SuperSaiyan3

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I thought that mRNA can only exist for a short while?

In a practice Kaplan test, they indicated that certain plants could store their information for protein synthesis in the form of mRNA.

But from what I know, mRNA gets degraded fairly quickly.

So what's going on? :confused:
 
May 8, 2009
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I thought that mRNA can only exist for a short while?

In a practice Kaplan test, they indicated that certain plants could store their information for protein synthesis in the form of mRNA.

But from what I know, mRNA gets degraded fairly quickly.

So what's going on? :confused:
mRNA usually gets degraded after a while. DNA is the "storage" for the information of protein synthesis.
 
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SuperSaiyan3

SuperSaiyan3

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mRNA usually gets degraded after a while. DNA is the "storage" for the information of protein synthesis.
SEE! That's what I thought as well! But the damn passage said otherwise!

I looked it up and it's Kaplan FL test #3, question number 138 of Passage #1.

I quote:

One explanation for the results of Experiment 4 is
that the instructions for forming the cap (A PROTEIN) are stored in
the apical tip of the stalk several weeks prior to stalk
formation. Which of the following pieces of evidence
best supports this explanation?

A. Isolation of DNA coding for cap-inducing pro-
teins from samples of Acetabularia taken sev-
eral weeks prior to stalk formation.
B. Exposure of a young Acetabularia to ribonucle-
ase, which cleaves RNA, blocks cap formation.
C. Exchange of nuclei between Acetabularia
crenulata and Acetabularia mediterranea leads
to formation of the cap associated with each
nucleus.
D. mRNA coding for cap-inducing proteins is
found to accumulate in the stalk apex.

ANSWER: D

mRNA is accumulated there. I would have said answer A..
 
Jul 3, 2009
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SEE! That's what I thought as well! But the damn passage said otherwise!

I looked it up and it's Kaplan FL test #3, question number 138 of Passage #1.

I quote:

One explanation for the results of Experiment 4 is
that the instructions for forming the cap (A PROTEIN) are stored in
the apical tip of the stalk several weeks prior to stalk
formation. Which of the following pieces of evidence
best supports this explanation?

A. Isolation of DNA coding for cap-inducing pro-
teins from samples of Acetabularia taken sev-
eral weeks prior to stalk formation.
B. Exposure of a young Acetabularia to ribonucle-
ase, which cleaves RNA, blocks cap formation.
C. Exchange of nuclei between Acetabularia
crenulata and Acetabularia mediterranea leads
to formation of the cap associated with each
nucleus.
D. mRNA coding for cap-inducing proteins is
found to accumulate in the stalk apex.

ANSWER: D

mRNA is accumulated there. I would have said answer A..
I think the reason why A is not the best answer here is because all cells contain the same DNA information. Whether that DNA is actually transcribed to mRNA and later translated into a useful protein is up to each specific cell to decide. So simply having the DNA information doesn't matter since all cells will have it. mRNA is quicker to degrade than DNA, yes, but at least finding some of this cap-inducing protein gene being transcribed in this cell shows that it is likely being made by this cell. Does that reasoning suffice?
 
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SuperSaiyan3

SuperSaiyan3

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I think the reason why A is not the best answer here is because all cells contain the same DNA information. Whether that DNA is actually transcribed to mRNA and later translated into a useful protein is up to each specific cell to decide. So simply having the DNA information doesn't matter since all cells will have it. mRNA is quicker to degrade than DNA, yes, but at least finding some of this cap-inducing protein gene being transcribed in this cell shows that it is likely being made by this cell. Does that reasoning suffice?
Answer explanation by Kaplan:

The question stem asks for support of the fact that the information for forming the cap is stored in the apical tip of the stalk. This information is in the form of mRNA which is used as the template for translation of the proteins which coordinate cap formation. The other choices may be true but they do not provide evidence for the storage of information in the apical tip.

Choice A is incorrect because the DNA coding for cap inducing proteins would be present at all times in the Acetabularia genome. This does not provide evidence for the storage of information in the stalk tip.



Looks like you were right...
 
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SuperSaiyan3

SuperSaiyan3

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I think the reason why A is not the best answer here is because all cells contain the same DNA information. Whether that DNA is actually transcribed to mRNA and later translated into a useful protein is up to each specific cell to decide. So simply having the DNA information doesn't matter since all cells will have it. mRNA is quicker to degrade than DNA, yes, but at least finding some of this cap-inducing protein gene being transcribed in this cell shows that it is likely being made by this cell. Does that reasoning suffice?
I still think this answer is kind of bad and debatable, because like you said:

- mRNA can be quickly degraded
- why wouldn't the cell just transcribe its genes again and just make fresh mRNA??

But I guess in retrospect, D is the only viable option because they say they have EVIDENCE of some sort of information stored... and it can't be DNA, thus leaving only mRNA.
 

raab32

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Because the question asked for direct evidence that storage of information occurs at the apical tip, it has to be mRNA. DNA doesn't really hang out outside the nucleus, but transcripts definitely could, allowing for fast, localized translation.

A similar method is used in neurons for synaptic communication. It's been shown that neurons rely on what is called local translation at the synapse... the transcripts for different receptor subunits and scaffolding proteins (among other synaptic components) are complexed in granules with associated proteins to increase the stability of the messages and anchor the granules to the cytoskeleton. Then when the neuron is stimulated through a synapse, the necessary components for long term potentiation and synapse strengthening are translated.
 

RogueUnicorn

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I still think this answer is kind of bad and debatable, because like you said:

- mRNA can be quickly degraded
- why wouldn't the cell just transcribe its genes again and just make fresh mRNA??

But I guess in retrospect, D is the only viable option because they say they have EVIDENCE of some sort of information stored... and it can't be DNA, thus leaving only mRNA.
this is what happens when you make assumptions... just because it can be/ is usually quickly degraded doesn't mean it always is.
 

neil100

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Also, guys don't forget. Drosophila (fruit flies) are notorious for mRNA being expressed to set A-P and D-V axis for the embryo.

And lot of other species. You might get a question like this for drosophila next time.
 

RogueUnicorn

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Also, guys don't forget. Drosophila (fruit flies) are notorious for mRNA being expressed to set A-P and D-V axis for the embryo.

And lot of other species. You might get a question like this for drosophila next time.
word. many animals' eggs in fact store maternal mRNA for quite some time, including humans. however, this information is not something that needs to be memorized; you will be expected to critically think your way through it.