Why genome seq A% ~= T% and G% ~= C%

Question

[fengbo]

I test 3 genome. Human, Yeast, E.coli All of them has the same result, G% = C%, A% = T%

result: https://github.com/zengfengbo/data

[fengbo]

Genome Seq is a single DNA.

Answer 1

[dariober]

Isn't your finding the Chargaff's rules?

[John]

Yup. I went years as a geneticist thinking it must be because C-G A-T, as the two posters below wrote, but that's not the case, or at least it's only half of the reason. The other reason is that when DNA moves around, it has a random insertion direction.

Answer 2

[Carlo Yague]

If on one strand of the genome you have about the same number of G and C, that means that the number of G on the + strand is about the same as the number of G on the - strand.

Now imagine that we start with a genome with more of G on the + strand. My guess is that due to multiple events of chromosome rearrangement (inversions) during evolution, the number of G is slowly reequilibrated until you have about the same number of G on both strands, meaning that you have about the same number of C and G on the same strand.

intial genome : 
GGGGAGGGAGTGGCGGGGG (+)       -> 1C and 15G
CCCCTCCCTCACCGCCCCC (-)

inversion -> toward equilibrium
AGGGG | CCCTCACCGCCCCC (+)    -> 11C and 4G  
TCCCC | GGGAGTGGCGGGGG (-) 

inversion -> toward equilibrium
AGGGGCCCTCACCGC | GGGG (+)    -> 7C and 9G  
TCCCCGGGAGTGGCG | CCCC (-)

[fengbo]

5'GGGGA3'\ /5'CCCTCACCGCCCCC3' (+) -> 11C and 4G
3'CCCCT5'/ \3'GGGAGTGGCGGGGG5' (-)

How A-3' linked with 3'-G?

[Carlo Yague]

You are right, I forgot to reverse the fragments. Will edit later. EDITED.

Answer 3

[beausoleilmo]

I would guess that this is because A is always paired with T and that C is always paired with G: https://en.wikipedia.org/wiki/DNA

Adenine pairs with thymine and guanine pairs with cytosine. It was represented by A-T base pairs and G-C base pairs

It might not perfectly match because there might be some imperfect sequencing or something similar.

[Carlo Yague]

If you take into account both strands of the genome, then your reasoning is correct and we would expect to have exactly the same number of G and C because of base pairing. However the answer might not be so simple because I think fengbo is looking at only one strand of the genome.