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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.


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

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
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
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
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.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar 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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