1. THE NUCLEAR ENVELOPE
I’m going to talk about the nuclear envelope.
The nuclear envelope is a complex structure,which consists of two nuclear membranes that
envelops the nucleus of an eukaryotic cell, an underlying nuclear lamina, and nuclear pore
complexes.Prokaryotic cells don’t need a nuclear envelope because they don’t have a well-
defined nucleus.
The nuclear envelope separates the contents of the nucleus from the cytoplasm,thereby
maintaining both as independent metabolic compartments.It also provides the structural
framework of the nucleus.
1. Nuclear Membrane:
It is made up of two layers, each composed of a lipid bilayer. It is perforated with
holes, called nuclear pores, to facilitate and regulate the exchange of materials (for
example,proteins and RNA) between the nucleus and the cytoplasm. In between these
two membranes is a space called perinuclear space.
The outer membrane is continuous with the endoplasmic reticulum.Continuity exists
between the lumen from the endoplasmatic reticulum and the perinuclear space. Its
composition is similar to the reticulum membran and both have ribossomes attached in
their cytoplasmatic face.
The inner membrane has proteins exclusively from the nucleus,called the nuclear
lamina, and it never has ribossomes attached.
2.Nuclear lamina: It is composed of intermediate filaments and membrane associated proteins.
It acts like a shield for the nucleus. Its principal functions are: to give stability to the nuclear
envolve and help create the nuclear envelope after the mitosis. Also it helps to unpack and pack
the chromatin during the mitosis. Finally, it helps in the replication of the DNA.
3. Nuclear pore complex:
The nuclear pore complexes are the only channels through which small polar molecules, ions,
and macromolecules (proteins and RNAs) are able to travel between the nucleus and the
cytoplasm.
Each complex is made of lots of proteins called nucleoproteins and in it we can perceive:
● 8 protein columns which make the lateral wall of the pore and are set into the perinuclear
space by anchoring proteins. Also 8 radial proteins placed in the middle of the columns
making a central canal,as a diaphragm
● An external cytoplasmic ring made of 8 proteics units and an inner nuclear ring. From
every ring fibrillar proteins start which are filaments that in the nuclear face form a
nuclear basket.
Each nuclear envelope from an animal cell can have between 3000 to 4000 pore complexes
which can change their number and diameter depending on the size of the macromolecules.
To conclude,I’m going to explain how the transport of substances through the pore complex is
possible.
The small molecules go through the nuclear pore by diffusion,but the biggest can only do it
when a specific import and export amino called nuclear localization signals appears which help
to the specific transporters to recognize the macromolecules and transport them through the
pore. This is an active transport process which consumes GTP.