Chapter at a Glance
- The period from birth to death is known as life span. The life span of the organisms varies from one organism to another organism.
- Life span depends upon the juvenility, maturity (stage when one starts to reproduce), ageing and senescence, and death.
- Reproduction is a biological process in which an organism gives rise to young ones similar to itself. It allows and ensures the species continuity generation after generation.
- Living organisms reproduce by three different modes of reproduction:
- (i) Asexual reproduction
- (ii) Vegetative reproduction
- (iii) Sexual reproduction
- Asexual reproduction is a process of producing individuals whose genes come from single parent only with the help of mitotic division.
- Offsprings produced are genetically and morphologically similar to each other and to the parent, hence they are also called as clones.
- Asexually reproducing organisms reproduce faster than sexually reproducing organisms.
- The biggest disadvantage of asexual reproduction is absence of diversity. Asexual reproduction occurs most commonly in unicellular organisms, in plants and in certain animals that have relatively simple organisations.
- Asexual reproduction takes place by following methods: binary fission; multiple fission; budding and spore formation, vegetative propagation.
- In binary fission a cell simply copies its DNA and then splits in two, giving a copy of its DNA to each daughter cell. For example, bacteria and archaebacteria.
- Multiple fission is a process in which the nucleus of the parent cell divides several times by amitosis, producing several nuclei. It occurs in many protists,e.g. sporozoans and algae.
- Budding is a process in which a new organism grows off the side of the parent through a part (called a bud, which means small growth). Bud detaches itself to form a new organism. Budding can occur both in unicellular and multicellular organisms. For example, bacteria yeast, protozoan, Hydra, etc.
- In spore formation, the parent body produce numerous spores (within a saclike structure called sporangium) which can produce new plants. It is surrounded by a hard-protective cover to withstand unfavourable conditions like high temperature and low humidity. Under favourable conditions, the spores germinate and grow into new plants. Moss, fern, molds and mushroom reproduce by using this method. The most common of these structures are zoospores.
- In fragmentation, a “parent” organism is split into multiple parts, each of which grows to become a complete, independent “offspring” organism. This process resembles budding and vegetative propagation, but with some differences.
- Vegetative propagation occurs in plants. In plants, the units of vegetative propagation like runner, rhizome, sucker, tuber, bulb are called vegetative propagules. All are capable of producing new offsprings.
- The main advantage of vegetative propagation methods is that the new plants contain the genetic material of only one parent, so they are essentially clones of the parent plant. This can also help to maintain consistent quality and taste in products made from plants or crops.
- Examples of vegetative propagules are rhizome of ginger, eyes of potato, bulbil of agave, leaf buds of Bryophyllum, sucker in apple and banana, offset of water hyacinth.
- Sexual reproduction involves the fusion of male and female gametes. It results in offspring that are not identical to the parents or to themselves.
- When male and female gametes are produced from male and female parents respectively, then it is called unisexual reproduction, e.g., man, cow, etc.
- When both male and female gametes are produced from same individual, then it is called bisexual reproduction, e.g., earthworm, tapeworm.
- Events in sexual reproduction can be grouped into three distinct stages: pre-fertilisation events, fertilisation and post-fertilisation events.
- Pre-fertilisation comprise all the events of sexual reproduction which takes place before the fusion of gametes. There are two main pre-fertilisation events-gametogenesis and gamete transfer.
- Gametogenesis is the process of formation of two types of gametes through meiosis. Gametes (male or female) are haploid cells.
- Gametes are of two types:
- Homogametes (also called isogametes) are morphologically similar in appearance and not categorised into male and female gametes. For example, algae (Cladophora ).
- Heterogametes are morphologically two unique female and male gametes. For example, antherozoid or sperm is the male gamete and egg or ovum is the female gamete.
- Plant may be bisexual (containing both male and female reproductive structure on same plant) or unisexual (containing male and female reproductive structure on different plant).
- In some plants and fungi, bisexual condition is also called as homothallic (or monoecious) and unisexual condition is called as heterothallic (or dioecious condition).
- The unisexual male flower is staminate, i.e., bearing stamens, while the female flower is pistillate i.e. bearing pistils.
- In some flowering plants, if both male and female flowers may be present on the same individual or on separate individuals are respectively called as monoecious (e.g, cucurbits and coconuts) or dioceous (e.g., papaya and date palm).
- Most animals are dioecious, for example, insects, spiders, fish, amphibians, reptilians, birds and mammals. They all exhibit sexual dimorphism.
- Hermaphroditism is a condition of having both male and female reproductive organs.
- Gametic transfer
- Gamete transfer is a process in which gametes are bring together to facilitate its fusion or the process of fertilisation.
- In most of the cases, male gamete is motile (algae, bryophytes & pteridophytes) and the female gamete is stationary except for few fungi and algae in which both types of gametes are motile.
- In seed plants, pollen grains are the carriers of male gametes and ovule has the egg or female gamete. Pollen grains produced in the anthers are transferred to the stigma by a process, known as pollination.
- Pollination is of two types: self pollination and cross pollination.
- Self pollination is the process of transfer of pollen grains from anther to stigma occurs within the same flower, e.g., pea.
- Cross-pollination is the process of transfer of pollen grains from anthers of male flower to the stigma offemale flower of different flowers through the agency of air, water or animals particularly insects.
Fertilisation (also called syngamy)
- It is the complete and permanent fusion of two gametes from different or same parent to form a diploid zygote.
- It may be external or internal on the basis of place of fusion of gametes.
- In external fertilisation, parents release sperms and eggs into the surrounding water, where fertilisation takes place and zygotes develop into offspring, e.g., Obelia, Nereis, Labeo and Frog.
- Internal fertilization is characterized by sperm fertilizing the egg within the female. Internal fertilization protects the fertilized egg or embryo from predation and harsh environments, which results in higher survival rates than can occur with external fertilization.
- There are some organisms in which the female gamete undergoes development to form new organisms without fertilisation. This process is known as parthenogenesis, e.g., rotifers, honeybees and some lizards.
- After the formation of the zygote, it develops into an embryo & the process is known as embryogenesis.
- During embryogenesis, a new individual is formed by the process of cell division and differentiation.
- On the basis of the development of zygote, the animals are of two types: oviparous and viviparous.
- The oviparous animals lay fertilized or unfertilized eggs. For example, reptiles &birds.
- The viviparous animals give birth to young ones. For example – the majority of mammals, including humans.
- In flowering plants, the zygote is formed inside the ovule of the ovary.
- The ovary develops into fruit which develops a thick wall called pericarp that is protective in function. Under favourable conditions, seeds germinate to produce new plants.
- NCERT Revision Notes for Class 12 (All Subjects)
- CBSE Class 12 Biology Notes
- NCERT Solutions for class 12th Biology Chapter 1 Reproduction in Organisms