Defining the cell cycle for the tachyzoite stage of Toxoplasma gondii

Mol Biochem Parasitol. 2001 Jul;115(2):165-75. doi: 10.1016/s0166-6851(01)00284-5.


Tachyzoite endodyogeny is characterized by a three phase cell cycle comprised of major G1 and S phases with mitosis following immediately upon the conclusion of DNA replication. Cytokinesis, which begins with the formation of daughter apical complexes, initiates in late S phase and overlaps mitosis. There is no evidence to support an extended G2 period in these parasites. In all strains, parasites with a 2 N DNA content are a relatively small subpopulation and when tachyzoites expressing a fluorescent nuclear marker (green-fluorescent-protein fused to proliferating-cell-nuclear-antigen) were observed by time-lapse microscopy, there appeared to be little delay between S phase and mitosis. Measurements of the DNA content of RH parasites by flow cytometry demonstrated that the G1 and S periods were approximately 60 and approximately 30% of a single division cycle, although these phases were longer in strains that display a slower growth rate. The overall length of S phase was determined by [3H]-thymidine autoradiography using transgenic parasites expressing herpes simplex thymidine kinase and validated by Northern analysis of S phase specific genes during synchronous growth. The fraction of S phase parasites by flow cytometry paralleled autoradiography, however, within S phase, the distribution of parasites was bimodal in all strains examined. Parasites containing a 1-1.7 N DNA complement were a small fraction when compared to the major S phase population which contained a near-diploid ( approximately 1.8 N) complement, suggesting parasites in late S phase have a slower rate of DNA replication. In lieu of a short or missing G2, where checkpoints are thought to operate in other eukaryotes, the bimodal replication of tachyzoite chromosomes may represent a distinct premitotic checkpoint associated with endodyogeny.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cell Cycle / physiology*
  • Cell Division
  • DNA, Protozoan / analysis
  • Flow Cytometry
  • Fluorescent Antibody Technique
  • G1 Phase
  • G2 Phase
  • Gene Expression
  • Mitosis
  • Proliferating Cell Nuclear Antigen / analysis
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • S Phase
  • Toxoplasma / cytology
  • Toxoplasma / genetics
  • Toxoplasma / growth & development*


  • DNA, Protozoan
  • Proliferating Cell Nuclear Antigen
  • RNA, Messenger