2+ DNA tests on same individual ?

[Externet]

Hello.

If a DNA sequence is taken from some tissue of a creature -say liver- is compared to another tissue -say kidney- of the same;  are they identical ?  Should they be ?

[CharonY]

4 minutes ago, Externet said:

Hello.

If a DNA sequence is taken from some tissue of a creature -say liver- is compared to another tissue -say kidney- of the same;  are they identical ?  Should they be ?

Yes they would be identical with the caveat that individual cells might have mutations. But unless you sequence individual cells, these won't show up in your final reads.

[Externet]

2 minutes ago, CharonY said:

unless you sequence individual cells

Thank you.  Does it mean individual cells of the same tissue/organ ?

4 minutes ago, CharonY said:

these won't show up in your final reads.

That I do not understand,  Would you please re-phrase ?

[CharonY]

Just now, Externet said:

Thank you.  Does it mean individual cells of the same tissue/organ ?

Would not matter. Cell can have small mutations due to errors in DNA replication or some aging effects. So if you take a few million cells from anywhere in your body, you will have quite a few cells that have an error here or there. Sometimes they lead to distinct phenotype. E.g. cancer cells usually carry quite a few mutations that makes them cancerous, but most will likely do nothing.

2 minutes ago, Externet said:

That I do not understand,  Would you please re-phrase ?

There are different techniques to sequence DNA, but most basically rely on reading out the sequences from a pool of DNA that you have isolated. This pool usually is derived from a mass of cells (e.g. bit of tissue). I.e. in most techniques you do not sequence a whole DNA molecule but bits and fragments derived from this pool.

Say you got in your sample one cell that has one mutation at a specific site, but in addition a million cells that do not carry this mutation, the likelihood of finding specific that mutated sequence is very low. 

[Genady]

If a mutation occurs in a cell of tissue A during early development, soon after it differentiated from cells of other tissues, wouldn't most or all of its daughter cells, which could be most or all of the tissue A cells, carry this mutation and thus have DNA different from DNA in other tissues? 

[CharonY]

51 minutes ago, Genady said:

If a mutation occurs in a cell of tissue A during early development, soon after it differentiated from cells of other tissues, wouldn't most or all of its daughter cells, which could be most or all of the tissue A cells, carry this mutation and thus have DNA different from DNA in other tissues? 

Mutations in early development could do that. But in early development tissues are not that clearly separated, either. I.e. we have a lot pluri-and totipotent cells doing the heavy lifting so, I do not think that mutations would necessarily be neatly contained (though it is not impossible, either).

[Sensei]

Physically, the molecules are practically unique. i.e. if there is not a single change in the atoms arrangement, there are still differences at nuclear level, Hydrogen occurs with several different isotopes (H-1, H-2, H-3), Carbon (C-12, C-13, C-14), Oxygen (O-16, O-17, O-18), Nitrogen (N-14, N-15).. The number of permutations in DNA is astronomical.. Of course, basic DNA test won't show this..

Targeted enrichment can be used in research:

https://www.nature.com/articles/s41598-017-04404-2

https://www.google.com/search?q=targeted+enrichment