You have to be careful with the meaning of the word “attach” here. In single-stranded DNA (or RNA) nucleotides are attached together like beads in a chain, by what chemists call “covalent” bonds, which for practical purposes you can think of as strong, “semi-permanent” bonds. But the two DNA strands that together make a double-helix are “attached” by weaker ionic bonds, which are more like an electrical “attraction” than a solid attachment. This is mediated by the attraction of the complimentary bases C to G, and A to T, (which I think you already know about). Therefore the two strands can fairly easily come apart (with some enzyme help) to allow transcription. When transcribing, the RNA polymerase replaces the absent DNA strand with ribonucleotides (rather than deoxyriboneucleotides), matching up the complimentary ribonucleotide (r-C, r- G, r-A, or r-U) to the bases on the single DNA strand. The enzyme connects the ribonucleotides together by covalent bonds, making a single-stranded RNA chain, but the RNA is only “attached” to the DNA template by the weaker attraction of the base-pairing, and this is only temporary, as the RNAs are pretty quickly processed and exported to the ribosomes from there. The RNA message never has a “hard” connection to the DNA template.
And yes, the separated DNA strands go back together, as the RNA polymerase moves along the DNA template.
Edited by OldCloner, 07 January 2019 - 06:42 AM.