• DNA found in chromosomes in the nucleus of the cells has the information to create proteins which lead to body design of an organism.
• If the organisms are to make exact copies of themselves, the DNA should replicate to make an exact copy of itself.
• DNA replicates in the cell with the help of various enzymes and this is accompanied by division of the basic unit of every organism, i.e., the cell.

• Variations allow organisms to exist in diverse habitats or niches. In its absence, a species may remain restricted to a particular area.
• If this area gets drastically altered due to various natural or man-made causes, the species may be wiped out.
• However, if some variations are present in few individuals, it would help them to colonise other habitats and survive.
• But, if variations are present in a single organism, there would be a very little chance for it to survive and species will be lost forever.

The organisms which reproduce through spore formation has the following advantages:

• Reproduction through spores is a simple and faster mode of reproduction.
• Being light in weight, spores can be dispersed very easily. This helps in dispersal of organism.
• As spores are covered with thick walls, they can survive under unfavourable
• conditions (Rhizopus spores remain viable upto 22 years).

• Multicellular organisms cannot reproduce cell-by-cell because they are not simply random collection of cells.
• They are formed of specialised cells organised as tissues and tissues into organs, which then have to be placed at different positions in the body.
• Multicellular organisms, therefore, require more complex ways of reproduction.
• Moreover, simple multicellular organisms can possess special type of cells which have the potential to grow into a new organism, but complex multicellular organisms have no such specialised cells.

Vegetative propagation is practiced for growing some types of plants because of the following reasons

• Plants that have lost their capability to produce seeds can be propagated by this method.
• It helps to grow plants bearing superior traits, as they are genetically identical to the parent plant.
• It is used for growing plants which require a longer time to grow and become mature.

• Chromosomes in the nucleus of a cell contain information for inheritance of features from parents to the next generation in the form of DNA, which is the source of information for making proteins.
• Therefore, a basic event in reproduction is creation of DNA copy for transfer into the next generation.

• Secretions from seminal vesicles and prostate gland provide nutrition to the sperms.
• Seminal vesicles secrete a viscous fluid which forms most of the part of semen.
• It also lubricates the passage through which the sperms travel.
• This fluid also protects the sperms from the acids normally present in the urethra and female reproductive duct.
• Prostate gland produces prostatic fluid which is discharged into urethra through more than two dozen fine ducts.

The females show development of breasts, darkening of nipples, onset or start of menstruation and growth of pubic hair.

After fertilisation the lining of uterus thickens and is richly supplied with blood to nourish the growing embryo.

• The embryo gets nutrition from the mother's blood with the help of a special tissue called placenta.
• This is a disc shaped tissue which is embedded in the uterine wall.
• Placenta contains villi on the embryo's side of the tissue and blood spaces on mother's side surrounding the villi.
• This provides a large surface area for food and oxygen to pass from mother to the embryo and waste products from embryo to mother.

• No, copper-T does not prevent the transmission of sexually transmitted diseases. Copper-T only prevents implantation.
• The only safe method that can be used to prevent the transmission of sexually transmitted diseases is condoms.

(b) Yeast.

(c) Vas deferens

(d) pollen grains.

• Sexual reproduction is considered to be superior over asexual reproduction as it brings about variations in the progeny. These variations allow organisms to live in diverse habitats with the help of adaptations.
• On the other hand, asexual reproduction does not bring about variations among the progeny.

• Testes are the sites for the formation of sperms.
• They also produce a hormone named testosterone, which is responsible for development of secondary sexual characters in males.

• Menstruation occurs when the egg is not fertilised.
• Every month, uterus prepares itself to receive a fertilised egg.
• To nourish the embryo, its lining becomes thick and spongy.
• In case, egg is not fertilised, this lining breaks and discharges out of the body
• through the vagina in the form of blood. This is referred to as menstruation.

Methods of Contraception :

The methods used for regulation of child birth are.

• Barrier method: In this method physical devices like condom, cervical cap and diaphragm is used to prevent sperms to reach up to the ovum.
• Chemical method: In this method specific drugs are used by females. These drugs may be (a) oral pills or (b) vaginal pills. Oral pills mainly contain hormones and are called oral contraceptives. They disturb hormonal balance so that eggs are not released and fertilised.
• Intra uterine (contraceptive devices (IUCDs): IUCD like copper-T is placed safely in the uterus by a skilled doctor. IUCDs prevent implantation of the fertilised ovum inside the uterus. This technique is not meant for males.
• Surgical method: This method is applicable to both males and females. In males, a small portion of vas deferens and in female a small portion of fallopian tube is surgically removed or ligated (tied). It is known as vasectomy in males which prevents release of sperms from the testes. In females, it is called tubectomy.

• Unicellular organisms are made up of only a single cell, which performs all the functions necessary for life. So, reproduction is done by simple, asexual methods. Unicellular organisms like Amoeba, Yeast, etc., mostly reproduce asexually. Fission, budding are the common modes of asexual reproduction used by unicellular organisms.
• In multicellular organisms various cells perform different functions. So, production of all these specialised cells is required and simple methods for reproduction are insignificant. Thus, they reproduce by sexual reproduction.
• Some primitive multicellular organisms also reproduce by asexual reproduction. Common modes of asexual reproduction found in multicellular organisms are fragmentation, regeneration, budding and sporulation.

• Reproduction induces variations in the population which help the population to
• tide over adverse environmental conditions and adapt to changing environment. Reproduction also helps to generate copies of individuals which are suited to a particular environment.
• During reproduction, copies of DNA pass from one generation to the next.
• This copying of DNA takes place with consistency in reproducing organisms.
• As, it is important for the maintenance of body design features (physiological as well as structural) which allows the organism to use that particular niche.
• Reproduction is therefore, linked to the stability of population of a species.

The reasons for adopting contraceptive methods are

• To avoid frequent pregnancies, which in turn helps in population control.
• To prevent the spread of Sexually Transmitted Diseases (STDs).