Final answer:
A hairpin loop forms when an RNA or DNA strand folds back on itself creating a double-stranded structure due to complementary base pairing. These structures play a key role in the termination of transcription by acting as signals that lead to dissociation of the RNA synthesis machinery.
Step-by-step explanation:
A hairpin loop is formed when a single strand of RNA folds back on itself to create a double-stranded structure due to intramolecular hydrogen bonds between complementary nucleotides. This typically occurs when an internally complementary sequence, particularly near the 3' end of an RNA transcript, folds to generate the secondary hairpin loop structure. Such structures are crucial in the termination of transcription, where they act as signals for the dissociation of RNA polymerase, template DNA, and the newly synthesized RNA transcript during the rho-independent termination process. In DNA, similar hairpin structures can form, creating a cruciform when present on both strands at a point of rotational symmetry. The palindromic sequences within strands allow for this intra-chain base pairing.
In the context of translation regulation, pre-miR21 and similar molecules mimic the hairpin loop structure and are involved in RNA processing by enzymes like Dicer. These structures, specifically the stable C-G-rich hairpins followed by A-T-rich regions, contribute to the rho-independent termination by causing RNA polymerase to stall and leading to the release of the mRNA transcript. In addition, such hairpin loops are involved in the design of self-assembling macromolecular scaffolds and are integral in various other DNA and RNA processing events.