Mechanical barriers during sexual act provide two benefits. The first benefit is avoidance of unwanted pregnancy and the second benefit is prevention of sexually transmitted diseases.

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.

Secretions from seminal vesicles and prostate gland provide nutrition to the sperms. This makes transportation of sperms easier by providing them a fluid medium.

The importance of DNA copying during reproduction are:
(i) It is responsible for the transmission of parental characteristics to the offsprings.
(ii) During DNA copying in reproduction, the changes occur due to the inheritance of traits from both the parents. This lead to certain genetic variations, which are useful for the evolution of species over a period of time.

Budding, fragmentation and regeneration are considered as asexual types of reproduction because gamete formation does not happen during these modes of reproduction and a single parent carries out the process of reproduction.
The above figures shows regeneration in planaria:
When the body of planaria is cut off into several part, each part regenerates its complementary part to develop into a new individual.

(a) Organisms need energy for survival which they obtain from life processes such as nutrition and respiration.
(b) reproduction needs a lot of energy
(c) Genetic material is transferred from one generation to the next as a result of reproduction through DNA copying.
(d) DNA copying takes place with high constancy and considerable variations, that is, advantages to the species for stability in the changing environment.

A disease which can be transmitted through sexual contact is called sexually transmitted disease or STD. Lack of hygiene is usually a major factor in providing conditions for spread of STDs. But unprotected sex with multiple and unknown partners is the biggest reason for the spread of STDs. Some of the ways to prevents STDs are as follows
(a) Being faithful to one's life partner
(b) Avoiding sexual contact with unknown person.
(c) Using condom during sexual intercourse
(d) Maintain personal hygiene.

There are various methods to avoid pregnancy. The physical barrier methods prevent the entry of sperm into the uterus. The hormonal pills prevent transfer of egg to the uterus. Copper-T is a method which prevents entry of sperms in the fallopian tube and thus prevents fertilization. Surgical methods are permanent methods to stop the chances of pregnancy.
Copper-T : Copper-T is in the shape of 'T' and is made of copper. Copper is known to have contraceptive properties. Moreover, copper-T makes a barrier in the at the opening of the fallopian tubes in the uterus and thus prevents sperms from entering the fallopian tubes to prevent fertilization.

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.

Two points of difference between asexual and sexual reproduction are as follows:
(a) A single parent is involved in asexual reproduction, while two parents are involved in sexual reproduction.
(b) Meiosis does not happen in asexual reproduction during gamete formation.
reason of variations in offspring produced by sexual reproduction: Two sets of genes are contributed by different parents, which bring different sets of characters in the offspring.