Reproduction in Organisms: One Mark Questions, Book Back Answers and Notes

About Reproduction in Organisms

Medium ~60 min study

Every species survives only because its members reproduce. This first unit of Class 12 Zoology sorts the many ways animals do this into two groups: asexual reproduction, where a single parent produces offspring that are genetically identical to it, and sexual reproduction, where two parents contribute gametes and the offspring carry new combinations of genes.

The chapter moves from simple to complex. It starts with fission and budding in single-celled and simple animals, goes on to gemmules in sponges, fragmentation and regeneration, and then turns to syngamy, conjugation and parthenogenesis. Each process comes with a named example, and those examples are exactly what the MCQs ask about.

The terms introduced here, such as haploid, diploid, gamete, zygote and parthenogenesis, come back in the units on human reproduction and genetics. Expect questions that match a type of fission to its organism, compare morphallaxis with epimorphosis, or ask what a honeybee drone develops from.

What you'll learn

• State the differences between asexual and sexual reproduction, with examples of each.
• Name the type of binary fission from its plane of division and match it to the right organism.
• Explain how gemmules help sponges survive and what apolysis means in tapeworms.
• Compare morphallaxis and epimorphosis using Hydra, lizard and starfish as examples.
• List the types of syngamy and explain how sexual reproduction produces variation.
• Explain complete and incomplete parthenogenesis and how it decides sex in honeybees.

Before you start

• Mitosis and meiosis, and how they differ.
• The meaning of haploid (n) and diploid (2n).
• Basic ideas of gametes, fertilization and animal body plans.

Reproduction in Organisms explained

How Animals Reproduce

Two Modes of Reproduction

Reproduction keeps a species going and, in its sexual form, produces the variation that evolution works on. The textbook lists features shared by all forms of reproduction: synthesis of RNA and proteins, replication of DNA, cell division, formation of reproductive units and, in sexual forms, their fusion. In asexual reproduction one parent gives rise to offspring that are clones of itself. In sexual reproduction two parents produce gametes that fuse, so each offspring is genetically unique. Neither mode is "better"; asexual reproduction is fast and needs no partner, while sexual reproduction creates variety.

Fission and Budding

Asexual reproduction is common in protists, bacteria, archaea and simple multicellular animals. In fission the parent body divides into two or more daughter individuals. Binary fission is named by the plane in which the cell divides: simple (irregular) binary fission in Amoeba, transverse fission in Paramecium, longitudinal fission in Euglena and oblique fission in dinoflagellates such as Ceratium. The chapter also covers multiple fission, sporulation and strobilation. In budding, a bud grows out from the parent, and it either separates to live on its own or stays attached. Gemmules are internal buds of freshwater sponges; they carry archaeocytes and let the sponge survive harsh conditions.

Fragmentation, Regeneration and the Life Cycle

In fragmentation the parent body breaks into pieces, and each piece can grow into a whole animal, as in sea anemones. In tapeworms, mature proglottids break off from the end of the body, a process called apolysis. Regeneration is the regrowth of injured tissue or lost parts. In morphallaxis a whole body grows from a small fragment, as in Hydra. In epimorphosis only the lost part is replaced, such as the tail of a wall lizard or the arm of a starfish. The chapter also describes three phases in the life of an organism: the juvenile phase of growth, the reproductive phase and the senescent phase, when the body's structure and function slowly break down.

Sexual Reproduction and Parthenogenesis

Sexual reproduction depends on syngamy, the fusion of a haploid male gamete with a haploid female gamete to form a diploid zygote. Fertilization may be external (outside the body, usually in water) or internal (inside the female). The types of syngamy you need to know are autogamy, exogamy, hologamy, paedogamy, merogamy, isogamy and anisogamy, along with conjugation in ciliates such as Paramecium. Parthenogenesis is the development of an egg into a complete individual without fertilization. In honeybees, unfertilized eggs develop into male drones (arrhenotoky), while fertilized eggs become queens and workers. In thelytoky, only females are produced from unfertilized eggs. So in bees the chromosome number of the egg decides sex; whether a female larva becomes a queen or a worker depends on how it is fed, not on its chromosomes.

Common mistakes to avoid

• Mixing up binary and multiple fission. Binary fission gives two daughter individuals; multiple fission gives many at the same time.
• Thinking every kind of regeneration makes a new animal. Morphallaxis does, but epimorphosis only replaces the lost part.
• Calling parthenogenesis ordinary asexual reproduction. The textbook treats it under sexual reproduction because the new individual develops from an egg, a gamete, even though there is no fertilization.
• Swapping external and internal fertilization. External fertilization takes place outside the body, usually in water, as in fishes and frogs; internal fertilization takes place inside the female, as in mammals.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is the difference between morphallaxis and epimorphosis?

In morphallaxis, a whole animal grows back from a small fragment of the body, as in Hydra. In epimorphosis, only the part that was lost is replaced, for example the tail of a wall lizard or the arm of a starfish. No new individual is formed in epimorphosis.

How does parthenogenesis occur in honeybees?

A honeybee queen lays both fertilized and unfertilized eggs. The unfertilized, haploid eggs develop into male drones; this is called arrhenotoky. The fertilized, diploid eggs develop into females, which become queens or workers. Because both kinds of eggs develop in the same species, this is an example of incomplete parthenogenesis.

What is strobilation in asexual reproduction?

Strobilation is repeated transverse fission seen in some multicellular animals, such as the jellyfish Aurelia. The polyp stage divides again and again across its body, forming a stack of disc-like pieces. Each piece is released as a small ephyra larva, which grows into an adult jellyfish.

What is the difference between seasonal and continuous breeders?

Seasonal breeders reproduce only at a particular time of year when conditions suit them, for example frogs, lizards and most birds. Continuous breeders can reproduce at any time throughout their sexually mature life, for example honeybees, rabbits and poultry.

Why are unicellular organisms considered immortal?

When a single-celled organism divides by binary fission, the parent does not die. Its nucleus and cytoplasm are simply shared between two daughter cells, so there is no dead body left behind and no natural death from ageing. The parent lives on in its two daughters.

What is syngamy and what are its types?

Syngamy is the complete fusion of two haploid gametes to form a diploid zygote. It is external in fishes and most amphibians, where gametes meet in water, and internal in reptiles, birds and mammals. Types named in the textbook include autogamy, exogamy, hologamy, paedogamy, merogamy, isogamy and anisogamy.