Re: Why can RNA leave the nucleus and DNA cannot?

Hi Maria, 

Thank you for your question.  Here's some information I hope will be 
helpful to you (please also refer to  http://cellbio.utmb.edu/cellbio/nucleus.htm
for a nice diagram and summary of nuclear structures)

The DNA resides permanently in the nucleus, where genes are 
periodically transcribed into messenger RNA, ribosomal RNA, and 
transfer RNA as the cell requires.  Proteins required for the 
transcription process are imported into the nucleus, and conversely,  
RNAs must be transported from the nucleus into the cytoplasm in 
order to perform their designated function.    These comings and 
goings are regulated by the nuclear pores--openings in the nuclear 
envelope at points where the inner and outer nuclear membrane 
fuse.  The transport of macromolecules into and out of the nucleus is 
a very important, and therefore highly regulated, process.  After all, 
you wouldn't want something to leave the nucleus before it was 
ready, or to enter the nucleus at an improper time, right?  Receptors 
on the nuclear pores are largely responsible for regulating transport, 
and only correctly "labelled" proteins and nucleic acids are allowed to 
pass in or out.  In the case of mRNA, this "label"  is present in the 
form of the 5' cap--a methylated G nucleotide added to the beginning 
(the end that was transcribed first, and where protein synthesis will 
initiate) of the RNA strand.  This modification occurs, along with other 
RNA processing events, in the nucleus, and once all processing is 
complete, the mRNA molecule can be identified by the nuclear pore 
as having "clearance" to leave the nucleus.  The other RNAs, rRNA 
and tRNA, don't have 5' caps, but instead must be assembled with 
proteins to form a complex which can then be exported through a 
nuclear pore.  DNA is never modified or labeled for transport, and 
therefore is never allowed to pass through the nuclear pores.   

I hope this answers your question.  Please feel free to write again if 
you need further information!  

Warm regards,

Jen