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Dna structure

At this point we now have a good picture of the chemical structure of the DNA molecule, now we need to begin placing it in the context of the cell. A typical eukaryotic chromosome contains from 1 to 20 cm of DNA. However, during metaphase of mitosis and meiosis, this DNA is packaged in a chromosome with a length of only 1 to 10 um. How is this amazing density achieved inside the cell?

DNA in the cell exists packed into a dense and regular structure called chromatin. Chromatin is composed of DNA, proteins, and a small amount of RNA. The proteins found in chromatin largely consist of histones, a basic protein which is positively charged at neutral pH, and nonhistone chromosomal proteins which are largely acidic at neutral pH. Histones have been highly conserved in all eukaryotes. There are five major histone types, called H1, H2a, H2b, H3, and H4, and which exist in specific molar ratios within the chromatin. Histones bind together with the DNA to form the basic structural subunit of chromatic, small ellipsoidal beads called nucleosomes which are around 11nm in diameter and 6nm high. Each nucleosome contains 146 nucleotide pairs which wrap around the histon protein complex 1 and 3/4 turns. The nucleosome complexes give the DNA molecula a packaging ratio of 6.

Histones

Beyond the nucleosome, there are two more levels of structural packaging. The second level of packing is the coiling of the nucleosome beads into a helical structure called the 30 nm fiber that is found in both interphase chromatin and mitotic chromosomes. This structure increases the packing ratio to about 40. The final packaging occurs when the fiber is organized in loops, scaffolds and domains that give a final packing ratio of about 1000 in interphase chromosomes and about 10,000 in mitotic chromosomes.

One important note is that DNA is not always packed into the super-dense chromosome structures evident during mitotic and meiotic replication. During interphase, or the general not-currently-reproducing phase of the cell where most of a cell's work is done, the chromatin, while still highly dense, is about 1/10 as dense as during cellular replication. This is important because it is believed that the highly-dense chromatic structure of DNA sterically inhibits transcription and thus gene expression. In order for genes to be expressed the chromatin structure must be relaxed so that the transcriptional proteins can gain access to the DNA molecule.

Now that we have a good grasp on the basic structure of DNA as a molecule, as well as in vivo, lets move on to the mechanisms of gene expression. The Central Dogma of genetics is: DNA is transcribed to RNA which is translated to protein. Protein is never back-translated to RNA or DNA, and except for retroviruses, DNA is never created from RNA. Furthermore, DNA is never directly translated to protein. DNA to RNA to protein.

DNA is the long term, stable, hard-copy of the genetic material; by way of analogy it is similar to the information on a computers hard-disk drive. RNA is a temporary intermediary between the DNA and the protein making factories, the ribosomes. To further extend our computer analogy, RNA could be compared to information in a cache, in that the lifetime of RNA is much shorter than that of either DNA or the average protein, and that RNA serves to carry information from the genome, located in the nucleus of the cell, to the ribosomes, which are located outside of the nucleus either in the cytosol or on the endoplasmic reticulum (which is a large set of folded membranes proximal to the nucleus that help manufacture proteins for extra-cellular export). To complete our analogy, proteins could be viewed as the programs of the cell. They are the physical representation of the abstract information contained within the genome. However, one caveat is that RNA does have some enzymatic activity and has other functions besides ferrying messages between the DNA and the ribosomes.

Questions & Answers

how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
can nanotechnology change the direction of the face of the world
Prasenjit Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Genefinding. OpenStax CNX. Jun 17, 2003 Download for free at http://cnx.org/content/col10205/1.1
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