What mechanism ensures correct joining of gene segments during rearrangement of the heavy and light chain loci? Recombination signal sequences-conserved sequences in regions just upstream or downstream of gene segments. Consist of a conserved heptamer and nonamer with a 12 or 23 bp spacer. The one-turn/two-turn rule(12/23 rule)-recombination occurs only between a segment with a 12 bp spacer and a segment with a 23 bp spacer.What mechanism ensures correct joining of gene segments during rearrangement of the heavy and light chain loci? Recombination signal sequences - conserved sequences in regions just upstream or downstream of gene segments. Consist of a conserved heptamer and nonamer with a 12 or 23 bp spacer. The one-turn/two-turn rule (12/23 rule) - recombination occurs only between a segment with a 12 bp spacer and a segment with a 23 bp spacer