Question

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.

Short answer

Answer: In the given Alpha and Beta chain sequences, one amino acid substitution (Proline to Serine) can occur as a result of a single nucleotide substitution.

Step-by-step solution

Divide the sequences into codons

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

Identify amino acids for each codon using a genetic code dictionary

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

Determine single nucleotide substitutions that lead to amino acid changes

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.

Calculate minimal mutational distance for cases where single nucleotide substitution does not result 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.

Key concepts

Amino Acid Substitution

Amino acid substitution occurs when the nucleotide sequence in the DNA changes and results in the replacement of one amino acid with another in a protein. This can significantly impact the protein's structure and function. Understanding amino acid substitution is vital in fields like genetics and molecular biology.

During protein synthesis, groups of three nucleotides, known as codons, encode each amino acid. Sometimes, a single nucleotide change, known as a point mutation, can lead to an amino acid substitution. For instance, in our given problem, a single nucleotide substitution changes the proline codon (CCA) into a serine codon (TCA). This results in the substitution of Proline (Pro) with Serine (Ser) in the hemoglobin beta chain.

These substitutions can have various effects:

• Neutral: No significant change in protein function.
• Beneficial: Enhances protein function or adaptation.
• Harmful: Reduces or destroys protein function.

Therefore, accurately identifying when and how amino acid substitutions occur is crucial to understanding genetic variation and its consequences.

Genetic Code Dictionary

The genetic code dictionary is a crucial tool in understanding the language of nucleic acids and their translation into proteins. It maps the sequence of nucleotides in DNA to their corresponding amino acids using codons. Each codon, a triplet of nucleotides, specifies a particular amino acid or a stop signal that ends protein synthesis.

For example, the codon ATG translates to methionine (Met), which is often the start signal for protein synthesis, setting the stage for subsequent amino acid sequences. Similarly, codons like TCA translate to serine (Ser), as seen in our example with the substitution challenge.

The genetic code is nearly universal across all organisms, highlighting the shared biochemistry of life on Earth. Key features of the genetic code include:

• Redundancy: Multiple codons can code for the same amino acid, providing a buffer against mutations.
• Specificity: Each codon maps to only one amino acid, preventing errors in protein synthesis.
• Start and stop signals: Specific codons initiate or terminate translation.

Using the genetic code dictionary, scientists can predict how nucleotide sequences translate into proteins and understand mutations that can lead to changes in protein structure and function.

Minimal Mutational Distance

Minimal mutational distance refers to the smallest number of nucleotide changes needed to alter one amino acid to another in a protein sequence when a single nucleotide substitution does not suffice. This concept is important for understanding the extent of mutations that can influence protein function and lead to evolutionary changes.

Consider the hypothetical example where converting Proline (CCA) in the alpha chain to Phenylalanine (Phe) requires multiple nucleotide changes. First, you change CCA (Pro) to UCA (Ser), then UCA to UUA (Leu), and finally UUA to UUU (Phe). This forms a sequence of three changes, giving a minimal mutational distance of 3 for this transformation.

Understanding minimal mutational distances helps in several ways:

• Evolutionary biology: Shows genetic variation and potential evolutionary paths.
• Genetic research: Identifies links between genetic mutations and alterations in proteins.
• Medical studies: Aids in understanding genetic disorders caused by multiple nucleotide mutations.

By studying minimal mutational distances, researchers can better grasp the complexity of genetic mutations and their cumulative impacts on organisms.