Welcome back Umesh, Show Why do mitochondria have their own genome? Where did it come from? After mitochondrial genomes were discovered in the early 1900s, researchers first hypothesized that mitochondria were likely evolutionary descendents from endosymbiotic bacteria. Endosymbionts are organisms that live within the body or cells of another organism. Today, this widely regarded theory is called endosymbiosis. Now that we know the sequences of mitochondrial and nuclear genomes from a wide range of organisms, scientists have confirmed that mitochondria are indeed the likely descendents of endosymbiotic bacteria, which are believed to have merged with eukaryotic cells around two billion years ago. Like today’s mitochondria, the endosymbiotic bacteria originally harbored a circular DNA genome. Why did endosymbiosis occur? During that time, life on Earth was exclusively unicellular, and it is believed that aerobically respiring bacteria were engulfed by anaerobic eukaryotes. Over time, the conditions on Earth changed slowly. One major change believed to have driven the rapid expansion of eukaryotes was the increase in the relative amount of atmospheric oxygen. This resulted in a need for eukaryotes to develop and maintain cellular mechanisms to cope with higher oxygen levels. Aerobic respiration from endosymbiotic bacteria provided eukaryotes with the ability to adapt to these new conditions. Over time, many of the genes from the bacterial endosymbionts were transferred to the eukaryotic nucleus. The mitochondrial organelle, however, retained a circular DNA genome encoding mitochondria-specific factors required for cellular aerobic respiration. Indeed, when comparing genome sequences of prokaryotes to eukaryotic mitochondrial genome sequences, a great deal of conservation can be found between bacterial genes encoding proteins involved in aerobic respiration and genes involved in mitochondrial respiration. Now, let’s end with a discussion of some interesting trivia related to mitochondria. Thanks to genes encoded by the mitochondrial genome, mitochondria build their own ribosomes, which are much more similar to bacterial ribosomes than to eukaryotic cytoplasmic ribosomes. Interestingly, this similarity leads to the sensitivity of eukaryotic cells to some antibiotics, which adversely affects bacterial ribosomes and mitochondrial ribosomes but not cytoplasmic ribosomes. You might also be interested to read that humans always inherit their mitochondria from their mothers — together with multiple copies of their mother’s circular mitochondrial DNA genome! As a result, your mitochondrial DNA will be very similar to the mitochondrial DNA of your mother’s mother, but it will not be similar at all to the mitochondrial DNA of your father’s mother. Additionally, disease-associated mutations in the mitochondrial genome exhibit a characteristic maternal inheritance pattern. Finally, mutations occur at a faster rate in mitochondrial DNA than in nuclear DNA. Because of its accelerated mutation rate, it turns out that mitochondrial DNA is a good yardstick by which to measure evolutionary relatedness! For general information about the cell biology of mitochondria, check out these links: http://www.nature.com/scitable/topicpage/mitochondria-14053590 http://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356 For more information regarding endosymbiosis and details about eukaryotic mitochondria, please check out these excellent articles and reviews: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1693097/?tool=pmcentrez http://www.ncbi.nlm.nih.gov/pmc/articles/PMC99014/?tool=pubmed http://www.nature.com/scitable/content/The-organization-and-inheritance-of-the-mitochondria-154 Follow these links to learn more about mitochondrial diseases and inheritance: http://www.nature.com/scitable/topicpage/mtdna-and-mitochondrial-diseases-903 http://www.nature.com/scitable/topicpage/non-nuclear-genes-and-their-inheritance-589 http://www.nature.com/scitable/topicpage/Somatic-Mosaicism-and-Chromosomal-Disorders-867 Regulation of gene expression and cell specializationHow different genes are expressed in different cell types. The big picture of eukaryotic gene regulation. Regulation of gene expression and cell specializationBiology is brought to you with support from the Amgen Foundation AP® is a registered trademark of the College Board, which has not reviewed this resource. What are cisIn eukaryotes, enhancers are a common type of cis-acting element. As its name implies, an enhancer promotes gene expression when the appropriate trans-acting element(s) binds to it. Trans-acting elements, also known as transcription factors, can either promote or inhibit gene expression.
What is a cisA noncoding DNA sequence in or near a gene required for proper spatiotemporal expression of that gene, often containing binding sites for transcription factors. Often used interchangeably with enhancer.
Which are cisCis-regulatory elements, such as promoters, enhancers, and silencers, are regions of non-coding DNA, which regulate the transcription of nearby genes. In contrast, trans-regulatory factors regulate (or modify) the expression of distant genes by combining with their target sequences [1, 2].
What is an example of eukaryotic gene regulation?An example is the TATA box, so named because it has a core sequence of TATAAA. This is a regulatory element that is part of the promoter of most eukaryotic genes. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex.
|
