The period from birth to the natural death of an organism represents its life span. Life span may be as short as a few days or as long as thousand years.
- Crow – 15 years
- Elephant – 60-90 years
- Dog – 15-20 years
- Cow – 20-25 years
- Horse – 60 years
- Crocodile – 60 years
- Tortoise – 150-250 years
- Fruit fly – 30 days
- Butter fly – 1-2 weeks
- Parrot – 140 years
- Rose – 5-7 years
- Banana tree – 25 years
- Rice plant – 3-4 months
- Banyan tree – 200 years
- Sequia and Redwood – 3000-4000 years
Whatever be the life span, death of every individual is certainty. Number of organisms should reduce due to death but there is a process in living organism that ensure continuity, i.e. Reproduction.
Reproduction is defined as a biological process in which an organism give rise to young one similar to itself. It ensure continuity of species of a particular organisms.
Asexual reproduction is the process by which an organism creates a genetically similar or identical copy of itself without a contribution of genetic material from another individual. This method is found in those organism in which single individual is capable of producing offspring.
Question1. Why asexual offspring are termed as clone?
Answer: Asexual organism produce offspring without involvement of two organism, fusion of gamete and crossing over, As a result, the offspring that are produced are not only identical to one another but are also exact copies of their parent. These offspring are genetically and morphologically identical and known as clone.
Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simply organizations. Asexual reproduction is the primary form of reproduction for single celled organisms such as the archaebacteria, eubacteria, and Protista. Many plants and fungi reproduce asexually as well. There are following modes of asexual reproduction:
- Binary Fission
- Spore Formation
- Vegetative propagation
In this type of method a cell divides into two halves and each rapidly grows into an adult. The nucleus divides first and the cytoplasm next. Due to both these division the mother cell splits into two equal sized daughter halves or cells. On the basis of cytoplasmic division Binary fission are of three types:
- When the plane of cytoplasmic division passes through any direction the fission is called simple binary fission. E.g. Amoeba.
- When the plane of cytoplasmic division passes through transverse axis of the individual is known as transverse binary fission. E.g. Paramecium.
- When the plane of cytoplasmic division passes through longitudinal axis of the individual is known as longitudinal binary fission. E.g. Euglena.
New individuals form by mitosis. Initially a small outgrowth of the parent's body develops. It then separates from the mother to lead a free life. E.g. Hydra, yeast. There are two types of budding: Exogenous budding (The bud grows externally on the surface of parent body. Example: Yeast) and Endogenous budding (The buds are formed within the parent's body. They are called gemmules. Gemmules are small group of cells in a protective covering. Example: sponges).
The body of the parent breaks into distinct pieces, each of which can produce an offspring. E.g. Hydra, some marine worms.
Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures. The most common of these structures are zoospores that usually are microscopic motile structures.
Other common asexual reproductive structures are conidia (penicillium), buds (Hydra) and gemmules (sponge).
Asexual Reproduction in plants usually termed as vegetative reproduction or vegetative propagation. In this type of reproduction plant used vegetative parts like root, stem and leaf for production of offspring. Since the formation of offspring doesn’t involve two parents, fusion of gamete, meiotic division, the process is type of asexual reproduction.
Natural Vegetative Method
In such methods of vegetative propagation, a portion of the plant gets detached from the mother plant and develops into a new independent platn under suitable environmental conditions. The detached parts may be roots, stems, leaves or flowers.
- Vegetative propagation by roots: In some plants like sweet potato, carrot, turnip, radish and Dahila, root develop adventitious buds which develop into new plants.
- Vegetative propagation by leaves: Leaves of many plants have adventitious buds. Such leaves when fall on the ground, their buds develop root and mature into individual plants. Example: Bryophyllum.
Vegetative propagation by stems: Stems of some plants modify and perform vegetative propagation. There are two modification in stem to perform vegetative propagation: Underground stems and Sub-aerial stems.
Sub-serial stems:Also known as creepers. They may be:
Underground stems:They may be:
- Sub-serial stems:Also known as creepers. They may be:
Artificial Vegetative Method
The capacity for regeneration in higher plants can be used for production of new individuals. Some common method of artificial methods of vegetative propagation are cutting, layering and grafting.
Question. What is Terror of Bengal?
Answer: Water hyacinth is an aquatic plant that grows well in water bodies like ponds and rivers. This plant was introduced in India because of its beautiful flowers and shapes of leaves. This plant can propagate vegetatively by producing runner called offset. It can spread all over the water body. It was known as "Terror of Bengal" as it grow fast and difficult to get rid of. It also drained O2 from the water, as a result of which, it led to the death of fishes.
Sexual reproduction is a process that creates a new organism by combining the genetic material of two organisms. Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. Sexual reproduction is an elaborate, complex and slow process as compared to asexual reproduction. In sexual reproduction the offspring receive a set of genome from each of the parents. Because of it offspring are not identical to the parents or amongst themselves. There are three main phase in sexually reproductive organism –
- Juvenile Phase – Pre reproductive phase
- Reproductive Phase
- Old age phase – Post reproductive phase
All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the Juvenile phase. It is known as vegetative phase in plants. The end of Juvenile or vegetative phase which marks the beginning of the reproductive phase.
Oestrus Cycle and Menstrual Cycle
The female of placental mammals exhibits cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase. In non-primate mammals like cows, sheep, rats, deer, dogs, tiger, etc., such cyclical changes during reproduction are called Oestrus Cycle. Where as in primates like monkeys, apes, and human it is called Menstrual Cycle.
Events in sexual reproduction
After attainment of maturity all sexually reproductive organisms follow a regular sequence. These regular sequences may be grouped into three stage –
- Pre- Fertilization Events
- Fertilization Events
- Post-Fertilization Events
These include all the events of sexual reproduction before the fusion of gametes. There are two main pre-fertilization events – gametogenesis and gamete transfer.
The process of formation of haploid gametes is known as gametogenesis. There are two type of gametes – male and female gamete. Gametes are haploid cells.
- If the two gametes are so similar in appearance that it is not possible to categories them into male and female gametes, then they are known asHomogametesorIsogametes. E.g. Some algae.
- If the two gametes are morphologically distinct to each and it is possible to categories them into male and female gametes, then they are known as Heterogametes. In such organisms the male gametes is called the antherozoids or sperm and the female gamete is called the egg or ovum. E.g. Human and majority of sexually reproductive organisms.
- If organism has both male and female structure, it is known as bisexual. If different members of an organism have male and female structure, it is known as unisexual,
- In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are used to denote the unisexual condition.
- In flowering plants, the unisexual male flower is staminate or bearing stamens, while the female is pistillate or bearing pistils.
- Animals that possess both male and female reproductive organs, are hermaphrodites. E.g. Earthworms, sponges.
Cell division during gamete formation
Organisms belonging to monera, fungi, algae and bryophytes have haploid plant body so they produce haploid gametes by mitosis. But organisms belonging to pteridophytes, gymnosperms, angiosperms and most of the animals including human beings, the parental body is diploid. These organisms produce haploid gametes by meiosis (reductive division).
After their formation, male and female gametes must be physically brought together to facilitate fusion. Successful transfer and coming together of gametes is essential for the most critical event in sexual reproduction.
- In majority of organisms, male gamete is motile and the female gamete is stationary.
- But in few fungi and algae in which both types of gametes are motile. There is a need for a medium through which the male gametes move. In several simple plants like algae, bryophytes and pteridophytes, water is the medium through which this gamete transfer takes place.
- A large number of the male gametes fail to reach the female gametes. To compensate this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.
- In seed plants, pollen grains are the carriers of male gametes and ovule have the egg. Pollen grains produced in anthers therefore, have to be transferred to the stigma for fertilization. The process of transfer of pollen grains to the stigma is known as pollination. There are two types of pollination self-pollination and cross pollination. After pollination pollen grain germinate on the stigma and the pollen tubes produced. Pollen tube discharge the male gamete in the ovule near the egg.
- In dioecious animal in which male and female gametes are formed in different individuals, the organism must has a special mechanism for gamete transfer.
The fusion of male and female gametes is known as syngamy and formation of a diploid zygote is known as fertilisation. Fertilisation are of two types on the basis "Where does syngamy occur". Two types are external fertilisation and internal fertilisation.
If the fusion of male and female gamete occurs in external medium or outside the body of the organism, then gametic fusion is known as external fertilization. In most aquatic organisms such as algae, fishes and some amphibians show external fertilization. These organisms show great synchrony between the sexes and release of large number of gametes into the surrounding medium in order to enhance the chances of syngamy. Frog and bony fishes show external fertilization.
If the fusion of male and female gamete occurs inside the body of the organism, then gametic fusion is known as internal fertilisation. Terrestrial organisms, fungi, higher animals such as reptile birds, mammals and bryophytes, and higher plants such as pteridophytes, gymnosperms and angiosperms show internal fertilisation.
A major disadvantage of external fertilization is survival of offspring because the offspring are exposed in front of predators.
In all these organisms, egg is formed inside the female body where they fuse with the male gamete. In these organism male gamete is motile and has to reach the egg in order to fuse with it. In seed plants non motile male gamete are carried to female gamete by pollen tube.
Question: Explain Parthenogenesis.
Answer:Parthenogenesis is a special form where an ovum develops into a new individual without being fertilised by the sperm. Parthenogenesis could be haploid (as in honey bees, ants, bees and wasps) or diploid (as in aphids), depending on the number of chromosomes present in the ovum. The organisms produced by parthenogenesis are identical to the mother. Other example: Rotifers and birds.
Post Fertilization Events
Events in sexual reproduction after the formation of zygote are called post-fertilisation events. Zygote and Embryogenesis are two main terms to study under post-fertilisation events.
After the fusion we got diploid zygote by fertilisation process. In external fertilisation zygote is formed in external medium and in internal fertilisation zygote is formed inside the body of the organisms. Zygote is the vital link that ensures continuity of species between organism of one generation and the next. Every sexual reproducing organism including human beings begin life as a single cell zygote.
The process of development of embryo from the zygote is known as embryogenesis. During embryogenesis, zygote undergoes mitosis cell division and cell differentiation. Cell division increase the number of cell in the developing embryo, cell differentiation helps to form specialised tissues and organs to form an organism.
Animals can be grouped into two categories based on how and where the development of the zygote takes place. These categories are:
- Oviparous: The fertilized egg is covered by a calcareous shell and is released into the outside environment. The development takes place inside the egg and the young one hatches out (example: birds and reptiles).
- Viviparous: The development of the zygote takes place inside the female body, and the developed young one is delivered outside (example: mammals, including humans).
In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower become dry and fall off. The pistil however, remains attached to the plant. The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develop a thick wall called pericarp that is protective in function.
- Zygote → Develops into → Embryo
- Ovule → Develops into → Seed
- Ovary → Develops into → Fruit → Contains → Seeds → Disperse and germinate to form new plants