# How do Organisms Reproduce?

## Class 10 NCERT Science

### NCERT

1   What is the importance of DNA copying in reproduction?

##### Solution :

DNA (Deoxyribonucleic acid) is the genetic material found in the chromosomes, which are present in the nucleus of a cell. The DNA is the information site for making proteins and each specific type of protein leads to a specific type of body design. Thus, it is the DNA molecule that determines the body design of an individual. DNA carries the genes responsible for all the characters of the individual including physiological processes. Therefore, it can be concluded that it is the DNA that gets transferred from parents to offspring and makes them look and behave in a similar manner.DNA determines body structure and function

2   Why is variation beneficial to the species but not necessarily for the individual?

##### Solution :

Variations are beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, all variations are not necessarily beneficial for the individual organisms.

3   How does binary fission differ from multiple fission?

In binary fission, a single cell divides into two equal cells. Amoeba and Bacteria divide by binary fission.$\\$ $\textbf{Binary fission in Amoeba}$$\\ In multiple fission, a single cell divides into many daughter cells simultaneously. Amoeba and Plasmodium divide by multiple fission.\\ \textbf{Multiple fission in Plasmodium} 4 How will an organism be benefited if it reproduces through spores? ##### Solution : Spores are dormant and hardy structures produced by some organisms in order to tide over unfavorable conditions. There are many advantages, if an organism reproduces through spores.\\ \textbf{Advantages of spore formation:}$$\\$ $\bullet$ Large numbers of spores are produced in one sporangium.$\\$ $\bullet$ Spores are distributed easily by air to far-off places to avoid competition at one place.$\\$ $\bullet$ Spores are covered by thick walls to prevent dehydration under unfavorable conditions.$\\$ It allows cells to germinate and form a new individual once favorable conditions return

5   Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?

##### Solution :

Simple organisms such as Hydra and Planaria are capable of producing new individuals through the process of regeneration. The process of regeneration involves the formation of new organisms from its body parts. Simple organisms can utilize this method of reproduction as their entire body is made of fewer and similar kind of cells in which any part of their body can be formed by growth and development.$\\$ However, complex organisms have organ-system level of organization. All the organ systems of their body work together as an interconnected unit. They can regenerate their lost body parts such as skin, muscles, blood, etc. However, they cannot give rise to new individuals through regeneration as a specific cell or tissue cannot give rise to completely different cell and tissue types. Therefore, it would not be possible to completely regenerate an entire new organism.

6   Why is vegetative propagation practiced for growing some types of plants?

##### Solution :

Vegetative propagation is the propagation of plants using their vegetative parts such as roots, stem, leaves, etc. This kind of propagation occurs independent of seeds. It has many advantages such as:$\\$ 1. Propagation of plants with no seeds or no viable seeds such as orchids, banana, etc.$\\$ 2. Propagation of a select superior variety of a plant as the plants produced will be genetically identical to the parent plant.$\\$ 3. A quicker method of propagation to obtain a large number of plants in a much lesser time.$\\$ 4. Introduction and propagation of plants in newer areas where seeds may not germinate due to unfavorable environmental or soil conditions.

7   Why is $DNA$ copying an essential part of the process of reproduction?

##### Solution :

$DNA$ (Deoxyribonucleic acid) copying is an essential part of reproduction as it passes genetic information from parents to offspring. It determines the body design of an individual. The reproducing cells produce a copy of their $DNA$ through some chemical reactions and result in two copies of $DNA$. The copying of DNA always takes place along with the creation of additional cellular structure. This process is then followed by division of a cell to form two cells.

8   How is the process of pollination different from fertilization?

##### Solution :

Pollination is the process of transfer of pollens from anther to stigma. It occurs with the help of certain pollinators such as air, water, birds, or some insects. Fertilization, on the other hand, is the fusion of the male and female gametes. It occurs inside the ovule and leads to the formation of zygote.

9   What is the role of the seminal vesicles and the prostate gland?

##### Solution :

The secretions from seminal vesicles and prostate glands lubricate the sperms and provide a fluid medium for easy transport of sperms. Their secretion also provides nutrient in the form of fructose, calcium, and some enzymes to enable the sperms to survive for a few days until they can fertilize the egg.

10   What are the changes seen in girls at the time of puberty?

##### Solution :

$\text{Secondary sexual characteristics in girls:}$$\\$ $\bullet$ Increase in breast size and darkening of skin of the nipples present at the tips of the breasts. $\bullet$ Appearance of hair in the genital area.$\\$ $\bullet$ Appearance of hair in other areas of skin like underarms, face, hands, and legs.$\\$ $\bullet$ Increase in the size of uterus and ovary.$\\$ $\bullet$ Beginning of menstrual cycle.$\\$ $\bullet$ More secretion of oil from the skin, which results in the appearance of pimples.$\\$ $\bullet$ Widening of the hips

11   How does the embryo get nourishment inside the mother’s body?

##### Solution :

The embryo develops inside the mother’s body for about nine months. Inside the uterus, the outer tissue surrounding the embryo develops finger-like projections called villi. These villi are surrounded by uterine tissue and maternal blood. They provide a large surface area for exchange of oxygen and nutrients. Also, there is a special tissue called placenta, which is embedded in the uterine wall. The embryo receives the oxygen and nutrients from the mother’s blood via the placenta. The waste materials produced by the embryo are also removed through the placenta

12   If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?

##### Solution :

No. Using a copper-T will not provide a protection from sexually transmitted diseases, as it does not prevent the entry of semen. It only prevents the implantation of the embryo in the uterus.

13   Asexual reproduction takes place through budding in

##### Solution :

(b) Asexual reproduction takes place through budding in yeast.

14   Which of the following is not a part of the female reproductive system in human beings?

##### Solution :

(c) Vas deferens is not a part of the female reproductive system in human beings.

15   The anther contains

##### Solution :

(d) The anther contains pollen grains.

16   What are the advantages of sexual reproduction over asexual reproduction?

##### Solution :

$\text{Advantages of sexual reproduction:}$ $\\$ $\bullet$ In sexual reproduction, more variations are produced. Thus, it ensures survival of species in a population.$\\$ $\bullet$ The new formed individual has characteristics of both the parents.$\\$ $\bullet$ Variations are more viable in sexual mode than in asexual one. This is because in asexual reproduction, DNA has to function inside the inherited cellular apparatus.$\\$ Progeny produced as a result of sexual reproduction are more hardy than those produced by asexual reproduction.

17   What are the functions performed by the testis in human beings?

##### Solution :

The testes are the male reproductive organs that are located outside the abdominal cavity within a pouch called scrotum.$\\$ $\text{Functions of testes:}$ $\\$ $\bullet$ Produce sperms$\\$ $\bullet$ Produce a hormone called testosterone, which brings about secondary sexual characters in boys.

18   Why does menstruation occur?

##### Solution :

Menstruation is a process in which blood and mucous flows out every month through the vagina. This process occurs every month because one egg is released from the ovary every month and at the same time, the uterus (womb) prepares itself to receive the fertilized egg. Thus, the inner lining of the uterus gets thickened and is supplied with blood to nourish the embryo. If the egg does not get fertilized, then the lining of the uterus breaks down slowly and gets released in the form of blood and mucous from the vagina.

19   Draw a labelled diagram of the longitudinal section of a flower.

##### Solution :

20   What are the different methods of contraception?

##### Solution :

Contraception is the method employed to prevent pregnancy. The contraceptive methods can be broadly divided into the following types:$\\$ $\bullet$ Natural method $\to$ It involves avoiding the chances of meeting of sperms and ovum. In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle because during this period, ovulation is expected and therefore, the chances of fertilization are very high.$\\$ $\bullet$ Barrier method $\to$ In this method, the fertilization of ovum and sperm is prevented with the help of barriers. Barriers are available for both males and females. Condoms are barriers made of thin rubber that are used to cover penis in males and vagina in females.$\\$ $\bullet$ Oral contraceptives $\to$In this method, tablets or drugs are taken orally. These contain small doses of hormones that prevent the release of eggs and thus fertilization cannot occur.$\\$ $\bullet$ Implants and surgical methods $\to$Contraceptive devices such as the loop or Copper-T are placed in uterus to prevent pregnancy. Some surgical methods can also be used to block the gamete transfer. It includes the blocking of vas deferens to prevent the transfer of sperms known as vasectomy. Similarly, fallopian tubes of the female can be blocked so that the egg will not reach the uterus known as Tubectomy.$\\$

21   How are the modes for reproduction different in unicellular and multicellular organisms?

##### Solution :

In unicellular organisms, reproduction occurs by the division of the entire cell. The modes of reproduction in unicellular organisms can be fission, budding, etc. whereas in multicellular organisms, specialised reproductive organs are present. Therefore, they can reproduce by complex reproductive methods such as vegetative propagation, spore formation, etc. In more complex multicellular organisms such as human beings and plants, the mode of reproduction is sexual reproduction.

22   How does reproduction help in providing stability to populations of species?

##### Solution :

Living organisms reproduce for the continuation of a particular species. It helps in providing stability to the population of species by producing a new individual that resembles the parents. This is the reason why cats give birth to only cats or dogs give birth to only dogs. Therefore, reproduction provides stability to populations of dogs or cats or any other species. In addition due to the presence of variants, the species is also better equipped to handle environmental changes and survive in the long run.

23   What could be the reasons for adopting contraceptive methods?

##### Solution :

Contraceptive methods are mainly adopted because of the following reasons: $\\$(i) To prevent unwanted pregnancies.$\\$ (ii) To control population rise or birth rate.$\\$ (iii) To prevent the transfer of sexually transmitted diseases.

24   Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?

##### Solution :

Geographical isolation can prevent the transfer of pollens among different plants. However, since the plants are self-pollinating, (which means that the pollens are transferred from the anther of one flower to the stigma of the same flower or of another flower of the same plant), geographical isolation cannot affect speciation in this case.