To get a more physical view of what's going on with DNA, and molecular genetics go here:
http://en.wikipedia.org/wiki/Chromatin
Blow the figure up enough to read the labels:
http://en.wikipedia.org/wiki/File:Chromatin_Structures.png
The words chromatin structure or chromatin template includes all of the above. On the far right we start with a DNA strand double helix coil.
Notice the words ``Add core histones''. Histones are small protein molecules that have a charged exterior that attaches to a section of the double helix. They create beads on the string and these units are called Nucleosomes. There are not a single wrap as shown, but usually multiple wraps.
Histone H1, H1, etc also has an electrically charged surface and it activates the histone to rotate, stick together into the slacks of with a DNA string wrap. The histone disks rotate and pull together into unit stacks.
The next packing step, ``add further scaffold proteins'' isn't well illustrated. The unit stacks leave unwrapped DNA strings between them. These strings and stacks are wrapped around the filament scaffold proteins in snake like fashion to form a bracelet. On chemical activation the scaffold filaments constrict together in coils to form an irregular shaped column. The column is the chromosome.
During RNA transcription, sections of this column are unwrapped, and exposed DNA strands are transcribed. This transcription process includes opening an exposed DNA strands, encoding the RNA template, then putting single DNA strand back together, then re-attaching the base pairs to form the double helix.
Although the illustration shows DNA as a strand, this is a simplification. The double strand is further looped together in segmented fashion by external binding proteins to form loops and clumps. In addition there are variously shaped attachments on the outside rim of the strand. One class of such attachments are referred to as DNA methylation. There are also attachments to the histone nuggets the DNA is wrapped around.
Go here for the picture of attachments to DNA and the Histone:
http://embryology.med.unsw.edu.au/MolDev/epigenetic.htm
Hill does a nice job of explaining what's going on
These attachments compose part of the epigenome. They technically are not part of the DNA code itself, but they suppress and modify or alter the activity of the RNA transcription process---just as if they were part of the DNA code. Here is a quote:
``In the mammalian lifecycle, the two periods of genome-wide epigenetic reprogramming are in the early embryo, when somatic patterns are set, and during germ cell development. ...we find that the methylation status of testicular DNA is highly distinct, displaying eightfold the number of hypomethylated loci relative to somatic tissues. .. These special properties of testicular DNA point to a broad, distinct epigenetic state that may be involved in maintaining a unique chromosomal structure in male germ cells.''
It needs to be stressed that the methylated DNA and attachments to histone and other epigenetic features are `normal'. It is thought that `abnormal' alterations of this epigenome are linked to `disease' of both a putatively genetic origin and more particularly alterations in the phenotypic expression of various cell lines over the course of the individual's lifetime. This explains why in studies of monozygote twins, one twin gets a disease like cancer and the other doesn't. It also explains why some diseases run in families.
Now something in the environment and behavior has changed large masses of people in the US and elsewhere to generate an epidemic of fat. IMHO whatever those changes are, they have likely effected the epigenetic systems of the people who struggle with their weight. Here is the relevant quote:
``You are what your mother eats, a study in mice shows that maternal diet can have transmittable epigenetic effects on the future offspring.
`Thus a mother's diet may have an enduring influence on succeeding generations, independent of later changes in diet. Although other reports have suggested such heritable epigenetic changes, this study demonstrates that a specific mammalian gene can be subjected to germ- line epigenetic change.' ''
To follow this up I found this:
http://www.neuroscience.cam.ac.uk/research/cameos/ObeseBrain.php
``...identical twins who are brought up in different families, will usually have very similar amounts, and distribution, of body fat in adult life, bearing little resemblance to the families into which they were adopted.''
Which means that you can eat `normally' and still get fat. I blame high fructose corn syrup, HBCS, livestock, poultry feeding systems that rely heavily on corn and include various hormones and three decades of bad regulation on the food industry.
CG
