Types

• The nuclear envelope is formed from two membranes, one within the other, that connect to each other at the nuclear pores. More than a hundred different proteins form a complex that contains a water-filled channel crossing the nuclear envelope; this water-filled channel is the nuclear pore.
  
  

Function

• Various proteins and other molecules need to enter or exit the nucleus in order to perform their functions. Once a messenger RNA (mRNA) has been synthesized in the nucleus, for example, it needs to travel out into the cell, where it can be used to make a protein. Proteins that help copy or read DNA, on the other hand, need to enter the nucleus. Nuclear pores regulate this traffic to ensure that only some molecules can enter and exit the nucleus of the cell.
  
  

Features

• While small water-soluble molecules can pass freely through the nuclear pore, larger molecules like proteins and RNAs can only do so if they possess a sorting signal that the proteins in the nuclear pore will recognize. RNAs, for instance, are modified before they leave the nucleus, and nuclear pores do not permit mRNAs to leave before these modifications take place.