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= = Function = =
The nucleus provides a site for genetic transcription that is segregated from the location of translation in the cytoplasm , allowing levels of gene regulation that are not available to prokaryotes . The main function of the cell nucleus is to control gene expression and mediate the replication of DNA during the cell cycle .
The nucleus is an organelle found in eukaryotic cells . Inside its fully enclosed nuclear membrane , it contains the majority of the cell 's genetic material . This material is organized as DNA molecules , along with a variety of proteins , to form chromosomes .
= = = Cell compartmentalization = = =
The nuclear envelope allows the nucleus to control its contents , and separate them from the rest of the cytoplasm where necessary . This is important for controlling processes on either side of the nuclear membrane . In most cases where a cytoplasmic process needs to be restricted , a key participant is removed to the nucleus , where it interacts with transcription factors to downregulate the production of certain enzymes in the pathway . This regulatory mechanism occurs in the case of glycolysis , a cellular pathway for breaking down glucose to produce energy . Hexokinase is an enzyme responsible for the first the step of glycolysis , forming glucose @-@ 6 @-@ phosphate from glucose . At high concentrations of fructose @-@ 6 @-@ phosphate , a molecule made later from glucose @-@ 6 @-@ phosphate , a regulator protein removes hexokinase to the nucleus , where it forms a transcriptional repressor complex with nuclear proteins to reduce the expression of genes involved in glycolysis .
In order to control which genes are being transcribed , the cell separates some transcription factor proteins responsible for regulating gene expression from physical access to the DNA until they are activated by other signaling pathways . This prevents even low levels of inappropriate gene expression . For example , in the case of NF @-@ κB @-@ controlled genes , which are involved in most inflammatory responses , transcription is induced in response to a signal pathway such as that initiated by the signaling molecule TNF @-@ α , binds to a cell membrane receptor , resulting in the recruitment of signalling proteins , and eventually activating the transcription factor NF @-@ κB . A nuclear localisation signal on the NF @-@ κB protein allows it to be transported through the nuclear pore and into the nucleus , where it stimulates the transcription of the target genes .
The compartmentalization allows the cell to prevent translation of unspliced mRNA . Eukaryotic mRNA contains introns that must be removed before being translated to produce functional proteins . The splicing is done inside the nucleus before the mRNA can be accessed by ribosomes for translation . Without the nucleus , ribosomes would translate newly transcribed ( unprocessed ) mRNA , resulting in malformed and nonfunctional proteins .
= = = Gene expression = = =
Gene expression first involves transcription , in which DNA is used as a template to produce RNA . In the case of genes encoding proteins , that RNA produced from this process is messenger RNA ( mRNA ) , which then needs to be translated by ribosomes to form a protein . As ribosomes are located outside the nucleus , mRNA produced needs to be exported .
Since the nucleus is the site of transcription , it also contains a variety of proteins that either directly mediate transcription or are involved in regulating the process . These proteins include helicases , which unwind the double @-@ stranded DNA molecule to facilitate access to it , RNA polymerases , which synthesize the growing RNA molecule , topoisomerases , which change the amount of supercoiling in DNA , helping it wind and unwind , as well as a large variety of transcription factors that regulate expression .
= = = Processing of pre @-@ mRNA = = =
Newly synthesized mRNA molecules are known as primary transcripts or pre @-@ mRNA . They must undergo post @-@ transcriptional modification in the nucleus before being exported to the cytoplasm ; mRNA that appears in the cytoplasm without these modifications is degraded rather than used for protein translation . The three main modifications are 5 ' capping , 3 ' polyadenylation , and RNA splicing . While in the nucleus , pre @-@ mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles ( hnRNPs ) . Addition of the 5 ' cap occurs co @-@ transcriptionally and is the first step in post @-@ transcriptional modification . The 3 ' poly @-@ adenine tail is only added after transcription is complete .
RNA splicing , carried out by a complex called the spliceosome , is the process by which introns , or regions of DNA that do not code for protein , are removed from the pre @-@ mRNA and the remaining exons connected to re @-@ form a single continuous molecule . This process normally occurs after 5 ' capping and 3 ' polyadenylation but can begin before synthesis is complete in transcripts with many exons . Many pre @-@ mRNAs , including those encoding antibodies , can be spliced in multiple ways to produce different mature mRNAs that encode different protein sequences . This process is known as alternative splicing , and allows production of a large variety of proteins from a limited amount of DNA .
= = Dynamics and regulation = =
= = = Nuclear transport = = =
The entry and exit of large molecules from the nucleus is tightly controlled by the nuclear pore complexes . Although small molecules can enter the nucleus without regulation , macromolecules such as RNA and proteins require association karyopherins called importins to enter the nucleus and exportins to exit . " Cargo " proteins that must be translocated from the cytoplasm to the nucleus contain short amino acid sequences known as nuclear localization signals , which are bound by importins , while those transported from the nucleus to the cytoplasm carry nuclear export signals bound by exportins . The ability of importins and exportins to transport their cargo is regulated by GTPases , enzymes that hydrolyze the molecule guanosine triphosphate to release energy . The key GTPase in nuclear transport is Ran , which can bind either GTP or GDP ( guanosine diphosphate ) , depending on whether it is located in the nucleus or the cytoplasm . Whereas importins depend on RanGTP to dissociate from their cargo , exportins require RanGTP in order to bind to their cargo .
Nuclear import depends on the importin binding its cargo in the cytoplasm and carrying it through the nuclear pore into the nucleus . Inside the nucleus , RanGTP acts to separate the cargo from the importin , allowing the importin to exit the nucleus and be reused . Nuclear export is similar , as the exportin binds the cargo inside the nucleus in a process facilitated by RanGTP , exits through the nuclear pore , and separates from its cargo in the cytoplasm .
Specialized export proteins exist for translocation of mature mRNA and tRNA to the cytoplasm after post @-@ transcriptional modification is complete . This quality @-@ control mechanism is important due to these molecules ' central role in protein translation . Mis @-@ expression of a protein due to incomplete excision of exons or mis @-@ incorporation of amino acids could have negative consequences for the cell ; thus , incompletely modified RNA that reaches the cytoplasm is degraded rather than used in translation .
= = = Assembly and disassembly = = =
During its lifetime , a nucleus may be broken down or destroyed , either in the process of cell division or as a consequence of apoptosis ( the process of programmed cell death ) . During these events , the structural components of the nucleus — the envelope and lamina — can be systematically degraded . In most cells , the disassembly of the nuclear envelope marks the end of the prophase of mitosis . However , this disassembly of the nucleus is not a universal feature of mitosis and does not occur in all cells . Some unicellular eukaryotes ( e.g. , yeasts ) undergo so @-@ called closed mitosis , in which the nuclear envelope remains intact . In closed mitosis , the daughter chromosomes migrate to opposite poles of the nucleus , which then divides in two . The cells of higher eukaryotes , however , usually undergo open mitosis , which is characterized by breakdown of the nuclear envelope . The daughter chromosomes then migrate to opposite poles of the mitotic spindle , and new nuclei reassemble around them .
At a certain point during the cell cycle in open mitosis , the cell divides to form two cells . In order for this process to be possible , each of the new daughter cells must have a full set of genes , a process requiring replication of the chromosomes as well as segregation of the separate sets . This occurs by the replicated chromosomes , the sister chromatids , attaching to microtubules , which in turn are attached to different centrosomes . The sister chromatids can then be pulled to separate locations in the cell . In many cells , the centrosome is located in the cytoplasm , outside the nucleus ; the microtubules would be unable to attach to the chromatids in the presence of the nuclear envelope . Therefore , the early stages in the cell cycle , beginning in prophase and until around prometaphase , the nuclear membrane is dismantled . Likewise , during the same period , the nuclear lamina is also disassembled , a process regulated by phosphorylation of the lamins by protein kinases such as the CDC2 protein kinase . Towards the end of the cell cycle , the nuclear membrane is reformed , and around the same time , the nuclear lamina are reassembled by dephosphorylating the lamins .
However , in dinoflagellates , the nuclear envelope remains intact , the centrosomes are located in the cytoplasm , and the microtubules come in contact with chromosomes , whose centromeric regions are incorporated into the nuclear envelope ( the so @-@ called closed mitosis with extranuclear spindle ) . In many other protists ( e.g. , ciliates , sporozoans ) and fungi , the centrosomes are intranuclear , and their nuclear envelope also does not disassemle during cell division .
Apoptosis is a controlled process in which the cell 's structural components are destroyed , resulting in death of the cell . Changes associated with apoptosis directly affect the nucleus and its contents , for example , in the condensation of chromatin and the disintegration of the nuclear envelope and lamina . The destruction of the lamin networks is controlled by specialized apoptotic proteases called caspases , which cleave the lamin proteins and , thus , degrade the nucleus ' structural integrity . Lamin cleavage is sometimes used as a laboratory indicator of caspase activity in assays for early apoptotic activity . Cells that express mutant caspase @-@ resistant lamins are deficient in nuclear changes related to apoptosis , suggesting that lamins play a role in initiating the events that lead to apoptotic degradation of the nucleus . Inhibition of lamin assembly itself is an inducer of apoptosis .
The nuclear envelope acts as a barrier that prevents both DNA and RNA viruses from entering the nucleus . Some viruses require access to proteins inside the nucleus in order to replicate and / or assemble . DNA viruses , such as herpesvirus replicate and assemble in the cell nucleus , and exit by budding through the inner nuclear membrane . This process is accompanied by disassembly of the lamina on the nuclear face of the inner membrane .
= = = Disease @-@ related dynamics = = =
Initially , it has been suspected that immunoglobulins in general and autoantibodies in particular do not enter the nucleus . Now there is a body of evidence that under pathological conditions ( e.g. lupus erythematosus ) IgG can enter the nucleus .
= = Nuclei per cell = =
Most eukaryotic cell types usually have a single nucleus , but some have no nuclei , while others have several . This can result from normal development , as in the maturation of mammalian red blood cells , or from faulty cell division .
= = = Anucleated cells = = =
An anucleated cell contains no nucleus and is , therefore , incapable of dividing to produce daughter cells . The best @-@ known anucleated cell is the mammalian red blood cell , or erythrocyte , which also lacks other organelles such as mitochondria , and serves primarily as a transport vessel to ferry oxygen from the lungs to the body 's tissues . Erythrocytes mature through erythropoiesis in the bone marrow , where they lose their nuclei , organelles , and ribosomes . The nucleus is expelled during the process of differentiation from an erythroblast to a reticulocyte , which is the immediate precursor of the mature erythrocyte . The presence of mutagens may induce the release of some immature " micronucleated " erythrocytes into the bloodstream . Anucleated cells can also arise from flawed cell division in which one daughter lacks a nucleus and the other has two nuclei .
In flowering plants , this condition occurs in sieve tube elements .
= = = Multinucleated cells = = =
Multinucleated cells contain multiple nuclei . Most acantharean species of protozoa and some fungi in mycorrhizae have naturally multinucleated cells . Other examples include the intestinal parasites in the genus Giardia , which have two nuclei per cell . In humans , skeletal muscle cells , called myocytes and syncytium , become multinucleated during development ; the resulting arrangement of nuclei near the periphery of the cells allows maximal intracellular space for myofibrils . Multinucleated and binucleated cells can also be abnormal in humans ; for example , cells arising from the fusion of monocytes and macrophages , known as giant multinucleated cells , sometimes accompany inflammation and are also implicated in tumor formation .
A number of dinoflagelates are known to have two nuclei . Unlike other multinucleated cells these nuclei contain two distinct lineages of DNA : one from the dinoflagelate and the other from a symbiotic diatom . Curiously the mitochondrion and the plastid of the diatom remain functional .
= = Evolution = =
As the major defining characteristic of the eukaryotic cell , the nucleus ' evolutionary origin has been the subject of much speculation . Four major hypotheses have been proposed to explain the existence of the nucleus , although none have yet earned widespread support .
The first model known as the " syntrophic model " proposes that a symbiotic relationship between the archaea and bacteria created the nucleus @-@ containing eukaryotic cell . ( Organisms of the Archaea and Bacteria domain have no cell nucleus . ) It is hypothesized that the symbiosis originated when ancient archaea , similar to modern methanogenic archaea , invaded and lived within bacteria similar to modern myxobacteria , eventually forming the early nucleus . This theory is analogous to the accepted theory for the origin of eukaryotic mitochondria and chloroplasts , which are thought to have developed from a similar endosymbiotic relationship between proto @-@ eukaryotes and aerobic bacteria . The archaeal origin of the nucleus is supported by observations that archaea and eukarya have similar genes for certain proteins , including histones . Observations that myxobacteria are motile , can form multicellular complexes , and possess kinases and G proteins similar to eukarya , support a bacterial origin for the eukaryotic cell .
A second model proposes that proto @-@ eukaryotic cells evolved from bacteria without an endosymbiotic stage . This model is based on the existence of modern planctomycetes bacteria that possess a nuclear structure with primitive pores and other compartmentalized membrane structures . A similar proposal states that a eukaryote @-@ like cell , the chronocyte , evolved first and phagocytosed archaea and bacteria to generate the nucleus and the eukaryotic cell .
The most controversial model , known as viral eukaryogenesis , posits that the membrane @-@ bound nucleus , along with other eukaryotic features , originated from the infection of a prokaryote by a virus . The suggestion is based on similarities between eukaryotes and viruses such as linear DNA strands , mRNA capping , and tight binding to proteins ( analogizing histones to viral envelopes ) . One version of the proposal suggests that the nucleus evolved in concert with phagocytosis to form an early cellular " predator " . Another variant proposes that eukaryotes originated from early archaea infected by poxviruses , on the basis of observed similarity between the DNA polymerases in modern poxviruses and eukaryotes . It has been suggested that the unresolved question of the evolution of sex could be related to the viral eukaryogenesis hypothesis .
A more recent proposal , the exomembrane hypothesis , suggests that the nucleus instead originated from a single ancestral cell that evolved a second exterior cell membrane ; the interior membrane enclosing the original cell then became the nuclear membrane and evolved increasingly elaborate pore structures for passage of internally synthesized cellular components such as ribosomal subunits .
= = Gallery = =
= Until the Whole World Hears =
Until the Whole World Hears is the fourth studio album by American Christian rock band Casting Crowns . Released on November 27 , 2009 , the album was produced by Mark . A Miller and features a sound that has been described as ' pure American rock ' , ' soft adult contemporary ' , and ' CCM ' . Lyrically , the album discusses Christian subjects such as God , Jesus , and salvation , with several songs being reinventions of classic hymns . Until the Whole World Hears sold over 167 @,@ 000 copies in its first week , Casting Crowns ' highest sales week to date , and debuted at No. 4 on the Billboard 200 and at No. 1 on the Billboard Christian Albums chart . In the United States , the album ranked as the 37th best @-@ selling album of 2010 and the 137th best @-@ selling album of 2011 ; it ranked as the first and third best @-@ selling Christian album in those years , respectively . It has sold over 1 @.@ 1 million copies and been certified Platinum by the Recording Industry Association of America ( RIAA ) .
Until the Whole World Hears received a mixed @-@ to @-@ positive reception from critics and was nominated for Pop / Contemporary Album of the Year at the 42nd GMA Dove Awards . Three singles were released from the album : the title track , " If We 've Ever Needed You " , and " Glorious Day ( Living He Loved Me ) " . The title track and " Glorious Day ( Living He Loved Me ) " both peaked atop the Billboard Christian Songs chart , while the latter also peaked at No. 2 on the Billboard Bubbling Under Hot 100 Singles chart ; " If We 've Ever Needed You " peaked at No. 5 on the Christian Songs chart . Two album cuts , " To Know You " and " Joyful , Joyful " , also appeared on the Christian Songs chart , peaking at numbers 27 and 3 , respectively .
= = Background and songwriting = =
Casting Crowns ' lead vocalist Mark Hall has stated that the band 's songs " have always come from our ministry in the church . They start as messages on Wednesday night , things we 're teaching our teenagers and their families " ; Until the Whole World Hears retains that same formula . Hall also stated that the musical sound of their songs is written so as not to conflict or detract from the lyrics , comparing the musical elements of his songs to a plate that the ' meat ' , the lyrics , are served on . A philosophy that " fuels " the album is the idea of putting " faith in action " ; Hall stated that he wanted believers to become more active in their faith , and to " get out of their pews and get involved in what God is doing " . " Always Enough " was written when a member of Hall 's church was killed in Afghanistan ; the band was unable to attend his funeral , as they were on the other side of the United States and couldn 't cancel their tour dates . Another song on the album , the title track , was inspired by the Biblical character of John the Baptist . Several songs on the album are reinventions of hymns , which Hall enjoyed experimenting with ; Hall retained the lyrics of the songs while reworking their melodies .
Until the Whole World Hears was produced by Mark A. Miller ; its executive producer was Terry Hemmings . It was recorded by Sam Hewitt , Michael Hewitt , and Dale Oliver at Zoo Studio in Franklin , Tennessee ; the string instrument tracks on the title track and " Always Enough " were recorded by Bobby Shin at Little Big Sound Studio in Nashville , Tennessee , while the string instrument tracks on " If We 've Ever Needed You " and " Joyful , Joyful " were recorded by John Painter and Leslie Richter at Ocean Way in Nashville . Crowd vocals on the title track and " Blessed Redeemer " were recorded by Carter Hassebroek , Darren Hughes and Billy Lord at Eagle 's Landing Baptist Church in McDonough , Georgia . Digital editing was handled by Michael Hewitt , while mixing was handled by Sam Hewitt . The album was mastered by Andrew Mendelson , Shelly Anderson , Natthaphol Abhigantaphand and Daniel Bacigalupi at Georgetown Masters in Nashville .
= = Composition = =
The overall sound of Until the Whole World Hears has been described as ' pure American rock ' , ' soft adult contemporary ' , and ' CCM ' . Robert Ham of Christianity Today regarded the album as comparable to the sound of rock bands Creed and Nickelback . One critic observed that most of the songs on the album " start off with chords plunked out on a piano or strummed slowly on a guitar , letting the song build slowly toward a massive wave of sound " . The title track has been described as having a " real rock vibe " infused by electric guitar riffs , while " Shadow of Your Wings " has been described as an " unashamed rock @-@ n @-@ roll jam " . " Joyful , Joyful " is driven by a " pulsing " and " driving " string section that " calls to mind Coldplay 's ' Viva la Vida ' " . " Mercy " and " Blessed Redeemer " feature female @-@ fronted vocals , the former sung by Megan Garrett and the latter sung by Melodee DeVevo ; on " At Your Feet " , Hector Cervantes and Juan DeVevo joining Mark Hall on vocals .
Every track on Until the Whole World Hears features references to Christian subjects such as God , Jesus , and salvation . " Joyful , Joyful " , " Blessed Redeemer " , and " Glorious Day ( Living He Loved Me ) " were adopted from classic hymns . " Holy One " and " Shadow of Your Wings " are taken almost word for word from the Book of Psalms . " If We 've Ever Needed You " and " Always Enough " are " darker inspirational anthems " , while other songs explore themes such as repentance and forgiveness .
= = Release and sales = =
Until the Whole World Hears had first @-@ week sales of 167 @,@ 000 copies , Casting Crowns ' best sales week to date ; the album 's high first @-@ week sales enabled a No. 4 debut on the Billboard 200 . It also debuted at No. 1 on the Billboard Christian Albums chart and at No. 12 on the Billboard Digital Albums chart . The Recording Industry Association of America ( RIAA ) certified the album Platinum , signifying shipments of over 1 @,@ 000 @,@ 000 copies . By April 2011 Until the Whole World Hears had sold over 800 @,@ 000 copies , and as of March 2014 the album has sold 1 @.@ 1 million copies . Billboard magazine ranked Until the Whole World Hears as the best @-@ selling Christian album and the 37th best @-@ selling album overall of 2010 . It also ranked as the 3rd best @-@ selling Christian album and the 137th best @-@ selling album overall of 2011 .
Three singles were released from Until the Whole World Hears . The title track was released as the album 's lead single and peaked at No. 1 on the Billboard Christian Songs chart and at No. 23 on the Billboard Heatseekers Songs chart . " If We 've Ever Needed You " , the second single released from the album , peaked at No. 5 on the Christian Songs chart . " Glorious Day ( Living He Loved Me ) " was released as the third single off the album and peaked atop the Christian Songs chart . It also peaked at No. 2 on the Bubbling Under Hot 100 Singles chart and at No. 20 on the Heatseekers Songs chart . Two other songs off the album , " To Know You " and " Joyful , Joyful " , appeared on the Christian Songs chart ; they peaked at numbers 27 and 3 , respectively .
= = Reception = =
= = = Critical reception = = =
Until the Whole World Hears met with an overall mixed to positive response from critics . Several critics regarded the musical aspects of the album as mediocre or sub @-@ par , while others praised the album 's lyrical content . Jared Johnson of Allmusic gave the album four out of five stars and described it as a " powerful worship experience " , but also stated that " some might wonder how a little more variety would sound from such experienced professionals ... the band 's core sound continues to land in the AC cross hairs " . Andrew Greer of CCM Magazine commented that the album " ups the musical ante a bit , with some borderline poetic verses and interesting musical riffs " , but also commented that " many of these tracks still suffer from the ' Crowns Cliché Syndrome , ' using lyrical Christian @-@ ese to produce trite rhymes that seem hard @-@ pressed to energize a ready @-@ to @-@ worship crowd " . Robert Ham of Christianity Today criticized the album for using what he deemed as a repetitive musical formula , but also praised the song " Joyful , Joyful " , which he compared to Coldplay 's song " Viva la Vida " , as well as Megan 's Garrett 's vocals on " Mercy " . He concluded that Until the Whole World Hears " feels like a step backward creatively " . At Cross Rhythms , Tony Cummings rated the album seven out of ten squares , saying " this album is a little disappointing considering some of the glories that preceeded [ sic ] it . "
Debra Akins of Gospel Music Channel.com said the album " follows successfully in the footsteps of its predecessors " and " should further solidify Casting Crowns as a staple artist for Christian music fans everywhere " . Roger Gelwicks of Jesus Freak Hideout gave Until the Whole World Hears two out of five stars , opining that " Casting Crowns has come down to a whole new low , such that it could be their most unremarkable record to date " and that " it is almost insulting to the listener to believe that one is supposed to find this project profound or listenable " . Paul Asay of Plugged In stated that " With appropriate apologies to the many talented and successful acts that straddle the secular and spiritual with their tunes , it 's great to have a band that speaks to the Christian heart with such power and eloquence — without apology " .
= = = Awards and accolades = = =
Until the Whole World Hears was nominated for the Pop / Contemporary Album of the Year at the 42nd GMA Dove Awards Two of its three singles also received award nominations ; its title track was nominated for Pop / Contemporary Record Song of the Year at the 41st GMA Dove Awards , and " Glorious Day ( Living He Loved Me ) " has been nominated for Song of the Year and Worship Song of the Year at the 43rd GMA Dove Awards . It was nominated for Top Christian Album at the 2012 Billboard Music Awards .