Fig. 2. Immunoelectron microscopy of RAD51/DMC1 (10 nm gold grains) and RPA (5 nm gold grains) in association with chromosome cores/SCs in an early and later meiotic prophase spermatocyte. (A) At early meiotic prophase, there is an abundance of RAD51/DMC1 foci but relatively few RPA foci in association with partially paired zygotene chromosome cores and SCs. (B; insert) At a later stage, pachytene, there are numerous RPA foci and very few RAD51/DMC1 foci. The RPA foci are concentrated on central element-associated nodules, TN. The width of the SC is about 200 nm. (C) At early pachytene, there are occasionally a few unpaired centromeric ends (15 nm gold grains). One of these `laggards' is shown to have RPA associated with a single, unpaired chromosome core, indicating that RPA is not always associated with RAD51/DMC1 protein. The image further demonstrates the presence of mixed foci containing RAD51/DMC1 and RPA antigen (mix). Incidental is the observation that RPA gold grains occur as a small loop (arrow) in this and other electron micrographs. The width of the SC is about 200 nm.