@ maryam 2009 NBME 12 block 4 Q 30 - docnok1

[ArchivalUser]

Please can you explain it to me this is about the sequence of the human beta globin gene and the answer is B. Thanks a lot.

[ArchivalUser]

please post the Q

[ArchivalUser]

There is a graph.

The sequence of the human beta globin gene, with the three exons in bold capital letters is shown in the figure. The translation start codon ATG at posistion 154-156 and the translation stop codon TAA at position 1575-1577 are underlined. WHICH of the following mutations (circled) will most likely lead to beta Thalassemia.

[ArchivalUser]

There is a graph.

The sequence of the human beta globin gene, with the three exons in bold capital letters is shown in the figure. The translation start codon ATG at posistion 154-156 and the translation stop codon TAA at position 1575-1577 are underlined. WHICH of the following mutations (circled) will most likely lead to beta Thalassemia?

A.C__T at position 1
B.G__A at position 246
C.G__A at position 672
D.G__A at position 2049

[ArchivalUser]

sorry,I have no access to the graph...

[ArchivalUser]

Here's the graph
http://img.ctrlv.in/4f3456270c705.jpg

[ArchivalUser]

By introducing G__A at position 246, you introduce a stop codon... before it was GTT, when you transcribe to mRNA its CAA. Now after the mutation its ATT, when you transcribe this you get UAA, which is a stop codon and thus stops transcribing the protein.

[ArchivalUser]

ok I understand the explanation but I can't get it from the graph for I can't read the graph to get your explanation for example why position 246? where I have to read to get the answer?

[ArchivalUser]

@img87 first you do the reverse complementary of the strart codon ATG___GTT and with the mutation you get ATT that will be transcribed to UAA (stop codon) As this happen 154__156 and the answer has 246 and that's why you choose 246?

[ArchivalUser]

I haven't done this form yet but still peaked into this Q



"B" (Splice Site mutation)


Position 1 and position 2049 are not part of the "Beta globin gene (154 - 1577)"

so option A and D are out.

-position 246 and 672 , both are located within the Beta globin gene.

-Beta-Thalassemia have "POINT Mutation" in

either "SPLICE Site" (* as at position 246 where the first Intron start)

or "Non-sense mutation" introducing a STOP Codon (but this is not the case here)



NOTE thatEXONS are in Bold Capital letters)

154--[EXON1]--245_*246--[Intron1]--*375_376--[EXON2]--598_*599--[Intron2]--*1448

149--[EXON 3]--1577


* are the Splice Sites and site at position 246 if get mutated, it will cz Beta-Thalassemia