Both mitochondria and chloroplasts contain a genome consisting of a circular DNA molecule that is presumably derived from the ancestral bacterial chromosome. Over evolutionary time, these organelle genomes have lost many genes that were unnecessary for life as an organelle inside a host cell. In addition, many genes that are still necessary have been transferred to the chromosomes in the nucleus. As a consequence, the mitochondria of animals have very little DNA left. For example, human mito-
endosymbiosis Form of symbiosis where one organism lives inside the other plastid Any organelle that is genetically equivalent to a chloroplast, whether functional in photosynthesis or not symbiosis Association of two living organisms that interact symbiotic theory Theory that the organelles of eukaryotic cells are derived from symbiotic prokaryotes urkaryote Hypothetical ancestor that provided the genetic information of the eukaryotic nucleus
Defects due to mutations in mitochondrial genes are inherited maternally.
chondrial DNA has only 13 protein encoding genes, together with genes for several rRNA and tRNA molecules (Fig. 19.03). However, the mitochondrial proteome, that is the complete set of proteins expressed in mitochondria, numbers approximately 400. The genes for most of these reside in the nucleus and these polypeptides must be imported into the organelle after synthesis on the ribosomes of the eukaryotic cytoplasm.
The chloroplasts of higher plants retain rather more DNA than mitochondria— approximately enough for a hundred genes—but this is still much less than their bacterial ancestors. It is estimated that 1,000 or more genes from the ancestral photosynthetic prokaryote have been transferred to the plant cell nucleus.
During sexual reproduction, mitochondria and chloroplasts are inherited maternally. When a sperm fertilizes an egg cell to create a zygote, the organelles of the sperm are lost. The new individual retains the organelles from the egg cell, i.e. those from the female parent only. Certain inherited defects of humans are due to mutations in the mitochondrial DNA. These affect the generation of energy by respiration and affect the function of muscle and nerve cells in particular. These defects are passed on through the maternal line as all children with the same mother inherit the same mitochondria.
Partial exceptions to the rule of maternal inheritance for organelles occur in a few single-celled eukaryotes. Chlamydomonas is a single-celled green alga whose cells contain a single chloroplast. During mating, about 5% of the zygotes have two chloro-plasts rather than one. In these cells recombination can occur between the two different chloroplast genomes. Division of the zygote gives cells with only a single chloroplast each. These may be examined to determine the outcome of the genetic crosses.
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