CBSE - How do Organisms Reproduce

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. Therefore, it can be concluded that it is the DNA that gets transferred from parents to offsprings and makes them look similar.

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.

Binary fission	Multiple fission
1. Splitting of unicellular organisms like amoeba into two equal halves during cell division is termed binary fission.	1. Division of single-celled organisms such as malarial parasites into many daughter cells simultaneously is termed multiple fission.
2. Amoeba and Bacteriadivide by binary fission.	2. Plasmodium divide by multiple fission

There are many advantages, if an organism reproduces through spores.

Advantages of spore formation:

Large numbers of spores are produced in one sporangium.

Spores are distributed easily by air to far-off places to avoid competition at one place.

Spores are covered by thick walls to prevent dehydration under unfavourable conditions.
 

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 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.

Vegetative propagation is the ability of the plants to reproduce by producing new plants from the vegetative plant parts such as leaf, stem, or roots under appropriate conditions. This method is the only means of reproduction for some seedless plant varieties such as banana, rose, and jasmine. However, this method of reproduction is also used for agricultural purposes in commercial production of some plants such as sugarcane, grapes, roses, etc.

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.

Pollination	Fertilisation
1. Pollination refers to the process of transfer of pollen grains from anther to stigma of flower.	1. Fertilisation refers to fusion of male and female gamete to form a zygote. .
2. It occurs with the help of certain pollinators such as air, water, birds, or some insects.	2. It occurs inside the ovule and leads to the formation of zygote.

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.

Secondary sexual characteristics in girls:

Increase in breast size and darkening of skin of the nipples present at the tips of the breasts.

Appearance of hair in the genital area.

Appearance of hair in other areas of skin like underarms, face, hands, and legs.

Increase in the size of uterus and ovary.

Beginning of menstrual cycle.

More secretion of oil from the skin, which results in the appearance of pimples.

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.

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.

Asexual reproduction takes place through budding in
 (a) Amoeba (b) Yeast
 (c) Plasmodium (d) Leishmania.
 
 ans (b) Yeast

Which of the following is not a part of female reproductive system in human beings?
 (a) Ovary (b) Uterus
 (c) Vas deferens (d) Fallopian tube
 
 ans : (c) Vas deferens

The anther contains
 (a) Sepals (b) Ovules
 (c) Carpel (d) Pollen grains.
 
 ans :(d) Pollen grains.

Advantages of sexual reproduction:

1.In sexual reproduction, more variations are produced. Thus, it ensures survival of species in a population.

2.The new formed individual has characteristics of both the parents.

3. 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.

The testes are the male reproductive organs that are located outside the abdominal cavity within a pouch called scrotum.

Functions of testes:

Produce sperms

Produce a hormone called testosterone, which brings about secondary sexual characters in boys.

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.

longitudinal section of a flower

Various methods used for regulation of child birth can broadly categories as:

(i) Barrier methods: In this method, physical devices such as condom, diaphragm, cervical cap and copper-T are used.
(ii) Chemical method: use of spermicidal jelly by woman, oral pills and vaginal pills.
(iii) Surgical method: In surgical method, a small portion of vas deferens in male and the oviduct of female, is surgically removed or ligated. It is called vasectomy in male and Tubectomy in females

In unicellular organisms, cell division, or fusion leads to the creation of new individuals.

In multicellular organisms with simple body organization budding, fragmentation may work but in complex multicellular organisms only sexual reproduction takes place

The consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to use the particular niche. Reproduction is, therefore, linked to the stability to populations of species.

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.

1. In the list of organisms given below, those that reproduce by the asexual method are
(i) banana
(ii) dog
(iii) yeast
(iv) Amoeba
(a) (ii) and (iv)
(b) (i), (iii) and (iv)
(c) (i) and (iv)
(d) (ii), (iii) and (iv)
Ans. (b) (i), (iii) and (iv)
Explanation: Dog never reproduces asexually. Yeast and amoeba always reproduce asexually. Cultivated varieties of banana always reproduce asexually.
2. In a flower, the parts that produce male and female gametes (germ cells) are
(a) stamen and anther
(b) filament and stigma
(c) anther and ovary
(d) stamen and style
Ans. (c) Anther and ovary

3. Which of the following is the correct sequence of events of sexual reproduction in a flower?
(a) pollination, fertilisation, seedling, embryo
(b) seedling, embryo, fertilisation, pollination
(c) pollination, fertilisation, embryo, seedling
(d) embryo, seedling, pollination, fertilization
Ans. (c) Pollination, fertilisation, embryo, seedling
Explanation: Fertilisation cannot happen without pollination. Embryo is formed after fertilization and seedling comes out of embryo.

4. Offspring formed by asexual method of reproduction have greater similarity among themselves because
(i) asexual reproduction involves only one parent
(ii) asexual reproduction does not involve gametes
(iii) asexual reproduction occurs before sexual reproduction
(iv) asexual reproduction occurs after sexual reproduction
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iv)
(d) (iii) and (iv) 
Ans. (a) (i) and (ii)
Explanation: Statements (iii) and (iv) are incorrect because sexual and asexual reproduction do not follow a particular sequence. Moreover, there are many organisms which reproduce asexually only. Additionally, there are many other organisms which never show asexual reproduction.

5. Characters transmitted from parents to offspring are present in
(a) cytoplasm
(b) ribosome
(c) golgi bodies
(d) genes
Ans. (d) Genes
Explanation: Genes are responsible for transmission of characters from one generation to the next generation.

6. Characters that are transmitted from parents to offspring during reproduction show
(a) only similarities with parents
(b) only variations with parents
(c) both similarities and variations with parents
(d) neither similarities nor variations
Ans. (c) Both similarities and variations with parents
Explanation: A child shows certain similarities and certain variations in characters from parents. This shows characters in a child can be both similar as well as different than parents.

7. A feature of reproduction that is common to Amoeba, Spirogyra and Yeast is that
(a) they reproduce asexually
(b) they are all unicellular
(c) they reproduce only sexually
(d) they are all multicellular
Ans. (a) They reproduce asexually
Explanation: Number of cells in an organism does not decide if the organism would reproduce asexually or sexually. Hence options ‘b’ and ‘d’ are incorrect. These organisms never reproduce sexually and hence option ‘c’ is incorrect.

8. In Spirogyra, asexual reproduction takes place by
(a) breaking up of filaments into smaller bits
(b) division of a cell into two cells
(c) division of a cell into many cells
(d) formation of young cells from older cells
Ans. (a) breaking up of filaments into smaller bits
Explanation: Spirogyra shows reproduction by fragmentation. 

9. The ability of a cell to divide into several cells during reproduction in Plasmodium
is called
(a) budding
(b) reduction division
(c) binary fission
(d) multiple fission
Ans. (d) Multiple fission

10. The correct sequence of reproductive stages seen in flowering plants is
(a) gametes, zygote, embryo, seedling
(b) zygote, gametes, embryo, seedling
(c) seedling, embryo, zygote, gametes
(d) gametes, embryo, zygote, seedling
Ans. (a) gametes, zygote, embryo, seedling.
Explanation: Gametes fuse during fertilization of form zygote. Zygote develops into embryo. Embryo produces seedling after germination of seed.

11. The number of chromosomes in parents and offsprings of a particular species remains constant due to 
(a) doubling of chromosomes after zygote formation
(b) halving of chromosomes during gamete formation
(c) doubling of chromosomes after gamete formation
(d) halving of chromosomes after gamete formation
Ans. (b) halving of chromosomes during gamete formation
Explanation: During gamete formation, number of chromosomes becomes half. Due to this, number of chromosomes in zygote becomes same as in somatic cell of the organism. This ensures constant number of chromosomes in a particular species.

12. In Rhizopus, tubular thread-like structures bearing sporangia at their tips are called
(a) filaments
(b) hyphae
(c) rhizoids
(d) roots
Ans. (b) Hyphae

13. Vegetative propagation refers to formation of new plants from
(a) stem, roots and flowers
(b) stem, roots and leaves
(c) stem, flowers and fruits
(d) stem, leaves and flowers
Ans. (b) Stem, roots and leaves
Explanation: Vegetative propagation happens from vegetative parts of a plant. Stem root and leaves are vegetative parts. 

14. Factors responsible for the rapid spread of bread mould on slices of bread are
(i) large number of spores
(ii) availability of moisture and nutrients in bread
(iii) presence of tubular branched hyphae
(iv) formation of round shaped sporangia
(a) (i) and (iii)
(b) (ii) and iv)
(c) (i) and (ii)
(d) (iii) and (iv)
Ans. (c) (i) and (ii)
Explanation: Large number of spores ensures survival of some spores even in adverse conditions. Moisture and nutrients in bread provide conducive environment for spores to produce new bread mould.

16. Which of the following statements are true for flowers?
(i) Flowers are always bisexual
(ii) They are the sexual reproductive organs
(iii) They are produced in all groups of plants
(iv) After fertilisation they give rise to fruits
(a) (i) and (iv)
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: Some flowers are unisexual and hence statement (i) is incorrect. Flowers are produced by angiosperms only and hence statement (iii) is incorrect.

17. Which among the following statements are true for unisexual flowers?
(i) They possess both stamen and pistil
(ii) They possess either stamen or pistil
(iii) They exhibit cross pollination
(iv) Unisexual flowers possessing only stamens cannot produce fruits
(a) (i) and (iv)
(b) (ii), (iii) and (iv)
(c) (iii) and (iv)
(d) (i), (iii) and (iv)
Ans. (b) (ii), (iii) and (iv)
Explanation: Since a unisexual flower has either stamen or pistil, hence cross pollination is necessary in them. Fruit is a mature ovary and hence a flower possessing only stamens cannot produce fruit.

18. Which among the following statements are true for sexual reproduction in
flowering plants?
(i) It requires two types of gametes
(ii) Fertilisation is a compulsory event
(iii) It always results in formation of zygote
(iv) Offspring formed are clones
(a) (i) and (iv)
(b) (i), (ii) and (iv)
(c) (i), (ii) and (iii)
(d) (i), (ii) and (iv)
Ans. (c) (i), (ii) and (iii)
Explanation: Offspring produced after sexual reproduction are never clones of their parents. Hence, statement (iv) is incorrect.

19. In Figure 8.1, the parts A, B and C are sequentially

(a) cotyledon, plumule and radicle
(b) plumule, radicle and cotyledon
(c) plumule, cotyledon and radicle
(d) radicle, cotyledon and plumula
Ans. (c) Plumule, cotyledon and radicle
Explanation:

20. Offspring formed as a result of sexual reproduction exhibit more variations because
(a) sexual reproduction is a lengthy process
(b) genetic material comes from two parents of the same species
(c) genetic material comes from two parents of different species
(d) genetic material comes from many parents
Ans. (b) genetic material comes from two parents of the same species
Explanation: In sexual reproduction, genetic materials are contributed by two parents.
Members of two different species cannot interbreed and hence option �c� is incorrect.
21. Reproduction is essential for living organisms in order to
(a) keep the individual organism alive
(b) fulfill their energy requirement
(c) maintain growth
(d) continue the species generation after generation
Ans. (d) continue the species generation after generation
Explanation: Other life processes are essential for keeping the organism alive. Reproduction is essential to continue the lineage of an organism.

22. During adolescence, several changes occur in the human body. Mark one change
associated with sexual maturation in boys
(a) loss of milk teeth
(b) increase in height
(c) cracking of voice
(d) weight gain
Ans. (c) cracking of voice
Explanation: Other changes; as shown in options happen in girls also.

23. In human females, an event that reflects onset of reproductive phase is
(a) growth of body
(b) changes in hair pattern
(c) change in voice
(d) menstruation
Ans. (d) Menstruation
Explanation: Other changes; as shown in options happen in boys also.

24. In human males, the testes lie in the scrotum, because it helps in the
(a) process of mating
(b) formation of sperm
(c) easy transfer of gametes
(d) all the above
Ans. (b) Formation of sperm
Explanation: The fact that testes lie in the scrotum ensures that the temperature of testes is below body temperature. It ensures optimum production of sperms.

25. Which among the following is not the function of testes at puberty?
(i) formation of germ cells
(ii) secretion of testosterone
(iii) development of placenta
(iv) secretion of estrogen
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Ans. (c) (iii) and (iv)
Explanation: These events take place in females and not in males. Testes are present in males.

26. The correct sequence of organs in the male reproductive system for transport of
sperms is
(a) testis → vasdeferens →urethra
(b) testis →ureter →urethra
(c) testis →urethra →ureter
(d) testis →vasdeferens →ureter
Ans. (a) testis →vasdeferens →urethra
Explanation: Ureter is connected to kidneys only.

27. Which among the following diseases is not sexually transmitted?
(a) Syphillis
(b) Hepatitis
(c) HIV - AIDS
(d) Gonorrhoea
Ans. (b) Hepatitis
Explanation: It is a water-borne disease.

When the stamens of a bisexual flower are removed artificially, cross pollination takes place. This ensures fertilization. Due to this, flower is able to produce fruit.

In unicellular organism, new individual is created after cell division. Creation of new individual is called reproduction. Hence, cell division in unicellular organism can be termed as type of reproduction.

A true copy of anything is called a clone. In case of asexual reproduction, genes are contributed by a single parent. Due to this, offsprings produced by asexual reproduction are clones of their parent

Sexual reproduction involves gamete formation. Number of chromosomes is halved during gamete formation. As a result, when two gametes fuse during fertilization, the number of chromosomes in zygote becomes same as in somatic cell of the organism. This ensures that offsprings produced by sexual reproduction have the same number of chromosomes as their parents.

Water does not provide nutrition to yeasts. But sugar solution provides the necessary nutrition to yeasts. Due to this, colonies of yeast fail to multiply in water, but multiply in sugar solution.

Spores of bread mould need nutrition and moisture for germination and further growth. A dry slice of bread can provide nutrition but it lacks moisture. Due to this, bread mould grow profusely on a moist slice of bread rather than on a dry slice of bread.

Two reasons for the appearance of variations among the progeny formed by sexual reproduction are:

(a) Genes are contributed by two parents
(b) Crossing over during meiosis also results in variations. 
 

Would a Planaria cut vertically into two halves regenerate into two individuals? Complete Figure 8.2 D and E by indicating the regenerated regions. 

Planaria has the capability of regeneration. Due to this, a cut portion of planaria would develop complementary portion to become a new individual. Following figures show new individuals from D and E:

From the internet, gather information about the chromosome numbers of five animals and five plants. Correlate the number with the size of organism and answer the following questions.

(a) Do larger organisms have more number of chromosomes/cells?

ANS: (a)  No, there is no relationship between size of organism and its chromosome number.

(b) Can organism with fewer chromosomes reproduce more easily than organisms with more number of chromosomes?

ANS : (b) No, process of reproduction follows a common pattern and is not dependent on the number of chromosomes

(c) More the number of chromosomes/cells greater is the DNA content. Justify.

ANS: (c) Yes, since the major component of chromosome is DNA, if there are more chromosomes in a cell, the quantity of DNA will also be more.

Number of chromosomes in both the gametes is equal. Hence, female gametes of tobacco plant will have 24 chromosomes. Number of chromosomes in zygote would double the number in gametes, hence it would be 48.

Pollination is the process by which pollen grains are transferred to the sigma. Once pollen grains land on stigma, they germinate to produce pollen tubes and subsequently to facilitate fertilization. Hence, fertilization cannot take place in flowers if pollination does not occur.

After the formation of zygote, further development takes place through mitosis. We know that after mitosis, the number of chromosomes in daughter cells is same as in mother cell. Due to this, number of chromosomes in embryo is same as in zygote. For the same reason, number of chromosomes in an organism is same as in zygote or embryo. So, it is clear that the chromosomes number of zygote, embryonal cells and adult of a particular organism is always constant.

After fertilization, zygote is located in the ovary.

Every organism faces a lot of competition for survival. Barring a very few, most of the organisms end up being food for some other organism. Competition and predation result in drastic reduction in population of a species. But reproduction ensures that population of a species is maintained at optimum level. So, it can be said that reproduction is linked to stability of population of a species.

General growth is about growth in size of an organism. Sexual maturation is about the accomplishment of ability to reproduce. General growth begins right from the time an organism comes into this world. Sexual maturity begins at a later stage. In most of the animals, general growth stops after a certain age and this coincides with attainment of sexual maturity.

Sperm comes out from testis into the vas deferens and then passes through urethra before ejaculation. The secretions of seminal vesicle and prostrate glands provide nutrition to the sperms and also facilitate their transport.

 Following changes take place in uterus if fertilization does not occur:
• The extra lining of uterus degenerates.
• Fragments of the uterine lining are discharged through vagina.
• Unfertilized egg is also discharged.
• Menstruation take place. 

Following changes take place in uterus after implantation of young embryo:
• Uterine lining thickens to support to developing embryo.
• Uterine lining is richly supplied with blood vessels so that nutrition and oxygen could be supplied to the developing foetus. 

Following are the benefits of mechanical barriers during sexual act:
• Prevention of unwanted pregnancy.
• Prevention of STDs. 

(a) Production of egg
(b) Site of fertilisation
(c) Site of implantation
(d) Entry of the sperms 

The ratio of chromosome number between an egg and its zygote is 1:2. Sperm has Y chromosome as the 23rd chromosome, while egg has X chromosome as the 23rd chromosome. Moreover, the genetic makeup of sperm is similar to father and that of egg similar to mother.

A single parent is involved and gamete formation does not take place in reproduction through budding, fragmentation and regeneration. Hence, all of these are considered as asexual types of reproduction.

Regeneration in Planaria: The body of planaria may get divided into many pieces. Each piece has the ability to develop complementary portion to become a new individual. The given figure shows a planaria getting divided into three pieces. Subsequently, each piece develops into a new individual. 

Asexual reproduction	Sexual reproduction
(a) Involves only one parent	(i) Often involves two parents
(b) Gametes are not produced	(ii) Gametes are produced
(c) No fertilisation and zygote formation	(iii) Fertilisaton and zygote formation is observed.
(d) Meiosis does not occur at anytime during reproduction	(iv) Meiosis occurs at the time of gamete formation

Variations are observed in the offspring formed by sexual reproduction, because of
following reasons:
• Gene pool is contributed by two parents.
• Crossing over during meiosis results in variations.
• DNA replication also plays some role in variation.

Difference between pollination and fertilization: Transfer of pollen grains from anther to stigma is called pollination. On the other hand, fusion of male and female gametes is called fertilization. Pollination is a physical process in which movement of pollen grains are attained by certain physical factors. Fertilization is a biological process. Ovary is the site of fertilization and embryo is the product of fertilization.

The following figure shows fertilization in plant:

Gamete is formed as a result of meiosis in gametogenetic cells. Zygote is formed after fusion 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.

• Ovary: Produces female gametes

• Anther: Produces male gametes

Placenta is a disc-like structure embedded in the uterine wall. It contains villi on the embryonal side. Blood spaces are present on the mother’s side. Following are the functions of placenta:
• Supplying nutrients to the foetus.
• Supplying oxygen to the foetus.
• Taking away carbon dioxide from the foetus.
• Taking away excretory products from the foetus. 

Various ways to avoid pregnancy are as follows:
 (a) Physical barrier
 (b) Copper -T
 (c) Hormone pills
 (d) Surgical procedure
Hormone Pills: Many hormones pills are available in the market. These pills stop ovulation and thus prevent conception. Many new pills are available which need to be taken in fewer number of doses. These are more convenient and have least side effects. 

In human beings, one egg is released from either of the ovaries once in a month. The egg is transferred to the fallopian tube. Sperms swim up to the fallopian tube. Only one sperm is able to penetrate the egg. That is how fertilization takes place. A menstrual cycle is composed of about 28 days. This means only one egg is available for fertilization in one menstrual cycle. Hence, it can be said that fertilization can occur only once in a month.

Survival of an individual depends on many factors. An individual can survive if he/she gets food and shelter. An individual can survive if he/she is not killed by a predator. An individual can survive if he/ she is not killed by a competitor. In biological sense, an individual can survive if all the life processes continue in the body. But reproduction creates a new individual and the new individual carries the lineage of the species even after the death of parents. Moreover, a good rate of reproduction ensures a large population which counterbalances mortality in the population. We know that a higher birth rate and lower mortality rate helps in growth of population. If mortality rate becomes more than birth rate, then a population can be wiped out. Thus, reproduction helps in maintaining the stability of a species.

Sexualy Transmitted Diseases: Diseases which spread from one person to another through sexual act are called sexually transmitted diseases. HIV, Gonorrhoea, Herpes, etc. are examples of STDs. Ways to prevents STDs: • Use of condoms or other physical barriers. • Avoiding sexual contacts with unknown partners.