All Topics  
Nuclear envelope

 

 
   Email Print
   Bookmark   Link
 

 

Nuclear envelope
 
 
  Topics Nuclear envelope Topic Home

The nuclear envelope (NE) (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that encloses the genetic material in eukaryotic
Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
 cells
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus (DNA
DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
 in particular) from the cytosol
Cytosol

The cytosol or intracellular fluid is the liquid found inside cell . In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion....
 (cytoplasm).

Numerous nuclear pore
Nuclear pore

Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double Endomembrane system surrounding the eukaryote cell cell nucleus....
s are inserted in the nuclear envelope and facilitate and regulate the exchange of materials (protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s such as transcription factors, and RNA
RNA

Ribonucleic acid is a type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate....
) between the nucleus and the cytoplasm.

Each of the two membranes is composed of a lipid bilayer
Lipid bilayer

A lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cell ....
.






Discussion
Ask a question about 'Nuclear envelope'
Start a new discussion about 'Nuclear envelope'
Answer questions from other users
Full Discussion Forum



Encyclopedia


Diagram Human Cell Nucleus
The nuclear envelope (NE) (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that encloses the genetic material in eukaryotic
Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
 cells
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus (DNA
DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
 in particular) from the cytosol
Cytosol

The cytosol or intracellular fluid is the liquid found inside cell . In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion....
 (cytoplasm).

Numerous nuclear pore
Nuclear pore

Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double Endomembrane system surrounding the eukaryote cell cell nucleus....
s are inserted in the nuclear envelope and facilitate and regulate the exchange of materials (protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s such as transcription factors, and RNA
RNA

Ribonucleic acid is a type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate....
) between the nucleus and the cytoplasm.

Each of the two membranes is composed of a lipid bilayer
Lipid bilayer

A lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cell ....
. The outer membrane is continuous with the rough endoplasmic reticulum while the inner nuclear membrane is the primary residence of several inner nuclear membrane proteins
Inner nuclear membrane proteins

Inner nuclear membrane proteins are proteins that are embedded in or associated with the inner membrane of the nuclear envelope. They have been postulated to function during NE formation at the conclusion of mitosis....
. The outer and inner nuclear membrane are fused at the site of nuclear pore complex insertion.

The inner nuclear membrane is connected to the nuclear lamina
Nuclear lamina

The nuclear lamina is a dense fibrillar network inside the Cell nucleus of an eukaryotic cell. It is composed of intermediate filaments and membrane associated proteins....
, a network of intermediate filament
Intermediate filament

Intermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of actin and microtubules, although they were initially designated 'intermediate' because their average diameter was between those of narrower mi...
s composed of various lamin
Lamin

Nuclear Lamins, also known as Class V intermediate filaments, are fibrous proteins providing structural function and transcriptional regulation in the cell nucleus....
s (A, B1, B2, & C). The lamina acts as a site of attachment for chromosomes and provides structural stability to the nucleus. The lamins have been associated with various genetic disorders collectively termed laminopathies.

The nuclear envelope has two membranes, each with the typical unit membrane structure. They enclose a flattened sac and are connected at the nuclear pore sites. The outermost membrane is continuous with the rough endoplasmic reticulum (ER) and has ribosomes attached (see figure to the left). The space between the outer and inner membranes is also continuous with rough endoplasmic reticulum space. It can fill with newly synthesized proteins just as the rough endoplasmic reticulum does. The nuclear envelope is enmeshed in a network of filaments for stability.

The nuclear envelope is shown in an electron micrograph in the figure to the right. The filaments outside the envelope are not visualized with these protocols. Also, the nuclear lamina just inside the nuclear envelope is not shown well (see paragraph below for description). However, one can see ribosomes on the outer membrane and the sac enclosed by the two membranes. Dense patches of Heterochromatin are seen just inside the inner membrane.

The space between the two membranes that make up the nuclear envelope is called the perinuclear space (also called the perinuclear cisterna, NE Lumen), and is usually about 20 - 40 nm wide.

The nuclear envelope has been postulated to play a role in the organization and transcriptional activity of chromatin
Chromatin

Chromatin is the complex combination of DNA, RNA, and protein that makes up chromosomes. It is found inside the cell nucleus of Eukaryote cell , and within the nucleoid in prokaryotic cells....
.

Nuclear envelope breakdown during mitosis in metazoans


During prophase
Prophase

Prophase is a stage of mitosis in which the chromatin condenses into a highly ordered structure called a chromosome in which the chromatin becomes visible....
 in mitosis
Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
, the chromatid
Chromatid

A chromatid is one among the two identical copies of DNA making up a replicated chromosome, which are joined at their centromeres, for the process of cell division ....
s begin condensing to form chromosome
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
s, and the nuclear envelope breaks down and is retracted into the mitotic endoplasmic reticulum
Endoplasmic reticulum

The endoplasmic reticulum is a eukaryote organelle that forms an interconnected network of tubules, vesicle , and cisternae within cell . The lacey membranes of the endoplasmic reticulum were first seen by Keith R....
. At metaphase
Metaphase

Metaphase, from the ancient Greek ?et? and f?s?? , is a stage of mitosis in the Eukaryote cell cycle in which condensed chromosomes, carrying DNA sequence, align in the middle of the cell before being separated into each of the two daughter cells....
, the nuclear envelope has been completely disassembled and absorbed by the ER allowing the chromosomes to be put togeather by spindle fibers attached to each chromosome at the kinetochore. Other eukaryotes such as yeast undergo closed mitosis, where the chromosomes segregate within the nuclear envelope, which then buds as the three daughter cells divide.

In the process of mitosis, the phospholipid bilayer must degrade during prometaphase. Without this step, the nuclear material would never be able to separate into two nuclei, and by extension, two cells. At the end of anaphase however, the chromosomes are now separated, and each set is in its respective half of the parent cell. In order to protect the genetic material, the nuclear membrane must re-form at this stage. This is done using membrane from the endoplasmic reticulum and proteins called lamins that guide the new envelope. (Alberts) Like every other organelle in the cell, the endoplasmic reticulum is composed of a phospholipid bilayer. This thin membrane is very flexible, and portions can be pinched off to form new organelles, vesicles, or in this case, nuclei. In 2007, researchers discovered that pieces of the ER break off and merge together to form the nuclear envelope. However, there must be a structural element that brings these vesicles together, because the nucleus is much more structurally complicated than the ER. This element is a layer of lamin between the chromatid and the nuclear envelope itself, known as the lamina. Lamins are long, fibrous, proteins. In prometaphase, they are phosphorylated (a phosphate group is added), causing the protein to change shape and lose its structural properties. This is what causes the breaking down of the nuclear membrane. However, towards the end of anaphase, the existing lamin is dephosphorylated, and even more is produced. Once the chromosomes have separated, the lamina begins to form again. Sometime at the beginning of mitosis, ends of the endoplasmic reticulum bound to the DNA, using lamin B receptor. (Duband-Goulet and Courvalin) Once the lamina begins to re-form, it forces tubules of ER to form a network across the surface of the chromatid. (Anderson and Hetzer) These tubes eventually are flattened out and merged, forming a solid nuclear membrane. It is not certain what mechanisms cause this to happen. The ER and the LBR eventually detach themselves from the DNA. In a mature cell, the lamina forms a continuous layer just inside the nuclear envelope, and is attached to the nucleus by proteins known as emerin. (http://www.mda.org/publications/images/q12-3_ed_nucleus.jpg)

External links