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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

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
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.
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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