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Lodish H, Berk A, Zipursky SL, et al. Molecular cell Biology. Fourth edition. Brand-new York: W. H. Freeman; 2000.

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Although the vast bulk of DNA in most eukaryotes is discovered in the nucleus, someDNA is present within the mitochondria the animals, plants, and also fungi and also within thechloroplasts of plants. These organelles space the main cellular sites because that ATPformation, throughout oxidative phosphorylation in mitochondria and photosynthesis inchloroplasts (Chapter 16). Many linesof evidence indicate the mitochondria and also chloroplasts progressed from bacteria thatwere endocytosed into ancestral cells containing a eukaryotic bio nucleus, formingendosymbionts. end evolutionary time, most of the bacte-rial genesencoding materials of the present-day organelles were transferred to the nucleus.However, mitochondria and also chloroplasts in today’s standard scale retaincircular DNAs encoding proteins crucial for organellar function as well as theribosomal and transfer RNAs forced for your translation. Thus eukaryotic cellshave multiple hereditary systems: a primary nuclear system and an additional systemswith their very own DNA in the mitochondria and chloroplasts.

Mitochondria contain Multiple mtDNA Molecules

Individual mitochondria are large enough to be watched under the irradiate microscopeand even the mitochondrial DNA (mtDNA) can be recognize by fluorescencemicroscopy. The mtDNA is located in the internal of the mitochondrion, theregion known as the matrix. Together judged by the number of yellow fluorescent“dots” the mtDNA, a Euglena gracilis cellcontains at the very least thirty mtDNA molecules (Figure 9-42).


Figure 9-42

Visualization that mitochondrial DNA in a farming Euglenagracilis cell. Cells to be treated with a mixture of 2 dyes: ethidium bromide,which binds to DNA and also emits a vermilion fluorescence, and DiOC6,which is included specifically right into mitochondria (more...)

Since the dyes used to visualize nuclear and mitochondrial DNA perform not influence cellgrowth or division, replication the mtDNA and department of the mitochondrialnetwork can be complied with in life cells utilizing time-lapse microscopy. Suchstudies present that mtDNA replicates throughout interphase. In ~ mitosis eachdaughter cell receives around the same variety of mitochondria, yet sincethere is no mechanism for apportioning precisely equal numbers of mitochondria tothe daughter cells, part cells contain more mtDNA 보다 others. All themitochondria in eukaryotic bio cells save multiple mtDNA molecules. Thus thetotal quantity of mtDNA in a cell counts on the variety of mitochondria, the sizeof the mtDNA, and the number of mtDNA molecules per mitochondrion.

Genes in mtDNA exhibit Cytoplasmic Inheritance and also Encode rRNAs, tRNAs, andSome Mitochondrial Proteins

Studies of mutants in yeasts and also other single-celled organisms an initial indicatedthat mitochondria exhibit cytoplasmic inheritance and thus mustcontain their own genetic system (Figure9-43). For instance, petite yeast mutants exhibitstructurally abnormal mitochondria and also are i can not qualify of oxidative phosphorylation. Together a result, petite cells grow more slowly than wild-typeyeasts and kind smaller colonies (hence the surname “petite”).Genetic crosses between different (haploid) yeast strains proved that thepetite mutation does not segregate with any known nucleargene or chromosome. In later on studies, many petite mutants were uncovered to containdeletions that mtDNA.


Figure 9-43

Cytoplasmic inheritance that the petite mutationin yeast. Petite-strain mitochondria are defective in oxidative phosphorylationdue come a deletion in mtDNA. (a) Haploid cell fuse to produce adiploid cell the undergoes meiosis, during which arbitrarily segregationof (more...)

Mitochondrial inheritance in yeasts is biparental: throughout the blend of haploidcells, both parents add equally come the cytoplasm the the diploid. Inmammals and most other animals, however, the sperm contributes small (if any)cytoplasm come the zygote, and also virtually all of the mitochondria in the embryo arederived native those in the egg, not the sperm. Studies in mice have presented that99.99 percent the mtDNA is maternally inherited, yet a small part (0.01 percent)is inherited indigenous the masculine parent. In higher plants, mtDNA is inheritedexclusively in a uniparental fashion through the female parent (egg), not themale (pollen).

The entire mitochondrial genome native a variety of different organisms has now beencloned and sequenced, and mtDNAs from every these sources have actually been uncovered toencode rRNAs, tRNAs, and also essential mitochondrial proteins. Every proteins encodedby mtDNA space synthesized on mitochondrial ribosomes. Every mitochondriallysynthesized polypeptides established thus far (with one possible exception) arenot complete enzymes but subunits that multimeric complexes offered in electron transfer or ATP synthesis. Most proteins localized in mitochondria, such together themitochondrial RNA and DNA polymerases, room synthesized on cytoplasmic ribosomesand room imported into the organelle by processes discussed in chapter 17.

The Size and Coding volume of mtDNA Vary significantly in DifferentOrganisms

Surprisingly, the size of the mtDNA, the number and also nature of the proteins itencodes, and even the mitochondrial hereditary code itself, vary considerably amongdifferent organisms. Human being mtDNA, a one molecule that has been completelysequenced, is amongst the smallest known mtDNAs, comprise 16,569 base pairs(Figure 9-44). It encodes the tworRNAs discovered in mitochondrial ribosomes and also the 22 tRNAs used to translatemitochondrial mRNAs. Human being mtDNA has actually 13 assignment that begin with one ATG(methionine) codon, end with a protect against codon, and are long sufficient to encode apolypeptide of an ext than 50 amino acids; every one of the possible proteins encoded bythese open reading frames have been identified. Mammalian mtDNA, in contrast tonuclear DNA, lacks introns and contains no long noncoding sequences.


Figure 9-44

Human mitochondrial DNA (mtDNA), which has been sequenced in itsentirety. Proteins and also RNAs encoded by every of the two strands room shownseparately. Warrior of the outer (H) strand wake up in theclockwise direction and of the inner (L) strand in (more...)

Invertebrate mtDNA is about the same size as human mtDNA, but yeast mtDNA isalmost 5 times as huge (≈78,000 bp). The mtDNAs from yeast and otherlower standard scale encode plenty of of the same gene products as mammalian mtDNA, aswell as others whose gene are uncovered in the nuclei of mammalian cells.

In contrast to various other eukaryotes, i beg your pardon contain a single form of mtDNA, plantscontain several varieties of mtDNA that appear to recombine through each other. PlantmtDNAs space much bigger and much more variable in size than the mtDNAs that otherorganisms. Also in a solitary family that plants, mtDNAs have the right to vary as lot aseightfold in size (watermelon = 330,000 bp;muskmelon = 2,500,000 bp). Uneven animal,yeast, and also fungal mtDNAs, plant mtDNAs contain gene encoding a 5S mitochondrialrRNA, i beg your pardon is present only in the mitochondrial ribosomes of plants, and also theα subunit of the F1 ATPase. The mitochondrial rRNAs ofplants are also considerably bigger than those of various other eukaryotes. The recentsequencing of among the the smallest plant mtDNAs has actually revealed that long, noncodingregions and duplicated order are mostly responsible because that the greater lengthof plant mtDNAs.

Differences in the size and coding capacity of mtDNA from miscellaneous organisms mostlikely reflect the activity of DNA in between mitochondria and also the cell nucleus duringevolution. Straight evidence for this movement originates from the monitoring thatseveral protein encoded by mtDNA in some varieties are encoded by nuclear DNA inothers. That thus appears that whole genes relocated from the mitochondrion come thenucleus, or angry versa, throughout evolution.

The most striking example of this phenomenon requires the gene coxII, i m sorry encodes subunit 2 of cytochrome coxidase. This gene is uncovered in mtDNA in all organisms studied other than for onespecies of legume, the mung bean: in this organism only, the coxII gene is nuclear. Numerous RNA transcripts of tree mitochondrialgenes room edited, mostly by the enzyme-catalyzed counter of selected Cresidues come U, and occasionally U come C. (RNA modifying is discussed in thing 11.) The atom coxII gene that mung p corresponds an ext closely come the edited cox IIRNA transcripts 보다 to the mitochondrial cox II genes discovered inother legumes. This facts are strong evidence that the cox IIgene relocated from mitochondrion to the nucleus throughout mung bean development by aprocess that involved an RNA intermediate. Presumably this movement connected areversetranscription mechanism comparable to that by which processed pseudogenesare created in the nuclear genome native nuclear-encoded mRNAs.

Products that Mitochondrial Genes room Not Exported

As far as is known, every RNA transcripts of mtDNA and also their translate into productsremain in the mitochondrion, and also all mtDNA-encoded proteins space synthesized onmitochondrial ribosomes. Mitochondria encode the rRNAs that form mitochondrialribosomes, although all but one or 2 of the ribosomal proteins (depending onthe species) room imported from the cytosol. In many eukaryotes, all of the tRNAsused for protein synthetic in mitochondria room encoded by mtDNAs. However, inwheat, in the parasitic protozoan Trypanosoma brucei (the causeof African sleeping sickness), and also in ciliated protozoa, every mitochondrial tRNAsare encoded by the atom DNA and imported right into the mitochondrion.

Reflecting the bacterial genealogy of mitochondria, mitochondrial ribosomesresemble bacter ribosomes and also differ native cytoplasmic ribosomes in your RNAand protein compositions, your size, and their sensitivity come certainantibiotics (see number 4-32). Forinstance, chloramphenicol block protein synthetic by bacterial and mostmitochondrial ribosomes, however not by cytoplasmic ribosomes. Conversely,cycloheximide inhibits protein synthesis by eukaryotic bio cytoplasmic ribosome butdoes not influence protein synthetic by mitochondrial ribosome or bacterialribosomes. In cultured mammalian cell the just proteins synthesized in thepresence that cycloheximide space encoded by mtDNA and produced by mitochondrialribosomes.

Mitochondrial hereditary Codes different from the traditional Nuclear Code

The hereditary code offered in animal and also fungal mitochondria is various from thestandard code supplied in all prokaryotic and eukaryotic atom genes; remarkably,the code even differs in mitochondria from different varieties (Table 9-4). Why and how this phenomenonhappened throughout evolution is mysterious. UGA, because that example, is normally a stopcodon, yet is review as tryptophan by human and also fungal mitochondrial translationsystems; however, in plant mitochondria, UGA is quiet a prevent codon. AGA and AGG,the conventional nuclear codons because that arginine likewise code for arginine in fungal andplant mtDNA, yet they are protect against codons in mammalian mtDNA and serine codons inDrosophila mtDNA.


As displayed in Table 9-4, plantmitochondria appear to utilize the standard genetic code. However, comparisonsof the amino mountain sequences of plant mitochondrial proteins with the nucleotidesequences of tree mtDNAs suggested that CGG might code foreither arginine (the “standard” aminoacid) or tryptophan. This noticeable nonspecificity that the tree mitochondrialcode is defined by editing of mitochondrial RNA transcripts, which deserve to convertcytosine residual water to uracil residues. If a CGG sequence is edited come UGG, thecodon states tryptophan, the typical amino mountain for UGG, whereas uneditedCGG codons encode the traditional arginine. Thus the translation mechanism in plantmitochondria does make use of the standard genetic code.

Mutations in Mitochondrial DNA cause Several Genetic illness in Man

The severity of condition caused by a mutation in mtDNA depends on the nature ofthe mutation and also on the relationship of mutant and also wild-type DNAs current in aparticular cabinet type. Generally, once mutations in mtDNA space found, cellscontain mixtures of wild-type and also mutantmtDNAs — a condition known asheteroplasmy. Each time a mammalian somatic or germ-linecell divides, the mutant and wild-type mtDNAs will segregate randomly right into thedaughter cells, as occurs in yeast cells (see figure 9-43). Thus, the mtDNA genotype fluctuates indigenous onegeneration and also from one cell department to the next, and can drift towardpredominantly wild-type or mainly mutant mtDNAs. Due to the fact that all enzyme forthe replication and also growth of mitochondria, such as DNA and RNA polymerases, areimported indigenous the cytosol, a mutant mtDNA should not be at a“replication disadvantage”; mutants that involve largedeletions that mtDNA might even be in ~ a selective advantage in replication.

All cells have actually mitochondria, yet mutations in mtDNA only affect some tissues.Those most usually affected are organization that have a high need for ATPproduced through oxidative phosphorylation, and tissues the require many or all ofthe mtDNA in the cell to synthesize functional mitochondrial proteins.Leber’s hereditary optic neuropathy (degeneration the the optic nerve,accompanied by boosting blindness), for instance, is resulted in by a missensemutation in the mtDNA gene encoding subunit 4 that the NADH-CoQ reductase. Any kind of ofseveral big deletions in mtDNA cause another collection of illness including chronicprogressive outside ophthalmoplegia and Kearns-Sayre syndrome, i beg your pardon arecharacterized by eye defects and, in Kearns-Sayre syndrome, additionally by abnormalheartbeat and central nervous mechanism degeneration. A third condition, causing“ragged” muscle yarn (with improperly assembledmitochondria) and also associated uncontrolled jerky movements, is because of a singlemutation in the TYCG loop that the mitochondrial lysine tRNA. As a result of thismutation, the translate in of several mitochondrial proteins apparently isblocked.

Chloroplasts Contain huge Circular DNAs Encoding much more Than a HundredProteins

As we discuss in chapter 16, thestructure of chloroplasts is comparable in many respects to the of mitochondria.Like mitochondria, chloroplasts save on computer multiple duplicates of the organellar DNAand ribosomes, i m sorry synthesize part chloroplast-encoded proteins making use of the“standard” hereditary code. Other chloroplast proteins arefabricated top top cytosolic ribosomes and also are incorporated into the organelle aftertranslation.

Chloroplast DNAs are circular molecules of120,000 – 160,000 bp, depending on the species.The finish sequences of numerous chloroplast DNAs have been determined,including those from liverwort (121,024 bp) and tobacco (155,844 bp). Theliverwort chloroplast genome has actually two reverse repeats, each consisting that 10,058bp, the contain the rRNA genes and also a few other copied genes. In spite of thedifference in size, the all at once organization and also gene ingredient of theliverwort and also tobacco DNAs are an extremely similar; the size differential is dueprimarily to the size of the reverse repeat in which some genes areduplicated.

Of the ≈120 gene in chloroplasts DNA, around 60 are affiliated in RNAtranscription and also translation, including genes for rRNAs, tRNAs, RNA polymerasesubunits, and also ribosomal proteins. About 20 genes encode subunits that thechloroplast photosynthetic electron carry complexes and also theF0F1 ATPase complex. Likewise encoded in the chloroplastgenome is the bigger of the 2 subunits the ribulose 1,5-bisphosphatecarboxylase, i m sorry is affiliated in the fixation of carbon dioxide duringphotosynthesis.

Reflecting the endosymbiotic origin of chloroplasts, some areas of chloroplastDNA space strikingly comparable to those of the DNA of contemporary bacteria. Forinstance, chloroplastic DNA encodes four subunits of RNA polymerase that arehighly homologous come the subunits that E. Coli RNA polymerase.One segment of chloroplasts DNA encodes eight proteins that room homologous toeight E. Coli ribosome proteins; the order of these gene isthe very same in the 2 DNAs.

Liverwort chloroplasts DNA has some gene that are not detect in the largertobacco chloroplasts DNA, and vice versa. Because the two varieties of chloroplastscontain practically the same collection of proteins, this data imply that part genesare current in the chloroplast DNA that one species and in the nuclear DNA of theother, denote that part exchange the genes between chloroplast and also nucleushas arisen during evolution.

 Most mtDNA is inherited native egg cellsrather than sperm, and mutations in mtDNA an outcome in a maternalcytoplasmic pattern of inheritance.
 Mitochondrial ribosom resemble bacterialribosomes in their structure, sensitivity to chloramphenicol, andresistance come cycloheximide.

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