IdeaWazaWiki - User contributions [en]

Archive:Hoag's Object

2007-08-26T03:43:16Z

68.190.6.197:

{{Galaxy
| name = Hoag's Object
| image = [[Image:Hoag'sobject.jpg|300px]]<br />
<small>Hoag's Object taken by the [[Hubble Space Telescope]].<br>
Courtesy of [[NASA]]/[[ESA]]</small>
| epoch = [[J2000]]
| ra = {{RA|15|17|14.4}}<ref name="ned">{{cite web
| title=NASA/IPAC Extragalactic Database
| work=Results for Hoag's Object
| url=http://nedwww.ipac.caltech.edu/
| accessdate=2006-12-21}}</ref>
| dec = {{DEC|+21|35|08}}<ref name="ned" />
| constellation name = [[Serpens Caput]]
| z = 12,740 ± 50 [[kilometer|km]]/[[second|s]]<ref name="OConnelletal1974">{{cite journal
| author=O'Connell, Robert W.; Scargle, Jeffrey D.; Sargent, W. L. W.
| title=The Nature of Hoag's Object . .
| journal=Astrophysical Journal
| year=1974
| volume=191
| pages=61-62
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...191...61O }}</ref><ref name="ned" />
| dist_ly = 600 ± 30 [[light-year|Mly]] (183 ± 8 [[parsec|Mpc]])<ref name="OConnelletal1974" />{{Ref_label|A|a|none}}
| type = SA0(r){{Fact|date=February 2007}}
| appmag_v = 16.0<ref name="ned" />
| size_v = 0′.28 × 0′.28<ref name="ned" />
| notes = Ring galaxy
| names = [[Principal Galaxies Catalogue|PGC]] 54559,<ref name="ned" /> PRC D-51<ref name="ned" />
}}

'''Hoag's Object''' is a non-typical [[galaxy]] of the type known as a [[ring galaxy]]. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy was discovered in [[1950]] by astronomer [[Art Hoag]] who identified it as either a [[planetary nebula]] or a [[peculiar galaxy]].<ref name="Hoag1950">{{cite journal
| author=Hoag, Arthur A.
| title=A peculiar object in Serpens
| journal=Astronomical Journal
| year=1950
| volume=55
| pages=170
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1950AJ.....55Q.170H
}}</ref>

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circle the older yellow nucleus of this ring galaxy ~600 million [[light-year]]s away in the constellation [[Serpens]]. The diameter of the 6″ inner core of the galaxy is about 17 ± 0.7 kly (5.3 ± 0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75 ± 3 kly (24.8 ± 1.1 kpc) and an outer 45″ diameter of 121 ± 4 kly (39.9 ± 1.7 kpc), which is slightly larger than the [[Milky Way Galaxy]].<ref name="OConnelletal1974" />{{Ref_label|A|a|none}} The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. As rare as this type of galaxy is, oddly another more distant ring galaxy can be seen, between the nucleus and the outer ring at the 0100 (one o'clock) position.

==History and formation==
Interestingly, even though Hoag's Object was clearly shown on the [[Palomar Star Survey|Palomar ''Star Survey'']], it was not included in either the ''[[Morphological Catalogue of Galaxies]]'', the ''[[Catalogue of Galaxies and Clusters of Galaxies]]'', nor the ''[[Catalogue of Galactic Planetary Nebulae]]''.<ref name="OConnelletal1974" />

In the initial announcement of his discovery, Art Hoag proposed the hypothesis that the visible ring was a product of [[gravitational lensing]]. This idea was later discarded due to the fact that the nucleus and the ring have the same [[redshift]], and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.<ref name="Schweizeretal1987">{{cite journal
| author=Schweizer, Francois; Ford, W. Kent, Jr.; Jederzejewski, Robert; Giovanelli, Riccardo
| title=The structure and evolution of Hoag's object
| journal=Astrophysical Journal, Part 1 (ISSN 0004-637X)
| year=[[September 15]] [[1987]]
| volume=320
| pages=454-463
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987ApJ...320..454S
}}</ref>

Many of the details of the galaxy remain a mystery, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.

It has been suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a [[barred spiral galaxy]]. Schweizer, et al. (1987)<ref name="Schweizeretal1987" /> claim that this is an unlikely hypothesis due to the fact that the nucleus of the object is spheroidal, whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily. Interestingly, a few galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.

While there is much still to be explained about the formation of Hoag's Object, it is suggested that a major "accretion event", in other words a collision or a capturing of one galaxy by another, caused the formation of the galaxy we see today. The event likely took place at least 2-3 thousand years in the past, and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]].

==External links==

* [http://heritage.stsci.edu/2002/21/big.html Hubble Space Telescope image of Hoag's Object.]
* http://nedwww.ipac.caltech.edu/level5/Rings/Rings17_2.html
* http://oposite.stsci.edu/pubinfo/PR/2002/21/index.html
*[http://www.spacetelescope.org/images/html/opo0221a.html ESA/Hubble image of Hoag's Object]

== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}1974 O'Connell paper<ref name="OConnelletal1974" /> assumes a [[Hubble constant]] of 75 this figure adjusts for 2006's 70{{±|2.4|3.2}} (km/s)/Mpc.</li>
</ol>
</div>

==References==

<div class="references-small"><references/></div>

[[Category:Ring galaxies]]
[[Category:Serpens constellation]]
[[Category:PGC objects|54559]]

[[de:Hoags Objekt]]
[[fr:Objet de Hoag]]
[[lb:Hoag's Objet]]
[[sk:Hoagov objekt]]

Archive:Hoag's Object

2007-08-26T03:41:59Z

68.190.6.197:

{{Galaxy
| name = Hoag's Object
| image = [[Image:Hoag'sobject.jpg|300px]]<br />
<small>Hoag's Object taken by the [[Hubble Space Telescope]].<br>
Courtesy of [[NASA]]/[[ESA]]</small>
| epoch = [[J2000]]
| ra = {{RA|15|17|14.4}}<ref name="ned">{{cite web
| title=NASA/IPAC Extragalactic Database
| work=Results for Hoag's Object
| url=http://nedwww.ipac.caltech.edu/
| accessdate=2006-12-21}}</ref>
| dec = {{DEC|+21|35|08}}<ref name="ned" />
| constellation name = [[Serpens Caput]]
| z = 12,740 ± 50 [[kilometer|km]]/[[second|s]]<ref name="OConnelletal1974">{{cite journal
| author=O'Connell, Robert W.; Scargle, Jeffrey D.; Sargent, W. L. W.
| title=The Nature of Hoag's Object . .
| journal=Astrophysical Journal
| year=1974
| volume=191
| pages=61-62
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...191...61O }}</ref><ref name="ned" />
| dist_ly = 600 ± 30 [[light-year|Mly]] (183 ± 8 [[parsec|Mpc]])<ref name="OConnelletal1974" />{{Ref_label|A|a|none}}
| type = SA0(r){{Fact|date=February 2007}}
| appmag_v = 16.0<ref name="ned" />
| size_v = 0′.28 × 0′.28<ref name="ned" />
| notes = Ring galaxy
| names = [[Principal Galaxies Catalogue|PGC]] 54559,<ref name="ned" /> PRC D-51<ref name="ned" />
}}

'''Hoag's Object''' is a non-typical [[galaxy]] of the type known as a [[ring galaxy]]. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy was discovered in [[1950]] by astronomer [[Art Hoag]] who identified it as either a [[planetary nebula]] or a [[peculiar galaxy]].<ref name="Hoag1950">{{cite journal
| author=Hoag, Arthur A.
| title=A peculiar object in Serpens
| journal=Astronomical Journal
| year=1950
| volume=55
| pages=170
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1950AJ.....55Q.170H
}}</ref>

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circle the older yellow nucleus of this ring galaxy ~600 million [[light-year]]s away in the constellation [[Serpens]]. The diameter of the 6″ inner core of the galaxy is about 17 ± 0.7 kly (5.3 ± 0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75 ± 3 kly (24.8 ± 1.1 kpc) and an outer 45″ diameter of 121 ± 4 kly (39.9 ± 1.7 kpc), which is slightly larger than the [[Milky Way Galaxy]].<ref name="OConnelletal1974" />{{Ref_label|A|a|none}} The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. As rare as this type of galaxy is, oddly another more distant ring galaxy can be seen, between the nucleus and the outer ring at the 0100 (one o'clock) position.

==History and formation==
Interestingly, even though Hoag's Object was clearly shown on the [[Palomar Star Survey|Palomar ''Star Survey'']], it was not included in either the ''[[Morphological Catalogue of Galaxies]]'', the ''[[Catalogue of Galaxies and Clusters of Galaxies]]'', nor the ''[[Catalogue of Galactic Planetary Nebulae]]''.<ref name="OConnelletal1974" />

In the initial announcement of his discovery, Art Hoag proposed the hypothesis that the visible ring was a product of [[gravitational lensing]]. This idea was later discarded due to the fact that the nucleus and the ring have the same [[redshift]], and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.<ref name="Schweizeretal1987">{{cite journal
| author=Schweizer, Francois; Ford, W. Kent, Jr.; Jederzejewski, Robert; Giovanelli, Riccardo
| title=The structure and evolution of Hoag's object
| journal=Astrophysical Journal, Part 1 (ISSN 0004-637X)
| year=[[September 15]] [[1987]]
| volume=320
| pages=454-463
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987ApJ...320..454S
}}</ref>

Many of the details of the galaxy remain a mystery, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.

It has been suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a [[barred spiral galaxy]]. Schweizer, et al. (1987)<ref name="Schweizeretal1987" /> claim that this is an unlikely hypothesis due to the fact that the nucleus of the object is spheroidal, whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily. Interestingly, a few galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.

While there is much still to be explained about the formation of Hoag's Object, it is suggested that a major "accretion event", in other words a collision or a capturing of one galaxy by another, caused the formation of the galaxy we see today. The event likely took place at least 2-3 billion years in the past, and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]]. Or you could believe that the universe is not old at all, and that God made a galaxy like this just to screw up everyone's theories to prove that none of this happened by chance.

==External links==

* [http://heritage.stsci.edu/2002/21/big.html Hubble Space Telescope image of Hoag's Object.]
* http://nedwww.ipac.caltech.edu/level5/Rings/Rings17_2.html
* http://oposite.stsci.edu/pubinfo/PR/2002/21/index.html
*[http://www.spacetelescope.org/images/html/opo0221a.html ESA/Hubble image of Hoag's Object]

== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}1974 O'Connell paper<ref name="OConnelletal1974" /> assumes a [[Hubble constant]] of 75 this figure adjusts for 2006's 70{{±|2.4|3.2}} (km/s)/Mpc.</li>
</ol>
</div>

==References==

<div class="references-small"><references/></div>

[[Category:Ring galaxies]]
[[Category:Serpens constellation]]
[[Category:PGC objects|54559]]

[[de:Hoags Objekt]]
[[fr:Objet de Hoag]]
[[lb:Hoag's Objet]]
[[sk:Hoagov objekt]]

Archive:Hoag's Object

2007-08-26T03:41:29Z

68.190.6.197:

{{Galaxy
| name = Hoag's Object
| image = [[Image:Hoag'sobject.jpg|300px]]<br />
<small>Hoag's Object taken by the [[Hubble Space Telescope]].<br>
Courtesy of [[NASA]]/[[ESA]]</small>
| epoch = [[J2000]]
| ra = {{RA|15|17|14.4}}<ref name="ned">{{cite web
| title=NASA/IPAC Extragalactic Database
| work=Results for Hoag's Object
| url=http://nedwww.ipac.caltech.edu/
| accessdate=2006-12-21}}</ref>
| dec = {{DEC|+21|35|08}}<ref name="ned" />
| constellation name = [[Serpens Caput]]
| z = 12,740 ± 50 [[kilometer|km]]/[[second|s]]<ref name="OConnelletal1974">{{cite journal
| author=O'Connell, Robert W.; Scargle, Jeffrey D.; Sargent, W. L. W.
| title=The Nature of Hoag's Object . .
| journal=Astrophysical Journal
| year=1974
| volume=191
| pages=61-62
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...191...61O }}</ref><ref name="ned" />
| dist_ly = 600 ± 30 [[light-year|Mly]] (183 ± 8 [[parsec|Mpc]])<ref name="OConnelletal1974" />{{Ref_label|A|a|none}}
| type = SA0(r){{Fact|date=February 2007}}
| appmag_v = 16.0<ref name="ned" />
| size_v = 0′.28 × 0′.28<ref name="ned" />
| notes = Ring galaxy
| names = [[Principal Galaxies Catalogue|PGC]] 54559,<ref name="ned" /> PRC D-51<ref name="ned" />
}}

'''Hoag's Object''' is a non-typical [[galaxy]] of the type known as a [[ring galaxy]]. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy was discovered in [[1950]] by astronomer [[Art Hoag]] who identified it as either a [[planetary nebula]] or a [[peculiar galaxy]].<ref name="Hoag1950">{{cite journal
| author=Hoag, Arthur A.
| title=A peculiar object in Serpens
| journal=Astronomical Journal
| year=1950
| volume=55
| pages=170
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1950AJ.....55Q.170H
}}</ref>

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circle the older yellow nucleus of this ring galaxy ~600 million [[light-year]]s away in the constellation [[Serpens]]. The diameter of the 6″ inner core of the galaxy is about 17 ± 0.7 kly (5.3 ± 0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75 ± 3 kly (24.8 ± 1.1 kpc) and an outer 45″ diameter of 121 ± 4 kly (39.9 ± 1.7 kpc), which is slightly larger than the [[Milky Way Galaxy]].<ref name="OConnelletal1974" />{{Ref_label|A|a|none}} The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. As rare as this type of galaxy is, oddly another more distant ring galaxy can be seen, between the nucleus and the outer ring at the 0100 (one o'clock) position.

==History and formation==
Interestingly, even though Hoag's Object was clearly shown on the [[Palomar Star Survey|Palomar ''Star Survey'']], it was not included in either the ''[[Morphological Catalogue of Galaxies]]'', the ''[[Catalogue of Galaxies and Clusters of Galaxies]]'', nor the ''[[Catalogue of Galactic Planetary Nebulae]]''.<ref name="OConnelletal1974" />

In the initial announcement of his discovery, Art Hoag proposed the hypothesis that the visible ring was a product of [[gravitational lensing]]. This idea was later discarded due to the fact that the nucleus and the ring have the same [[redshift]], and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.<ref name="Schweizeretal1987">{{cite journal
| author=Schweizer, Francois; Ford, W. Kent, Jr.; Jederzejewski, Robert; Giovanelli, Riccardo
| title=The structure and evolution of Hoag's object
| journal=Astrophysical Journal, Part 1 (ISSN 0004-637X)
| year=[[September 15]] [[1987]]
| volume=320
| pages=454-463
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987ApJ...320..454S
}}</ref>

Many of the details of the galaxy remain a mystery, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.

It has been suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a [[barred spiral galaxy]]. Schweizer, et al. (1987)<ref name="Schweizeretal1987" /> claim that this is an unlikely hypothesis due to the fact that the nucleus of the object is spheroidal, whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily. Interestingly, a few galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.

While there is much still to be explained about the formation of Hoag's Object, it is suggested that a major "accretion event", in other words a collision or a capturing of one galaxy by another, caused the formation of the galaxy we see today. The event likely took place at least 2-3 billion years in the past, and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]]. Or you could believe that the universe is not old at all, and that God made a galaxy like this just to screw up everyone's theories to prove that non of this happened by chance.

==External links==

* [http://heritage.stsci.edu/2002/21/big.html Hubble Space Telescope image of Hoag's Object.]
* http://nedwww.ipac.caltech.edu/level5/Rings/Rings17_2.html
* http://oposite.stsci.edu/pubinfo/PR/2002/21/index.html
*[http://www.spacetelescope.org/images/html/opo0221a.html ESA/Hubble image of Hoag's Object]

== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}1974 O'Connell paper<ref name="OConnelletal1974" /> assumes a [[Hubble constant]] of 75 this figure adjusts for 2006's 70{{±|2.4|3.2}} (km/s)/Mpc.</li>
</ol>
</div>

==References==

<div class="references-small"><references/></div>

[[Category:Ring galaxies]]
[[Category:Serpens constellation]]
[[Category:PGC objects|54559]]

[[de:Hoags Objekt]]
[[fr:Objet de Hoag]]
[[lb:Hoag's Objet]]
[[sk:Hoagov objekt]]

Eukaryotic transcription

2006-10-23T04:42:37Z

68.190.180.186: /* Searching for a full understanding of transcription */

[[Image:Label RNA pol II.png|thumb|right|200px|'''Figure 2'''. Eukaryotic RNA polymerase II in a complex with DNA and mRNA <ref name="Gnatt2001">[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=11313499 Structural basis of transcription: an RNA polymerase II elongation complex at 3.3 A resolution] by Averell L. Gnatt, Patrick Cramer, Jianhua Fu, David A. Bushnell and Roger D. Kornberg in [[w:Science (journal)|Science]] (2001) Volume 292, pages 1876-1882.</ref>.]]

The Nobel Prize in Chemistry in 2006 was awarded to [[w:Roger D. Kornberg|Roger D. Kornberg]] for his work on the molecular basis of '''eukaryotic transcription''' <ref name=Nobel2006">2006 award at the [http://nobelprize.org/nobel_prizes/chemistry/laureates/2006/index.html Nobel Prize website].</ref>.

This [[Portal:Learning Projects|learning project]] allows exploration of scientific research that is related to 2006 Nobel Prize in Chemistry. If you have questions, leave them on the [[Talk:Eukaryotic transcription|discussion page]].

==Transcription==
[[Image:Information flow.png|thumb|left|386px|'''Figure 1'''. Information flow in living cells from DNA to RNA to protein.]]
[[w:Eukaryotic|Eukaryotic]] organisms such as humans store their genetic information in the structure of DNA molecules. Most of the genetic instructions in genes are used to specify the structures of proteins (Figure 1). The many proteins of cells function as molecular machines that produce the living state. Intermediate between DNA and protein is RNA. The process by which cells convert the structure of a gene into the structure of a mRNA molecule is called transcription. In eukaryotes, a large molecular complex of specialized proteins is required to achieve the careful control of which subset of genes is transcribed in each type of cell. At the core of this complex is the [[w:Enzyme|enzyme]] RNA polymerase II.

Roger Kornberg's laboratory has used biochemical techniques to isolate the large complex of proteins that allow RNA polymerase II to produce messenger RNA. Working with [[w:Yeast|yeast]] cells as an experimental system, Kornberg's research team has isolated and purified the functioning RNA pol II protein complex with attached template
[[w:DNA|DNA]], product [[w:mRNA|mRNA]] and substrate [[w:Nucleotide|nucleotides]] and captured images of the complex using [[w:Electron microscope|electron microscopy]] and [[w:X-ray crystallography|X-ray crystallography]] <ref name="Thelander2006">Advanced information on the Nobel Prize in Chemistry 2006: [http://www.kva.se/KVA_Root/files/newspics/DOC_200610494343_28494252634_chemadv06.pdf Molecular basis of eukaryotic transcription] by Lars Thelander.</ref> (see Figure 2)

[[Image:Phylogenetic_tree.svg|thumb|left|300px|'''Figure 3'''. A [[w:phylogenetic tree|phylogenetic tree]] of living things, based on [[w:rRNA|rRNA]] sequence data, showing the separation of [[w:bacterium|bacteria]], [[w:archaea|archaea]], and [[w:eukaryote|eukaryotes]] <ref name="Woese1990">"Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya" by C. R. Woese, O. Kandler, and M. L. Wheelis in [[w:Proceedings of the National Academy of Sciences|Proceedings of the National Academy of Sciences U.S.A.]] (1990) Volume 87, pages 4576-4579. [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=2112744 Full text online].</ref>.]]
==General transcription factors==
There are three major forms of life on Earth, bacteria, archaea and eukaryotes (Figure 3). RNA polymerase in bacteria is less complex than RNA polymerase in eukaryotes. Some of the increased complexity of RNA polymerase in eukaryotes reflects differences between DNA in eukaryotes and DNA in bacteria. Two important differences are that eukaryotes organize their DNA into [[w:Nucleosome|nucleosomes]] and have more complex mechanisms for regulation of gene transcription.

[[Image:Nucleosome rasmol.JPG|thumb|right|'''Figure 4'''. The DNA of eukaryotes wraps around histone proteins forming nucleosomes.
<ref> McDonald D, "Milestone 9, (1973-1974) [http://www.nature.com/milestones/geneexpression/milestones/articles/milegene09.html The nucleosome hypothesis: An alternative string theory], Nature Milestones: Gene Expression. (2005)</ref>.]]
Nucleosomes are a complex of DNA and [[w:Histone|histone]] proteins (Figure 4). In order for transcription to occur, DNA must be released from being tightly coiled in nucleosomes. Bacteria do not have nucleosomes. Another complication of eukaryotic gene expression regulation is that gene sequences controlling transcription are often distant from the DNA site where transcription starts. The [[w:RNA polymerase#RNA polymerase in bacteria|RNA polymerase of bacteria]] is relatively small with a core of five protein subunits and one additional protein that recognizes the start points for transcription<ref name="">[http://www.pubmedcentral.gov/pagerender.fcgi?tool=pmcentrez&pageindex=1&artid=38558 The sigma subunit of Escherichia coli RNA polymerase senses promoter spacing] by A. J. Dombroski, B. D. Johnson, M. Lonetto, and C. A. Gross in [[w:Proceedings of the National Academy of Sciences|Proceedings of the National Academy of Sciences U.S.A.]] (1996) Volume 93, pages 8858–8862.</ref>. In contrast, RNA polymerase II of yeast (Baker’s yeast [[w:Saccharomyces cerevisiae|Saccharomyces cerevisiae]]) has 12 protein subunits<ref name="Gnatt2001" /> and requires five [[w:General transcription factor|general transcription factor]] proteins (TFIIB, D, E, F and H). The general transcription factors are complex, for example, TFIIH has at least six protein subunits in various eukaryotic organisms from yeast to mammals.

==Searching for a full understanding of transcription==
As discussed above, Kornberg has made important contributions to on-going attempts to discover the full complexity of transcription in eykaryotes.In addition to the polymerase core and its associated general transcription factors, another large protein complex called Mediator is involved in the control of RNA polymerase II <ref name="Kim1994">"A multiprotein mediator of transcriptional activation and its interaction with the C-terminal repeat domain of RNA polymerase II" by Y. J. Kim, S. Bjorklund, Y. Li,, M. H. Sayre and R. D. Kornberg in [[w:Cell (journal)|Cell]] (1994) Volume 77, pages 599-608.</ref>. In some cells, Mediator is about 4 times larger than the RNA polymerase core complex (with as many as 20 different protein subunits) and is important for transmitting the effects of positive and negative regulators of gene transcription (often quite distant from the transcription start site) to the core polymerase. Efforts continue to reveal the details of how the Mediator complex functions.

==Learning project; where to next?==
Explore one of the following questions and then describe what you learn on this page:
*How many types of [[w:RNA polymerase|RNA polymerase]] are there in humans and what does each type do?
*What is known about transcription in archaea? ([http://www.jbc.org/cgi/reprint/M605209200v1.pdf hint])
*How do [[w:HIV|HIV]] proteins interaction with RNA polymerase II and induce transcription from the HIV-1 promoter? (hint: [http://www.ncbi.nlm.nih.gov search here])

==References==
<references/>

==See also==
*This lesson on eukaryotic transcription is part of the [[Nobel Prize in Chemistry]] learning project.
*The [[Nobel Prize in Physiology or Medicine]] learning project.

==External links==
*[[w:RNA polymerase II|RNA polymerase II]] article at Wikipedia.

[[Category:Nobel Prize in Chemistry]]