Answer Key
Transcription and Translation
Study Question 1:
Why do we need to transcribe DNA to mRNA at all in order to make
protein? Why don't our cells just keep their ribosomes inside the nucleus and
translate the DNA directly? Try to come up with a plausible answer to this
question.
(Hint: You can think about it evolutionarily, or you can think about how
many copies we have of a given piece of DNA, you can make an argument based
on DNA's structure... there are many ways to answer this!)
There are a few good lines of reasoning you could use.
1) If we tried to translate directly off of DNA, we would be limited in how
many copies we could make of a protein at once. By transcribing many mRNA
copies from DNA, we can translate many strands of RNA simultaneously and
make more protein. This is a teleological answer, however - it doesn't give a real
evolutionary explanation.
2) DNA's double helical structure would make it difficult or impossible to
fit into the ribosome. This is a practical answer. However, why wouldn't the
ribosome evolve to fit around DNA?
3) RNA likely evolved before DNA; DNA just provided a more stable way
to store information. Translation of protein therefore likely predates the
existence of DNA. There was no way for evolution to dramatically change the
underlying basis of all biology, so translation of RNA -> protein cannot be
replaced by DNA -> protein. I consider this the most satisfying answer.
4) Very large proteins synthesized inside the nucleus, such as certain
muscle proteins, might be unable to fit through the nuclear pores to leave it. This
is practically quite true in us, but is a less satisfying answer, as it does not
explain why bacteria also translate RNA to protein instead of DNA to protein.
Study Question 2:
Draw a picture of a gene being transcribed. Include RNA polymerase, the
gene, the promoter region, transcription factors, and enhancers in the picture.
Briefly explain the function of each of these parts.
You may need to combine one or two figures to answer this. Here are some
sources you could use:
http://www.cambia.org/daisy/promoters/224/version/default/part/ImageDa
ta/data
http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/enhancer.GIF
http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/1
4711116/U2CP3-2_GenePromoter_ksm.jpg
Answer Key
Transcription and Translation
Study Question 3:
One of the more common mutations that causes the disease hereditary
pancreatitis is shown below:
mRNA SEQUENCE OF NORMAL:
AUG [...….] / CAA /CGC / CCG / CGU / GUC/ …….
mRNA SEQUENCE OF MUTANT:
AUG[…….] / CAA /CAC / CCG / CGU / GUC/ …….
Using the genetic dictionary in your lab manual (or a codon table from the
Internet), please explain what effect this mutation would have on the amino acid
sequence produced. Would it be a missense mutation or a nonsense mutation?
The sequence of the normal version (not counting the methionine) translates to
Gln-Arg-Arg-Val, and the mutant makes Gln-His-Arg-Val. Since a single amino
acid was replaced, this is a one AA missense mutation.
Study Question 4:
In order to try to figure out how a protein, which we'll call Studyase, moves
around in cells, I try to make a Green Fluorescent Protein (GFP) fusion. That is to
say, I make a DNA sequence that combines the Studyase sequence with the GFP
sequence, separated by a 27-base pair linker that codes for 9 amino acids. (A
linker is a short sequence that acts like a spacer between the two proteins,
keeping them from interfering with each other.) In this new protein, the GFP
comes after the Studyase.
*
Would it be possible to make a human being express this fusion just by
injecting them with the DNA? Why or why not?
Definitely not! First, the gene wouldn't actually be in the genome of any cells; it
would not be taken up from the bloodstream into the nucleus. Second (and less
importantly), I didn't mention any kind of promoters or enhancers in this
construct to activate its transcription.
*
For some reason, I make a mistake in designing the linker; I make it 26
nucleotide bases long. That is to say, between the end of the Studyase sequence
and the start codon of the GFP sequence, I place 26 "letters" of genetic code.
Would the Studyase still be functional? Would it glow from the GFP? Explain.
A 26-bp linker would not be functional here. The 27-base-pair linker would
insert 9 amino acids between the Studyase and GFP proteins in the fusion... but
the 26-bp linker would insert 8 amino acids, then have 2 extra base pairs. This
would cause a frame shift! The first base of the GFP sequence would be
Answer Key
Transcription and Translation
translated in the same codon with the last two base pairs of the linker. Then all
of the subsequent codons of the GFP would be misread!