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Mitochondria and Plastids are membrane-bound organelles that convert energy from foodstuffs (mitochondria and non-photosynthetic plastids) or sunlight (chloroplasts) into cellular energy. Some plastids may also be used for starch storage and the synthesis of fatty acids and terpenes. Organelles likely evolved from bacteria that were endocytosed by nucleated ancestral cells. Metazoan, plant, fungal, and other mitochondrial and plastid genomes tend to vary greatly in size and gene content with some of the genes required for the energy creating processes being made by the nuclear genome and imported. Many organisms use one genetic code to translate nuclear mRNAs, and a second one for their mitochondrial and plastid mRNAs. Organelles have their own independent genome that encodes a range of genes directly related to producing energy for the cell.

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 with varying levels of review compared to the primary record used to build the RefSeq. Additionally, protein clusters for the metazoan and plastid genomes proteins can be reviewed with Entrez Protein Clusters.

Mitochondria are small, oval shaped organelles surrounded by two specialized membranes. Mitochondria are the sites of aerobic respiration, and are the major energy production center in eukaryotes.Metazoan mitochondrial genomes are circular, about 16 kB in length, and typically encode 13 proteins used for energy production, as well as 22 tRNAs and 2 ribosomal RNAs. The number of proteins and tRNAs can vary in specific phylums. The low mutation rate in metazaon mtDNA sequence makes these genomes useful for scientists assessing genetic relationships of individuals or groups within a species and for the study of evolutionary relationships.

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 about 120-200 kB in length. Their genes encode proteins mainly involved in photosynthesis, tRNAs, and ribosomal RNAs. Chloroplast mRNAs are translated with translation_table 11, although they often undergo extensive RNA editing, so it is difficult to predict the protein translations from genomic sequence. Annotation of chloroplast genes can also be complicated since these genomes may use trans-splicing to create different forms of certain genes.
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: February 25, 2009


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