Molecular Genetics

Facts

• Each cell can have multiple organelles.
• Within each organelle, DNA is located in identifiable regions called nucleoids.
• Each nucleoid may consist of several genomes.
• As a result, cells are multiploid with respect to organelle genomes.
• The condition in which all mitochondrial or plastid genomes in a cell are identical is called homoplasmy. When they are not, the condition is heteroplasmy.
• Cells can become heteroplasmic by mutation in one mitochondrial DNA or by zygotic fusion.

Interpretations

• Mendelian genetics cannot apply to organelle trait inheritance. Rather, the traits obey principles deduced from population genetics.
• If heteroplasmy is due to a mixture of organelles of different genotypes, cells may tend to homoplasmy upon repeated cell division due to loss of one type of organelle. If the differing genomes are within the same organelle, slight differences in replication efficiency can lead to a shift toward homplasmy.
• Identical twins need not be genetically identical due to different drifts in the population of organelle genomes.

Further information

• Variegation in the leaves of some plant species is due to drift to homoplasmy in some of the leaf cells.
• Recent evidence indicates that the contents of different organelles can mix through fusion of the organelles.

This is page 1374 of Molecular Genetics by Ulrich Melcher, © 1997-8, 2003-4