78. What is the minimum number of tRNAs required to recognize all six codons of serine (UCU, UCA, UCG, UCC,
AGU and AGC)?
(1) 2
(2) 3
(3) 4
(4) 6
Introduction:
The genetic code is made up of codons, which are sequences of three nucleotides that specify particular amino acids. Serine, an amino acid, has six codons: UCU, UCA, UCG, UCC, AGU, and AGC. The question arises – how many tRNAs are required to recognize all six codons of serine? Let’s dive into the mechanics of tRNA recognition and how the redundancy of the genetic code reduces the number of tRNAs needed.
Understanding tRNA Recognition:
Transfer RNAs (tRNAs) are crucial in translating mRNA codons into amino acids during protein synthesis. Each tRNA molecule has an anticodon that binds to a complementary codon on the mRNA. The uniqueness of the tRNA anticodon allows it to recognize specific codons. However, due to the redundancy of the genetic code (where multiple codons can code for the same amino acid), some tRNAs can recognize more than one codon.
For serine, the six codons are:
-
UCU
-
UCA
-
UCG
-
UCC
-
AGU
-
AGC
These six codons are all part of the same codon family and encode the same amino acid: serine.
Codon Family and Wobble Hypothesis:
The key to understanding how a single tRNA can recognize multiple codons lies in the “wobble hypothesis.” This hypothesis, proposed by Francis Crick, suggests that the third nucleotide of a codon (known as the wobble position) is less stringent in its base-pairing. This flexibility allows one tRNA to recognize multiple codons that differ only in their third nucleotide.
For example:
-
The codons UCU, UCC, UCA, and UCG all differ at the third position but code for serine. A single tRNA with the anticodon “AGA” can recognize all four of these codons.
-
The codons AGU and AGC also code for serine and differ only at the third position. A tRNA with the anticodon “UCA” can recognize both of these codons.
Minimum Number of tRNAs:
Given the redundancy of the codons for serine, we do not need a separate tRNA for each of the six codons. A total of two tRNAs can recognize all six codons:
-
One tRNA with the anticodon “AGA” can recognize UCU, UCC, UCA, and UCG.
-
Another tRNA with the anticodon “UCA” can recognize AGU and AGC.
Thus, the minimum number of tRNAs required to recognize all six serine codons is 2.
Correct Answer:
(1) 2
Conclusion:
The redundancy of the genetic code and the flexibility in tRNA codon recognition (via the wobble hypothesis) allow fewer tRNAs to recognize multiple codons. For the six serine codons (UCU, UCA, UCG, UCC, AGU, and AGC), only two tRNAs are required, which is a great example of the efficiency of the genetic code. This phenomenon of codon redundancy and tRNA versatility is a key aspect of how cells optimize protein synthesis.
Understanding how tRNAs work and recognizing the importance of codon redundancy is essential in the fields of genetics and molecular biology.
