A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants. Wild-type polypeptide N . . . Thr–His–Ser–Gly–Leu–Lys–Ala . . . C Mutant 1 N . . . Thr–His–Ser–Val–Leu–Lys–Ala . . . C Mutant 2 N . . . Thr–His–Ser–C Mutant 3 N . . . Thr–Thr–Leu–Asp–C Mutant 4 N . . . Thr–Gln–Leu–Trp–Ile–Glu–Gly . . . Determine the wild-type mRNA sequence.

Verified step by step guidance

1. Identify the codons for the wild-type amino acids: Thr (Threonine), His (Histidine), Ser (Serine), Gly (Glycine), Leu (Leucine), Lys (Lysine), Ala (Alanine).
2. Use a codon table to find possible mRNA codons for each amino acid: Thr (ACU, ACC, ACA, ACG), His (CAU, CAC), Ser (UCU, UCC, UCA, UCG, AGU, AGC), Gly (GGU, GGC, GGA, GGG), Leu (UUA, UUG, CUU, CUC, CUA, CUG), Lys (AAA, AAG), Ala (GCU, GCC, GCA, GCG).
3. Compare the wild-type sequence with Mutant 1, which has a single amino acid change from Gly to Val. This suggests a single nucleotide change in the Gly codon to a Val codon (GUU, GUC, GUA, GUG).
4. Analyze Mutant 2, which ends after Ser, indicating a possible stop codon introduced after Ser. This suggests a mutation in the Gly codon to a stop codon (UAA, UAG, UGA).
5. Examine Mutant 3 and Mutant 4 for additional insights, noting that Mutant 3 has a different sequence starting from the second amino acid, suggesting a frameshift or early stop, and Mutant 4 has a completely different sequence, indicating a possible frameshift mutation.

Key Concepts

Genetic Code

The genetic code is a set of rules that defines how the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a polypeptide. Each amino acid is specified by a codon, which is a sequence of three nucleotides. Understanding the genetic code is essential for determining the mRNA sequence that corresponds to a given polypeptide sequence.

Transcription and Translation

Transcription is the process by which the DNA sequence of a gene is copied into mRNA, while translation is the subsequent process where ribosomes synthesize proteins based on the mRNA sequence. To determine the wild-type mRNA sequence from the polypeptide, one must understand how these processes work together to convert genetic information into functional proteins.

Mutations

Mutations are changes in the nucleotide sequence of DNA that can lead to alterations in the corresponding polypeptide. Analyzing the differences between the wild-type and mutant polypeptide sequences helps identify specific mutations and their potential effects on protein function. This understanding is crucial for reconstructing the wild-type mRNA sequence from the provided polypeptide data.