Question

In the coding strand of DNA for the alpha gene of normal hemoglobin \((\mathrm{HbA}),\) the three bases that correspond to codon 142 of the mRNA are TAA and the alpha chain has 141 amino acids. In the coding strand of the gene for the alpha chain of Hemoglobin Constant Spring, the three bases are CAA and the chain contains 172 amino acids. Explain the mutation that has occurred.

Short answer

Answer: The specific mutation in Hemoglobin Constant Spring changed the 142nd codon in the DNA coding strand from TAA (a stop codon) to CAA (which codes for the amino acid glutamine). This mutation leads to an elongated alpha chain, containing 172 amino acids instead of the normal 141 amino acids found in HbA.

Step-by-step solution

Understand the role of DNA, mRNA, and amino acids

DNA contains the genetic information for creating proteins, which are made up of amino acids. The process of turning this genetic information into proteins involves two main steps: transcription, where DNA is transformed into mRNA, and translation, where mRNA is transformed into the amino acid sequence. The mRNA carries the information from the DNA in the form of three-letter sequences called codons, which code for specific amino acids during translation.

Compare the codons between normal hemoglobin and Hemoglobin Constant Spring

For normal hemoglobin (HbA), the 142nd mRNA codon is derived from the DNA coding strand with bases TAA. In Hemoglobin Constant Spring, the three bases in the coding strand at the same position are CAA. This change represents a point mutation in the DNA coding strand.

Identify the change in amino acids

The normal hemoglobin (HbA) contains 141 amino acids in its alpha chain, while the Hemoglobin Constant Spring contains 172 amino acids. There is an increase of 31 amino acids in the alpha chain of Hemoglobin Constant Spring compared to HbA.

Determine the specific mutation

The change in the DNA coding strand from TAA to CAA for the 142nd codon leads to different mRNA codon and might affect amino acid coding. In this case, the mutation from TAA to CAA has caused a change from a stop codon, which signals the end of translation, to a codon that codes for an amino acid (glutamine). Since the mutation has changed a stop codon to a coding codon, the translation process does not stop at the 141st amino acid as it does in normal HbA, but continues, ultimately forming a longer alpha chain with 172 amino acids.

Conclusion

The mutation that occurred in Hemoglobin Constant Spring has changed the 142nd codon in the DNA coding strand from TAA (which is a stop codon) to CAA (which codes for the amino acid glutamine). As a result, the alpha chain of Hemoglobin Constant Spring has been elongated, containing 172 amino acids instead of the normal 141 amino acids found in HbA.