Reproduction in Organisms Important Questions for CBSE Class 12 Biology Sexual Reproduction
1.All organisms have to reach a certain stage of growth and maturity in their life cycle before they reproduce sexually.
2.Life cycle of an organism is divided into the following three phases:
(i)Juvenile or vegetative phase – Pre-reproductive period
(ii)Reproductive phase — Reproductive period
(iii)Senescent phase – End of reproductive phase and ageing.
3.Reproduction is a seasonal phenomenon, in few organisms.
(i)Based on the seasonality in reproduction in majority of plants, following categories can be made:
- Monocarpic plants Flowering occurs once in their life cycle, bear fruits and die, e.g. annuals like rice, wheat; biennials like radish, henbane, etc.
- Polycarpic plants Flowering occurs every year in a particular season after reaching maturity, e.g. mango, apple, orange, etc.
Similarly, in animals, the categories are:
- Seasonal breeders Breed at a particular time of the year, e.g. frogs, lizards, birds, etc.
- Continuous breeders Reproduce throughout their lifespan of sexual maturity, e.g. cattle, poultry, rabbit, etc.
(ii)Cyclic changes in reproductive system of females in mammals during reproductive phase are:
- In non-primate mammals, e.g. cow, sheep, rat, deer, dog, tiger, etc., these cyclic changes during reproduction are termed as oestrus cycle.
- In primate mammals, e.g. monkeys, apes and humans, these cyclic changes are termed as menstrual cycle.
4.The events in sexual reproduction occur in the following three stages:
5.Pre-fertilisation events occur in two stages
A.Gametogenesis It is the formation of male and female gametes.
Types of gametes are:
(a) Homogametes or isogametes Gametes which are not differentiated as male and female gametes, e.g. algae such as Ulothrix.
(b)Heterogametes Gametes which are morphologically distinct, e.g. gametes in human, i.e. sperm (male) and ovum (female).
On the basis of gametes, sexuality in organisms can be classified as follows:
Sexuality in Plants
(i)Homothallic or monoecious plants Both male and female flowers are present on same plant body (bisexual condition), e.g. in several fungi and plants like cucurbits and coconuts.
(ii)Heterothallic or dioecious plants Male and female flowers are present on separate plant body (unisexual condition), e.g. papaya and date palm. In flowering plants, the unisexual male flower is staminate while female is pistillate.
Diversity of sexuality in organisms
(a) Monoecious plant iChara) (b) Dioecious plant tMarchantia) (c) Bisexual flower (sweet potato)
Sexuality in Animals
(i) Bisexual animals have both male and female organs, e.g. earthworm, sponge, leech, tapeworm, etc. These are also called as hermaphrodites.
(ii) Unisexual animals have male and female organs in different individuals, e.g. cockroach, human, dog, etc.
Cell Division during Gamete Formation
(i) Gametes are always haploid.
(ii) The organisms belonging to Monera, fungi, algae and bryophytes have haploid plant body. These organisms produce gametes by mitotic division.
(iii)In pteridophytes, gymnosperms, angiosperms and most of the animals including humans, the parental body is diploid. In these organisms, specialised cells called meiocytes or gamete mother cells undergo meiosis that results in formation of haploid gametes.
B.Gamete transfer It occurs in various ways in different organisms:
- In most organisms, male gamete is motile and female gamete is non-motile.
- Both gametes are motile in few fungi and algae.
- In algae, bryophytes and pteridophytes, gamete transfer takes place through water by producing several male gametes to ensure fertilisation.
- In self-fertilising plants, e.g. peas, gamete transfer is easy as the anthers and stigma are closely located. In cross-fertilising plants, gamete transfer occurs by pollination.
- In bisexuals or dioecious plant, special mechanisms are evolved to transfer gametes.
6.Fertilisation (syngamy) is the process of fusion of male and female gametes, which
leads to the formation of a diploid zygote.
Fusion of gametes occur in various ways as given below:
(i)In some lizards, birds, honeybees and rotifers, female gamete undergoes development to form new offsprings without fertilisation. This is called parthenogenesis.
(ii)In some animals, external fertilisation occurs (fusion of gametes takes place outside the body), e.g. bony fishes, frogs, etc. A large number of gametes are released in the surrounding medium by such animals.
(iii)In some organisms, internal fertilisation occurs (fusion of gametes takes place inside the body), e.g. fungi, higher animals such as birds, mammals and majority of plants. The number of ova produced is less, but a large number of male gametes are formed, as many of them fail to reach the ova.
7.Post-fertilisation events occur after zygote formation.
(i)Zygote is the beginning of new life. It is always diploid and ensures the continuity of race from generation to generation. Organism such as fungi, develops a thick wall around it that is resistant to external damaging factors. Zygote divides by meiosis to form haploid spores that grow into haploid individuals in case of organisms which lead haplontic life cycle. In organisms with diplontic life cycle, zygote undergoes mitotic division and develops into an embryo.
(ii)Embryogenesis is the process of development of embryo from zygote.
The events which occur during embryogenesis are:
(a)Mitosis (cell division) occurs leading to the growth of embryo.
(b)Cell undergoes differentiation and attains specific shape, size and function.
(c)Cell differentiation leads to production of different tissues, organs and organ systems.
(d)Depending on the development of embryo the animals can be:
- Oviparous animals, e.g. reptiles and birds, embryo develops outside the body of female (eggs).
- Viviparous animals,e. g. majority of mammals including humans, embryo develops inside the body of female.
(e)The events that occur in flowering plants after fertilisation are:
- Sepals, petals and stamens wither and shed off.
- Pistil remains attached to the plant.
- Zygote develops into embryo and the ovules develop into ceed.
- Ovary develops into the fruit.
Pericarp is produced as the wall of ovary. Seeds disperse by different agents and germinate into new plants after getting suitable conditions
8.Organisms such as aphids, slime moulds, sea-anemones and many plants switch from asexual to sexual mode of reproduction, when environmental conditions are unfavourable. Sexual reproduction leads to variation in offsprings, thus providing a mechanism for selective adaptations to occur.
Previous Year Examination Questions
1 Mark Questions
1.Give one example of an animal which exhibits oestrus cycle. [Delhi 2014 c]
Ans. Example of an animal that exhibits oestrus cycle is cow.
2.Write the two pre-fertilisation events from the list given below: Syngamy, Gametogenesis, Embryogenesis, Pollination[All India 2014 C]
Ans.Gametogenesis and pollination are pre-fertilisation events from the list.
3.In which two of the following organisms is the fertilisation external?
Bony fishes, Ferns, Frogs, Birds.[Delhi 2014 C]
Ans.Bony fishes and frogs are two organisms with external fertilisation.
4.Differentiate between oviparous and viviparous animals. [Delhi 2014 c]
Ans. In oviparous animals, embryo develops outside the body of female in eggs, e.g. reptiles On the contrary in viviparous animals, embryo develops inside body of female. They give birth to babies, e.g. humans.
5.Name the phenomenon and one bird, where the female gamete directly develops into a new organism.[All India 2013]
Ans. Parthenogenesis is the phenomenon in which a female gamete directly develops into a new organism. It is seen in some birds (Turkey), rotifers and honeybee.
6.Give the name of the common phenomenon with reference to reproduction in rotifers, honeybees and Turkeys. [Delhi 2013 c]
Ans. Parthenogenesis is the common phenomenon of with reference to reproduction in rotifers, honeybees and Turkeys.
7.Cucurbits and papaya plants bear staminate and pistillate flowers. Mention the categories they are put under separately on the basis of the type of flowers they bear.[HOTS; Delhi 2012]
Ans.Papaya is dioecious because the staminate and pistillate flowers are borne on two different plants, while cucurbits such as cucumber is monoecious because it bears both staminate and pistillate flowers on the same plant.
8.A list of three flowering plants is given below. Which ones out of them are:
(ii)bearing pistillate flowers
List Date palm, Cucurbits and Pea [Foreign 2011]
Ans. (i)Cucurbits are monoecious.
(ii) Date palm is dioecious, in which an individual plant bears pistillate flowers
9.Name the type of cell division that takes place in the zygote of an organism exhibiting haplontic life cycle. [Delhi 2011]
Ans. Meiosis is the type of cell division that takes place in zygote of an organism exhibiting haplontic life cycle.
10.A moss plant produces a large number of antherozoids but relatively only a few egg cells. Why? [Delhi 2010]
Ans.The antherozoids or male gametes in moss plants are motile and depend on water for transport towards female gametes (non-motile). During gamete transfer, large number of male gametes are lost. Thus, to ensure fertilisation,large number of male gametes are released to reach the non-motile female gametes.
11.Why are papaya and date palm plants said to be dioecious, whereas cucurbits and coconut palms monoecious, inspite of all of them bearing unisexual flowers?[HOTS; Foreign 2010]
Ans.Papaya and date palm plants are said to be dioecious because male and female flowers are borne on separate plants, whereas cucurbits and coconut palms are monoecious because male and female flowers are borne on the same plant.
12.What is the major difference you observe in the offsprings produced by asexual reproduction and in the progeny produced by sexual reproduction? [Delhi 2008]
Ans.Offsprings -produced by asexual reproduction are genetically identical to parents and to each other. On the contrary, progeny produced as a result of meiosis and gametic fusion in sexual reproduction show variations and differences from the two parents as well as among themselves.
2 Marks Questions
13.Coconut palm is monoecious,while date palm is dioecious. Why are they so called? [Delhi 2014]
Ans.Coconut palm is monoecious because both male and female flowers are borne on the same plant.Date palm is dioecious because these plants bears exclusively either male or female flowers.
14.Name any two organisms and the phenomenon involved where the female gamete undergoes development to form new organisms without fertilisation.[Foreign 2014]
Ans.Parthenogenesis is the phenomenon in which a female gamete directly develops into a new organism. It is seen in some birds (Turkey), rotifers and honeybee.
15.Why do algae and fungi shift to sexual mode of reproduction just before the onset of adverse conditions? [Delhi 2014]
Ans.Organisms such as fungi and algae switch to sexual mode of reproduction during adverse conditions because asexual reproduction produces a large population that may not survive due to lack of resources. Sexual reproduction also brings variation into the individuals, some of which might help the individuals to better adapt to the changed conditions and survive. This ensures the continuity of species.