(a) Describe how genetic information is carried in chloroplasts and mitochondria.
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Answer:
The biosynthesis of mitochondria and plastids requires contributions from two separate genetic systems. Most of the proteins in mitochondria and chloroplasts are encoded by special genes devoted to this purpose in nuclear DNA. These proteins are imported into the organelle from the cytosol after they have been synthesized on cytosolic ribosomes. Other organelle proteins are encoded by organelle DNA and synthesized on ribosomes within the organelle, using organelle-produced mRNA to specify their amino acid sequence (Figure 14-51). The protein traffic between the cytosol and these organelles seems to be unidirectional, as no known proteins are exported from mitochondria or chloroplasts to the cytosol. An exception occurs under special conditions when a cell is about to undergo apoptosis. The release of intermembrane space proteins (including cytochrome c) from mitochondria through the outer mitochondrial membrane is part of a signaling pathway that is triggered in cells undergoing programmed cell death
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The DNA molecules observed or present in mitochondria and chloroplasts are small and circular, like the DNA of a simple bacteria. There are generally many copies of DNA in a single mitochondria or chloroplast.
Explanation:
Chloroplasts and mitochondria are intracellular bioenergetic organelles with unique genomes and genetic systems. DNA replication and transfer to daughter organelles generates a cytoplasmic inheritance of traits associated with key events of photosynthesis and respiration. The prokaryotic ancestors of chloroplasts and mitochondria were endosymbionts whose genes were copied into the genome of the cellular host. These copies gave rise to nuclear chromosomal genes encoding cytosolic and precursor proteins synthesized in the cytosol for import into the endosymbiont-developed organelle. Mitochondria, like chloroplasts, tend to be inherited from just one parent or the other (or at least, to be unequally inherited from the two parents). In the case of humans, it is the mother who contributes mitochondria to the zygote, or one-celled embryo, by way of the egg's cytoplasm. Sperm do contain mitochondria, but they are not usually inherited by the zygote. There has been a reported case of paternal inheritance of mitochondria in a human, but this is extremely rare. What explains gene retention for complete synthesis of very few protein subunits in chloroplasts and mitochondria? One hypothesis is that the expression of genes encoding protein subunits of energy-converting enzymes is controlled by the physical environment through direct and unconditional regulatory control (controlled by changes in the redox state of the corresponding gene products). It means that we have to respond to change. This hypothesis proposes that the entire redox regulatory system must remain in its original membrane-bound compartment to maintain function. Colocation of genes and gene products for redox regulation of genes [deleted] CoRR) is a hypothesis that is consistent with the results of various experiments designed to test it, and there does not seem to be any other satisfactory explanation.
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