Chapter

Mitochondria

Moyra Smith

in Phenotypic Variation

Published on behalf of Oxford University Press

Published in print February 2011 | ISBN: 9780195379631
Published online October 2012 | e-ISBN: 9780199975211 | DOI: http://dx.doi.org/10.1093/med/9780195379631.003.0006
Mitochondria

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Through pioneering studies in the 1960s and 1970s by A. Lehninger (1979), E. Racker (1975), and by 1997 Nobel laureates J. Walker and P. Boyer, we gained insight into the key role of mitochondria in energy generation. In the electron-transport chain, electrons derived from reduced nicotinamide adenine dinucleotide (NADH) and reduced flavin adenine dinucleotide (FADH2) generated in metabolism of glucose and fatty acids, are transferred to molecular oxygen and these electrons serve as energy source. Soluble carriers such as coenzyme Q and cytochrome C transfer electrons through the four mitochondrial electron complexes. Cytochrome C oxidase transfers electrons to oxygen. Coincident with electron-transfer protons are transferred to the space that exists between the inner and the outer mitochondrial membranes leading ultimately to an electrochemical proton gradient across the mitochondrial membrane. The proton translocating adenosine triphosphate (ATP) synthase complex (complex V) harnesses the free energy generated by this gradient. This complex synthesizes ATP from adenosine diphosphate (ADP) when free energy and inorganic phosphate are available. Disruption of the correct electron flow can lead to generation of reactive oxygen species (ROS). The mitochondrial electron-transport chain and oxidative phosphorylation are schematically represented in Fig. 6–1.

Sequencing of the mitochondrial genome revealed that it is 16.5 kb in length, it encodes 13 proteins, 22 transfer RNAs (tRNAs), and two ribosomal RNAs (Attardi, 1986). A map of the mitochondrial genome is depicted in Fig. 6–2.

Each cell has hundreds of mitochondria. Mitochondria with deleterious mutations often coexist with wild-type mitochondria; this is referred to as heteroplasmy. The phenotype is determined by the ratio of wild-type to mutant mitochondria (Wallace et al., 1988). Over the past decade we have gained insight into the more than 1,000 nuclear genome-encoded proteins that function within the mitochondria (Wright et al., 2009).

Chapter.  8658 words.  Illustrated.

Subjects: Clinical Genetics ; Clinical Cytogenetics and Molecular Genetics

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