Final answer:
There are fewer tRNAs than mRNA codons due to wobble base pairing and the redundancy of the genetic code, which allows one tRNA to pair with multiple codons.
Step-by-step explanation:
The best explanation of why there are 45 tRNAs yet 61 possible mRNA codons that specify an amino acid is due to the concept of wobble base pairing and the redundancy of the genetic code. Although there are 64 codons (including three stop codons), the 61 that code for amino acids do not each require a unique tRNA. Some tRNAs can recognize more than one codon for their specific amino acid due to a relaxation of the base-pairing rules at the third position of the codon, a phenomenon referred to as wobbling. This allows a single tRNA to pair with multiple codons, encoding for the same amino acid. Therefore, fewer tRNAs are needed to translate the genetic code into proteins.