IGCSE Biology Paper-4: Specimen Questions with Answers 139 - 140 of 279

Passage

Cotton, Gossypium hirsutum, is grown for the fibers that form within the fruits after fertilization, as shown in Fig.

Fibers from the fruits of cotton plants are used in the textile industry.

The Fibres
  • Cotton plants have been genetically engineered to produce a protein that is toxic to the caterpillars of several insect pests. This gives the cotton plants resistance to the pests.
  • The cry gene for pest resistance was isolated from the bacterium Bacillus thuringiensis and inserted into the cells of cotton plants as shown in Fig.
Bacterial DNA

Question 139 (2 of 5 Based on Passage)

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An enzyme cuts the cry gene from the DNA of B. thuringiensis.

State the name of the regions labelled A on Fig.

Explanation

Restriction enzymes cut the strand of DNA a little away from the center of the palindrome sites but between the same two bases of the opposite strands. This leaves single stranded unpaired bases at cut ends. These ends with unpaired bases are called sticky ends or cohesive ends. The latter are named so because they form hydrogen bonds with their complementary cut counter parts. The sticky ends facilitate the action of the enzyme DNA ligase.

Endonuclease and Sticky Ends

Question 140 (3 of 5 Based on Passage)

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The plasmids containing the cry gene are inserted into the cells of cotton plants.

Outline how the cells of cotton plants use the cry gene to make the toxic protein.

Explanation

BT cotton was created through the addition of genes encoding toxin crystals in the Cry group of endotoxins. When insects attack and eat the cotton plant the Cry toxins or crystal protein are dissolved due to the high pH level of the insect՚s stomach. Once an insect ingests the inactive toxin it is converted into an active form of toxin due to the alkaline pH of the alimentary canal that solubilizes the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores which cause cell swelling and lysis and finally cause death of the insect.

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