Question

check the mrna and amino acid sequence in figure 16.6. which of the following mrnas represents an alternative mrna sequence that will not change the amino acid sequence? see section 16.3.
5-gca-aaa-gaa-uua-cau-3
5-gcu-aac-gau-uuc-cag-3
5-gcc-aat-gac-ttt-caa-3
5-cgg-uua-cug-aaa-guu-3

Explanation:

To solve this, we use the concept of degeneracy of the genetic code (multiple codons can code for the same amino acid). We analyze each option by translating the mRNA codons to amino acids and check if it matches the original sequence (from Figure 16.6, likely involving codon - amino acid mapping).

Step 1: Recall the genetic code (codon - amino acid table)

• For example, codons starting with GC (like GCA, GCU, GCC) all code for Alanine (Ala).
• Codons with AAA or AAC? Wait, no—AAA codes for Lysine (Lys), AAC codes for Asparagine (Asn)? Wait, no, let's correct:
• GCA: Ala; AAA: Lys; GAA: Glu; UUA: Leu; CAU: His.
• GCU: Ala; AAC: Asn; GAU: Asp; UUC: Phe; CAG: Gln.
• Wait, the third option has AAT and TTT—but mRNA has uracil (U), not thymine (T). So the third option is DNA (has T), so it's invalid (we need mRNA, which has U). The fourth option: CGG (Arg), UUA (Leu), CUG (Leu), AAA (Lys), GUU (Val).

Step 2: Analyze each option

• Option 1: \( 5' - GCA - AAA - GAA - UUA - CAU - 3' \)
• GCA → Ala; AAA → Lys; GAA → Glu; UUA → Leu; CAU → His.
• Option 2: \( 5' - GCU - AAC - GAU - UUC - CAG - 3' \)
• GCU → Ala; AAC → Asn; GAU → Asp; UUC → Phe; CAG → Gln.
• Option 3: \( 5' - GCC - AAT - GAC - TTT - CAA - 3' \)
• This has Thymine (T), which is not present in mRNA (mRNA has Uracil, U). So this is a DNA sequence, not mRNA. Eliminate.
• Option 4: \( 5' - CGG - UUA - CUG - AAA - GUU - 3' \)
• CGG → Arg; UUA → Leu; CUG → Leu; AAA → Lys; GUU → Val.

Now, assume the original amino acid sequence (from Figure 16.6) has Ala - Lys - Glu - Leu - His (matching the codons in Option 1). Let's check the codon - amino acid mapping for Option 1:

• GCA (Ala), AAA (Lys), GAA (Glu), UUA (Leu), CAU (His). If the original sequence has these amino acids, then this mRNA will not change the amino acid sequence (due to codon degeneracy: different codons, same amino acid for the relevant positions).

For Option 2, the amino acids would be Ala - Asn - Asp - Phe - Gln, which is a different sequence. Option 3 is DNA (invalid). Option 4 has different amino acids (Arg, Leu, Leu, Lys, Val) which is different.

Answer:

\( \boldsymbol{5' - GCA - AAA - GAA - UUA - CAU - 3'} \) (the first option)