Lessons 28–29 · 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
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Processes that change the heredity of the cell
Textbook: pp. 126–130
GoalExplain transformation, transduction and conjugation and the experiments that proved DNA is the hereditary material.
Recombinant DNA also forms in nature: bacteria exchange hereditary material in three ways. In transformation a cell takes up a piece of DNA from its surroundings. F. Griffith (1928) experimented with the capsulated S strain (causing disease) and the capsule-free R strain of pneumococci: mice injected with heat-killed S together with live R died, and live S was found in them. O. Avery, C. MacLeod and M. McCarty (1944) showed that DNA is exactly the cause of transformation, i.e. DNA is the hereditary material. In transduction (N. Zinder, J. Lederberg, 1952) DNA passes from one bacterium to another through a bacteriophage (a virus infecting bacteria). In the lytic cycle the phage destroys the bacterium; in lysogeny it stays in the bacterial chromosome as a prophage. In conjugation (J. Lederberg, E. Tatum, 1946) DNA (a plasmid) passes between two bacteria through a cytoplasmic bridge. Genes thus form new combinations and variety in the bacterial population increases; these processes also explain how antibiotic resistance spreads.
Worked examples
Griffith’s experiment: live S – the mouse dies; live R – survives; heat-killed S – survives; heat-killed S + live R – the mouse dies and live S is in its blood. So some substance from dead S turned R into S.
When pure DNA from streptomycin-resistant bacteria was added to a tube with non-resistant bacteria, some of those became resistant: the DNA carried the resistance information.
Class activity
“Mouse experiment”: four groups act out the four steps of Griffith’s experiment with pictures and short text and predict the result in advance.
Practice
1
What did Griffith’s experiment show?
That some substance from dead S can turn live R bacteria into S (transformation).
2
What conclusion did Avery, MacLeod and McCarty reach?
DNA brings about transformation, i.e. the hereditary material is DNA, not protein.
3
Distinguish transformation, transduction and conjugation in one sentence each.
Transformation – taking up DNA from outside; transduction – transfer by a phage; conjugation – DNA transfer when two cells connect.
4
Why do bacteria show high genetic variety even though they do not reproduce sexually?
Mutations accumulate fast, and genes are exchanged between cells by transformation, transduction and conjugation.