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The Nucleus And Its Envelope Biology Essay

This study highlights the mechanisms which are involved in altering the nuclear shape and nuclear size. This study also highlights the mechanisms which are involved in nuclear envelope assembly and in nuclear envelope disassembly.

Eukaryotes are those organisms that have membrane bounded nucleus as well as membrane bounded organelles. They can by either multicellular or single-cell organisms. Eukaryote organisms include animals, plants, fungi, and protists.

Cells are functional units of living organisms. Eukaryotic cells have a distinct nucleus which is composed of protein and DNA.

Nucleus is made up of nuclear envelope that surrounds the nucleus to protect its genetic material, nucleolus that codes for the synthesis of ribosomal RNA, DNA which is the genetic information of cell and chromatin made up of DNA and protein. It is associated with DNA replication and its repair.

Nucleus serves as a controlling centre of the cell. It stores the cell's hereditary material and coordinates the cell's activities which include; growth, metabolism, protein synthesis, and reproduction. It accounts for about 10% of total mammalian cell volume. The nucleus was the first organelle to be observed in 1682 as a lumen in red blood cells of salmon fish by a Dutch scientist called Thonius Philips van Leeuwenhoek. Later in 1831, it was given a name cell nucleus by Robert Brown, a Scottish Botanist (Mazzarello, Paolo. 1999). In animals it is the largest cellular organelle. It is a membrane enclosed organelle. A nuclear envelope is located around the nucleus composed of double lipid membranes which protects the cell's genetic material within the cell’s nucleus. Within the nucleus, the genetic material is organised in long molecules of DNA with other proteins such as histones to form chromosomes.

The cell cycle and mitosis:

The eukaryotic cell cycle is divided into four phases (Geoffrey, M. Cooper 2000):

G1 phase (a period for cell growth)

S phase (a period for DNA synthesis)

G2 phase (a rapid cell growth and protein synthesis period)

M phase (Mitosis period)

The G1 phase, S phase and G2 phase all have a collective name known as “interphase” which is involved in the preparation for cell division.

Mitosis is a process that enables a eukaryotic cell to separate the chromosomes into 2 identical sets in two separate nuclei. Cytokinesis initiates immediately after mitosis to split a mitotic cell into two cells. The organelles, cell membrane, cytoplasm and nuclei get equally divided into 2 daughter cells. So M phase of cell division in eukaryotic cell cycle is defined by mitosis and cytokinesis together, which involves a division of a mother cell into two genetically identical daughter cells. They are also genetically identical to their parent cell.

Within mitosis there are 4 stages of cell division (Alberts et al. 2000)::-

Prophase; Early prophase and Late prophase

Early prophase: This stage involves the condensing of chromatins into discrete chromosomes.

Late prophase: At this stage, a nuclear envelope starts breaking down and formation of spindles takes place at opposite poles of mitotic cell. The chromosomes continue to condense and gradually become short and thick. Nucleolus also disappears at this stage.

Metaphase: During this stage, mitotic spindle forms. Each chromosome gets attached to that spindle fibre. Then chromosomes align on metaphase plate.

Anaphase; Early anaphase, Late anaphase

Early anaphase: This stage involves the separation of the sister chromatids.

Late anaphase: This stage involves the elongation of the microtubules. During this phase the chromosomes get pulled further apart. The process of cytokinesis starts from this stage. The nuclear membrane also begins to reform at opposite ends of cell around each group of chromosomes. The nucleoli also reappear at this stage.

Telophase: This is the last stage of mitosis where nuclear envelope reformation completes as it involves the final steps for the completion of total formation of a membrane between each of the new daughter cells. So that the cytokinesis process can yield two separate new cells. Hence telephase is then followed by cytokinesis.

Cytokinesis: It is a process followed by a telophase in which a mitotic cell splits into 2 daughter cells. It ensures that the chromosome number is maintained from one generation to the next generation. It is the last stage in cell division after mitosis.

2a) Variation in mitosic process:

The process of mitosis varies in different spices. In higher eukaryotes, e.g. in metazoans open mitosis takes place which involves a complete disassembling of nuclear envelope at G2/M cell phase transition which then reassembles in G1 phase once DNA gets segregated. Whereas in lower eukaryotes e.g. in fungi closed mitosis takes place where nuclear envelope remains intact and mitosis takes place in nucleus (De Souza, C.P. and Osmani S.A. 2007).

2b) Importance of nuclear envelope:

The nuclear envelope was discovered in the 1950s (Martin, Hetzer. 2010). It is located around the nucleus in a eukaryotic cell. It is composed of double lipid membranes which protects the cell's genetic material within the cell’s nucleus. It acts as a physical barrier to separate the contents of the nucleus from the cytoplasm in eukaryotic cells. It helps to maintain the shape of the nucleus. It has many nuclear pores complexes made from the building blocks of nucleoporins within its structure. Jennifer Lippincott-Schwartz is the first lady who visualised the mitotic nuclear envelope and Golgi breakdown in live cells. In 1990, Jennifer Lippincott-Schwartz reported that nuclear envelope and Golgi components redistribute into the endoplasmic reticulum once the mitosis begin and then reappear out of the endoplasmic reticulum once the mitosis ends (Lippincott-Schwartz et al. 1990).

During each cell division cycle, the nuclear envelope gets disassembled and then reassembled for cell proliferation. A recent study done by Martin Hetzer (2010) highlights that it also has major role in genome organization and in gene expression.

Nuclear pores were first discovered in the mid twentieth century (Martin, Hetzer. 2010). Nuclear pore complexes facilitate and regulate the water-soluble macromolecular nucleocytoplasmic exchange i.e. RNA which moves from the nucleus to the cytoplasm.

The study done by Martin Hetzer (2010) also highlights that nuclear pores are also involved in genes regulation. Martin Hetzer (2010) discovered through his research that the parts of the nuclear pore complex can migrate into the nucleus and then regulate gene expression within the nucleus.

2b i) Nuclear envelope membranes (Chi et al 2009):-

Inner nuclear membrane is a primary residence of several inner nuclear membrane proteins. It is found connected to the nuclear lamina, facing towards nucleoplasm.

Outer nuclear membrane is continuous with the rough endoplasmic reticulum, which is indicated with ribosome attachment.

Pore membrane allows the joining of inner nuclear membrane to the outer nuclear membrane.

Nuclear lamins are fibrous proteins involved in organization of chromatin and supporting the embedding of nuclear pore complexes in the nuclear envelope. During mitosis, they get phosphorylated by Mitosis promoting factor which leads the disassembly of the lamina and disassembly of the nuclear envelope.

Whereas at the end of mitosis cycle i.e. after chromosome segregation and when the pores get re-generated within the double membrane of nuclear envelope to serve their roles in transporting material between the cytoplasm and the nucleus, then at the same time nuclear lamins dephosphorylation also takes place in order to promote the reassembly of the nuclear envelope.

They are divided into three different subtypes which are lamin type A and lamin type B and lamin type C. Lamin type B is the one which constitutively expressed in all cell types of metazoans (Li-Chuan, T. and Rey-Huei, C. 2011)

There are number of associated proteins with nuclear envelope which can be divided into three groups according to their positions (Chi et al 2009).

2bii) Associated protein groups (Chi et al 2009):-

Trans-nuclear membrane proteins found in nuclear pore complex. These proteins serve their roles in transporting material between the cytoplasm and the nucleus.

Integral nuclear membrane proteins, embedded in the inner nuclear membrane which provides a physical barrier.

Nesprin proteins are found on the outer nuclear membrane which are involved in nucleocytoplasmic coupling.

Nuclear envelope associated proteins which are situated beneath the nuclear membrane. These proteins have their roles in many nuclear activities such as chromatin segregation, RNA processing, cell cycle progression, gene transcription.

2c) Importance of normal nuclear shape and size:

Abnormal nuclear shape and size both affect cell’s function. Most cells have an oval or round nucleus. There are various factors which lead to an abnormal nuclear shape such as aging and various diseases e.g. Hutchinson-Gilford Progeria Syndrome. Zinc and colleagues (2004) have associated an abnormal nuclear shape with cancer in their literature. According to the publication of He and colleagues (2008), abnormal nuclear shape brings a change in chromosome organisation which lead a change in gene expression which may results into a cancer.

Alteration in nuclear shape causes alteration in nucleus rigidity which can be beneficial for those cells which require a bit of a squeeze through tight space.

Nuclear size is also equally important. It varies not only within different species but also within different types of cells in the same species. In mammalians, the average size of normal nucleus is 6 micrometres in diameter. The nuclear size does matter because overall it is the size of a nucleus which contains most of the cell's genetic material and is responsible for running most of the cell’s functions. Its size increases from the moment it forms until interphase where it reaches its final size. It has been proposed by Roca-Cusachs and colleagues (2008) that the progression of cell’s cycle depends on its nuclear size. It was found by Schmidt and Schibler (1995) that larger the nuclear size, the higher the level of RNA transcription. A large nucleus may also be important in provide sufficient space for maintaining of its nuclear compartments such as nucleolus. Cancerous cells always develop large nuclei due to being malignant.

There should be some mechanisms which are involved in controlling the change of nuclear shape, nuclear size and involved in controlling the nuclear envelope assembly and nuclear envelope disassembly. Hence this study focuses on the mechanisms behind which lead to change nuclear shape, size and the roles of any proteins that contribute to ER membrane dynamics which control the nuclear envelope assembly and nuclear envelope disassembly.

3) The mechanism behind the change in nuclear shape:

There are 2 different mechanisms exist, either one of them plays its role behind the change in nuclear shape.

Changes in nuclear lamina:

This mechanism involves any change arises in nuclear lamina. An example of neutrophils for this type of mechanism has been described in the study done by Hoffmann and colleagues (2007). It has been shown that a mutation in the lamin B receptor caused the conversion of the multi lobed nuclei of neutrophils into hypolobulated nuclei i.e. changed neutrophil’s nuclear shape.

Changes due to microtubule-generated forces:

This mechanism involves an involvement of microtubules which lead alteration in nuclear shape. An example of Drosophila embryo for this type of mechanism has been described in the study done by

Pilot and colleagues (2006). It has been shown that nuclear shape of Drosophila embryo was changed from spherical to ellipsoid due to cytoplasmic microtubules and Charleston. Charleston is an inner nuclear membrane protein that plays its role in generating chromatin-NE associations. That chromatin-NE association causes transduction of the forces utilized by cytoplasmic microtubules into a change of nuclear shape.

It is also important to note that some cells utilize these microtubule-generated forces in maintaining their normal nuclear shape.

3a) The mechanism behind the change in nuclear size:

The mechanism behind the change in nuclear size was discovered by Heald and Levy (2010). The both researchers took an initiative to find that why two spices of African clawed frogs have different sized nucleus.

Heald and Levy (2010) found that more material get sucked in by the large nucleus compared to the small nucleus. Both researchers also reported a discovery of 2 important proteins which play their roles in the change of nuclear size.

One of the proteins was the importing protein that plays its role in importing the structural material. That structural material builds the lamin proteins web, which strengthens the nuclear shell from inside. So faster the importing of structural material and lamins, the faster they get combined with the underside of nuclear envelope which causes inflation in the nucleus.

The other protein was karyopherin alpha protein which was found at nuclear pore complexes. Most of the small cargos such as ions can passive diffuse themselves through nuclear pore complexes whereas the large cargo can’t pass through the nuclear pore complexes. So the role of karyopherin alpha protein is to transport particular large cargos through a nuclear pore complex. In other words, karyopherin alpha protein acts as a carrier molecule for large cargo to get it into the nucleus from the cytoplasm through nuclear pore complexes.

3b) Why nuclear envelope breakdown?

During cell division, the nuclear envelope gets broken down which starts off with the stretching and tearing of the nuclear membrane with the assistance of spindle microtubules to allow the DNA in dividing cell’s nucleus to be partitioned into two daughter cells (Li-Chuan, T. and Rey-Huei, C. 2011).

Before mitosis starts, the cell duplicates its chromosomes. Then during mitosis, those replicated chromosomes should be partitioned to opposite poles of the dividing cell. To achieve this partitioning, the sister chromatids attach to spindle (composed of microtubules) which are linked to centrosomes. Centrosomes are located outside the nucleus whereas the sister chromatids are inside nucleus. Hence the nuclear envelope surrounding the nucleus gets dismantled during late prophase to allow the microtubules attachment to the chromatids (Lippincott-Schwartz. 2002).

3b i) The mechanism of nuclear envelope breakdown:

The nuclear envelope breakdown process takes place during late prophase (phase of mitosis) which can be divided into the following steps (Guttinger et al 2009) :-

Nuclear pore complex disassembly

Disassembly and depolymerization of the nuclear lamina

Clearance of nuclear envelope membranes

Roles of mitotic Kinases: Mitotic kinases control the activation of nuclear pore complexes disassembly, lamina depolymerisation and the dissociation of inner nuclear membrane proteins from the nuclear binding sites (Guttinger et al. 2009).

Role of microtubules: During propase, those microtubules attached to the outer face of the nucleus exert their pulling forces to cause nuclear envelope invaginations around the centrosomes which lead to the formation of a few holes on the nucleus distal side. As these holes are formed as a result of mechanical stretching of the nuclear lamina hence they cause discontinuities in that nuclear lamina (Guttinger et al. 2009). Beaudouin and colleagues (2002) have shown that this nuclear envelope tearing process driven by microtubules does require dynein which is a cytoplasmic minus end directed microtubule motor as it associates with nuclear envelope at the end of cell division phase G2. Once the microtubules have played their role in causing discontinuities in the nuclear lamina, the nuclear envelope disassembly continues lead by lamin phosphorylation which will be discussed later.

Nuclear pore complex disassembly

The nuclear envelope breakdown process starts in late prophase and gets completed within minutes. It starts with the disassembly of nuclear pore complexes which stops the passive diffusion of nuclear and cytoplasmic components due to the loss of nuclear pore’s function (Guttinger et al. 2009). The nucleoporins disassociate from the nuclear pore complex whereas the scaffold proteins remain stable within the nuclear envelope in the form of cylindrical ring complexes. This was confirmed through an investigation based on confocal time-lapse microscopy of single dividing cells (Guttinger et al. 2009).

Disassembly and depolymerization of the nuclear lamina

Once the nuclear pore complex gets disassembled and microtubules played their role in causing discontinuities in the nuclear lamina, the nuclear envelope disassembly continues which is caused by lamin phosphorylation which results the filaments to break down into individual lamin dimers. Phosphorylation is catalyzed by the Cdc2 protein kinase (CDK1) which is a highly conserved protein that induces depolymerization of the nuclear lamina in vitro. Any mutation of CDK1 phosphorylation site in lamin, leads to the blocking of lamina disassembly process (Guttinger et al. 2009).

During this stage, nuclear inner membrane proteins also detach from lamins and chromatin. The detachment depends on CDK1 phosphorylation because the CDK1 targets are lamina associated proteins lAP2α, lAP2β37 and lamin B receptor at the inner nuclear membrane. The nuclear envelope membrane proteins also retract into the endoplasmic reticulum membrane system at this stage (Guttinger et al. 2009).

There are a few more known Kinases include; Kinase C, Aurora A and Polo-like kinase 1 which contribute to the nuclear envelope breakdown process in various species (Guttinger et al. 2009).

Table 1: Few more known Kinases and their functions

Kinases

Name

Function

Reference

Kinase C

It plays its role in modulating membrane structure events.

Guttinger et al. (2009)

Aurora A

It is implicated with the mechanism of nuclear envelope breakdown. During mitosis, it is vital for the proper spindle assembly.

Guttinger et al. (2009)

Polo-like kinase 1

Absence of Polo-like kinase 1 delays the mechanism of nuclear envelope breakdown. It is involved in G2/M transition.

Guttinger et al. (2009)

Clearance of nuclear envelope membranes

The nuclear pore complex disassembly, disassembly and depolymerization of the nuclear lamina, the phosphorylation of inner membrane proteins and chromatin proteins allows a complete absorption of nuclear envelope and their resident proteins into the mitotic endoplasmic reticulum (Richard, R. and Ruth, W. 2008).

The following diagram illustrates the mechanism of nuclear envelope breakdown process:-

3c) Why Nuclear envelope reforms?

Telophase is the last stage of mitosis where nuclear envelope reformation completes because at this stage; all the effects of prophase and pro-metaphase get reversed. Two daughter nuclei form containing nucleoli each hence nuclear membrane needs to be reformed which was broken during late prophase. So it can act as a physical barrier to separate the contents of two daughter nuclei from the cytoplasm.

3c i) The mechanism of nuclear envelope reformation:

The nuclear envelope reformation starts in late anaphase around each segregated mass of chromatin and the nuclear reassembly completes in telophase. So the mechanism of nuclear envelope reformation can be divided into the following steps (Guttinger et al 2009):-

Nuclear envelope assembly

Preparation of chromatin for nuclear envelope reassembly

Nuclear pore complex reassembly

Starting reformation of nuclear envelope membrane

Ending nuclear envelope formation and nuclear protein complex insertion

Sealing of newly formed nuclear envelope

Nuclear envelope assembly

During nuclear envelope assembly the nuclear pore complexes and other nuclear envelope components are recruited. The production of RanGTP (a protein) on the chromatin surface helps to release the nuclear pore complex components with the help of importin proteins from inhibitory complexes (Guttinger et al. 2009).

Preparation of chromatin for nuclear envelope reassembly

During this stage which takes place in late anaphase, the chromosomes become compact to ensure the reassembly of nuclear envelope around mass of chromatin. RCC1, a chromatin binding protein is required for the compaction of chromosomes and for nuclear envelope assembly. It is also known to be involved in the conversion of RanGDP to RanGTP during nuclear envelope assembly.

Protein phosphate 1 controls anaphase chromosome structure, chromatin de-condensation and lamina B de-phosphorylation (Guttinger et al. 2009).

Nuclear pore complex reassembly

This stage also takes place in late anaphase which involves the recruitment of the NUP107-160 complex and then it’s binding to the chromatin to achieve reassembly of nuclear pore complex. Walther et al (2003) have reported in their study that a depletion in NUP107-160 complex leads to a reduction in the number of nuclear pore complexes in somatic cells so their study proves that NUP107-160 complex has a very significant role in nuclear pore complex reassembly.

Starting reformation of nuclear envelope membrane

Reticulons (RTNs) are integral membrane bending proteins which found to be residing on endoplasmic reticulum. They are a group of evolutionary conservative proteins. They have many important roles such as promoting curvature of membrane. Hence they are associated with tubular endoplasmic reticulum.

Each reticulon has its own reticulon homology domain on its C-terminal end. That reticulon homology domain has 2 short hairpin transmembrane domains which basically generate membrane curvature through increasing the cytoplasmic leaflet area. The short hairpin transmembrane domains are soluble in the cytoplasm hence they occupy outer membrane bilayer leaflet area. Then those short hairpin transmembrane domains expand their occupied area into the inner leaflet of the bilayer in order to generate the membrane curvature in endoplasmic reticulum tubule (Zurek and colleagues 2010).

So this stage of starting reformation of nuclear envelope membrane starts off with the binding of the tips of endoplasmic reticulum tubules with chromatin. Many pore membrane proteins which normally found to be residing on nuclear envelope membranes e.g. NDC1 is present on those tips to mediate chromatin association.

The recruitment of endoplasmic reticulum tubules leads to the formation of flattened nuclear envelope patches on chromatin. The binding of inner nuclear membrane proteins with the chromatin strengthens the attachment of membrane sheets to the chromatin.

The conversion of chromatin associated tubules to membrane sheets is kinetically controlled by removal of “endoplasmic reticulum tubule forming proteins” (Guttinger et al. 2009).

Richard et al. (2008) have revealed in their study that tube-forming reticulons membrane proteins cause delay in nuclear envelope closure when overexpressed. On the other hand, Richard et al.(2008) have also revealed in their study that a reduction in reticulons by RNA interference does speeds up the nuclear envelope closure. Hence they do get ejected during telophase from the chromatin associated endoplasmic reticulum tubules for the reformation of nuclear envelope. Richard et al.(2008) also reported that this process takes roughly 10 minutes from the beginning of anaphase till the complete closure of the re-formed nuclear envelope.

During early telophase, different chromatin regions attract different nuclear envelope proteins.

The inner nuclear membrane proteins play their roles in stabilizing the association of membrane sheets with chromatin. Ulbert and colleagues (2006) have shown that many inner nuclear membrane proteins bind to DNA and that binding makes the efficient recruitment of membranes to chromatin.

Ending nuclear envelope formation and nuclear protein complex insertion

During this stage, nuclear envelope formation is ended in order to re-establish its boundary function i.e. to act as a physical barrier to separate the contents of the cell’s nucleus from the cell’s cytoplasm. The pores get re-generated within the double membrane of nuclear envelope to serve their roles in transporting material between the cytoplasm and the nucleus (Guttinger et al. 2009).

How these pores get re-generated?

There have been 2 models proposed by the researchers (Guttinger et al. 2009):-

The first model proposes that a membrane fusion event takes place between the inner nuclear membrane and the outer nuclear membrane which causes the insertion of pores into the patches of double flattened membrane on chromatin surface but this mechanism hasn’t been detected yet.

The second model proposes that a chromatin associated pre-pore made by NuP107-160 complex during anaphase attracts the membrane and then fuse with the membrane for pore formation. This mechanism has been detected during nuclear pore assembly in metazoan cells and in yeast during interphase.

More research is still on-going in this area to report a definite mechanism which exists for pores re-generation.

Once the nuclear pore complexes get regenerated, they allow the nuclear import of lamins to complete the nuclear lamina assembly in order to promote the reassembly of the nuclear envelope.

Sealing of newly re-formed nuclear envelope

This is the last stage of nuclear envelope reformation process which involves the sealing of newly re-formed nuclear envelope. This requires the formation of a continuous sheet of double membrane around chromatin. The annular membrane fusion takes place to close any remaining holes which connect the inner nuclear membrane and outer nuclear membrane. The exact mechanism for annular membrane fusion is unknown but it does involve a membrane ring constriction to separate the two membranes in nuclear envelope (Guttinger et al. 2009).

The following diagram illustrates the mechanism of nuclear envelope reformation process:-

Summary:

In eukaryotes, nucleus is the largest cellular organelle. It contains the cells genetic material hence it is known to be a controlling centre of the cell. Nucleus coordinates the cell's growth, metabolism, protein synthesis, and reproduction. The genetic material stored within a nucleus is protected by a double lipid membrane called nuclear envelope. Nuclear envelope separates the contents of the nucleus, acting as a physical barrier to separate the contents of the nucleus from the cytoplasm. Hence it is vital for a nucleus. Nuclear pores within its membranes facilitate and regulate the water-soluble macromolecular nucleocytoplasmic exchange i.e. RNA which moves from the nucleus to the cytoplasm.

Mitosis is a type of cell division which is essential for multicellular organisms to grow and repair damaged tissue. In higher eukaryotes such as in metazoans, open mitosis takes place whereas in lower eukaryotes such as in fungi, closed mitosis takes place.

Within mitosis there are 4 stages of cell division; prophase, metaphase, anaphase and telophase which is then followed by cytokinesis. Mitosis produces 2 daughter cells which are exactly identical to the parent cell. So throughout a cell division, dynamic changes take place in the cellular compartments to prevent any errors in the transmission of genetic material to the daughter cells. One of the changes includes the breaking down of nuclear envelope during a late prophase and then its reformation starting in a late anaphase.

Normal nuclear shape is absolutely crucial for the proper functioning of a cell. A normal nucleus in a cell is either found to be in an oval shape or in a round shape. There are various causes which lead abnormal nuclear shape. An abnormal nuclear shape leads a change in gene expression through a change in chromosomes organisation which may result into a cancer.

The nuclear shape can either be changed due to the changes in nuclear lamina or can be either due the changes lead by microtubule-generated forces. Charleston is the key inner nuclear membrane protein that plays its role in generating chromatin-NE associations. That chromatin-NE association causes transduction of the forces utilized by cytoplasmic microtubules into a change of nuclear shape.

Normal nuclear size is also absolutely crucial for the proper functioning of a cell because nucleus is the organelle which is responsible for running most of the cell’s functions. Nucleus size varies within different species even within different types of cells in same species. In mammalians, the average size of normal nucleus is 6 micrometres in diameter. Cancerous cells always develop large nuclei due to being malignant. The mechanism behind the change in nuclear size was discovered by Heald and Levy (2010) who discovered that 2 important proteins which play their roles in the change of nuclear size. One of the proteins is the importing protein that plays its role in importing the structural material to build the lamin proteins web which then leads the inflation of nucleus. In addition, more material get sucked in by the large nucleus compared to the small nucleus. The other protein is the karyopherin alpha protein which plays its role in transporting large cargo which cannot get pass through the nuclear pore complexes from the cytoplasm into the nucleus. So karyopherin alpha protein acts as a carrier molecule for large cargo to get it into the nucleus from the cytoplasm. So together these two different proteins account for the difference in nucleus size among different types of cells in same species and within species.

The breaking down of the nuclear envelope is important because during cell division the replicated chromosomes should be partitioned to opposite poles of the dividing cell. To achieve this partitioning, the sister chromatids attach to spindle (composed of microtubules) which are linked to centrosomes. Centrosomes are located outside the nucleus whereas the sister chromatids are inside nucleus. Hence the nuclear envelope surrounding the nucleus gets dismantled during late prophase to allow the microtubules attachment to the chromatids.

The nuclear envelope breakdown process starts during late prophase (phase of mitosis) which is then gets completed after the following stages :-

Nuclear pore complex disassembly

Disassembly and depolymerization of the nuclear lamina

Clearance of nuclear envelope membranes

It can be concluded that for a complete breakdown of nuclear envelope; the nuclear pore complex disassembly, disassembly and depolymerization of the nuclear lamina, the phosphorylation of inner membrane proteins and chromatin proteins is absolutely vital.

Likewise, the formation of nuclear envelope is equally important because during the last stage of mitosis called “telophase”. All the effects of prophase and pro-metaphase get reversed. Two daughter nuclei form along nucleoli each hence nuclear membrane needs to be reformed which was broken during late prophase. So it can act as a physical barrier to separate the contents of two daughter nuclei from the cytoplasm.

The nuclear envelope reformation starts in late anaphase around each segregated mass of chromatin which then completes in telophase. So the stages of nuclear envelope reformation are as follow:-

Nuclear envelope assembly

Preparation of chromatin for nuclear envelope reassembly

Nuclear pore complex reassembly

Starting reformation of nuclear envelope membrane

Ending nuclear envelope formation and nuclear protein complex insertion

Sealing of newly formed nuclear envelope

It can be concluded that for a complete reformation of nuclear envelope; the binding of the tips of endoplasmic reticulum tubules with chromatin, the nuclear pore complex reassembly and nuclear import of lamins is absolutely vital to complete the nuclear lamina assembly.

Reticulons are integral membrane bending proteins residing on endoplasmic reticulum. They have a role in promoting curvature of membrane hence they are associated with tubular endoplasmic reticulum. The recruitment of endoplasmic reticulum tubules leads to the formation of flattened nuclear envelope patches on chromatin. It has been observed that overexpressed reticulons cause delay in nuclear envelope closure, whereas a reduction in reticulons by RNA interference does speeds up the nuclear envelope closure. That is why reticulons do get ejected from the chromatin associated endoplasmic reticulum tubules during telophase to complete the reformation of nuclear envelope.

The nuclear envelope breakdown process is very complex, where phosphorylation is absolutely crucial. A cytoplasmic dynein (molecular motor protein) is used to tear up the nuclear envelope but there are still many undiscovered mechanisms which can answer many of the questions. e.g. the mechanism is still unknown for the attachment of a cytoplasmic dynein to nuclear envelope to facilitate nuclear envelope breakdown.

On the other hand, the role of protein phosphatases in nuclear reassembly is completely understood. Many recent studies including a study done by Ulbert and colleagues (2006) have shown that the changes in chromatin influence the re-assembly of nuclear envelope.

The mechanism behind the change in nuclear shape and change in nuclear size has been completely identified and understood. However, more research is still on-going to identify few more hidden mechanisms which are involved in nuclear envelope disassembly such as the mechanism is for the attachment of a cytoplasmic dynein to nuclear envelope to tear up the nuclear envelope to facilitate the nuclear envelope breakdown. Likewise for nuclear envelope reformation, a definite mechanism for nuclear pores re-generation within the membranes during nuclear envelope reformation still needs to be uncovered. More experimental analysis may contribute to bring up new factors which may serve in future towards the development of cancer treatment drugs.

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