Mitochondria is one of the defining features of eukaryotic cells. It is a key innovation in life’s evolution. Here in this video by BioVisions, HarvardX outlines this organelle’s fascinating biology with informative visualizations that help grasp its molecular nature. However, before you continue, you might be interested in a visualization by XVIVO as a great primer to this video for cytological reasons. XVIVO has teamed up with Google to create a stunning 3-D immersive visualization of subcellular compartments inside a generalized animal cell.
There’s compelling evidence that mitochondria and chloroplasts were once free living organisms. The endosymbiont hypothesis provides an explanation for how these organelles evolved to become the driving force for the eukaryotization of cells. In animals mitochondria are inherited from maternal parent. This phenomenon was quite informative especially in understanding ancestral origins of anatomically modern Humans.
Mitochondria have a genome of their own containing 37 highly conserved genes. Mitochondrial genomes also contain a “hypervariable” region. The pace of change in this region is traceable and provides a molecular clock. Based on this molecular clock we can calculate time passed since the appearance of anatomically modern humans. The following video neatly compiles shared features of mitochondria. Again please note that the generalized circular genome representation does not apply to plant mitochondria since they recombine and compartmentalize their gene content in wildly diverse ways. For instance soybean mitochondria can reorganize up to 760 different mitochondrial “subgenomes”.
Note however that here in this video we are seeing an animal-centric eukaryotic model. Organelle biology in plants show striking differences and mitochondrial genomes can be quite diverse. For instance, as a proto-chloroplast analog structural biologists have resolved the photosynthetic chromatophores of the purple bacteria. These buckminsterfullerene structures contain light harvesting, electron transport chain, proton pump and ATPsynthase complexes. Except light harvesting, the rest of the components are shared with mitochondria.
Mitochondria used to have a full genome comprising thousands of genes. However almost all of them moved into the nucleus and variation in what remained appears to be quite very influential and can even drive speciation.
Regulation of transcription, RNA processing and translation in organelles are largely dependent on nuclear genes. Due to their asexual clonal nature with asymmetrical uniparental inheritance modes, organelle genomes evolve at different rates. Chloroplast genomes of land plants evolve at much higher rates compared to their nuclear genomes. Unlike highly conserved and size constrained animal mitochondria, plant mitochondrial genomes are very large, enriched with DNA repeat sequences and are species specific. Evidence suggests that there are RNA-binding “mediator” proteins encoded in the nuclear genome that interact with transcripts in the organelles. These proteins with specific RNA-binding abilities stabilize and even edit nucleotides in the organellar transcripts.
Land plant genomes revealed large numbers of co-evolving nuclear encoded RNA-binding proteins. These proteins most probably serve roles as RNA-level resuscitation of lost functions in organelles and is being encapsulated as “genome debugging hypothesis.” A great majority of fertility restoring nuclear loci in cytoplasmically sterile male plants with incompatible mitochondrial genes correspond to these RNA-binding proteins.
You can learn more about chemistry of biological molecules in the Molecular Nature category.
2 Comments