Re: Can anything other than specific types of RNA go through the nuclear pores?

This is a very good question!!
The nucleus, as you know, holds the DNA of the cell and is surrounded by a 
double-membrane nuclear envelope.  Because this is where the DNA is located, 
therefore, this is also the site of all mRNA transcription and DNA 
replication before a cell divides.  Yes, these processes require many 
proteins, such as enzymes that synthesize the mRNA and new DNA called 
polymerases.  So, as you asked, how do all these proteins get into the 
nucleus through the envelope if they are translated on ribosomes in the 
cytoplasm?  Your guess was right, through the nuclear pore complex (NPC).  
The NPC is a very large complex of proteins embedded in the envelope, and it 
is believed to be the same "gate" for any molecules entering or leaving the 
nucleus (RNA and Protein).  There is a small pore in the center of the NPC 
that is 9 nanometers in diameter, that allows any molecule small enough to 
passively diffuse through the pore.
For larger molecules and proteins over a certain size, they must be actively 
transported through the NPC (using ATP).  That means, the same NPC can both 
act as a barrier to large molecules (acting as a 9nm pore), and it can let 
specific large proteins in with the hydrolysis of ATP. -But the NPC does not 
let just any protein in my active transport.  Only proteins which contain a 
short amino acid sequence that acts as a Nuclear Import or Nuclear Export 
signal can be actively transported in or out, respectively.  The cell, 
therefore, can use this signal mechanism to decide which large proteins it 
wants to be in the nucleus and which it wants to exclude from the nucleus.

I hope this answers your question.  Nuclear transport is a very interesting 
field of study, and it still not very well known how these "cargo" molecules 
are transported though the NPC.