NCERT Solutions for Class 10 Science Chapter 7 Exercise

Answer: DNA carries genetic information from one generation to the other and this helps in producing organisms of its own types. DNA copying is a must for inheriting the traits from parents. Any variations in DNA copying will give rise to the origin of new species.

Answer: The reason why the variation is beneficial to the species rather than individuals is because sometimes the climatic changes have a drastic effect on the species, which makes their survival difficult. For example, if the temperature of the water body increases, then there might be certain species of microorganisms which might die. This may result in disturbance in the environment. So, variation is beneficial to species and not for the individuals.

Answer: In binary fission a single cell divides into two equal halves. Amoeba and bacteria divide by binary fission. In multiple fission, a single cell divides into many daughter cells simultaneously. Plasmodium divides by multiple fission.

Answer: An organism is benefited by reproducing through the spores because spores are surrounded by a thick layer which protects them in adverse conditions. When favourable conditions occur, these spores start to grow again.

Answer: Organisms at higher complex levels cannot give rise to new individuals through regeneration because they have organization of their organs system at different levels. All these organ systems are interconnected and work in full coordination. They can regenerate a few of their lost body parts like skin, blood, muscles, etc. but can’t give rise to new individuals.

Answer: Vegetative propagation is practiced for growing some types of plants because –
(i) It is used to grow a plant in which viable seeds are not formed.
(ii) It helps to introduce plants in new areas where the seed germination fails to produce mature plants.
(iii) A good quality of a race or variety can be preserved by the method of vegetative propagation.

Answer: DNA copying is an essential part of the process of reproduction because it carries the genetic information from the parents to offspring. A copy of DNA is produced through some chemical reactions resulting in two copies of DNA. Along with the additional cellular structure, DNA copying also takes place which is then followed by cell division into two cells.

Answer: 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 is the fusion of the male and female gametes. It occurs inside the ovule and leads to the formation of zygote.

Answer: Seminal vesicle and prostate gland secretes fluids which form a part of the semen. The fluid makes the path smooth through which the sperms travel. This fluid protects the sperms from the acids present in the urethra. and provides nutrition to sperms in the form of fructose, calcium and some enzymes.

Answer: The changes seen in girls at the time of puberty are –
(i) Increase in breast size and darkening of skin of the nipples present at the tips of the breasts.
(ii) Appearance of hair in the genital area.
(iii) Appearance of hair in other areas of skin like underarms, face, hands, and legs.
(iv) Increase in the size of uterus and ovary.
(v) Beginning of menstrual cycle.
(vi) More secretion of oil from the skin, which results in the appearance of pimples.

Answer: After fertilization the lining of the 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. It 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 from mother to the embryo and waste products from embryo to mother.

Answer: Copper-T is a contraceptive method which prevents implantation of the zygote inside the uterus. It cannot prevent a woman from sexually transmitted diseases.

Answer: (b) yeast.

Answer: (c) Vas deferens.

Answer: (d) pollen grains.

Answer: More variations are produced in sexual reproduction which ensures the survival of species in a population. The newly formed individual in sexual reproduction gains characteristics of both the parents. Variations are more viable in sexual mode than in asexual one.

Answer: Two main functions performed by the testis in human beings are –
(i) Producing male gametes called sperms.
(ii) Producing a hormone called testosterone.

Answer: 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 fertilised, then the lining of the uterus breaks down slowly and gets released in the form of blood and mucous from the vagina.

Answer: Diagram of the longitudinal section of a flower —

Answer: Different methods of contraception are –
(i) Natural method – Meeting of sperms and ovum can be avoided by abstaining from mating from 10th to 17th day of the menstrual cycle.
(ii) Barrier method – Use of a condom by male or female works as a barrier to stop the meeting of sperms and ovum.
(iii) Oral method – Birth control pills are taken orally. These pills contain small portions of hormones that block the eggs from joining sperms.
(iv) Surgical method – The fallopian tubes are blocked in females to stop flow of eggs and vas deferens is blocked in men to stop the flow of sperms.

Answer: Reproduction in unicellular organisms occurs by the division of the whole cell. In unicellular organisms, the modes of reproduction are often fission, budding. etc. whereas specialized reproductive organs are present in multicellular organisms. Therefore, reproduction can take place by complex reproductive methods like vegetative propagation, spore formation, etc. The mode of the reproduction will be sexual reproduction in more complex multicellular organisms like human beings and plants.

Answer: Reproduction is the process of producing the same kind of species by the existing species. This is done so as to maintain the population of that species and also to take forward their species to next generations. Stability is maintained by keeping a check of rate of births and rate of deaths.

Answer: Reasons for adopting contraceptive methods –
(i) To prevent unwanted pregnancies.
(ii) To control population rise or birth rate.
(iii) To prevent the transfer of sexually transmitted diseases.