A model for effective pairing and recombination at meiosis based on early replicating sites (R-bands) along chromosomes.

A model for meiotic pairing is proposed in which early replicating sites (R-band equivalent) along chromosomes are envisaged as sites for synaptic initiation. Only within such sites will "effective" pairing for recombination be established. Pairing in later replicating (G- and C-band equivalent) regions will be "ineffective" and will not provide for the stringent requirements of the crossover process. Exchange events might be predetermined at S-phase, and possibly at junctions between early and later replicating sequences, these being seen as vulnerable sites for breakage. Temporal shifts in replication from early to late S, are postulated to produce localized pairing disruption and lowering of crossover values as regions of chromatin shift from being effectively to ineffectively paired.