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

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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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Short Answer▾

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.

Question 140 (3 of 5 Based on Passage)

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Short Answer▾

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

(i) State the name of the enzyme that cuts DNA.

Explanation

Restriction Endonucleases

  • They cut the DNA strands at specific base sequence in palindrome site. Thus, they function as ‘molecular scissors’ or chemical scalpels. The restriction endonuclease inspects the length of a DNA sequence. Once it recognizes specific sequence, it binds to the DNA and cuts each of the two strands of the double helix at specific points in their sugar phosphate back bones. Special sequence in the DNA recognized by restriction endonuclease is called palindromic nucleotide sequence.
  • The palindromes are groups of letters that form the same words when read in both directions forward and backward. Ex:

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