If you took a DNA sample from someone's brain and that same persons liver at more or less the same time, would that DNA - all else being equal (e.g. no mutations from radiation) - be exactly the same?
If you took a DNA sample from an embryo, and then that embryo went on to be born, live a long life for 80 years and become an old man/woman; if you were to then take another DNA sample of this person, would that sample be exactly the same as the one which you took when he/she was in the womb? (again, all things being equal - so dismissing mutations due to radiation for example).
No. even if you exclude the changes in DNA sequences that originate from mismatch errors in DNA replication, mutation etc. Because
1.As a person gets old , the short repeat sequences in the telomere of a chromosome in each cell may be lost or be reduced in amounts leading to a sequence difference.Moreover this changes in telomere sequences may cause cancer/cell apoptosis . Telomere
2.Do not forget transposons (consisted of DNA transposons and retrotransposons). They change their position in genome and multiply their DNA over time.So they also lead to sequence differences in a person(comparing his/her younger and older DNA) Transposons These were the answers to question 2 .You can also think that there might be a slight difference in DNA of two different tissue of the same individual (because of the tolemere and transposons)
As a zygote starts to multiply itself ,slight changes of sequence will happen to the offspring cells .After a dozens of cell division you may encounter a little deviation compared to the original zygote sequence.