There are 61 mRNA codons that specify an amino acid, but only 45 tRNAs. This is best explained by the fact that
A. some tRNAs have anticodons that recognize four or more different codons.
B. competitive exclusion forces some tRNAs to be destroyed by nucleases.
C. the rules for base pairing between the third base of a codon and tRNA are flexible.
D. many codons are never used, so the tRNAs that recognize them are dispensable.
E. the DNA codes for all 61 tRNAs but some are then destroyed.
Ans: C. the rules for base pairing between the third base of a codon and tRNA are flexible.
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During days 8 and 9 after fertilization in humans, ____
a. amnion cleavage begins b. a blastocyst forms c. an embryonic disk is formed d. implantation occurs e. gastrulation occurs
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