What characteristic of hagfishes differentiates them from lancelets and tunicates?
A) Gills
B) Skull surrounding the brain
C) Dorsal nerve cord
D) Bony jaws
E) Post-anal tail
B
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Which of the following statements are fundamental to the clonal-selection theory of how the adaptive immune system functions?
I) Each lymphocyte has a unique membrane receptor that recognizes one antigen. II) When the lymphocyte binds an antigen, it is activated and begins dividing to form many identical copies of itself. III) Cloned lymphocytes have slight differences and are selected by the spleen for removal if they do not bind an antigen. IV) Cloned cells descend from an activated lymphocyte and persist even after the pathogen is eliminated. A) only I and III B) only II and IV C) only I, II, and IV D) only II, III, and IV E) I, II, III, and IV
In some organisms, a special tRNA recognizes the UAG stop codon but incorporates the amino acid
A) methionine. B) selenocysteine. C) glycine. D) pyrrolysine. E) none of the above
Cotyledons develop as part of _____
a. carpels c. embryo sporophytes b. accessory fruits d. petioles
The thymus
a. controls metabolism. b. causes T-lymphocyte cells to mature. c. controls heart rate. d. produces antibodies. e. deposits calcium into bones.