IGCSE Biology Paper-4: Specimen Questions with Answers 137 - 138 of 279

Question 137

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Mammals also have a lymphatic system.

Outline the functions of the lymphatic system.

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Explanation

The lymphatic system has three functions:

  • The removal of excess fluids from body tissues. This process is crucial because water, proteins, and other substances are continuously leaking out of tiny blood capillaries into the surrounding body tissues. If the lymphatic system didn՚t drain the excess fluid from the tissues, the lymph fluid would build up in the body՚s tissues, and they would swell.
  • Absorption of fatty acids and subsequent transport of fat, chyle, to the circulatory system.
  • Production of immune cells (such as lymphocytes, monocytes, and antibody producing cells called plasma cells) .

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 138 (1 of 5 Based on Passage)

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

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

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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: