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Finally, we turn to chloroplast DNA (cpDNA), examining its characteristics, organization, and function.

The Biology of Mitochondria and Chloroplasts

Mitochondria and chloroplasts are membrane-bounded organelles located in the cytoplasm of eukaryotic cells (iFigure 20.2). Mitochondria are present in almost all eukaryotic cells, whereas chloroplasts are found in multicel-lular plants and some algae. Both organelles generate ATP, the universal energy carrier of cells.

Mitochondrion and Chloroplast Structure

Mitochondria are from 0.5 to 1.0 micrometer (^m) in diameter, about the size of a typical bacterium; chloroplasts are typically from about 4 to 6 ^m in diameter. Both are surrounded by two membranes that enclose a region (called the matrix in mitochondria and the stroma in chloroplasts) that contains enzymes, ribosomes, RNA, and DNA. In mitochondria, the inner membrane is highly folded; embedded within it are the enzymes that catalyze electron transport and oxidative phosphorylation. Chloroplasts have a third membrane, called the thylakoid membrane, which is highly folded and stacked to form aggregates called grana. This membrane bears the pigments and enzymes required for photophos-phorylation. New mitochondria and chloroplasts arise by the division of existing organelles (I Figure 20.3). Mitochondria

20.3 New mitochondria arise by division of existing mitochondria. (a) DNA molecules within the mitochondria segregate randomly in organelle division. (b) Electron micrograph of a dividing mitochondrion from a liver cell. (T. Kanaseki and D. Fawcett/Visuals Unlimited.)

Mitochondrion

Mitochondrial DNA

Mitochondrion

Mitochondrial DNA

A mitochondrion grows, and its DNA replicates.

Organelle division starts with constriction of the outer membrane.

During division, cellular DNA segregates randomly.

A mitochondrion grows, and its DNA replicates.

Organelle division starts with constriction of the outer membrane.

During division, cellular DNA segregates randomly.

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and chloroplasts possess DNA that encodes polypeptides used by the organelle, as well as rRNAs and tRNAs needed for the translation of these proteins.

The Genetics of Organelle-Encoded Traits

Mitochondria and chloroplasts are present in the cytoplasm and are usually inherited from a single parent. Thus traits encoded by mtDNA and cpDNA exhibit uniparental inheritance. In animals, mtDNA is inherited almost exclusively from the female parent, although occasional male transmission of mtDNA has been documented. Paternal inheritance of organelles is common in gymnosperms and occurs occasionally in angiosperms as well. Some plants even exhibit biparental inheritance of mtDNA and cpDNA.

Individual cells may contain from dozens to hundreds of organelles, each with numerous copies of the organelle genome; so each cell typically possesses from hundreds to thousands of copies of mitochondrial and chloroplast genomes (IFigure 20.4). A mutation arising within one organellar DNA molecule generates a mixture of mutant and wild-type DNA sequences within that cell. The occurrence of two distinct varieties of DNA within the cytoplasm of a single cell is termed heteroplasmy. When a heteroplas-mic cell divides, the organelles segregate randomly into the two progeny cells in a process called replicative segregation (I Figure 20.5), and chance determines the proportion of mutant organelles in each cell. Although most progeny cells will inherit a mixture of mutant and normal organelles, just by chance some cells may receive organelles with only mutant or only wild-type sequences; this situation is known as homoplasmy.

20.4 Individual cells may contain many mitochondria, each with several copies of the mitochondrial genome. Shown is a cell of Euglena gracilis, stained so that the nucleus appears red, mitochondria green, and mtDNA yellow. (From Y. Huyashi and K. Veda, Journal of Cell Sciences 93, 1989, 565.)

A heteroplasmic cell has two distinct varieties of DNA contained in its organelles.

-WThe organelles segregate randomly in cell division,.

Replication of mitochondria

A heteroplasmic cell has two distinct varieties of DNA contained in its organelles.

-WThe organelles segregate randomly in cell division,.

Replication of mitochondria

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