Class 12 Biology Chapter 1 Notes PDF Download
Biology is the study of life and its processes. It is a fascinating subject that covers a wide range of topics, from the molecular level to the ecosystem level. One of the most important topics in biology is reproduction, which is the process by which living organisms produce new individuals of their own kind. In this article, we will provide you with the notes for Class 12 Biology Chapter 1, which deals with reproduction in organisms. You can download the PDF of these notes for free and use them for your exam preparation.
class 12 biology chapter 1 notes pdf download
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Introduction
Reproduction is a vital phenomenon for all living organisms, as it ensures the continuity of their species from generation to generation. It also leads to the creation of genetic variation, which is essential for adaptation and evolution. Reproduction can be classified into two types: asexual and sexual. Asexual reproduction involves only one parent and produces offspring that are genetically identical to the parent. Sexual reproduction involves two parents of opposite sex and produces offspring that are genetically different from both the parents.
What is reproduction?
Reproduction is defined as the biological process by which an organism gives rise to another organism similar to itself. It involves the transfer of genetic material from one or more parents to the offspring. Reproduction can be either natural or artificial. Natural reproduction occurs without any human intervention, while artificial reproduction involves human manipulation of the reproductive process, such as cloning, in vitro fertilization, etc.
Types of reproduction
There are two main types of reproduction: asexual and sexual. Let us look at each type in detail.
Asexual reproduction
Asexual reproduction is a type of reproduction in which only one parent is involved and no gametes (sex cells) are formed. The offspring produced by asexual reproduction are called clones, as they are genetically identical to the parent. Asexual reproduction is common in simple organisms, such as bacteria, algae, fungi, protozoa, etc. It is also seen in some plants and animals with relatively simple organization.
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Sexual reproduction
Sexual reproduction is a type of reproduction in which two parents of opposite sex participate and gametes (sex cells) are formed. The male gamete (sperm) fuses with the female gamete (egg) to form a zygote, which develops into an offspring. The offspring produced by sexual reproduction are genetically different from both the parents, as they inherit traits from both of them. Sexual reproduction is seen in most plants and animals with complex organization.
Reproduction in Organisms
In this section, we will discuss how different organisms reproduce by asexual or sexual methods. We will also look at some examples of each type of reproduction.
Asexual Reproduction in Animals
The most commonly seen modes of asexual reproduction in animals are as follows:
Fission
Fission is a mode of asexual reproduction in which an organism splits into two or more independent organisms after attaining a certain size or age. Fission can be binary or multiple. Binary fission occurs when an organism divides into two equal halves, each forming a new individual. For example, amoeba, parame cium, etc. Multiple fission occurs when an organism divides into many small individuals at once. For example, plasmodium, sporozoans, etc.
Budding
Budding is a mode of asexual reproduction in which an outgrowth or bud develops on the body of the parent organism and detaches to form a new individual. The bud may remain attached to the parent for some time before separating. Budding is seen in organisms with high regenerative capacity, such as hydra, yeast, sponges, etc.
Regeneration
Regeneration is a mode of asexual reproduction in which an organism can regrow a lost or damaged part of its body and form a new individual. Regeneration is also a means of repairing injuries and maintaining homeostasis. Regeneration is seen in organisms with simple body organization and high cellular differentiation, such as planaria, starfish, earthworms, etc.
Parthenogenesis
Parthenogenesis is a mode of asexual reproduction in which an unfertilized egg develops into a new individual without the involvement of a male gamete. Parthenogenesis can be natural or induced. Natural parthenogenesis occurs in some animals that can switch between sexual and asexual modes of reproduction, such as honeybees, aphids, rotifers, etc. Induced parthenogenesis occurs when an egg is artificially stimulated to develop without fertilization, such as by chemical or physical means.
Asexual Reproduction in Plants
The most commonly seen modes of asexual reproduction in plants are as follows:
Vegetative propagation
Vegetative propagation is a mode of asexual reproduction in which a vegetative part of a plant, such as root, stem, leaf, or bud, gives rise to a new plant. Vegetative propagation can be natural or artificial. Natural vegetative propagation occurs when plants produce specialized structures that can grow into new plants, such as runners, rhizomes, tubers, bulbs, corms, etc. Artificial vegetative propagation occurs when humans manipulate the vegetative parts of plants to produce new plants, such as by cutting, grafting, layering, tissue culture, etc.
Spore formation
Spore formation is a mode of asexual reproduction in which plants produce spores that can germinate and grow into new plants. Spores are microscopic structures that are produced by the sporophyte generation of plants. Spore formation is seen in lower plants, such as algae, fungi, bryophytes, pteridophytes, etc.
Fragmentation
Fragmentation is a mode of asexual reproduction in which a plant breaks into fragments that can develop into new plants. Fragmentation is seen in plants with simple body organization and high regenerative capacity, such as algae, mosses, liverworts, etc.
Sexual Reproduction in Animals
The main steps involved in sexual reproduction in animals are as follows:
Gametogenesis
Gametogenesis is the process by which gametes (sex cells) are produced from germ cells (precursor cells) by meiosis (a type of cell division that reduces the chromosome number by half). Gametogenesis occurs in the gonads (reproductive organs) of animals. In males, gametogenesis is called spermatogenesis and produces sperms. In females, gametogenesis is called oogenesis and produces eggs.
Fertilization
Fertilization is the process by which the male gamete (sperm) fuses with the female gamete (egg) to form a zygote (fertilized egg). Fertilization can be external or internal. External fertilization occurs when the gametes are released into the external environment and fuse there. External fertilization is seen in aquatic animals, such as fish of angiosperms, in which two fertilization events take place inside the ovule. One male gamete fuses with the egg cell to form a diploid zygote, which develops into an embryo. The other male gamete fuses with the central cell, which contains two polar nuclei, to form a triploid primary endosperm nucleus, which develops into an endosperm. The endosperm is a nutritive tissue that provides food for the developing embryo and seedling.
Embryo development and seed formation
Embryo development is the process by which the zygote undergoes a series of cell divisions and differentiation to form an embryo. The embryo consists of a radicle (embryonic root), a plumule (embryonic shoot), and one or two cotyledons (seed leaves). The cotyledons store food for the embryo and may also perform photosynthesis after germination. The embryo is enclosed by a protective covering called the seed coat, which is derived from the integuments of the ovule. The seed coat helps in seed dispersal and dormancy. The seed is the mature ovule that contains the embryo and the endosperm.
Conclusion
In this article, we have learned about the different types and modes of reproduction in organisms. We have also discussed the various steps and examples of asexual and sexual reproduction in animals and plants. Reproduction is a vital process for all living organisms, as it ensures their survival and evolution. Reproduction also leads to genetic variation, which is the basis of biodiversity and adaptation.
Summary of the chapter
The main points covered in this chapter are:
Reproduction is the biological process by which an organism gives rise to another organism similar to itself.
Reproduction can be classified into two types: asexual and sexual.
Asexual reproduction involves only one parent and produces offspring that are genetically identical to the parent.
Sexual reproduction involves two parents of opposite sex and produces offspring that are genetically different from both the parents.
Asexual reproduction is common in simple organisms, such as bacteria, algae, fungi, protozoa, etc. It is also seen in some plants and animals with relatively simple organization.
Sexual reproduction is seen in most plants and animals with complex organization.
The most commonly seen modes of asexual reproduction in animals are fission, budding, regeneration, and parthenogenesis.
The most commonly seen modes of asexual reproduction in plants are vegetative propagation, spore formation, and fragmentation.
The main steps involved in sexual reproduction in animals are gametogenesis, fertilization, development, and birth.
The main steps involved in sexual reproduction in plants are flower structure and function, pollination and pollen-pistil interaction, double fertilization and endosperm development, embryo development and seed formation.
Importance of reproduction for organisms and biodiversity
Reproduction is important for organisms and biodiversity for the following reasons:
Reproduction ensures the continuity of life from generation to generation.
Reproduction leads to genetic variation among offspring, which is essential for adaptation and evolution.
Reproduction maintains the population size and balance of different species in an ecosystem.
Reproduction creates new combinations of traits that may result in new species or varieties.
Reproduction enhances the diversity of life forms on Earth.
FAQs
Here are some frequently asked questions about this chapter:
What is the difference between gamete and zygote?
A gamete is a haploid (n) sex cell that is produced by meiosis from a germ cell. A zygote is a diploid (2n) cell that is formed by the fusion of two gametes during fertilization.
What is the difference between self-pollination and cross-pollination?
Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower or plant. Cross-pollination is the transfer of pollen grains from the anther to the stigma of a different flower or plant.
What is the difference between oviparous, ovoviviparous, and viviparous development?
Oviparous development occurs when the offspring is hatched from an egg laid outside the body of the mother. Ovoviviparous development occurs when the offspring is hatched from an egg retained inside the body of the mother. Viviparous development occurs when the offspring is born alive from the body of the mother.
What is the difference between spore and seed?
A spore is a microscopic structure that is produced by the sporophyte generation of lower plants, such as algae, fungi, bryophytes, pteridophytes, etc. A seed is a macroscopic structure that is produced by the sporophyte generation of angiosperms (flowering plants). A spore contains only a single cell, while a seed contains an embryo and an endosperm.
What is the difference between direct and indirect development?
Direct development occurs when the offspring resembles the adult form at birth. Indirect development occurs when the offspring undergoes a larval stage before reaching the adult form.
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