bases evolution URGENT!

[Cátia Calé]

Hi! could you answer me to the next questions, please?
- Why DNA´s uracil base evoluted  to timine; do you think that there is some
relation between this evolution and repair mechanisms? or  with CpG islands-
Hotspots in wich there is desmetilation of metilated citosine that origines
timine...?
- what´s the difference between holliday model and the branch in double
strand? this is related with the number of Holliday structures formed or
with the inital splicing?

thanks