Plastids and Mitochondria

Both Plastids and Mitochondria are belived to result from primitive prokaryotic cells - this is termed the endosymbiont theory. An example is the Chloroplast which is believed to have evolved from the Cyanobacteria.

Evidence for this includes the fact that these contain their own DNA and the mechanisms to transcribe proteins - despite the fact that the vast majority of proteins within these organelles are also contained within the cell\'s nuclear DNA.

Plastids and Mitochondria are classed as semi-autonomous organelles because although they are capable of synthesis of their own proteins and contain their own DNA, within the cell they are partially reliant upon cellular metabolism in a way that cyanobacteria, for example, are not.

Centrioles

Centrioles form Centrosomes, which are nucleation points for microtubule organisation centres (MTOCs), which promotes polymerisation of alpha- and beta- tubulin dimers such that microtubles are formed. This however is not a protein synthesis proceedure, and the centrioles themselves are produced from nuclear DNA.

During the cell cycle, the two centrioles which form the centrosome change orientation, and a new daughter centriole is synthesised in the vicinity of each pre-existing centriole. This occurs at the G1/S boundary, and extension of the daughter centrioles continues into the S and G2 phases.

The control of centriole synthesis is cell mediated, being precisely matched to the cell cycle, meaning that the centriole does not have control over its own metabolism. The centriole does not contain genetic material, and is unable to synthesis tubulin, the essential monomer for centriole and MT production. This means that the centriole is:

1) Unable to synthesis its constituent proteins
2) Unable to supply DNA relating to itself
3) Unable to divide without significant support from other organelles

This means that the centriole, despite being seemedly able to divide, is not autonomous.

References

Fukasawa K : Introduction : Oncogene : 9 September 2002, Volume 21, Number 40, Pages 6140-6145