what is a nucleus envelope

Full Answer

What is the envelope of the nucleus?

The nuclear envelope (NE) is a highly regulated membrane barrier that separates the nucleus from the cytoplasm in eukaryotic cells. It contains a large number of different proteins that have been implicated in chromatin organization and gene regulation.

What does nucleolus envelope do?

The nuclear envelope keeps the contents of the nucleus, called the nucleoplasm, separate from the cytoplasm of the cell. The all-important genetic material, mainly the DNA is kept separate and relatively safe from the chemical reactions taking place in the cytoplasm.

What is nuclear envelope made of?

The nuclear envelope is a double membrane composed of an outer and an inner phospholipid bilayer. The thin space between the two layers connects with the lumen of the rough endoplasmic reticulum (RER), and the outer layer is an extension of the outer face of the RER.

What are the two main functions of nuclear envelope?

The nuclear envelope has several functions. First, it provides a physical barrier between the cytosol and the nuclear contents. Second, it is attached to the lamina, which gives the nucleus its sturdy structure and its shape.

What does an envelope do in a cell?

The cell envelope provides structural integrity to the cell. In prokaryotes, it protects the cell from the internal turgor pressure caused due to a high concentration of macromolecules inside the cell. There are two types of bacterial cell envelopes: Gram-positive cell wall.

What does the nuclear envelope protect DNA from?

In eukaryotic cells the nuclear envelope isolates and protects DNA from molecules that could damage its structure or interfere with its processing.

Is the nuclear envelope a single layer?

The nuclear envelope is a double membrane composed of an outer and an inner phospholipid bilayer. The thin space between the two layers connects with the lumen of the rough endoplasmic reticulum (RER), and the outer layer is an extension of the outer…

Is the nuclear envelope an organelle?

It is a double-membraned organelle and this double membrane is referred to as nuclear envelope (also called nuclear membrane, plasmalemma, sor karyotheca). The nuclear envelope surrounds most of the eukaryotic cell's genetic material as opposed to the genetic material of the prokaryotes that is not membrane bound.

Why is it called nuclear envelope?

The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surrounds the nucleus, which encloses the genetic material. The nuclear envelope consists of two lipid bilayer membranes: an inner nuclear membrane and an outer nuclear membrane.

How is the nuclear envelope formed?

In a current model for reassembly after mitosis, the nuclear envelope forms by a reshaping of ER tubules. For the assembly of pores, two major models have been proposed. In the insertion model, nuclear pore complexes are embedded in the nuclear envelope after their formation.

What is the difference between the nuclear envelope and the cell membrane?

The key difference between the cell membrane and nuclear membrane is that cell membrane encloses the cytoplasm and the cell organelles and is a lipid bilayer while nuclear membrane encloses the nucleus and it is made up of double lipid bilayer.

What happens to the nuclear envelope and nucleolus?

At the beginning of mitosis, the chromosomes condense, the nucleolus disappears, and the nuclear envelope breaks down, resulting in the release of most of the contents of the nucleus into the cytoplasm.

Why a nuclear envelope is so important for maintaining the integrity of DNA?

The main role of the nuclear envelope (NE) is to compartmentalize and protect the unfolded genomic DNA from the cytoplasm in eukaryote cells. It is composed of a lipid bilayer reinforced in its inner side with a sheet-like structure of proteins called the nuclear lamina.

What is the nuclear envelope?

The nuclear envelope (NE) is a highly regulated membrane barrier that separates the nucleus from the cytoplasm in eukaryotic cells. It contains a large number of different proteins that have been implicated in chromatin organization and gene regulation. Although the nuclear membrane enables complex levels of gene expression, it also poses a challenge when it comes to cell division. To allow access of the mitotic spindle to chromatin, the nucleus of metazoans must completely disassemble during mitosis, generating the need to re-establish the nuclear compartment at the end of each cell division. Here, I summarize our current understanding of the dynamic remodeling of the NE during the cell cycle.

What is the role of the NE in the genome?

In summary, the NE fulfills a critical role in shielding the genome from cytoplasmic components, but also represents a highly specialized membrane that provides anchoring sites for chromatin and the cytoskeleton (D'Angelo and Hetzer 2006).

What are the proteins that connect the NE to the cytoskeleton?

Topology of the NE. Inner and outer nuclear membranes (INM and ONM, respectively) are separated by the ER lumen or perinuclear space (PNS). The nuclear lamina interacts with NE proteins and chromatin. INM proteins link the NE to chromatin and the lamina. ONM proteins provide a connection from the nucleus to the cytoskeleton. The lamin B receptor (LBR) interacts both with B-type lamins and chromatin-associated heterochromatin protein 1 (HP1) in conjunction with core histones. Members of the LEM (lamina-associated protein 2 [LAP2], emerin, MAN1)-domain family (pink) bind to lamins and interact with chromatin through barrier-to-autointegration factor (BAF). SUN proteins (SUN 1 and 2) interact with nesprins in the ONM, thereby forming so-called LINC complexes that establish connections to actin and intermediate filaments in the cytoplasm. Nurim is a multi-pass membrane protein with unknown function. Proteomic approaches have identified ∼60 putative transmembrane proteins (NETs), most of which remain uncharacterized.

What is the NE membrane?

The NE, a hallmark of eukaryotic cells, is a highly organized double membrane that en closes the nuclear genome (Kite 1913). Early electron microscopy (EM) images revealed that the inner (INM) and outer nuclear membranes (ONM) are continuous with the endoplasmic reticulum (ER) (Watson 1955). Despite the lipid continuity between the NE and the ER, both ONM and INM are comprised of diverse groups of proteins that are typically not enriched in the ER (Hetzer et al. 2005) (Table 1). The firstgroup consists of ∼30 different polypeptides, called nucleoporins or Nups, which form the ∼40–70 MD nuclear pore complexes (NPCs) (Tran and Wente 2006; D'Angelo and Hetzer 2008). NPCs are aqueous channels that show eightfold rotational symmetry with an outer diameter of ∼100 nm and a central transport channel measuring 40 nm in diameter, through which bidirectional exchange of proteins, RNA, and ribonucleoprotein complexes between the nucleoplasm and cytoplasm occurs (Beck et al. 2004; Beck et al. 2007; Terry et al. 2007). A subset of Nups is stably embedded in the NE, forming a scaffold structure or NPC core (Rabut et al. 2004; D'Angelo et al. 2009), which is thought to stabilize the highly curved and energetically unfavorable pore membrane (Alber et al. 2007; Boehmer et al. 2008). This core includes the Nup107/160 complex (Nup84 complex in yeast) and the Nup205 complex (yeast Nup170), which together constitute ∼50% of the entire NPC (Fig. 1) (Brohawn et al. 2009). Attached to this scaffold are peripheral Nups, many of which contain phenylalanine-glycine (FG) rich repeats that establish a permeability barrier and also mediate active, receptor-dependent transport across the NE (Peters 2009). A secondgroup of NE proteins, specifically localizes to the INM (Fig. 1) (Schirmer and Gerace 2005). Although most of these >60 integral membrane proteins (also referred to as NE transmembrane proteins or NETs [Schirmer et al. 2003]) remain largely uncharacterized, interaction with lamins (see later) and chromatin have been shown for some of them, such as lamin B receptor (LBR), lamina-associated polypeptide (LAP) 1, LAP2, emerin, and MAN1 (Akhtar and Gasser 2007; Dorner et al. 2007; Schirmer and Foisner 2007). It is becoming increasingly clear that INM proteins play vital and diverse roles in nuclear function such as chromatin organization, gene expression, and DNA metabolism (Mattout et al. 2006; Heessen and Fornerod 2007; Reddy et al. 2008). Importantly, improper localization and function of INM proteins have been linked to numerous human diseases, which has sparked considerable interest in NE biology over the last decade (Vlcek and Foisner 2007; Worman and Bonne 2007; Neilan 2009).

What is NE remodeling?

NE remodeling in proliferating cells is a highly dynamic process that involves a vast number of molecular players (Figs. 2,​,3).3). By the end of interphase in G2, the nuclei have duplicated their genome, doubled the number of NPCs, and increased the surface area of the NE. The surrounding ER network is continuous with the NE, but not enriched in NE proteins. The entry of mitosis, i.e., prophase, is marked by NE breakdown (NEBD) and the loss of the nucleo-cytoplasmic compartmentalization (Burke and Ellenberg 2002). Between NEBD and early anaphase, when chromosomes align in the metaphase plate and subsequently segregate, chromatin is essentially free of membranes (Puhka et al. 2007; Anderson and Hetzer 2008a). During these cell-cycle stages, the majority of soluble NE proteins are distributed throughout the cytoplasm and transmembrane NE proteins reside in the mitotic ER (Ellenberg et al. 1997; Anderson and Hetzer 2007; Puhka et al. 2007) (Fig 2). In anaphase, ER membranes begin to reassociate with and rapidly enclose the chromatin mass (Anderson and Hetzer 2008a). Chromatin association of a subset of Nups, decondensation of chromatin, and the assembly of new NPCs occurs concomitantly with NE formation (Anderson and Hetzer 2008c). At the end of cell division, the NE has reformed as a closed membrane barrier and re-establishes the nuclear compartment by enabling selective nucleo-cytoplasmic transport (Dultz et al. 2008). After its formation, the NE expands and undergoes additional structural changes necessary for cell-cycle progression and transcription, including the assembly of new NPCs (Winey et al. 1997; D'Angelo et al. 2006) (Fig. 3).

Why do NPCs double in interphase?

In order for cells to properly progress through multiple cell divisions, the number of NPCs doubles in interphase (Maul et al. 1972). It is unclear if the increase simply reflects the necessity to double pores for daughter cells of the next division cycle, or whether an increase in NPC number is important for interphase cell-cycle progression, e.g., efficient replication or transcription. Interphase nuclear pore assembly is particularly interesting because NPC formation occurs from both sides of the NE (D'Angelo et al. 2006). Thus, the question arises, what is the mechanism of communication between the double membranes? Interestingly, Nup133, a member of the Nup107/160 complex, contains an ALPS-like motif including an amphipathic α-helical domain that has been shown to act as a membrane curvature sensor in vitro (Drin et al. 2007). It is possible that this domain is involved in targeting the Nup107/160 complex to membranes during NE formation. A prediction from this idea is that the membrane hole is formed before Nup107/160 is recruited. Whereas the fusion of INM and ONM remains elusive in mammalian cells, significant progress has been made in deciphering this process in yeast.

How is NE formed?

Because membranes are typically delicate structures that are easily disrupted during cell fractionation and fixation, it is easy to see how data obtained from different systems lead to the postulation of sometimes opposing models. There are essentially two different ideas about how the NE is formed: (1) by vesicle fusion and (2) by reshaping of ER into NE sheets.

What is the nuclear envelope?

The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surrounds the nucleus, which encases the genetic material .

What is the origin of the nucleus?

A study of the comparative genomics, evolution and origins of the nuclear membrane led to the proposal that the nucleus emerged in the primitive eukaryotic ancestor (the “prekaryote”), and was triggered by the archaeo -bacterial symbiosis.

How does the nuclear membrane break down?

In mammals, the nuclear membrane can break down within minutes, following a set of steps during the early stages of mitosis. First, M-Cdk's phosphorylate nucleoporin polypeptides and they are selectively removed from the nuclear pore complexes. After that, the rest of the nuclear pore complexes break apart simultaneously. Biochemical evidence suggests that the nuclear pore complexes disassemble into stable pieces rather than disintegrating into small polypeptide fragments. M-Cdk's also phosphorylate elements of the nuclear lamina (the framework that supports the envelope) leading to the disassembly of the lamina and hence the envelope membranes into small vesicles. Electron and fluorescence microscopy has given strong evidence that the nuclear membrane is absorbed by the endoplasmic reticulum—nuclear proteins not normally found in the endoplasmic reticulum show up during mitosis.

What are the proteins that connect the nucleoskeleton to the nuclear membrane?

All four nesprin proteins (nuclear envelope spectrin repeat proteins) present in mammals are expressed in the outer nuclear membrane. Nesprin proteins connect cytoskeletal filaments to the nucleoskeleton. Nesprin-mediated connections to the cytoskeleton contribute to nuclear positioning and to the cell’s mechanosensory function. KASH domain proteins of Nesprin-1 and -2 are part of a LINC complex (linker of nucleoskeleton and cytoskeleton) and can bind directly to cystoskeletal components, such as actin filaments, or can bind to proteins in the perinuclear space. Nesprin-3 and-4 may play a role in unloading enormous cargo; Nesprin-3 proteins bind plectin and link the nuclear envelope to cytoplasmic intermediate filaments. Nesprin-4 proteins bind the plus end directed motor kinesin-1. The outer nuclear membrane is also involved in development, as it fuses with the inner nuclear membrane to form nuclear pores.

Which membrane is continuous with the endoplasmic reticulum membrane?

The outer nuclear membrane is continuous with the endoplasmic reticulum membrane. The nuclear envelope has many nuclear pores that allow materials to move between the cytosol and the nucleus. Intermediate filament proteins called lamins form a structure called the nuclear lamina on the inner aspect of the inner nuclear membrane ...

What stage of mitosis does the nuclear membrane break down?

In other eukaryotes (animals as well as plants), the nuclear membrane must break down during the prometaphase stage of mitosis to allow the mitotic spindle fibers to access the chromosomes inside. The breakdown and reformation processes are not well understood.

What happens to the nuclear membrane during cell division?

Cell division. During the G2 phase of interphase, the nuclear membrane increases its surface area and doubles its number of nuclear pore complexes. In eukaryotes such as yeast which undergo closed mitosis, the nuclear membrane stays intact during cell division.

What is the nuclear envelope?

The nuclear envelope protects the cell’s genetic material from the chemical reactions that take place outside the nucleus. It also contains many proteins that are used in organizing DNA and regulating genes.

What is the nuclear membrane?

The nuclear membrane, also called the nuclear envelope, is a double membrane layer that separates the contents of the nucleus from the rest of the cell. It is found in both animal and plant cells. A cell has many jobs, such as building proteins, converting molecules into energy, and removing waste products. The nuclear envelope protects the cell’s genetic material from the chemical reactions that take place outside the nucleus. It also contains many proteins that are used in organizing DNA and regulating genes.

What are the differences between plant and animal cell nuclear membranes?

Much more is known about animal and yeast cell nuclear membranes than those of plant cells, but the knowledge gap is decreasing thanks to recent research. Plant nuclear membranes lack many of the proteins that are found on the nuclear membranes of animal cells , but they have other pore membrane proteins that are unique to plants. Animal cells have centrosomes, structures that help organize DNA when the cell is preparing to divide; plants lack these structures and appear to rely entirely on the nuclear membrane for organization during cell division. With further research, scientists may better understand the uniqueness of plant cell nuclear membranes.

How many proteins are in a nuclear pore?

Each nuclear pore is made up of about 30 different proteins that work together to transport materials. They also connect the outer and inner membranes. During cell division, more nuclear pores are formed in the nuclear membrane in preparation for cell division.

How do nuclear pores work?

Nuclear pores pass through both the outer and inner membranes of the nuclear membrane. They are made up of large complexes of proteins and allow certain molecules to pass through the nuclear membrane. Each nuclear pore is made up of about 30 different proteins that work together to transport materials . They also connect the outer and inner membranes.

What is the function of the inner membrane?

The inner membrane contains proteins that help organize the nucleus and tether genetic material in place. This network of fibers and proteins attached to the inner membrane is called the nuclear lamina. It structurally supports the nucleus, plays a role in repairing DNA, and regulates events in the cell cycle such as cell division and the replication of DNA. The nuclear lamina is only found in animal cells, although plant cells may have some similar proteins on the inner membrane.

How does the double layer of the nucleus protect the cell?

Only certain proteins can physically pass through the double layer. This protects genetic information from mixing with other parts of the cell, and allows different cellular activities to occur inside the nucleus and outside the nucleus in the cytoplasm, where all other cellular structures are located.

Overview

Structure

Cell division

Origin of the nuclear membrane

External links

Nuclear Membrane Definition

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Function of The Nuclear Membrane

Differences Between Nuclear Membranes in Plant and Animal Cells

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