reproduces by budding, in which small buds grow out and separate from the parent cell. These modes depend on the body design of the organism.

• Binary fissionparent cell divides into two daughter cells.
• Multiple fissionparent cell divides into many daughter cells simultaneously.
• Buddinga small outgrowth separates and grows into a new individual.

Multicellular organisms with simple body organisation can reproduce by fragmentation. Spirogyra, for example, breaks into smaller pieces on maturation, and each fragment grows into a new individual. More complex organisms cannot simply divide cell by cell because their cells are organised into tissues and organs. Some simple animals such as Hydra and Planaria can regenerate a complete organism from cut body parts, but regeneration is not the same as reproduction because organisms do not normally depend on being cut to reproduce. Hydra also reproduces by budding, where repeated cell division at one site forms a bud that matures and detaches. Plants use vegetative propagation, in which roots, stems or leaves develop into new plants. This is used in layering and grafting for sugarcane, roses and grapes, and allows plants such as banana, orange, rose and jasmine that have lost the ability to produce seeds to be grown. Plants raised this way bear flowers and fruits earlier and are genetically similar to the parent.

• Fragmentationbody breaks into pieces, each growing into a new individual.
• Regenerationspecialised cells proliferate and develop into different tissues.
• Vegetative propagationnew plants from root, stem or leaf parts.
• Tissue culturecells from the growing tip are grown in an artificial medium to form plantlets.

In many simple multicellular organisms, specific reproductive parts can be seen. In the bread mould Rhizopus, the thread-like hyphae are not reproductive parts. The tiny blob-on-a-stick structures are sporangia, which contain spores. Each spore can develop into a new Rhizopus individual. Spores are covered by thick walls that protect them until they reach a moist surface where they can begin to grow. All the modes discussed so far allow new generations to be created from a single individual, and together they are called asexual reproduction.

• Sporangia contain spores.
• Thick walls protect spores until conditions are suitable.
• Spores can grow into new individuals on a moist surface.

Sexual reproduction involves two individuals and combines DNA from both, creating new combinations of variations. In flowering plants, the reproductive parts are in the flower. Stamens are the male reproductive parts and produce pollen grains. The pistil is the female reproductive part and has three parts: the ovary at the bottom, the style in the middle and the stigma at the top. The ovary contains ovules, and each ovule has an egg cell. Pollen must be transferred from the stamen to the stigma; this is called pollination. If pollen lands on the stigma of the same flower, it is self-pollination; if it is transferred to another flower, it is cross-pollination. Agents like wind, water or animals help in cross-pollination. After pollination, a pollen tube grows through the style to reach the ovary. The male germ-cell fuses with the egg cell; this fusion is fertilisation and forms the zygote. The zygote divides to form an embryo inside the ovule. The ovule becomes a seed with a tough coat, and the ovary ripens into a fruit. The seed germinates into a seedling under suitable conditions.

• Stamenmale reproductive part, produces pollen grains.
• Pistilfemale reproductive part, made of ovary, style and stigma.
• Pollinationtransfer of pollen from stamen to stigma.
• Fertilisationfusion of male and female germ-cells to form a zygote.

Humans reproduce sexually. During early teenage years, the body undergoes changes that are not just growth but sexual maturation. Common changes include thick hair in armpits and the genital area, thinner hair on legs, arms and face, oily skin and pimples. In girls, breast size increases and menstruation begins. In boys, facial hair grows and the voice cracks. These changes occur at different times and rates in different people. The period during adolescence when reproductive tissues mature is called puberty. The male reproductive system includes testes, which produce sperms and secrete testosterone. Testes are located in the scrotum outside the abdominal cavity because sperm formation requires a lower temperature. Sperms travel through the vas deferens, which joins the urethra. Glands like the prostate and seminal vesicles add secretions that make sperm transport easier and provide nutrition. The female reproductive system includes ovaries, which produce eggs and some hormones, oviducts or fallopian tubes, the uterus and the vagina. The uterus opens into the vagina through the cervix.

• Pubertythe period of sexual maturation during adolescence.
• Testesproduce sperms and secrete testosterone.
• Ovariesproduce eggs and some hormones.
• Fallopian tubesite where fertilisation usually occurs.

During sexual intercourse, sperms enter through the vagina and travel to the oviduct, where one may fuse with an egg. The fertilised egg, or zygote, divides to form a ball of cells called an embryo. The embryo is implanted in the lining of the uterus and continues to develop into a foetus. The uterus lining thickens every month and is richly supplied with blood to nourish the embryo. The embryo gets nutrition and oxygen from the mother's blood through the placenta, a disc embedded in the uterine wall. Waste substances from the embryo are also removed through the placenta. Development takes about nine months, and the child is born by rhythmic contractions of the uterine muscles. If the egg is not fertilised, it lives for about one day. The thickened uterine lining is no longer needed and breaks down, coming out through the vagina as blood and mucous. This cycle, called menstruation, occurs roughly every month and lasts about two to eight days.

• Zygotefertilised egg.
• Embryoball of cells formed by division of the zygote.
• Placentaprovides nutrition and oxygen to the embryo and removes waste.
• Menstruationmonthly shedding of the uterine lining when the egg is not fertilised.

Sexual maturation does not mean that the body or mind is ready for sexual acts or for raising children. Many diseases can be transmitted during sexual contact, including bacterial infections like gonorrhoea and syphilis, and viral infections like warts and HIV-AIDS. Using a condom during sex helps prevent transmission of many of these infections to some extent. Pregnancy places major demands on a woman's body and mind, so various contraceptive methods are used to avoid it. Mechanical barriers such as condoms prevent sperm from reaching the egg. Hormonal methods, usually oral pills, change hormonal balance so that eggs are not released. Devices like the loop or copper-T are placed in the uterus. Surgical methods block the vas deferens in males or the fallopian tube in females. Surgery can also be used for removal of unwanted pregnancies, but it may be misused for illegal sex-selective abortion of female foetuses. For a healthy society, the female-male sex ratio must be maintained, and prenatal sex determination is prohibited by law.

• Sexually transmitted diseases include gonorrhoea, syphilis, warts and HIV-AIDS.
• Contraceptive methods include barriers, hormonal pills, copper-T and surgical blocks.
• Prenatal sex determination is prohibited by law.