Organelle Genetics
• Maternal inheritance• Non-Mendelian inheritance
– Ratios do not fit those proposed by Mendel
• Cytoplasmic inheritance• Nonchromosomal inheritance
Terms are not necessarily equivalent!
Plasmon = genetic elements of the cytoplasm
Examples of non-Mendelian inheritance:• Variegated-shoot phenotypes in four o’clocks (Correns, 1908)
Normal chloroplastGreenphotosynthetic
Mutant chloroplastWhitenon-photosynthetic
Mixed chloroplastsWhite/green
Maternal inheritance
• Experiments were performed by Correns on the four o'clock plant Green, variegated (white and green) or white leaves
Normal flowers develop at different locations on the plant
Crosses were made among the flowers associated with:
Female Male Phenotype Progeny Phenotype Phenotype
Green Green, variegated or white Green
Variegated Green, variegated or white Variegated
White Green, variegated or white White The progeny cross always exhibited the color of the leaf of the female Trait expresses maternal inheritance
Most plants, including corn, Arabidopsis, wheat and tomato show strict maternal inheritanceof organellar (chloroplast and mitochondrial) DNA.
The female phenotype in a cross is always expressed in its offspring
Maternal inheritance: with occasional paternal transmission
Plants showing "strict" maternal inheritance (albomaculatus)
Test for male transmission in Antirrhinum majus:
Normal x yellow-green (prasinizans) 41,203 normal:13 variegated
yellow-green (prasinizans) x Normal 42,285 yellow-green:8 varieg
Mutation in female parent unlikely. a) Mutation rate is low.
Only 1/436,000 plants was spontaneously variegated. b) Same phenotype seen for all variegated progeny
Biparental inheritance
• Inheritance of variegation suggested both parents contribute, but ratios are non-Mendelian.
• Pollen transmission of plastids found for several species
– e.g. Pelargonium (geranium), Oenothera, Medicago, Phaseolus, Acacia
• Some plants have regular paternal transmission of plastids through pollen.
• Nuclear genes can affect “strength” of pollen transmission of organelles.
– e.g. Oenothera, Pelargonium (geranium), Phaseolus, Hypericum
Origins of mitochondria & chloroplastsBoth mitochondria and chloroplasts are believed to be derived from
endosymbiotic bacteria.
Endosymbiotic bacteria = free-living prokaryotes that invaded ancestral eukaryotic cells and established a mutually beneficial relationship.
Mitochondria - believed to be derived from a photosynthetic purple bacterium that entered a eukaryotic cell > billion years ago.
Chloroplasts - believed to be derived from a photosynthetic cyanobacterium.
Many required mitochondria and chloroplast proteins also are coded by nuclear genes.
numtDNA = nuclear mtDNA (mtDNA transposed to the nucleus)
Mitochondrial genome size and genic content
Size and gene content of mitochondrial genomes compared with a Proteobacterial (Rickettsia) genome
Circles and lines represent circular and linear genome shapes, respectively. For genomes >60 kbp, the DNA coding for genes with known function (red) is distinguished from that coding for unidentified ORFs and intergenic sequences (blue).
M. W. Gray, G. Burger, and B. F. Lang Science 1999: 1476-1481. Mitochondrial Evolution
mtDNAs --- Overview
• mtDNAs occur in (almost) all aerobic eukaryotic cells & generate energy for cells by oxidative phosphorylation (producing ATP).
• Most mtDNA genomes are circular and supercoiled (linear mtDNAs occur in some protozoa and some fungi).
• mtDNAs lack histone proteins.
• Copy number is high, multiple genomes per mitochondria and many mitochondria per cell (can easily PCR).
• Sizes of mtDNA varies widely.
• Humans and other vertebrates ~17 kb(all of mtDNA codes gene products)
• Yeast ~80 kb
• Plants ~100 kb to 2 Mb
(lots of non-coding mtDNA)
Human Mitochondrial Genomes
• Human Nuclear Genome: About 30,000 genes on 23 chromosomes (3.3 billion base pairs/haploid cell)
• Mitochondrial Genome contains 37 genes:– 13 code for some of the proteins involved in
oxidative respiration– 22 tRNA genes– 2 rRNA genes
– 16,569 base pairs, circular, very compact, filled with genes
Human Mitochondrial Diseases
Are maternally inherited: only offspring of affected mothers are affected
Show deficiency in mitochondrial function
Are caused by a mutation in a mitochondrial geneExamples:
• myoclonic epilepsy and ragged red fiber disease (MERRF)
• Deafness, dementia, seizures
• Point Mutation in a mitochondrial tRNA
• Leber’s Hereditary Optic Neuropathy (LHON)
• Sudden bilateral blindness
• Point mutation in small subunit of NADH dehydrogenase
• Kearns-Sayre Syndrome (KSS)
• Symptoms in eyes, muscles, heart, brain
• Deletion mutation in mtDNA
Why aren’t all mitochondrial mutations lethal?
Heteroplasmy
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HETEROPLASMY: Normal mitochondria with normal DNA vs Mitochondria with mutant DNA "Homoplasmic Cell. Healthy people have homoplasmic cells -- that is, each cell has normal mitochondrial DNA. People with mitochondrial DNA mutations have heteroplasmic cells. Each cell has a mixture of good and bad mitochondria.
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70% mutant mitochondria =severe symptoms?
30% mutant mitochondria =mild symptoms
Heteroplasmic CellsHomoplasmic Cell
http://www.mitoresearch.org/mitodiseases.html
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http://www.mitoresearch.org/mitodiseases.html
http://www.mitoresearch.org/treatmentdisease.html
Commonly Affected Systems in Mitochondrial Disorders
Maize mitochondrial genome“NB” genotype
from the inbred B37 line
569,630 nt58 identified genes33 known proteins21 tRNAs (for 14 diff aa)* 3 rRNAs
*A tRNA is carried on a 2 kb linear plasmid
From Clifton et al.2004, Plant Physiology
What is in the NB maize mitochondrial genome ?
Based on genome complexity (one copy of large repeats removed)520 kb for maize NB359 kb for rice
Exons
Introns
rRNAs
ctDNA
tRNAs
???
ORFs
pseudo
From Clifton et al.2004, Plant Physiology
Half the Arabidopsis mitochondrial DNA sequences are both non-coding and novel
Marienfeld, Unseld and Brennicke,1999, TIPS 4:495-502
- In sugar beet, 55% of mt genome has no recognizable function or database homology- Only 21% of sugar beet mtDNA is shared with Arabidopsis (Kubo et al. NAR 2000)
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