Organelle Genetics

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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

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• Examples of non-Mendelian inheritance
• Variegated-shoot phenotypes in four oclocks
  (Correns, 1908)

Mixed chloroplasts White/green
Mutant chloroplast White non-photosynthetic
Normal chloroplast Green photosynthetic
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Maternal inheritance
The female phenotype in a cross is always
expressed in its offspring

• 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 inheritance of organellar (chloroplast
and mitochondrial) DNA.
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Maternal inheritance with occasional paternal
transmission
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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

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Origins of mitochondria chloroplasts

• Both 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 gt 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)

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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
gt60 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
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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)

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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

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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 gene
• Examples
• myoclonic epilepsy and ragged red fiber disease
  (MERRF)
• Deafness, dementia, seizures
• Point Mutation in a mitochondrial tRNA

• Lebers 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

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Why arent all mitochondrial mutations
lethal?Heteroplasmy
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Heteroplasmic Cells
Homoplasmic Cell
70 mutant mitochondria severe symptoms?
30 mutant mitochondria mild symptoms
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.
http//www.mitoresearch.org/mitodiseases.html
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http//www.mitoresearch.org/mitodiseases.html
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Commonly Affected Systems in Mitochondrial
Disorders
http//www.mitoresearch.org/treatmentdisease.html
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Maize mitochondrial genomeNB genotypefrom the
inbred B37 line
58 identified genes 33 known proteins 21 tRNAs
(for 14 diff aa) 3 rRNAs A tRNA is carried
on a 2 kb linear plasmid
569,630 nt
From Clifton et al. 2004, Plant Physiology
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What is in the NB maize mitochondrial genome ?
Based on genome complexity (one copy of large
repeats removed) 520 kb for maize NB 359 kb for
rice
From Clifton et al. 2004, Plant Physiology
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Half the Arabidopsis mitochondrial DNA sequences
are both non-coding and novel
- 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)
Marienfeld, Unseld and Brennicke,1999, TIPS
4495-502