Lessons 15–16 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology, Grade 10, 1st edition. “Sharq” publishing and printing joint-stock company, Tashkent, 2017
15
Reproduction of organisms. Asexual reproduction
Textbook: pp. 66–68
GoalList the methods of asexual reproduction, give examples, and assess its advantages and disadvantages.
Reproduction is the ability of a living organism to create similar offspring using hereditary information. In asexual reproduction only one parent takes part, no sex cells are needed, it is based on mitosis, so offspring are genetic copies (clones) of the parent. In unicellular organisms: binary fission (in bacteria the circular DNA is copied and the cell splits; in amoeba, ciliates and Euglena it is based on mitosis), budding (yeast), multiple fission – schizogony (the malaria Plasmodium), and spores (Chlamydomonas, fungi). In multicellular organisms: fragmentation (Spirogyra, flatworms, starfish), budding (Hydra, sponges), spores (mosses, ferns, cap fungi) and vegetative reproduction of plants (tubers, rhizomes, cuttings, runners). In polyembryony the embryo splits into several parts, each developing into a separate organism (identical twins in humans). The advantage is fast and abundant offspring in favourable conditions; the drawback is very little genetic variety, so if the environment changes all individuals may be equally weak.
Worked examples
If one bacterium divides every 30 minutes, it divides 6 times in 3 hours and gives 2 · 2 · 2 · 2 · 2 · 2 = 64 bacteria.
If a tuber piece with an “eye” (bud) is planted, an exact copy of the parent plant grows; this is why the traits of a variety are kept.
Class activity
“Method of reproduction”. Picture cards (yeast, Hydra, Spirogyra, fern, strawberry) are placed in columns matching the method of reproduction.
Practice
1
State three features of asexual reproduction.
Only one parent takes part; it is based on mitosis and somatic cells; offspring are genetically identical to the parent.
2
Which method of reproduction do these use: yeast, Spirogyra, fern, malaria Plasmodium?
One bacterium divides every 20 minutes. How many bacteria are there after 2 hours (6 divisions)? (Number only.)
64
4
Why can a population that reproduces only asexually die out completely when the environment changes sharply?
The individuals are genetically identical, so one disease or condition affects all of them in the same way; there is no hereditary variety that would allow adaptation.