Questions about DNA molecules

Answer from the beginning. "Should single-stranded DNA be one molecule or two molecules after melting?" -Each single chain is one molecule, and one * * * is two molecules. "Is the hairpin structure universally possible?" -Hairpin structures are everywhere. As long as the conditions of base complementarity in a certain region are met, single-stranded molecules can form hairpin structures, whether DNA or RNA. The hairpin structure of DNA is less, because DNA exists in the form of double strands, and single-stranded DNA (such as viral DNA and DNA temporarily twisted into single strands due to replication or other reasons) also has hairpin structure. Even double-stranded DNA can form a hairpin structure locally, forming an intramolecular or intermolecular triple strand. "After DNA melting, can single strands be complementary?" -When the double strands are untied, the two single strands are complementary. "Are the motifs of these two chains the same?" -Focus on this. Contrary to other people's views, unfortunately, from your description, I don't think your idea is correct. Not necessarily the same base sequence. The 5'-3' sequence of the first strand is not necessarily the same as the 5'-3' sequence of the second strand, nor is it necessarily the same as the 3'-5' sequence of the second strand. The most extreme example is that one strand is AAAAAA, then its complementary strand is TTTTTT, which obviously cannot be the same base sequence from any angle. Only DNA with complete palindrome structure can have the same base sequence in the same direction of two strands, such as 5'-gg tacc-3', and its complementary strand is 3'-CCATG-5', and the 5'-3' sequences of these two strands are the same. Natural DNA is very long, and local palindromes are common (which is also a necessary condition for hairpin structure), while complete palindromes are impossible. Therefore, after double-stranded unwinding, only a part of single-stranded DNA can form a double-stranded structure (hairpin), and other parts can only exist in single-stranded form. If you have any questions or I have misunderstandings, please keep asking questions =)