How do Organisms Reproduce?

Reproduction is not essential for the survival of an individual, but it is essential for the continuation of a species. At its core, reproduction involves making copies of DNA, the information source for building the body, along with extra cellular machinery. Since DNA copying is not perfectly accurate, small variations arise, and these variations are the basis for the survival of species in changing environments. In this chapter you will study modes of asexual reproduction in single-celled and multicellular organisms: binary and multiple fission, fragmentation, regeneration, budding, vegetative propagation and spore formation. You will then learn about sexual reproduction, which combines DNA from two individuals. In flowering plants, you will study the parts of a flower, pollination, fertilisation and the formation of seeds and fruits. In human beings, you will learn about the changes at puberty, the male and female reproductive systems, fertilisation, the role of the placenta and what happens when the egg is not fertilised. Finally, you will study reproductive health, sexually transmitted diseases and methods of contraception.

Detailed Worksheet: How do Organisms Reproduce? Section A - Definitions (10 marks) 1. What is reproduction? Why is DNA copying an essential part of it? (2 marks) 2. Distinguish between binary fission and multiple fission, giving one example of each. (2 marks) 3. What is vegetative propagation? Give two examples of plants propagated in this way. (2 marks) 4. Define pollination. Distinguish between self-pollination and cross-pollination. (2 marks) 5. What is the placenta? State its function. (2 marks) Section B - Calculations and Applications (15 marks) 6. An Amoeba divides by binary fission once every 24 hours under favourable conditions. Starting with one Amoeba, how many will there be after 5 days? Write a general expression for the number after n divisions. (3 marks) 7. A bacterium divides by binary fission every 20 minutes. Starting with 10 bacteria, how many will be present after 2 hours, assuming none die? Why is such growth not seen indefinitely in nature? (3 marks) 8. A gardener plants 120 rose stem cuttings and 120 rose seeds. 96 cuttings and 54 seeds grow into plants. Calculate the success percentage of each method and give two advantages of vegetative propagation suggested by the result. (3 marks) 9. The menstrual cycle in a woman is about 28 days long, and an egg is released roughly on the 14th day of the cycle. If a cycle starts on 1 March, on which date is ovulation expected, and on which date is the next menstruation expected if the egg is not fertilised? Explain why the uterine lining breaks down. (3 marks) 10. India's child sex ratio (0-6 years) in Census 2011 was about 919 girls per 1000 boys. Calculate the number of girls expected in a town with 50,000 boys aged 0-6 at this ratio, and the number of 'missing' girls compared with a ratio of 1000 : 1000. Name one law that bans prenatal sex determination. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the longitudinal section of a flower showing the sepals, petals, stamen (anther and filament), pistil (stigma, style and ovary) and ovule. (4 marks) 12. Draw labelled diagrams showing (i) binary fission in Amoeba and (ii) budding in Hydra. (3 marks) 13. Draw a labelled diagram of the human female reproductive system showing the ovary, oviduct, uterus, cervix and vagina. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Why is variation beneficial to a species but not necessarily to an individual? Explain how the consistency of DNA copying helps an organism fit its niche, and how variation helps a population survive when the niche changes drastically, for example due to rising water temperature. (5 marks) 15. Compare the advantages and disadvantages of the different methods of contraception: barrier, hormonal, intrauterine devices (Copper-T) and surgical methods. Which method also protects against sexually transmitted diseases, and why? (5 marks) 16. Explain why more complex organisms cannot give rise to new individuals through regeneration, while Planaria can. What special cells are involved in regeneration? Describe how Rhizopus reproduces by spore formation and why spores are able to survive harsh conditions. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Use correct scientific names wherever possible.