Biology 9700 · AS & A Level · Gene control

Gene control — practice question

Several mutations affect how fetal haemoglobin, HbF, and normal adult haemoglobin, HbA, are made. • The HbA allele encodes the normal $\beta$-globin polypeptide of haemoglobin. • The HbS allele, which results from a base substitution mutation, encodes an abnormal $\beta$-globin polypeptide. • Because of the base substitution, the amino acid glutamine, with a polar R group, is substituted by valine, with a non-polar R group, in the polypeptide. When the partial pressure of oxygen is low ($pO_2$), the abnormal haemoglobin molecules (HbS) become fibres. These fibres make red blood cells sickle-shaped, and the cells may obstruct blood capillaries. People whose adult haemoglobin is entirely abnormal (HbS) have sickle cell anaemia. This is a long-term painful disorder that may be fatal.
(a)[2]

Explain why this mutation makes HbS assemble into fibres.

(b(i))[2]

Fetal haemoglobin, HbF, is made by the fetus until just before birth, after which adult haemoglobin begins to be synthesised. By six months old, adult haemoglobin has taken the place of most of the HbF. This switch happens when the genes for HbF are turned off and the genes for adult haemoglobin are turned on. A base substitution, British-198, causes fetal haemoglobin to keep being produced. Under normal conditions, by the age of six months the HbF concentration falls to below 1% of total haemoglobin. With the British-198 mutation, HbF in an adult may be as high as 20% of total haemoglobin. HbF has a higher affinity for oxygen at low $pO_2$ than adult haemoglobin. People who have both sickle cell anaemia and British-198 mutation show less severe symptoms of sickle cell anaemia. Suggest why having the British-198 mutation lowers the symptoms of sickle cell anaemia.

(b(ii))[1]

In adults carrying the British-198 mutation, the gene for a fetal haemoglobin polypeptide stays switched on because a protein that regulates gene expression is present. State the term used for a protein that regulates gene expression.

(c(i))[4]

Gel electrophoresis can be used to check which haemoglobin variants an individual has: HbA, HbS and HbF. An alkaline buffer is used so that every haemoglobin molecule becomes negatively charged. Compared with HbA, HbS molecules carry one more positive charge. Describe and explain how gel electrophoresis is used to diagnose sickle cell anaemia.

(c(ii))[1]

Give the term used for a protein that regulates gene expression.

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