Alpha-1 antitrypsin deficiency is an inherited disorder caused by a mutation in DNA. In the most common severe form, a single base substitution occurs in the SERPINA1 gene that codes for the alpha-1 antitrypsin (AAT) polypeptide. Explain how this mutation causes a change in the structure of the protein produced. Use your knowledge of DNA, transcription, and translation.

The abnormal AAT protein fails to fold correctly, becomes trapped inside liver cells where it is synthesised, and cannot be released into the blood. Consequently, the lungs lose protection from elastase, an enzyme that breaks down elastic fibres in alveoli. Suggest one symptom a person with alpha-1 antitrypsin deficiency may experience.

Alpha-1 antitrypsin deficiency can be treated using stem cell therapy. Healthy liver stem cells (hepatoblasts) from a donor are transplanted into the patient's liver. Explain why using stem cells from a healthy donor can treat alpha-1 antitrypsin deficiency.