Mitochondria replication

Because mitochondria replicate more often than do nuclei, the relative proportion of mutant and wild-type mtDNAs may change within a cell cycle. 

There is a second factor that helps to preserve rapidly dividing cells their fast turnover. When a cell divides, it must reproduce its cytoplasm and mitochondria, as well as its DNA. This means that mitochondria replicate faster in rapidly dividing cells than in non-dividing cells, even if the latter are packed with mitochondria. In most cells, the mitochondrial population varies from good condition to completely shredded. Undamaged mitochondria replicate faster than damaged mitochondria. Each time a cell divides, then, the new pool of mitochondria is derived from the least-damaged survivors of the last pool, and this helps to replenish... 

Gap (or growth) 2 phase (G2 phase). The cell grows in size, mitochondria replicate, chromosomes condense and proteins (e.g. microtubular proteins) needed for mitosis are made during this phase. The G2 checkpoint confirms the integrity of DNA and the cell volume. Providing the check is satisfactory, the mitosis phase is triggered by the active cdkl/cyclin B complex. 

Regoes A, Zourmpanou D, Leon-Avila G, van der Giezen M, Tovar J, Hehl AB (2005) Protein import, replication and inheritance of a vestigial mitochondrion. J Biol Chem 280 30557-30563... 

FIGURE 24-6 A dividing mitochondrion. Some mitochondrial proteins and RNAs are encoded by one of the copies of the mitochondrial DNA (none of which are visible here). The DNA (mtDNA) is replicated each time the mitochondrion divides, before cell division. 

While a DNA molecule may exist as a straight rod, the two ends are often covalently joined. Thus, the chromosomes of E. coli and of other bacteria are single closed circles. Circular DNA molecules are also found in mitochondria, chloroplasts, and many viruses. Further complexity arises from the fact that the circles of DNA are sometimes interlocked in chainlike fashion (catenated). An unusual example of this phenomenon is the presence of thousands of small catenated DNA circles in the single mitochondrion of a trypanosome (Fig. 5-16).183 Sometimes circular DNA is knotted as in Fig. 5-17.184-186 Knots and catenanes often appear as intermediate forms during replication and recombination, especially involving circular DNA.187 188... 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

MITOCHONDRION A membrane-bound organelle of eukaryotic organisms that replicates independently of the cell nucleus and contains its own ENA and its own protein-synthesizing apparatus its function is to provide energy to the cell in the form of adenosine triphosphate by oxidative phosphorylation. 

In mammalian cells, some 1% of the total cellular DNA is found in the mitochondria. This DNA is double stranded, circular, and small, with a molecular weight of about 10 million, which is in the same range as that of viral DNAs. Some four to ten molecules of DNA per mitochondrion, along with some ribosomes, are found in the matrix space. DNA replication, transcription, and synthesis of some mitochondrial proteins take place in the matrix space. This protein synthesis very much resembles that of bacteria. The mitochondrial genetic code differs from the "universal" genetic code (Chapter 12) used for nuclearly encoded proteins and bacteria. The reasons for this are unknown. 

The structure of replication forks in eukaryotes is essentially the same as in bacteria. Chain growth is continuous on the leading strand and discontinuous on the lagging strand. There are equivalents of the polymerases, helicase, primase, SSB, etc., but there are clearly some differences. For example, two different polymerases, DNA polymerase 8 and DNA polymerase a, function on the leading and lagging strand, respectively. Also, the mitochondrion has its own DNA polymerase. 

The mitochondrion of all eukaryotes and the chloroplasts of plant cells are the only organelles that have their distinct genomes. These genomes are made of a single, circular DNA molecnle denoted mtDNA in mitochondrion and ctDNA in chloroplast. The replication and the mode of inheritance of organelle genomes are distinct from the nuclear genomes. 

Probably, the ultimate case of symbiosis involves cellular inclusions called mitochondria and chloroplasts. Mitochondria are small cylindrical bodies within eukaryotic cells that function as the chemical powerhouses of these cells (see Sections 5.3.7 and 5.5.1). It is in the mitochondrion that ATP is formed through the biochanical reactions of the Krebs cycle (see Section 3.10). Mitochondria are self replicating within the cell and their numbers increase as cellular energy needs increase. They contain their own DNA (mtDNA) separate from the DNA in the ceU nucleus. 

Another cellular constituent, the mitochondrion, which is essentially responsible for the energetics of the cell, also contains RNA and DNA, but in small quantities (less than 1% of the protein by weight). It is probable that the mitochondrial DNA and RNA constitute the genetic equipment for the replication of these plasmids and control some of... 

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