Molecular Genetics

Facts

ORGANELLE GENOMES				Organelle genomes are principally circular DNAs. Linear DNAs are also found. Some organelles also contain linear "plasmids". Concatenated circles are found in kinetoplasts of trypanosomes and related organisms. They can be classed as minicircles and maxicircles and form an extensive network. Circular and linear DNAs of many different sizes are found in plant mitochondria (ref). Lariat and branched forms are also seen. The sizes of many plant mitochondrial genomes have been measured best by kinetic complexity. Compare their sizes to those of bacterial genomes.
group	species	kbp	topology
Mitochondria
mammalian	human	16.5	circular
fungal	S.cerevisiae	74-85	circular
	P. anserina		linear
	Candida rhagii		linear
Paramecia			linear
Acetabularia sp.			linear
plant	B. campestris	218	multiple circles
	Z. mays	570	multiple circles
	muskmelon	2500	multiple circles
Plastids
	most	130-150	circles
	range	80-200

Interpretations

• Consistent with the presence of heritable material in mitochondria and plastids, they contain DNA.
• Organelle genomes, especially those of mitochondria, vary widely in size and form.
• The smallest organelle genomes (mammalian mitochondria) are so small that they can only contain a very small set of genes, while the largest are as large as, or larger than those of many bacteria.

Further information

• Mitochondrial genomes are targets of genome projects as are plastid genomes.
• Recent evidence suggests that the majority of plastid DNA is actually in the form of complex branched linear molecules, consisting of several genomes worth of DNA (ref).
• Recombination and rolling circle replication likely play roles in maintaining the diversity of plant mitochondrial DNAs.
• Animal parasites in the Apicomplexan group contain an organelle whose genome sequence indicates that it was derived from plastids (rev).

This is page 1371 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2001, 2003-4