 Nuclear pores , which provide aqueous channels through the envelope , are composed of multiple proteins , collectively referred to as nucleoporins . The pores are about 125 million daltons in molecular weight and consist of around 50 ( in yeast ) to several hundred proteins ( in vertebrates ) . The pores are 100 nm in total diameter ; however , the gap through which molecules freely diffuse is only about 9 nm wide , due to the presence of regulatory systems within the center of the pore . This size selectively allows the passage of small water @-@ soluble molecules while preventing larger molecules , such as nucleic acids and larger proteins , from inappropriately entering or exiting the nucleus . These large molecules must be actively transported into the nucleus instead . The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope , each of which contains an eightfold @-@ symmetric ring @-@ shaped structure at a position where the inner and outer membranes fuse . Attached to the ring is a structure called the nuclear basket that extends into the nucleoplasm , and a series of filamentous extensions that reach into the cytoplasm . Both structures serve to mediate binding to nuclear transport proteins . 
 Nuclear pores , which provide aqueous channels through the envelope , are composed of multiple proteins , collectively referred to as nucleoporins . The pores <a href='drop_aux' title='1'><mark>are</mark></a>about 125 million daltons in molecular weight and consist of around 50 ( in yeast ) to several hundred proteins ( in vertebrates ) . The pores <a href='drop_aux' title='2'><mark>are</mark></a>100 nm in total diameter ; however , the gap through which molecules freely diffuse <a href='drop_aux' title='3'><mark>is</mark></a>only about 9 nm wide , due to the presence of regulatory systems within the center of the pore . This size selectively <a href='uninflect' title='4'><mark>allows</mark></a> the passage of small water @-@ soluble molecules while preventing larger molecules , such as nucleic acids and larger proteins , from inappropriately entering or exiting the nucleus . These large molecules must be actively transported into the nucleus instead . The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope , each of which <a href='uninflect' title='5'><mark>contains</mark></a> an eightfold @-@ symmetric ring @-@ shaped structure at a position where the inner and outer membranes fuse . Attached to the ring <a href='drop_aux' title='6'><mark>is</mark></a>a structure called the nuclear basket <a href='null_relcl' title='7'><mark>that</mark></a><a href='uninflect' title='8'><mark>extends</mark></a> into the nucleoplasm , and a series of filamentous extensions <a href='null_relcl' title='9'><mark>that</mark></a>reach into the cytoplasm . Both structures serve to mediate binding to nuclear transport proteins . 
Nuclear pores , which provide aqueous channels through the envelope , r composed of multiple proteins , collectively referred to as nucleoporins . da pores about 125 million daltons in molecular weight and consist of around 50 ( in yeast ) to several hundred proteins ( in vertebrates ) . The pores 100 nm in total diameter ; however , the gap through which molecules freely diffuse only bout 9 nm wide , due to the presence of regulatory systems within the center of the pore . dis size selectively allow the passage of small water @-@ soluble molecules while preventing larger molecules , such as nucleic acids and larger proteins , from inappropriately entering or exiting tdanucleus . dem large molecules must be actively transported into the nucleus instead . The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope , each of which contain an eightfold @-@ symmetric ring @-@ shaped structure at a position where thdanner and outer membranes fuse . Attached to the ring a structure called the nuclear basket extend into the nucleoplasm , and a series of filamentous extensions reach into the cytoplasm . Both structures serve to mediate binding to nuclear transport proteins .