Final answer:
All thymine residues are not necessarily methylated, the statement is false. In DNA, cytosine may be methylated but not uniformly. Bases in DNA pair specifically, adenine with thymine (A=T) and guanine with cytosine (G=C), as dictated by Chargaff's rules.
Step-by-step explanation:
The statement that all thymine residues are methylated is false. In DNA, methyl groups may be added to cytosine residues in a process called DNA methylation, but this does not occur on all cytosine residues. Thymine itself is already a methylated form of uracil, which is found in RNA.
Regarding the base pairing question, cytosine always pairs with guanine in DNA following Chargaff's rules. So, if part of a DNA chain has the sequence of bases ATTG, the corresponding sequence that it binds to on the other chain would be TAAC.
Erwin Chargaff's research revealed important patterns in DNA composition among different species. He found that the amount of adenine equals the amount of thymine, and the amount of cytosine equals the amount of guanine (A=T and G=C). Nonetheless, the concentration of these nucleotide bases varies from species to species, though not within a species, which can aid in species identification.