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  #1  
Old 12-29-2015
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Default USMLE sample question question

Hey there,

i have been going through the sample questions and came across one i am a bit confused about. it is this:

94. During an experiment, an investigator finds a point mutation (CGG→TGG) in the gene encoding the protein kinase regulatory subunit. The genetic code is shown in the figure. This mutation is most likely to alter the amino acid sequence with which of the following changes?
(A) Arg→Trp (B) Gly→Arg (C) Gly→Trp (D) Thr→Gly (E) Trp→Arg

They do not tell you if it is the coding/template strand or not which is why i got confused. if it were the coding strand then surely you would read it backwards seeing as the DNA is read 3' --> 5' whilst RNA is made 5' --> 3'. then you would get CCG --> CCU, but that is not an option. i then assumed it was the template strand and got A. If this were to come up in the real exam do i just assume it is the template strand?
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Old 12-29-2015
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Quote:
Originally Posted by smetin92 View Post
Hey there,

i have been going through the sample questions and came across one i am a bit confused about. it is this:

94. During an experiment, an investigator finds a point mutation (CGG→TGG) in the gene encoding the protein kinase regulatory subunit. The genetic code is shown in the figure. This mutation is most likely to alter the amino acid sequence with which of the following changes?
(A) Arg→Trp (B) Gly→Arg (C) Gly→Trp (D) Thr→Gly (E) Trp→Arg

They do not tell you if it is the coding/template strand or not which is why i got confused. if it were the coding strand then surely you would read it backwards seeing as the DNA is read 3' --> 5' whilst RNA is made 5' --> 3'. then you would get CCG --> CCU, but that is not an option. i then assumed it was the template strand and got A. If this were to come up in the real exam do i just assume it is the template strand?
You always assume it is the coding strand unless told otherwise. In this case mRNA would read 5' CGG 3'-->5' UGG 3'. I do not know which amino acids those codons correspond to.
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My answer would have been A.
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Old 12-29-2015
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Quote:
Originally Posted by drmachado View Post
My answer would have been A.
thanks, i am just a bit confused though. is my reasoning as to why the codons would be CCG --> CCU correct if the DNA is read 3' --> 5'? or am i chatting rubbish?
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When they give you the 3bp codon you just have to look for that combination in the DNA codon table (I'm hoping they will give it to us in the test because I'm not planning on learning it all lol). The codons will be the same (CGG, TGG or if they give you the RNA table it would be CGG, UGG). That's my understanding. Let's see what other input we get here in the forum to your question.
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Quote:
Originally Posted by smetin92 View Post
thanks, i am just a bit confused though. is my reasoning as to why the codons would be CCG --> CCU correct if the DNA is read 3' --> 5'? or am i chatting rubbish?
DNA is read 5' to 3'. RNA is synthesized and read 5' to 3'. Coding strand is the same as mRNA except for uracil instead of thymine.

To explain a little further. mRNA is synthesized complementary to the template strand, not the coding strand. mRNA synthesis starts at the promoter region of the gene. Since the two strands of DNA are antiparallel, one of them is going to have a 5' end at the promoter and the second strand is going to have a 3' end. Coding strand is the strand with the 5' end at the promoter. RNA polymerase will make RNA in a 5' to 3' direction. It will start at the promoter and make RNA that is complementary and antiparallel to the template strand. Thats because synthesis has to happen in a 5' to 3' direction and it has to start at the promoter. It will transcribe a strand that has a 3' end at the promoter so RNA can have a 5' end at the promoter (antiparallel).

Coding strand is antiparallel and complementary to the template strand, mRNA is antiparallel and complementary to the template strand. Essentially, coding strand of the DNA=mRNA but with one important exception, uracil instead of thymine.

I hope this clears things up.
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Quote:
Originally Posted by nodo View Post
dna is read 5' to 3'. Rna is synthesized and read 5' to 3'. Coding strand is the same as mrna except for uracil instead of thymine.

To explain a little further. Mrna is synthesized complementary to the template strand, not the coding strand. Mrna synthesis starts at the promoter region of the gene. Since the two strands of dna are antiparallel, one of them is going to have a 5' end at the promoter and the second strand is going to have a 3' end. Coding strand is the strand with the 5' end at the promoter. Rna polymerase will make rna in a 5' to 3' direction. It will start at the promoter and make rna that is complementary and antiparallel to the template strand. Thats because synthesis has to happen in a 5' to 3' direction and it has to start at the promoter. It will transcribe a strand that has a 3' end at the promoter so rna can have a 5' end at the promoter (antiparallel).

Coding strand is antiparallel and complementary to the template strand, mrna is antiparallel and complementary to the template strand. Essentially, coding strand of the dna=mrna but with one important exception, uracil instead of thymine.

I hope this clears things up.
i get it now!
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