Why are there only 45 different tRNA for 61 codons?

Why are there only 45 different tRNA for 61 codons?

• A. Because of wobble, the specificity for the third nucleotide base in a codon is relaxed
• B. Because there are no more tRNA available
• C. Because the genetic code has only 45 codons
• D. Because ribosome can read codons without tRNA

Answer: (A)

The main idea is that the genetic code is read with flexibility at the third base of the codon. Because the last position can pair in a looser way, a single tRNA molecule can recognize multiple codons that differ there, so you don’t need a separate tRNA for every codon. This concept is known as wobble base pairing. For example, a tRNA with inosine at the wobble position of its anticodon can pair with codons ending in U, C, or A. With this wobble flexibility, a smaller set of tRNA molecules can cover all 61 sense codons, which is why you see about 45 different tRNAs suffice. It’s not due to a lack of tRNAs or the ribosome skipping codons; it’s the relaxed pairing at the third base that expands codon recognition by each tRNA.

The main idea is that the genetic code is read with flexibility at the third base of the codon. Because the last position can pair in a looser way, a single tRNA molecule can recognize multiple codons that differ there, so you don’t need a separate tRNA for every codon. This concept is known as wobble base pairing. For example, a tRNA with inosine at the wobble position of its anticodon can pair with codons ending in U, C, or A. With this wobble flexibility, a smaller set of tRNA molecules can cover all 61 sense codons, which is why you see about 45 different tRNAs suffice. It’s not due to a lack of tRNAs or the ribosome skipping codons; it’s the relaxed pairing at the third base that expands codon recognition by each tRNA.