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Mitochondria and chloroplasts (a type of plastid) are membrane-bound organelles that convert energy from foodstuffs (mitochondria) or sunlight (chloroplasts) into forms that can be used by the cell.
The organelles likely evolved from bacteria that were endocytosed more than one billion years ago. Although the organelles maintain their own genomes, many genes encoding mitochondrial and chloroplast proteins are found in the cell nucleus.
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Mitochondria are small, oval shaped organelles surrounded by two highly specialized membranes. Mitochondria are the sites of aerobic respiration, and are generally the major energy production center in eukaryotes. Animal mitochondrial genomes are normally circular, ~16 kB in length, and encode 13 proteins used for energy production, as well as 22 tRNAs and 2 rRNAs. Plant mitochondrial genomes tend to be 10 - 150 times larger and contain additional genes. Many organisms use one genetic code to translate nuclear mRNAs, and a second one for their mitochondrial mRNAs.
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Chloroplasts are larger than mitochondria, and are surrounded by two specialized membranes. In plants and some other eukaryotes, chloroplasts are the sites of photosynthesis, a process in which atmospheric carbon dioxide is "fixed" into organic compounds, and oxygen is released into the atmosphere. Chloroplast genomes are ~120 - 200 kB in length. Their ~120 genes encode ribosomal RNAs and proteins, tRNAs, and proteins involved in photosynthesis. Chloroplast mRNAs
are translated with the standard genetic code, although they often undergo
extensive RNA editing, so it is difficult to predict the protein
translations from genomic sequence.
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The organelle genomes on this site are part of the NCBI Reference Sequence (RefSeq) project that provides curated
sequence data and related information for the community to use as a standard. The animal (metazoan) mitochondrial records are considered "reviewed", that is, they have been manually curated by the NCBI staff. Other mitochondrial and chloroplast genome records are "provisional" and are presented as found in the source GenBank records used to create them.
The mitochondion and chloroplast images are courtesy of The Biology Project.
Last modified: May 24, 2000
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