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When the DNA from the nucleus is transcribed to an mRNA, the mRNA is spliced by an enzyme before it goes outside through the nuclear pore. What is the name of this enzyme and how does it recognize where to splice the DNA so only the introns remain inside? Would it not need an instruction set itself to recognize exactly where to cut so the sequence does not change?

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There are two factors that involve the ability of enzymes to process RNA.
1) Structure see wikipedia
2) Binding affinitya

Let's take a look at the splicing process: enter image description here

The active 'sites' (GU,A & AG) need to be in spatial proximity (point one), and the enzyme needs to be able to bind there, aka forming hydrogen bonds with the nucleotides, which is mostly the case if:
a) the nucleotides are 'free' as in they are not paired with other nucleotides
b) it is energetically favorable for the enzyme to bind and do its work, which factors like temperature, and of course on the correct nucleotides

If there were some mutations, the splicing might not work.

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  • $\begingroup$ Comment for the enzyme: It's a complex, the splicosome. There is a cool movie about it: dnalc.org/resources/3d/rna-splicing.html $\endgroup$ – yapphdorlw Jan 22 '16 at 14:12
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    $\begingroup$ Simply add the additional information into the answer by using the edit function below it and do not put it in the comments. $\endgroup$ – Chris Jan 22 '16 at 14:14
  • $\begingroup$ There are several determinants. As you mentioned 5'-GU--//--[---]--//--AG-3' is the typical sequence of an intron. There is also a polypyrimidine tract in the intron. $\endgroup$ – WYSIWYG Jan 22 '16 at 17:32

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