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= Principe Amedeo @-@ class ironclad =
The Principe Amedeo class was a pair of ironclad warships built for the Italian Regia Marina ( Royal Navy ) in the 1870s and 1880s . They were the culmination of a major naval construction program designed to give Italy a powerful fleet of ironclads . The two ships , Principe Amedeo and Palestro , were the last Italian ironclads to feature sailing rigs and wooden hulls . They were armed with a battery of six 10 @-@ inch ( 254 mm ) guns and were capable of a speed in excess of 12 knots ( 22 km / h ; 14 mph ) . The ships had uneventful careers , spending much of it in Italy 's colonial empire . By the late 1880s , they were withdrawn from service and employed in secondary roles , first as headquarters ships for harbor defenses . Principe Amedeo was converted into a depot ship in 1895 and was discarded in 1910 , while Palestro was used as a training ship from 1894 to 1900 before being scrapped in 1902 – 04 .
= = Design = =
In 1862 , the Italian government under Prime Minister Urbano Rattazzi and his naval minister Carlo Pellion di Persano made the decision to build a fleet of ironclad warships . The Italian fleet had already acquired a pair of small , French @-@ built armored frigates of the Formidabile class , and two more vessels of the Re d 'Italia class had been ordered from the United States . Five more ironclads were ordered from foreign shipyards , three wooden steam frigates already under construction were converted into armored ships , and four more ironclads were ordered from Italian shipyards . The two Principe Amedeo @-@ class ships were the last two of this first generation of Italian ironclads . The design for Principe Amedeo was prepared by Inspector Engineer Giuseppe De Luca . He had initially planned on using entirely wooden hulls for the ships , but had changed to composite wood and iron construction by the time the ships were laid down .
= = = General characteristics and machinery = = =
The two ships differed slightly in size . Principe Amedeo was 79 @.@ 73 meters ( 261 @.@ 6 ft ) long between perpendiculars , while Palestro was 78 @.@ 82 m ( 258 @.@ 6 ft ) long . Principe Amedeo had a beam of 17 @.@ 4 m ( 57 ft ) and a draft of 7 @.@ 9 m ( 26 ft ) ; Palestro 's beam measured 17 @.@ 3 m ( 57 ft ) , and she had a draft of 8 m ( 26 ft ) . Both ships displaced 5 @,@ 761 metric tons ( 5 @,@ 670 long tons ; 6 @,@ 350 short tons ) normally , but Principe Amedeo displaced 6 @,@ 020 t ( 5 @,@ 920 long tons ; 6 @,@ 640 short tons ) at full load and Palestro reached 3 @,@ 218 t ( 3 @,@ 167 long tons ; 3 @,@ 547 short tons ) . They had a crew of 548 officers and men .
The ships ' propulsion system consisted of one single @-@ expansion steam engine that drove a single screw propeller , with steam supplied by six coal @-@ fired , cylindrical fire @-@ tube boilers . The boilers were trunked into a single funnel . The lead ship 's engine produced a top speed of 12 @.@ 2 knots ( 22 @.@ 6 km / h ; 14 @.@ 0 mph ) at 6 @,@ 117 indicated horsepower ( 4 @,@ 561 kW ) , while Palestro made 12 @.@ 85 kn ( 23 @.@ 80 km / h ; 14 @.@ 79 mph ) at the same horsepower . They could steam for 1 @,@ 780 nautical miles ( 3 @,@ 300 km ; 2 @,@ 050 mi ) at a speed of 10 knots ( 19 km / h ; 12 mph ) . The ships were barque @-@ rigged to supplement the steam engine ; Principe Amedeo and her sister were the last rigged ironclad to be built by Italy . The ships ' sail area were 36 @,@ 738 square feet ( 3 @,@ 413 @.@ 1 m2 ) for Principe Amedeo and 37 @,@ 361 sq ft ( 3 @,@ 471 @.@ 0 m2 ) for Palestro .
= = = Armament and armor = = =
Palestro and Principe Amedeo were both armed with a main battery of six 10 in ( 254 mm ) guns , though they were mounted differently in each ship . Principe Amedeo carried hers in a single armored casemate located amidships , while Palestro 's guns were mounted in three armored casemates . The first was located forward , toward the bow , the second and third were placed close to the stern on each side of the ship . Both ships also carried an 11 in ( 279 mm ) gun that was mounted forward as a bow chaser . Later in her career , Principe Amedeo received a secondary battery of six 2 @.@ 9 in ( 74 mm ) guns and six machine guns , along with two torpedo tubes .
The two ships were protected by iron belt armor that was 8 @.@ 7 in ( 221 mm ) thick and extended for the entire length of the hull . The casemates were protected with 5 @.@ 5 in ( 140 mm ) of iron plating , and the small conning tower had 2 @.@ 4 in ( 61 mm ) thick iron plates .
= = Ships = =
= = Service history = =
Neither ship had a particularly eventful career . They were completed too late to take part in the final stages of the wars of Italian unification . Instead , they were assigned to the Italian colonial empire , with occasional stints in the main Italian fleet . In 1880 , Palestro took part in a naval demonstration off Ragusa in an attempt to force the Ottoman Empire to comply with the terms of the Treaty of Berlin and turn over the town of Ulcinj to Montenegro . The following year , Principe Amedeo was involved in a collision with the ironclad Roma during a hurricane , though neither ship was damaged .
In the late 1880s , both ships were withdrawn from frontline service and employed as headquarters ships for the defense of Taranto — Principe Amedeo — and La Maddalena — Palestro . Principe Amedeo was stricken from the naval register in 1895 and used as an ammunition depot ship in Taranto until 1910 , when she was sold for scrap . Palestro was employed as a training ship between 1894 and 1900 , when she too was stricken from the register . She was broken up between 1902 and 1904 .
= Cell nucleus =
In cell biology , the nucleus ( pl. nuclei ; from Latin nucleus or nuculeus , meaning kernel ) is a membrane @-@ enclosed organelle found in eukaryotic cells . Eukaryotes usually have a single nucleus , but a few cell types , such as mammalian red blood cells , have no nuclei , and a few others have many .
Cell nuclei contain most of the cell 's genetic material , organized as multiple long linear DNA molecules in complex with a large variety of proteins , such as histones , to form chromosomes . The genes within these chromosomes are the cell 's nuclear genome and are structured in such a way to promote cell function . The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression — the nucleus is , therefore , the control center of the cell . The main structures making up the nucleus are the nuclear envelope , a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm , and the nuclear matrix ( which includes the nuclear lamina ) , a network within the nucleus that adds mechanical support , much like the cytoskeleton , which supports the cell as a whole .
Because the nuclear membrane is impermeable to large molecules , nuclear pores are required that regulate nuclear transport of molecules across the envelope . The pores cross both nuclear membranes , providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions . Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes . Although the interior of the nucleus does not contain any membrane @-@ bound sub compartments , its contents are not uniform , and a number of sub @-@ nuclear bodies exist , made up of unique proteins , RNA molecules , and particular parts of the chromosomes . The best @-@ known of these is the nucleolus , which is mainly involved in the assembly of ribosomes . After being produced in the nucleolus , ribosomes are exported to the cytoplasm where they translate mRNA .
= = History = =
The nucleus was the first organelle to be discovered . What is most likely the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek ( 1632 – 1723 ) . He observed a " lumen " , the nucleus , in the red blood cells of salmon . Unlike mammalian red blood cells , those of other vertebrates still contain nuclei .
The nucleus was also described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London . Brown was studying orchids under microscope when he observed an opaque area , which he called the " areola " or " nucleus " , in the cells of the flower 's outer layer .
He did not suggest a potential function . In 1838 , Matthias Schleiden proposed that the nucleus plays a role in generating cells , thus he introduced the name " cytoblast " ( cell builder ) . He believed that he had observed new cells assembling around " cytoblasts " . Franz Meyen was a strong opponent of this view , having already described cells multiplying by division and believing that many cells would have no nuclei . The idea that cells can be generated de novo , by the " cytoblast " or otherwise , contradicted work by Robert Remak ( 1852 ) and Rudolf Virchow ( 1855 ) who decisively propagated the new paradigm that cells are generated solely by cells ( " Omnis cellula e cellula " ) . The function of the nucleus remained unclear .
Between 1877 and 1878 , Oscar Hertwig published several studies on the fertilization of sea urchin eggs , showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus . This was the first time it was suggested that an individual develops from a ( single ) nucleated cell . This was in contradiction to Ernst Haeckel 's theory that the complete phylogeny of a species would be repeated during embryonic development , including generation of the first nucleated cell from a " monerula " , a structureless mass of primordial mucus ( " Urschleim " ) . Therefore , the necessity of the sperm nucleus for fertilization was discussed for quite some time . However , Hertwig confirmed his observation in other animal groups , including amphibians and molluscs . Eduard Strasburger produced the same results for plants in 1884 . This paved the way to assign the nucleus an important role in heredity . In 1873 , August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity . The function of the nucleus as carrier of genetic information became clear only later , after mitosis was discovered and the Mendelian rules were rediscovered at the beginning of the 20th century ; the chromosome theory of heredity was therefore developed .
= = Structures = =
The nucleus is the largest cellular organelle in animal cells . In mammalian cells , the average diameter of the nucleus is approximately 6 micrometres ( µm ) , which occupies about 10 % of the total cell volume . The viscous liquid within it is called nucleoplasm , and is similar in composition to the cytosol found outside the nucleus . It appears as a dense , roughly spherical or irregular organelle .
= = = Nuclear envelope and pores = = =
The nuclear envelope , otherwise known as nuclear membrane , consists of two cellular membranes , an inner and an outer membrane , arranged parallel to one another and separated by 10 to 50 nanometres ( nm ) . The nuclear envelope completely encloses the nucleus and separates the cell 's genetic material from the surrounding cytoplasm , serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm . The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum ( RER ) , and is similarly studded with ribosomes . The space between the membranes is called the perinuclear space and is continuous with the RER lumen .
Nuclear pores , which provide aqueous channels through the envelope , are composed of multiple proteins , collectively referred to as nucleoporins . The pores are about 125 million daltons in molecular weight and consist of around 50 ( in yeast ) to several hundred proteins ( in vertebrates ) . The pores are 100 nm in total diameter ; however , the gap through which molecules freely diffuse is only about 9 nm wide , due to the presence of regulatory systems within the center of the pore . This size selectively allows the passage of small water @-@ soluble molecules while preventing larger molecules , such as nucleic acids and larger proteins , from inappropriately entering or exiting the nucleus . These large molecules must be actively transported into the nucleus instead . The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope , each of which contains an eightfold @-@ symmetric ring @-@ shaped structure at a position where the inner and outer membranes fuse . Attached to the ring is a structure called the nuclear basket that extends into the nucleoplasm , and a series of filamentous extensions that reach into the cytoplasm . Both structures serve to mediate binding to nuclear transport proteins .
Most proteins , ribosomal subunits , and some DNAs are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins , whereas those that mediate movement out of the nucleus are called exportins . Most karyopherins interact directly with their cargo , although some use adaptor proteins . Steroid hormones such as cortisol and aldosterone , as well as other small lipid @-@ soluble molecules involved in intercellular signaling , can diffuse through the cell membrane and into the cytoplasm , where they bind nuclear receptor proteins that are trafficked into the nucleus . There they serve as transcription factors when bound to their ligand ; in the absence of ligand , many such receptors function as histone deacetylases that repress gene expression .
= = = Nuclear lamina = = =
In animal cells , two networks of intermediate filaments provide the nucleus with mechanical support : The nuclear lamina forms an organized meshwork on the internal face of the envelope , while less organized support is provided on the cytosolic face of the envelope . Both systems provide structural support for the nuclear envelope and anchoring sites for chromosomes and nuclear pores .
The nuclear lamina is composed mostly of lamin proteins . Like all proteins , lamins are synthesized in the cytoplasm and later transported to the nucleus interior , where they are assembled before being incorporated into the existing network of nuclear lamina . Lamins found on the cytosolic face of the membrane , such as emerin and nesprin , bind to the cytoskeleton to provide structural support . Lamins are also found inside the nucleoplasm where they form another regular structure , known as the nucleoplasmic veil , that is visible using fluorescence microscopy . The actual function of the veil is not clear , although it is excluded from the nucleolus and is present during interphase . Lamin structures that make up the veil , such as LEM3 , bind chromatin and disrupting their structure inhibits transcription of protein @-@ coding genes .
Like the components of other intermediate filaments , the lamin monomer contains an alpha @-@ helical domain used by two monomers to coil around each other , forming a dimer structure called a coiled coil . Two of these dimer structures then join side by side , in an antiparallel arrangement , to form a tetramer called a protofilament . Eight of these protofilaments form a lateral arrangement that is twisted to form a ropelike filament . These filaments can be assembled or disassembled in a dynamic manner , meaning that changes in the length of the filament depend on the competing rates of filament addition and removal .
Mutations in lamin genes leading to defects in filament assembly cause a group of rare genetic disorders known as laminopathies . The most notable laminopathy is the family of diseases known as progeria , which causes the appearance of premature aging in its sufferers . The exact mechanism by which the associated biochemical changes give rise to the aged phenotype is not well understood .
= = = Chromosomes = = =
The cell nucleus contains the majority of the cell 's genetic material in the form of multiple linear DNA molecules organized into structures called chromosomes . Each human cell contains roughly two meters of DNA . During most of the cell cycle these are organized in a DNA @-@ protein complex known as chromatin , and during cell division the chromatin can be seen to form the well @-@ defined chromosomes familiar from a karyotype . A small fraction of the cell 's genes are located instead in the mitochondria .
There are two types of chromatin . Euchromatin is the less compact DNA form , and contains genes that are frequently expressed by the cell . The other type , heterochromatin , is the more compact form , and contains DNA that is infrequently transcribed . This structure is further categorized into facultative heterochromatin , consisting of genes that are organized as heterochromatin only in certain cell types or at certain stages of development , and constitutive heterochromatin that consists of chromosome structural components such as telomeres and centromeres . During interphase the chromatin organizes itself into discrete individual patches , called chromosome territories . Active genes , which are generally found in the euchromatic region of the chromosome , tend to be located towards the chromosome 's territory boundary .
Antibodies to certain types of chromatin organization , in particular , nucleosomes , have been associated with a number of autoimmune diseases , such as systemic lupus erythematosus . These are known as anti @-@ nuclear antibodies ( ANA ) and have also been observed in concert with multiple sclerosis as part of general immune system dysfunction . As in the case of progeria , the role played by the antibodies in inducing the symptoms of autoimmune diseases is not obvious .
= = = Nucleolus = = =
The nucleolus is a discrete densely stained structure found in the nucleus . It is not surrounded by a membrane , and is sometimes called a suborganelle . It forms around tandem repeats of rDNA , DNA coding for ribosomal RNA ( rRNA ) . These regions are called nucleolar organizer regions ( NOR ) . The main roles of the nucleolus are to synthesize rRNA and assemble ribosomes . The structural cohesion of the nucleolus depends on its activity , as ribosomal assembly in the nucleolus results in the transient association of nucleolar components , facilitating further ribosomal assembly , and hence further association . This model is supported by observations that inactivation of rDNA results in intermingling of nucleolar structures .
In the first step of ribosome assembly , a protein called RNA polymerase I transcribes rDNA , which forms a large pre @-@ rRNA precursor . This is cleaved into the subunits 5.8S , 18S , and 28S rRNA . The transcription , post @-@ transcriptional processing , and assembly of rRNA occurs in the nucleolus , aided by small nucleolar RNA ( snoRNA ) molecules , some of which are derived from spliced introns from messenger RNAs encoding genes related to ribosomal function . The assembled ribosomal subunits are the largest structures passed through the nuclear pores .
When observed under the electron microscope , the nucleolus can be seen to consist of three distinguishable regions : the innermost fibrillar centers ( FCs ) , surrounded by the dense fibrillar component ( DFC ) , which in turn is bordered by the granular component ( GC ) . Transcription of the rDNA occurs either in the FC or at the FC @-@ DFC boundary , and , therefore , when rDNA transcription in the cell is increased , more FCs are detected . Most of the cleavage and modification of rRNAs occurs in the DFC , while the latter steps involving protein assembly onto the ribosomal subunits occur in the GC .
= = = Other subnuclear bodies = = =
Besides the nucleolus , the nucleus contains a number of other non @-@ membrane @-@ delineated bodies . These include Cajal bodies , Gemini of coiled bodies , polymorphic interphase karyosomal association ( PIKA ) , promyelocytic leukaemia ( PML ) bodies , paraspeckles , and splicing speckles . Although little is known about a number of these domains , they are significant in that they show that the nucleoplasm is not a uniform mixture , but rather contains organized functional subdomains .
Other subnuclear structures appear as part of abnormal disease processes . For example , the presence of small intranuclear rods has been reported in some cases of nemaline myopathy . This condition typically results from mutations in actin , and the rods themselves consist of mutant actin as well as other cytoskeletal proteins .
= = = = Cajal bodies and gems = = = =
A nucleus typically contains between 1 and 10 compact structures called Cajal bodies or coiled bodies ( CB ) , whose diameter measures between 0 @.@ 2 µm and 2 @.@ 0 µm depending on the cell type and species . When seen under an electron microscope , they resemble balls of tangled thread and are dense foci of distribution for the protein coilin . CBs are involved in a number of different roles relating to RNA processing , specifically small nucleolar RNA ( snoRNA ) and small nuclear RNA ( snRNA ) maturation , and histone mRNA modification .
Similar to Cajal bodies are Gemini of Cajal bodies , or gems , whose name is derived from the Gemini constellation in reference to their close " twin " relationship with CBs . Gems are similar in size and shape to CBs , and in fact are virtually indistinguishable under the microscope . Unlike CBs , gems do not contain small nuclear ribonucleoproteins ( snRNPs ) , but do contain a protein called survival of motor neuron ( SMN ) whose function relates to snRNP biogenesis . Gems are believed to assist CBs in snRNP biogenesis , though it has also been suggested from microscopy evidence that CBs and gems are different manifestations of the same structure . Later ultrastructural studies have shown gems to be twins of Cajal bodies with the difference being in the coilin component ; Cajal bodies are SMN positive and coilin positive , and gems are SMN positive and coilin negative .
= = = = RAFA and PTF domains = = = =
RAFA domains , or polymorphic interphase karyosomal associations , were first described in microscopy studies in 1991 . Their function remains unclear , though they were not thought to be associated with active DNA replication , transcription , or RNA processing . They have been found to often associate with discrete domains defined by dense localization of the transcription factor PTF , which promotes transcription of small nuclear RNA ( snRNA ) .
= = = = PML bodies = = = =
Promyelocytic leukaemia bodies ( PML bodies ) are spherical bodies found scattered throughout the nucleoplasm , measuring around 0 @.@ 1 – 1 @.@ 0 µm . They are known by a number of other names , including nuclear domain 10 ( ND10 ) , Kremer bodies , and PML oncogenic domains . PML bodies are named after one of their major components , the promyelocytic leukemia protein ( PML ) . They are often seen in the nucleus in association with Cajal bodies and cleavage bodies . PML bodies belong to the nuclear matrix , an ill @-@ defined super @-@ structure of the nucleus proposed to anchor and regulate many nuclear functions , including DNA replication , transcription , or epigenetic silencing . The PML protein is the key organizer of these domains that recruits an ever @-@ growing number of proteins , whose only common known feature to date is their ability to be SUMOylated . Yet , pml- / - mice ( which have their PML gene deleted ) cannot assemble nuclear bodies , develop normally and live well , demonstrating that PML bodies are dispensable for most basic biological functions .
= = = = Splicing speckles = = = =
Speckles are subnuclear structures that are enriched in pre @-@ messenger RNA splicing factors and are located in the interchromatin regions of the nucleoplasm of mammalian cells . At the fluorescence @-@ microscope level they appear as irregular , punctate structures , which vary in size and shape , and when examined by electron microscopy they are seen as clusters of interchromatin granules . Speckles are dynamic structures , and both their protein and RNA @-@ protein components can cycle continuously between speckles and other nuclear locations , including active transcription sites . Studies on the composition , structure and behaviour of speckles have provided a model for understanding the functional compartmentalization of the nucleus and the organization of the gene @-@ expression machinery splicing snRNPs and other splicing proteins necessary for pre @-@ mRNA processing . Because of a cell 's changing requirements , the composition and location of these bodies changes according to mRNA transcription and regulation via phosphorylation of specific proteins . The splicing speckles are also known as nuclear speckles ( nuclear specks ) , splicing factor compartments ( SF compartments ) , interchromatin granule clusters ( IGCs ) , B snurposomes . B snurposomes are found in the amphibian oocyte nuclei and in Drosophila melanogaster embryos . B snurposomes appear alone or attached to the Cajal bodies in the electron micrographs of the amphibian nuclei . IGCs function as storage sites for the splicing factors .
= = = = Paraspeckles = = = =
Discovered by Fox et al. in 2002 , paraspeckles are irregularly shaped compartments in the nucleus ' interchromatin space . First documented in HeLa cells , where there are generally 10 – 30 per nucleus , paraspeckles are now known to also exist in all human primary cells , transformed cell lines , and tissue sections . Their name is derived from their distribution in the nucleus ; the " para " is short for parallel and the " speckles " refers to the splicing speckles to which they are always in close proximity .
Paraspeckles are dynamic structures that are altered in response to changes in cellular metabolic activity . They are transcription dependent and in the absence of RNA Pol II transcription , the paraspeckle disappears and all of its associated protein components ( PSP1 , p54nrb , PSP2 , CFI ( m ) 68 , and PSF ) form a crescent shaped perinucleolar cap in the nucleolus . This phenomenon is demonstrated during the cell cycle . In the cell cycle , paraspeckles are present during interphase and during all of mitosis except for telophase . During telophase , when the two daughter nuclei are formed , there is no RNA Pol II transcription so the protein components instead form a perinucleolar cap .
= = = = Perichromatin fibrils = = = =
Perichromatin fibrils are visible only under electron microscope . They are located next to the transcriptionally active chromatin and are hypothesized to be the sites of active pre @-@ mRNA processing .