Organelles and the Molecular Biology of Eukaryotes Help
Mitochondria vs. Chloroplasts
Mitochondria and chloroplasts have evolved for specialized functions in eukaryotic cells. Mitochondria are thought of as the powerhouse of the cell because many enzymes involved in cellular respiration and ATP production are located there. Chloroplasts serve as the site of photosynthesis in plants. Both mitochondria and chloroplasts share several characteristics with modern prokaryotic cells. All three generally have a circular double-stranded DNA genome (exceptions include some protozoans such as Paramecium and Tetrahymena that have linear mitochondrial DNA molecules). Their genomes are neither enclosed within a nuclear membrane nor associated with histone proteins (hence no nucleosome organization). They each code for part of their own protein-synthesizing systems (all rRNAs, tRNAs, and at least some of the ribosomal proteins). Many of the enzymes and other proteins that function in these organelles, however, are encoded by nuclear genes, synthesized on 80S ribosomes, and transported into these organelles. Their ribosomes are usually 70S or smaller and are sensitive to antibiotics and other substances that have no effect on the 80S eukaryotic cytoplasmic ribosomes. Protein synthesis is initiated by formyl-methionyl-tRNA. The nucleus, mitochondrion, and chloroplast are each bounded by a double-membrane envelope, but only the nuclear membrane contains pores. Mitochondria and chloroplasts grow in size and then seem to split in two, in a process akin to binary fission in bacteria.
Mitochondria are organelles found in the cytoplasm of both plants and animals. They contain the enzymes of the electron-transport chain that carry out oxidative phosphorylation in the production of adenosine triphosphate (ATP, the main source for energy-requiring biochemical reactions). Unlike chloroplasts, the mitochondrial genome (mtDNA) varies markedly in length between species. For example, in fungi, such as the yeast Saccharomyces cerevisiae, the mtDNA is about 86 kb, and inmostmetazoan (multicellular) animals it is on average 16 kb.Much of themtDNAof fungi and plants is thought to be noncoding (perhaps "junk" or "selfish" DNA). Animal mitochondrial genomes typically encode the same 37 proteins: 2 rRNA genes, 22 tRNA genes, and 13 protein-coding genes. The proteins encoded are involved in respiration (i.e., cytochrome oxidase), DNA replication, transcription, and translation (e.g., ribosomal proteins). Mitochondrial genomes are also typically AT rich (~ 70%).
One or more mitochondrial DNA molecules resides within each of the several nucleoid regions within the mitochondrion. If a cell contains 250 mitochondria, each with 5 mtDNA molecules, there will be 1250 mtDNA copies in that cell. Mitochondrial ribosomes are also highly variable between species (e.g., 55S in animals, 73S in yeast). There is also some interspecific variation in mitochondrial tRNAs. Some codons are read differently by mitochondrial tRNAs than by nuclear-encoded tRNAs. For example, AUA codes for methionine (not isoleucine) andUGAcodes for tryptophan (not translation termination) inmammalian mitochondria. MitochondrialmRNAs of fungi and higher plants contain introns, but those of animals lack introns and are transcribed as polycistronic mRNAs that become cut into monocistronic mRNAs before translation. Mitochondria have no DNA repair systems. Hence, the mutation rate of mictochondrial DNA is much higher than that of nuclear DNA.
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