Shown below are two homologous lengths of the alpha and beta chains of human hemoglobin. Consult a genetic code dictionary (Figure 13.7 ) and determine how many amino acid substitutions may have occurred as a result of a single nucleotide substitution. For any that cannot occur as a result of a single change, determine the minimal mutational distance.

We'll divide the given sequences into codons - groups of 3 nucleotides each that correspond to an amino acid. Alpha chain: ATG CCA GTG Beta chain: ATG TCA GTG

Using a genetic code dictionary (Figure 13.7), we will identify the corresponding amino acids for each codon. Alpha chain: Met Pro Val Beta chain: Met Ser Val

Now we need to analyze the differences in amino acid sequences caused by single nucleotide substitutions. For the second amino acid, we have Proline (Pro) in the Alpha chain and Serine (Ser) in the Beta chain. Let's consult the genetic code dictionary to see if there is a single nucleotide substitution that can cause this change. Proline (Pro) codons: CCU, CCC, CCA, CCG Serine (Ser) codons: UCU, UCC, UCA, UCG, AGU, AGC It's easy to notice that by changing the second nucleotide of the Proline codon CCA (Alpha chain) to a T, we get the Serine codon TCA (Beta chain). So, it's a single nucleotide substitution which results in an amino acid change.

In this particular example, all observed amino acid changes can occur as a result of single nucleotide substitutions. However, let's consider a hypothetical case where an amino acid change does not occur with a single change: Alpha chain: Met Pro Val Beta chain: Met Phe Val Phenylalanine (Phe) codons: UUU, UUC In this case, there is no single nucleotide substitution that can convert a Proline codon to a Phenylalanine codon. To find the minimal mutational distance, we need to find the shortest number of nucleotide changes required for the conversion: - CCA (Pro) -> UCA (Ser) -> UUA (Leu) -> UUA (Phe) (3 changes) So, the minimal mutational distance in this hypothetical case is 3. In conclusion, for the given Alpha and Beta chain sequences, there is one amino acid substitution (Pro->Ser) that can occur as a result of a single nucleotide substitution. For other cases where single substitution does not lead to amino acid change, it may be necessary to calculate the minimal mutational distance as shown in the example.