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Organelle genomes. Small but essential genomes Multiple organelles per cell; multiple genomes per organelle (20 – 20,000 genomes per cell, depending on cell type) Organized in nucleo -protein complexes called nucleoids - PowerPoint PPT Presentation
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Organelle genomesSmall but essential genomes
Multiple organelles per cell; multiple genomes per organelle (20 – 20,000 genomes per cell, depending on cell type)
Organized in nucleo-protein complexes called nucleoids
Non-Mendelian inheritance; usually but not always maternally inherited
Encode necessary but insufficient information to elaborate a fully functional organelle
Many nuclear gene products required for organelle function
Considerable cross-talk between nuclear and organelle genetic systems
The fate of endosymbiotic genomes Reduced coding content of organelle genomes Functional gene transfer to nucleus with protein targeted back to organelleEvolution of mitochondrial genome coding contentGenome Protein
coding genes
Rikettsia prowazekii (smallest proteobacterial genome)
832
Reclinomonas americana mitochondria(protozoan; most mitochondrial genes)
62
Marchantia polymorpha mitochondria1.9 x 10 5 bp(liverwort, non-vascular plant )
64
Arabidopsis thaliana mitochondria3.7 x 10 5 bp(vascular plant)
57
Homo sapiens mitochondria 13
Evolution of plastid genome coding content
Genome Protein coding genes
Synechococcus (cyanobacteria) 3,300Paulinella chromatophoraphotosynthetic body(endosymbiont cyanobacteria)
867
Porphyra purpurea plastid(red alga)
209
Chlamydomonas reinhardtii plastid(green alga)
63
Marchantia polymorpha plastid(liverwort, non-vascular plant)
67
Arabidopsis thaliana plastid(vascular plant)
71
Epifagus virginiana plastid (non-photosynthetic parasitic plant)
42
Plastid genome coding contentChloroplast Genome Database: http://chloroplast.cbio.psu.edu/
Generally conserved among land plants, more variable among algae
Genes for plastid gene expression rRNAs, tRNAs ribosomal proteins RNA polymerase
Genes involved in photosynthesis 28 thylakoid proteins
Photosystem I (psa)Photosystem II (psb)ATP synthase subunits (atp)NADH dehydrogenase subunits (nad)Cytochrome b6f subunits (pet)
RUBISCO large subunit (rbcL)(rbcS is nuclear encoded)
Group II introns, which are widely believed to be the progenitors of the nuclear splicing machinery (the spliceosome) and its substrates, comprise a large ribozyme (catalytic RNA) and the coding sequence of a reverse transcriptase. Group II introns are found in mitochondrial, chloroplast and bacterial genomes and a majority of them behave as retrotransposons.
The mitochondrial genome
HUMAN NUCLEAR GENOME24 chromosomes (haploid)3200 Mbp30,000 genes
Mitochondrial genome16569 bp37 genes
Human genome = nuclear genome + mitochondrial genome
• 1-10 m small• Mitochondria are
present in the cytoplasm of all eukaryote cells of animals and higher plants and also in some microorganisms (algae, fungi, protozoa).
Mitochondrial Genome
• Small circular genome• >1000 copies/ cell• 16569 bp 44%
G+C• H- Strand
Guanines• L- Strand
Cytosines• D- Loop 7S DNA
Mitochondrion plays a role in:
• Energy production Oxidative phosphorilation (OXPHOS)
• Maintaining the intracellular homeostasis• Protecting the rest of the cell from reactive
oxygen species (ROS)• Apoptosis important development and
disease
Mitochondria-the point of no return-to live or to die
casp9Bcl2 Apaf1ATP Caspase 3
Apoptosis
AIF
substratesAIF
Nuclear apoptosis
Smac/Diablo
XIAP
Pro-caspase 3
Genome Structure• The mitochondrial genome is a circle, 16.6
kb of DNA. A typical bacterial genome is 2-4 Mbp.
• The two strands are notably different in base composition, leading to one strand being “heavy” (the H strand) and the other light (the L strand).
• Both strands encode genes, although more are on the H strand.
• A short region (1121 bp), the D loop (D = “displacement”), is a DNA triple helix: there are 2 overlapping copies of the H strand there.
• The D loop is also the site where most of replication and transcription is controlled.
• Genes are tightly packed, with almost no non-coding DNA outside of the D loop. In one case, two genes overlap: they share 43 bp, using different reading frames. Human mitochondrial genes contain no introns, although introns are found in the mitochondria of other groups (plants, for instance).
The Human Mitochondrial Genome• Circular
• ~ 16 kb (some plants ~100 kb!)
• Crowded (~40 genes)
• 13 genes involved in oxidative phosphorylation + other genes (DNA pol, rDNAs, tRNAs)
• Most proteins in mitochondria are imported from cytoplasm
• 100,000 copies of mitochondrial DNA in ovum
2 - 10 copies/mitochondrion
Organization of the human genome
Limited autonomy of mt genomesmt encoded nuclear
NADH dehydrog 7 subunits >41 subunitsSuccinate CoQ red 0 subunits 4 subunitsCytochrome b-c1 comp 1 subunit 10 subunitsCytochrome C oxidase 3 subunits 10 subunitsATP synthase complex 2 subunits 14 subunitstRNA components 22 tRNAs nonerRNA components 2 components noneRibosomal proteins none ~80 Other mt proteins none mtDNA pol, RNA pol
etc.
The Human Mitochondrial Genomeexpression unlike nucleus genome…
• Transcription controlled by nuclear proteins:3 promoters- * H1: H-strand; complete transcription of one strand of mtDNA
* L: L-strand; complete transcription of light strand of mtDNA
* H2: Synthesis of 2 rRNAs
• Transcripts then procesed into individual genes prior to translation
Coding- Non-coding
37 genes 9 genes28 genes
L- strandH- strand
24 genes specify a mature RNA product
2 mitochondrial rRNA molecules (23S and 16S)
22 tRNA molecules
13 genes specify polypeptides
H strand enriched in G
L strand enriched in C
Mitochondrial Genetic code is somewhat different…
Human Mito Standard AGA Ter Arg AGG Ter Arg AUA, AUU Met Ile UGA Trp Ter
Plastid genetic code: GUG, UUG, AUU, CUG can initiate translation
UGA encodes trp at low efficiency in E. coli
Mitochondrial inheritance pattern - uniparentalmaternal in animals
Paternal inheritance in gymnosperms, some angiosperms
Endosymbiont Hypothesis• endosymbiont hypothesis: originally proposed in 1883 by Andreas Schimper, but
extended by Lynn Margulis in the 1980s.• Mitochondrial ribosomal RNA genes and other genes show that the original
organism was in the alpha-proteobacterial family (similar to nitrogen-fixing bacteria)
• Evidence: – mitochondria have their own DNA (circular)– the inner membrane is more similar to prokaryotic membranes than to eukaryotic. By
the hypothesis, the inner membrane was the original prokaryotic membrane and the outer membrane was from the primitive eukaryote that swallowed it.
– mitochondria make their own ribosomes, which are of the prokaryotic 70S type, not the eukaryotic 80S type.
– mitochondria are sensitive to many bacterial inhibitors that don’t affect the rest of the eukaryotic cell, such as streptomycin, chloramphenicol, rifampicin.
– mitochondrial protein synthesis starts with N-formyl methionine, as in the bacteria but unlike eukaryotes.
• Most of the original bacterial genes have migrated into the nucleus.• Eukaryotes that lack mitochondria generally have some mitochondrial genes in
their nucleus, evidence that their ancestors had mitochondria that were lost during evolution.
