Rey Williams was a small but healthy baby girl. The family pediatrician wasn’t w

Question

Rey Williams was a small but healthy baby girl. The family pediatrician wasn’t worried as both her parents were petite. However, at about the age of 13 months Rey stopped running around, tired easily, was pale and lost her appetite- symptoms that could not be explained by a recent flu bug going around as it lasted for months. Concerned the family pediatrician Dr. Wright decided to draw Rey’s blood to check for inherited blood-disorders.

At the next office visit, Dr. Wright sat Rey’s mother down and gently gave her the bad news. As he suspected it was determined that Rey had Thalassemia- an inherited disorder resulting from a defective for ?-globin protein- and thus defective Hemoglobin.

?-thalassemia patients are treated with frequent transfusions requiring many hospital visits, but this is not a cure. Individuals with severe forms of ?-thalassemia often die in mid-teens. Dr.Wright suggested to Rey’s parents that she may be a candidate for experimental gene therapy but first they needed a quick test to see if Rey had the kind of defect that would qualify for gene therapy. They isolated mature mRNA from Rey’s RBCs and from a person without Thalassemia(N). They then mixed the mRNA with single stranded genomic DNA sequences of the Beta Globin gene to allow for hybridization and observed it by electron microscopy. Double-stranded regions(hybridized) are represented by thick lines, and single-stranded regions (un-hybridized) are represented by thin lines.

Q1. What do the single stranded regions represent?

A. Introns

B. Exons

C. 3’UTR

D. 5’ UTR

Q2) Based on the hybridization experiments what can the pediatrician deduce about the

molecular nature of the defect in Rey’s globin gene?

[Note: In writing your answer also make a prediction about what kind(s) of mutations could

result in that change seen and what that does to the B-globin protein]

Sequencing a region of the Beta-globin gene covering exon 2- intron 2- exon 3 revealed

the following mutations within intron 2 when compared to normal beta globin gene

(mutations underlined). The corresponding mRNA therefore would look like below.

Explanation / Answer

Ans 1) Ans d) 5’UTR

It is the untranslated area which is also known as 5’-untranslated region and it is also known as leader RNA at times. It is the mRNA region which is located upstream from the initiation codon and helps in the process of regulation of translation of a transcript.

Ans 2) On making the comparison, there are two major mutations that marks the difference between the normal and mutated gene.

The first mutation that was observed was the conversion of the G to C and the second mutation was the conversion of U to G. The first mutation is transversion where purine (guanine) is converted to pyrimidine (cytosine). The other type of mutation is conversion of uracil to guanine which is pyrimidine to purine. This is again an example of transversion type of mutation.

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