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27. The correct answer is (C). Thymidine is one of the nucleotides found in DNA. The various phases
of the cell cycle are mitosis (M), gap 1 (G
1
), replication or synthesis of DNA (S), and gap 2 (G
2
).
During the gap phases, the cell synthesizes the various cellular products that are necessary. Note that
only during the S phase is DNA being synthesized. Thus, thymidine will be utilized during this phase,
and radio-labeled thymidine will be incorporated.
28. The correct answer is (C). Active transport is an energy-dependent process for cellular transport.
Specific types of active transport include the Na
+
– K
+
ATPase and axonal transport. Active transport
allows for the movement of a particle against its concentration gradient. This is in contrast to diffusion
and osmosis, which are passive processes: movement of a particle occurs down its concentration gradi-
ent (from higher to lower concentrations). Cytoplasmic streaming has been well-Characterized in algae.
This process involves the movement of cellular contents by microfilaments and utilizes energy.
29. The correct answer is (D). Reabsorption of water is not a direct function of the vertebrate liver. The
vertebrate digestive tract (intestines) absorbs water and the kidney functions to maintain salt and
water balance. The other answer choices represent several of the functions served by the liver.
30. The correct answer is (D). Recall the classification hierarchy employed by taxonomists:
Kingdom, Phylum, Class, Order, Family, Genus, Species
The more closely related two organisms are, the more common their classification (i.e., same king-
dom, phylum, class, order, etc.) In examining the answer choices, one choice clearly depicts disparate
organisms. The sea lily is a member of the kingdom Animalia, while the iris belongs to the kingdom
Plantae. Do not be misled by their names. The iris is a monocot flower, while the sea lily is a primitive
marine animal. The other answer choices are related as follows: squid-octopus, same class; crayfish-
shrimp, same class; shark-ray, same class; and lemur-monkey, same order. Note that it necessarily
follows that organisms of the same class belong to the same phylum and kingdom.

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