Figure 1. The cell biological context of P. falciparum intraerythrocytic (IE) mitotic recombination.

The first mitotic division of single IE P. falciparum genomes occurs from around hours 20–28 of postmerozoite invasion (PI). The three rows illustrate different staining methods used to visualise parasites, nuclei, and chromosomes before, during, and immediately after the first of the parasite's asynchronous mitotic divisions [18]. Giemsa: the developing trophozoite visualised after methanol fixation and Giemsa staining. CPO: Fluorescent staining following Coriphosphine O (CPO) uptake by parasitized erythrocytes. CPO, an acidic acridine derivative, is membrane permeable but shows no fluorescence when free in red blood cells and gives strong green emission using the argon laser (488 nm) when bound to parasite DNA in intact IE. FISH Differential Interference Contrast (DIC): Fluorescence in situ hybridisation (FISH) using cy3-labelled P. falciparum Rep 20 sequences, plus DAPI fluorescence, superimposed on the differential interference contrast image of intact IE parasites. Host cell membranes and vacuolar haemozoin are apparent. Rep 20 hybridises to repetitive sequences at both telomeres of each of the 14 chromosomes. Telomere clustering is evident before, during, and after the nuclear division. It is noteworthy that var genes are often found in tandem arrays immediately adjacent to the telomere repeats, thus telomere clustering could potentially align them for more efficient recombination. The most likely mitotic recombination scenario would appear to be that both DNA secondary structure-induced double strand breaks and microhomology mediated var recombination-associated repair occur during the DNA replication process itself.