DNA Sequence

[Ajay]

Hi ...

I got a basic doubt . I jus want to know wether the entire Dna sequence present in each cell in the body is the same . I mean specifically the entire Dna sequence not the gene sequence.

AJAY

[MrMistery]

Yes, the sequence is the same.

[sdekivit]

Yes, the sequence is the same.

no not true

--> there are chimeras and mosaics with different DNA-sequences due to chromosome abnormalities and beside that, somatic mutations can occur.

Also maternal chromosomes can differ in repetitive non-coding DNA from the paernal chromosomes (via a few mechanisms)

[Sepals]

The gametes (eggs and sperm) have half the amount of DNA compared to all other cells in the body hence these are haploid unlike the somatic cells which are diploid.

[Ajay]

Ok let me be specific.... Wether the DNA sequence is same for all cells in Humans (Homo sapiens) ? Im asking this question because during cell multiplication by mitosis a process called crossing over takes place between two non sister chromatids....so is there is chance that the sequence may change ?


AJAY

[Sepals]

Ok let me be specific.... Wether the DNA sequence is same for all cells in Humans (Homo sapiens) ? Im asking this question because during cell multiplication by mitosis a process called crossing over takes place between two non sister chromatids....so is there is chance that the sequence may change ?


AJAY

That only happens in gametes in the process called meiosis, not mitosis. You got the two confused. So these gamates have different DNA sequences for that reason, also because mutation can also occur and the reason I stated above. Also mutation can happen in somatic cells (non-gametes) which can lead to cancer.

[Ajay]

Yeah you are right mr.Sepals .... Which also leads to another doubt... So you mean to say every gamete is unique in its sequence?

[CRbomber650]

yeah, i am wondering the same thing... I would guess so. at least if crossing over takes place in all of the gametes created

[Sepals]

That's Miss Sepals. Yes all gamates are unique not just because of crossing over,which I think always happens with most chromsomes, but also because of the reassortment of chromosomes that happens when these line up the second time (anaphase II) before the cell split into what becomes gamates. Here by the maternal and paternal chromosomes line up in the centre of the spindle to spilt and the order and side these line up is simply by chance. You should look up meiosis to get more detail. Here could be enough info if not it's also avaliable in many other sources.

[Khaiy]

The specific mechanism for that reassortment is called "independant assortment", and that along with crossing over lead the odds of producing identical gametes in the billions, if not trillions.

However, somatic cells are basically the same (there are exceptions, but they are just that). The functional unit is generally identical (again, there are some exceptions). But on a teleomeric segment mutations are basically irrelevant, so you have a smaller portion of DNA variety to worry about.

[stellar876]

only 1% (approximately) of the human genome codes for proteins, RNA, etc. the other 99% is non-coding, and the same for all beings. It is the variability in that 1 percent that leads to variations in the phenotype of an individual. Also, there is a small degree of crossing over that takes place in mitosis, but it is very insignificant.

[MrMistery]

Yes, i remember a genetics professor saying that if one person has eyes of different colors, that is the result of mitotic crossing-over. Never made much sense to me though...