How do we know each cell has the same DNA?

Question

Since the discovery by Watson and Crick of DNA, how have we been able to show that all cells have the same? For example we know that human gametes have longer telomeres and only half the chromosomes, we know chimeras can have different sets of DNA and we also have a microbiome that exceeds the number of human cells. Did projects try to sequence cell DNA from many parts of the human body and conclude there are no significant variations?I mean it seems a bit too convenient and it&rsquo;s hard to prove a negative. Based on what we have found in other areas of bioinformatics can&rsquo;t we expect the situation to be a lot more complex than previously assumed?

nabla9 · Accepted Answer

We know that not every cell has the same DNA.
First are the well known exceptions like sperm and eggs, B cells and mature red blood cells. B cells have altered DNA to make antibodies. Then there are less known small differences discovered from tissue samples around the body. There can exist several non-disease minority forms that are selected for the conditions.
But but for the 99% of cases each cell has the same DNA, random tissue sample gives match sans mutations and diseases (like cancer).

drocer88 · Answer

The DNA in a human is not the same in all cells. Here's a paper : https://www.pnas.org/content/109/44/18018"Somatic mosaicism is the presence of genetically distinct populations of somatic cells within an organism and even within the same tissue. Somatic rearrangements in various cancer cell types are well documented, and more than 30 Mendelian diseases have been associated with somatic mosaicism (13). Somatic mosaicism is well known to occur in normal germline cells as the process of meiotic recombination and in cells of the immune system within Ig and T-cell receptor genes as a result of V(D)J recombination that provides diversity in immune responses. Other studies have revealed age-related structural variation in human blood cells (14), aneuploidy and retrotransposition in the human brain (15, 16), and aneuploidy in human preimplantation embryos (17). Evidence of copy number variation (CNV) in monozygotic twins (18) also reveals the presence of somatic mosaicism in tissues arising from the same zygote."

rolph · Answer

DNA is very often damaged or modified by environmental as well as physiological factors, and is then "corrected" so that DNA is in flux at the primary sequence, but has a central tendency that is chased.
DNA is also wound into a physical conformation that is also in flux, as well as the complexing of DNA whith protiens and other regions of DNA, this is the foundation of our current understanding of epigenetics.
the spatial distribution of DNA in the nucleus also is dynamic, the tendency is for DNA segments coding for transcripts that are to be exported from the nucleus are in proximity to nuclear membrane and a nuclear pore.
these factors vary throughout the various cell and tissue types of the body and over different physiological states.