The period when the rate of general body growth begins to slow down and reproductive tissues begin to mature, is called puberty.
Two changes observed in girls at the time of puberty are:
(i) The breast size begin to increase, 
(ii) Menstruation starts.

Sexually transmitted disease caused due to
(i)Bacterial infection is gonorrhoea, and
(ii)Viral infection id AIDS (Acquired Immune Deficiency Syndrome).These diseases can be prevented by responsible sexual behaviour such as of condom during intercourse, etc.

A process in which a DNA molecule produces two similar copies of itself in a reproducing cell through chemical reaction is a called DNA copying.
1.It transmits the characteristics from parents to the next generation (offspring)
2.It causes increased the variation in the population.

Reproduction is the process that helps in the growth of population of an organism.

Artificial methods of vegetative propagation like layering is used for growing jasmine plant.

Artificial method of vegetative propagation like cutting are used to grow rose plants.

DNA copying is not perfect accurate and the resultant errors are a source of variations in populations of organisms.

When a mature spirogyra filament attains considerable length, it breaks up into smaller fragments or pieces, and each fragment grows into a new filament or individual

When the colony of yeast is in water, it does not get nutrition, because water does not have nutrients. Sugar, on the other hand, provides nutrition. Once the yeast gets nutrition, it grows and begins producing buds. That is why, colonies of yeast fail to multiply in water, but multiply in sugar solution.

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.

The significance of reproductive health of society are:
1. regular medication and check-ups have led to development of reproductive health. Healthy mothers give birth to healthy children.
2. Reproductive health should be maintain in order to prevent sexually transmitted diseases (STDs).
3. Family planning by using various contraceptives enables a couple to decide on the number of children they want to have and when to have them. If a couple has less number of children they can provide good food, clothes and education to each child. So a small family is a happy family.
4. Having fewer children also keeps the mother in good health. This will reduce the cases of maternal mortality as well as new born mortality.

Gamete is formed as a result of meiosis in gametogenic cells. Zygote is formed after fusion of two different kinds of gametes. Different types of gametes are formed by different parents. Zygote is formed in the female reproductive part only.
Role of gamete in sexual reproduction: Gamete formation results in halving of number of chromosomes which is necessary to maintain the number of chromosomes in an organism which reproduces sexually.
Role of zygote in sexual reproduction: Zygote formation makes the number of chromosome equal to the somatic cells. further, it is the zygote which undergoes mitosis to develop into an embryo and subsequently into a new individual.