01. Reproduction in Organisms
Chapter-1: Reproduction in Organisms
Main Points To Cover:
Reproduction, a characteristic feature of all organisms for continuation of species; modes of reproduction – asexual and sexual reproduction; asexual reproduction – binary fission, sporulation, budding, gemmule formation, fragmentation; vegetative propagation in plants.
- The life span of an organism refers to the period for which it remains alive.
- Reproduction refers to the process by which each organism ensures that the species continues to exist.
- Reproduction may be described as the process by which organisms give rise to young ones, who in turn, become mature and produce their own young.
- The process of reproduction occurs in two ways:
- Asexual: A single organism is involved in reproduction.
- Sexual: Two organisms (female and male) are involved in reproduction.
- One parent is involved, which gives rise to its offspring.
- These offspring are identical to each other as well as to their parent. Thus, they are said to be clones of each other.
- This process is observed in (unicellular organisms) Protistans and Monerans.
- The manner of reproduction is cell division.
Means of asexual reproduction:
- Binary fission: Seen in Amoeba and Paramecium, this process involves a parent cell dividing in half, and then each half growing into an adult.
- Budding: Seen in yeast, this process involves buds developing from a parent cell (unequal in size to the parent), which then separate and become a mature adult.
- Formation of specialized structures:
- Conidia: Seen in Penicillium
- Gemmules: Seen in sponges
- Buds: Seen in Hydra
- Zoospores: Seen in Algae, these appear in the form of motile, microscopic spores.
- Vegetative propagation: Seen in plants, this process involves different parts of a plant developing into new plants.
- Runner: Seen in Gladiolus
- Rhizome: Seen in ginger
- Tuber: Seen in potato
- Bulb: Seen in onion
Sexual reproduction: Pre-fertilization Events
- In sexual reproduction, male or female gametes are formed, either in the same organism or in different organisms. These gametes join to form a zygote, from which a new organism develops.
- The new organisms are not identical to the parent organisms or to each other. Thus, sexual reproduction ensures diversity.
- Such organisms go through two phases:
- Juvenile phase: A non-reproductive phase where growth takes place
- Vegetative/reproductive phase
- Non-primate mammals, such as tigers, cows, rats, sheep, and dogs undergo a cyclic change in the functions of the ovaries and the oviduct. This change is known as the oestrus cycle. Primates, such as apes, monkeys, and humans, also undergo such a cycle, which is called the menstrual cycle.
- Continuous breeders are mammals which can reproduce throughout the reproductive phase of their life. Seasonal breeders are mammals which can reproduce only in favorable seasons.
Events in Sexual Reproduction
Certain events, called pre-fertilization events, fertilization, and post-fertilization events, are exhibited by sexually reproducing organisms.
Pre-fertilization Events: These events take place before the gametes fuse and consist of gametogenesis and gamete transfer.
- Male and female gametes are formed through the process of gametogenesis.
- These gametes are haploid.
- In organisms like Algae, the gametes are similar (isogametes or homogametes) and cannot differentiated into male gametes or female gametes.
- In other organisms, the gametes are different (heterogametes) from each other, morphologically and physiologically. They can be classified as antherozoid or sperm (male) and egg or ovum (female).
- Organisms in which both sexes exist in the same being are called homothallic or monoecious organisms, whereas organisms in which each sex exists in separate beings are called heterothallic or dioecious organisms.
- In unisexual flowers, female flowers are termed pistillate, whereas male flowers are termed staminate.
- Gametes are formed by the process of cell division. When the parent organisms are haploid, cell division occurs by mitosis. When the parent organisms are diploid, cell division occurs by meiosis and specialized cells called meiocytes undergo meiosis.
- Gametes need to be transferred for the process of joining to take place.
- Most organisms have motile male gametes and non-motile female gametes, wherein male gametes require a medium for movement. Most male gametes fail to reach the female gametes, thus large numbers of male gametes are produced to make up for the loss.
- In angiosperms, pollen grains carry the male gamete and ovules carry the female gamete.
- The anther produces pollen grains, which must be transferred to the stigma for fertilization to occur. In monoecious plants, this transfer takes place easily because the anther and stigma are located close to each other. In dioecious plants, the transfer occurs by pollination.
- The most crucial event in sexual reproduction is fertilization.
- The process of fertilization is known as syngamy and results in the formation of a zygote.
- A zygote may be formed without fertilization in some organisms like honeybees, some lizards, and rotifers. This process is called parthenogenesis.
- External fertilization refers to fertilization that occurs outside the body of an organism, usually in water. E.g. aquatic organisms and amphibians. The eggs and the young ones of such organisms are vulnerable to predators and they are at constant risk of death until they reach adulthood.
- Internal fertilization refers to fertilization that occurs within the body of an organism, usually the female. Motile male gametes reach the female gamete and fuse with it to form a zygote. Male gametes are produced in thousands. E.g. most terrestrial animals.
Post-fertilization Events: These events take place after fertilization has occurred.
- In all organism, haploid gametes join to form diploid zygote.
- Zygotes are formed in an external medium in external fertilization, whereas zygotes are formed inside an organism in internal fertilization.
- The formation of a zygote is related to the life cycle of an organism and its external environment. In some organisms, the zygote does not develop immediately after fertilization. Instead, it surrounds itself with a thick wall, which protects it against desiccation and damage.
- It refers to the process of formation of an embryo from a zygote.
- The zygote undergoes cell division and then differentiation.
- Cell division helps increase the number of cells in the embryo, whereas cell differentiation helps in the formation of specialized tissues and organs.
- Depending on where and how a zygote develops, organisms are classified into:
- Oviparous:g. reptiles, birds. The fertilized egg is surrounded by a calcareous shell and is released to the external environment. The offspring develops inside the egg and hatches out of it.
- Viviparous: E.g. mammals, including humans. The zygote forms inside the female’s body and the developed offspring are delivered to the external environment.
- In flowering plants, zygote formation occurs inside an ovule.
- The zygote develops into an embryo.
- The ovule develops into a seed.
- The ovary develops into a fruit, which contains seeds. These seeds are dispersed, which germinate to become new plants.
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