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Lodish H, Berk A, Zipursky SL, et al. Molecular cabinet Biology. Fourth edition. New York: W. H. Freeman; 2000.

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Although the vast bulk of DNA in many eukaryotes is uncovered in the nucleus, someDNA is present within the mitochondria of animals, plants, and fungi and also within thechloroplasts that plants. These organelles are the key cellular sites for ATPformation, throughout oxidative phosphorylation in mitochondria and photosynthesis inchloroplasts (Chapter 16). Numerous linesof evidence indicate the mitochondria and chloroplasts advanced from bacteria thatwere endocytosed into genealogical cells include a eukaryotic bio nucleus, formingendosymbionts. over evolutionary time, many of the bacte-rial genesencoding contents of the present-day organelles were moved to the nucleus.However, mitochondria and chloroplasts in today’s eukaryotes retaincircular DNAs encoding proteins important for organellar role as well as theribosomal and also transfer RNAs compelled for your translation. Therefore eukaryotic cellshave multiple hereditary systems: a primary nuclear system and second systemswith their own DNA in the mitochondria and chloroplasts.

Mitochondria save on computer Multiple mtDNA Molecules

Individual mitochondria are big enough to be viewed under the irradiate microscopeand even the mitochondrial DNA (mtDNA) deserve to be recognize by fluorescencemicroscopy. The mtDNA is located in the interior of the mitochondrion, theregion well-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 molecule (Figure 9-42).


Figure 9-42

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

Since the dyes offered to visualize nuclear and also mitochondrial DNA perform not impact cellgrowth or division, replication that mtDNA and department of the mitochondrialnetwork have the right to be complied with in life cells using time-lapse microscopy. Suchstudies present that mtDNA replicates transparent interphase. In ~ mitosis eachdaughter cabinet receives around the same number of mitochondria, but sincethere is no device for apportioning exactly equal numbers of mitochondria tothe daughter cells, part cells contain an ext mtDNA than others. Every themitochondria in eukaryotic cells contain multiple mtDNA molecules. Hence thetotal lot 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 Encode rRNAs, tRNAs, andSome Mitochondrial Proteins

Studies the mutants in yeasts and also other single-celled organisms very first indicatedthat mitochondria exhibit cytoplasmic inheritance and thus mustcontain their own hereditary 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 cell grow much more slowly 보다 wild-typeyeasts and kind smaller nests (hence the name “petite”).Genetic crosses in between different (haploid) yeast strains confirmed that thepetite mutation does no segregate with any known nucleargene or chromosome. In later studies, many petite mutants were found to containdeletions of mtDNA.


Figure 9-43

Cytoplasmic inheritance the the petite mutationin yeast. Petite-strain mitochondria space defective in oxidative phosphorylationdue come a deletion in mtDNA. (a) Haploid cell fuse to develop adiploid cell the undergoes meiosis, throughout which random segregationof (more...)

Mitochondrial inheritance in yeasts is biparental: during the blend of haploidcells, both parents add equally to the cytoplasm that the diploid. Inmammals and most various other animals, however, the sperm contributes small (if any)cytoplasm to the zygote, and also virtually every one of the mitochondria in the embryo arederived indigenous those in the egg, not the sperm. Researches in mouse have shown that99.99 percent that mtDNA is maternally inherited, yet a small component (0.01 percent)is inherited native the masculine parent. In greater plants, mtDNA is inheritedexclusively in a uniparental fashion through the female parent (egg), not themale (pollen).

The whole mitochondrial genome indigenous a variety of different organisms has now beencloned and sequenced, and also mtDNAs from all these sources have been uncovered toencode rRNAs, tRNAs, and also essential mitochondrial proteins. All proteins encodedby mtDNA room synthesized on mitochondrial ribosomes. Every mitochondriallysynthesized polypeptides figured out thus far (with one feasible exception) arenot finish enzymes yet subunits of multimeric complexes supplied in electron transfer or ATP synthesis. Most proteins localized in mitochondria, such as themitochondrial RNA and DNA polymerases, space synthesized ~ above cytoplasmic ribosomesand are imported into the organelle by processes disputed in thing 17.

The Size and also Coding volume of mtDNA Vary significantly in DifferentOrganisms

Surprisingly, the dimension of the mtDNA, the number and also nature the the protein itencodes, and even the mitochondrial hereditary code itself, vary considerably amongdifferent organisms. Person mtDNA, a circular molecule that has been completelysequenced, is among the smallest known mtDNAs, include 16,569 basic pairs(Figure 9-44). That encodes the tworRNAs discovered in mitochondrial ribosomes and the 22 tRNAs supplied to translatemitochondrial mRNAs. Human being mtDNA has 13 sequences that start with an ATG(methionine) codon, end with a protect against codon, and also are long sufficient to encode apolypeptide of more than 50 amino acids; every one of the feasible proteins encoded bythese open reading frames have actually been identified. Mammalian mtDNA, in comparison tonuclear DNA, lacks introns and contains no lengthy noncoding sequences.


Figure 9-44

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

Invertebrate mtDNA is around the same size as human being mtDNA, yet yeast mtDNA isalmost 5 times as large (≈78,000 bp). The mtDNAs from yeast and otherlower standard scale encode plenty of of the very same gene products as mammalian mtDNA, aswell together others whose genes are uncovered in the nuclei that mammalian cells.

In contrast to other eukaryotes, i m sorry contain a single type of mtDNA, plantscontain several species of mtDNA that appear to recombine v each other. PlantmtDNAs are much larger and an ext variable in dimension than the mtDNAs of otherorganisms. Also in a solitary family the plants, mtDNAs deserve to vary as much aseightfold in dimension (watermelon = 330,000 bp;muskmelon = 2,500,000 bp). Unlike animal,yeast, and fungal mtDNAs, plant mtDNAs contain gene encoding a 5S mitochondrialrRNA, i m sorry is current only in the mitochondrial ribosome of plants, and theα subunit the the F1 ATPase. The mitochondrial rRNAs ofplants are additionally considerably larger than those of various other eukaryotes. The recentsequencing of one of the the smallest plant mtDNAs has revealed that long, noncodingregions and duplicated assignment are mostly responsible for the better lengthof plant mtDNAs.

Differences in the size and coding volume of mtDNA from various organisms mostlikely reflect the motion of DNA between mitochondria and also the cell nucleus duringevolution. Straight evidence for this movement comes from the monitoring thatseveral proteins encoded by mtDNA in some species are encoded by atom DNA inothers. It thus appears that whole genes relocated from the mitochondrion to thenucleus, or vice versa, during evolution.

The many striking instance of this phenomenon requires the gene coxII, which encodes subunit 2 the cytochrome coxidase. This gene is discovered in mtDNA in every organisms studied except for onespecies the legume, the mung bean: in this organism only, the coxII gene is nuclear. Numerous RNA transcripts of tree mitochondrialgenes space edited, greatly by the enzyme-catalyzed counter of selected Cresidues come U, and also occasionally U to C. (RNA editing and enhancing is questioned in chapter 11.) The atom coxII gene of mung bean corresponds more closely come the edited cox IIRNA transcripts than to the mitochondrial cox II genes found inother legumes. These facts are solid evidence the the cox IIgene relocated from mitochondrion come the nucleus throughout mung bean evolution by aprocess that connected an RNA intermediate. May be this movement involved areversetranscription mechanism comparable to that whereby processed pseudogenesare generated in the nuclear genome native nuclear-encoded mRNAs.

Products of Mitochondrial Genes are Not Exported

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

Reflecting the bacterial genealogy of mitochondria, mitochondrial ribosomesresemble bacter ribosomes and differ from cytoplasmic ribosom 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, but not by cytoplasmic ribosomes. Conversely,cycloheximide inhibits protein synthesis by eukaryotic bio cytoplasmic ribosomes butdoes not affect protein synthesis by mitochondrial ribosomes or bacterialribosomes. In cultured mammalian cell the just proteins synthesized in thepresence the cycloheximide room encoded through mtDNA and also produced by mitochondrialribosomes.

Mitochondrial genetic Codes different from the typical Nuclear Code

The hereditary code offered in animal and fungal mitochondria is different from thestandard code supplied in every prokaryotic and eukaryotic atom genes; remarkably,the code even differs in mitochondria indigenous different species (Table 9-4). Why and how this phenomenonhappened during evolution is mysterious. UGA, because that example, is usually a stopcodon, however is review as tryptophan by human and fungal mitochondrial translationsystems; however, in tree mitochondria, UGA is quiet a stop codon. AGA and also AGG,the standard nuclear codons because that arginine additionally code for arginine in fungal andplant mtDNA, but they are protect against codons in mammalian mtDNA and also serine codons inDrosophila mtDNA.


As shown in Table 9-4, plantmitochondria appear to utilize the standard genetic code. However, comparisonsof the amino acid sequences of tree mitochondrial proteins v the nucleotidesequences of plant mtDNAs said that CGG might code foreither arginine (the “standard” aminoacid) or tryptophan. This apparent nonspecificity the the tree mitochondrialcode is defined by editing and enhancing of mitochondrial RNA transcripts, which deserve to convertcytosine residual water to uracil residues. If a CGG succession is edited to UGG, thecodon states tryptophan, the typical amino mountain for UGG, vice versa, uneditedCGG codons encode the typical arginine. Thus the translation device in plantmitochondria does utilize the standard genetic code.

Mutations in Mitochondrial DNA reason Several Genetic illness in Man

The severity of disease caused by a mutation in mtDNA depends on the nature ofthe mutation and on the proportion of mutant and also wild-type DNAs existing in aparticular cabinet type. Generally, when mutations in mtDNA room found, cellscontain mixtures of wild-type and 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 cell (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 mostly mutant mtDNAs. Since all enzymes forthe replication and also growth that mitochondria, such together DNA and also RNA polymerases, areimported native the cytosol, a mutant mtDNA should not be at a“replication disadvantage”; mutants the involve largedeletions that mtDNA could even be in ~ a selective advantage in replication.

All cells have actually mitochondria, yet mutations in mtDNA only impact some tissues.Those many usually affected are organization that have actually a high requirement for ATPproduced through oxidative phosphorylation, and also tissues the require many or every ofthe mtDNA in the cell to synthesize practical mitochondrial proteins.Leber’s hereditary optic neuropathy (degeneration the the optic nerve,accompanied by increasing blindness), for instance, is resulted in by a missensemutation in the mtDNA gene encoding subunit 4 that the NADH-CoQ reductase. Any ofseveral big deletions in mtDNA cause another set of conditions including chronicprogressive outside ophthalmoplegia and also Kearns-Sayre syndrome, which arecharacterized by eye defects and, in Kearns-Sayre syndrome, additionally by abnormalheartbeat and main nervous device degeneration. A 3rd condition, causing“ragged” muscle fibers (with improperly assembledmitochondria) and associated untreated jerky movements, is because of a singlemutation in the TYCG loop of the mitochondrial lysine tRNA. As a result of thismutation, the translation of number of mitochondrial proteins supposedly isblocked.

Chloroplasts Contain large Circular DNAs Encoding much more Than a HundredProteins

As we discuss in thing 16, thestructure of chloroplasts is similar in countless respects to the of mitochondria.Like mitochondria, chloroplasts save on computer multiple duplicates of the organellar DNAand ribosomes, i beg your pardon synthesize part chloroplast-encoded proteins making use of the“standard” hereditary code. Other chloroplast protein arefabricated ~ above cytosolic ribosomes and also are included into the organelle aftertranslation.

Chloroplast DNAs are circular molecules of120,000 – 160,000 bp, depending upon the species.The finish sequences of several chloroplast DNAs have been determined,including those indigenous liverwort (121,024 bp) and also tobacco (155,844 bp). Theliverwort chloroplast genome has two reverse repeats, every consisting the 10,058bp, the contain the rRNA genes and also a few other replicated genes. In spite of thedifference in size, the all at once organization and gene composition of theliverwort and also tobacco DNAs are really similar; the dimension differential is dueprimarily to the size of the turning back repeat in which some genes areduplicated.

Of the ≈120 gene in chloroplastic DNA, about 60 are affiliated in RNAtranscription and also translation, consisting of genes for rRNAs, tRNAs, RNA polymerasesubunits, and ribosomal proteins. About 20 genes encode subunits that thechloroplast photosynthetic electron deliver complexes and also theF0F1 ATPase complex. Also encoded in the chloroplastgenome is the larger of the 2 subunits that ribulose 1,5-bisphosphatecarboxylase, which is affiliated in the fixation of carbon dioxide duringphotosynthesis.

Reflecting the endosymbiotic beginning of chloroplasts, some regions of chloroplastDNA space strikingly similar to those that the DNA of contemporary bacteria. Forinstance, chloroplasts DNA encodes 4 subunits the RNA polymerase the arehighly homologous to 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 exact same in the 2 DNAs.

Liverwort chloroplastic DNA has actually some genes that space not detected in the largertobacco chloroplast DNA, and vice versa. Since the two species of chloroplastscontain virtually the same set of proteins, this data imply that some genesare existing in the chloroplastic DNA the one species and in the atom DNA the theother, indicating that some exchange the genes in between chloroplast and nucleushas arisen during evolution.

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

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