The Chromosomes of Organelles Outside the Nucleus Exhibit NonMendelian Patterns of Inheritance

1
The Chromosomes of Organelles Outside the Nucleus
Exhibit Non-Mendelian Patterns of Inheritance
2
Outline of Chapter 15

• The structure and function of mitochondrial and
  chloroplast genomes, including a description of
  their size, shape replication, and expression
• How genetic transmission revealed and explained
  non-Mendelian patterns of inheritance
• A comprehensive example of mutations in
  mitochondrial DNA that affect human health

3
Mitochondrial and chloroplasts are organelles of
energy conversion that carry their own DNA

• Chloroplasts capture solar energy and store it
  in carbohydrates
• Mitochondria release energy from nutrients and
  convert it to ATP

4
Mitochondria are sites of the Krebs cycle and an
electron transport chain that carries out the
oxidative phophorylation of ADP to ATP
Fig 15.2
5
Two stages by which mitochondria convert food to
energy

• Krebs cycle
• Metabolize pyruvate and fatty acids
• Produce high-energy electron carriers NADH and
  FADH2
• Oxydative phosphorylation
• Reactions that create ATP
• Molecular complexes I, II, III, IV form a chain
  that transports electrons from NADH and FADH2 to
  the final electron acceptor, oxygen
• Complex V uses the energy released by the
  electron transport chain to form ATP

6
Chloroplasts are sites of photosynthesis

• Capture, conversion, and storage of solar energy
  in bonds of carbohydrates

Fig. 15.3
7
Photosynthesis takes place in two parts

• Light trapping phase
• Solar energy is trapped and boosts electrons in
  chlorophyll
• Electrons are conveyed to electron transport
  systeme to convert water to oxygen and H
• Electron transport forms NADPH and drives
  synthsis of ATP
• Sugar-building phase
• Calvin cycle enzymes use ATP and NADPH to fix
  atmospheric carbon dioxide into carbohydrates
• Energy is stored in carbohydrate bonds

8
The genomes of mitochondria

• Location
• mtDNA lies within matrix of the organelle in
  structures called nucleoids
• mtDNA of most cells does not reside in single
  location

9
The size and gene content of mtDNA vary from
organism to organism
10
Unusually organized mtDNAs of Trypanosoma,
Leishmania, Crithidia

• Protozoan parasites with single mitochondrial
  called kinetoplast
• mtDNA exists in one place within kinetoplast
• Large network of 10-25,000 minicircles 0.5 2.5
  kb in length interlocked with 50-100 maxicircles
  21-31 kb long
• Maxicircles contain most genes
• Minicircles involved in RNA editing

11
Human mtDNA carries closely packed genes

• 16.5 kb in length, or 0.3 of total genome length
• Carries 37 genes
• 13 encode polypeptide subunits that make up
  oxydative phosphorylation apparatus
• 22 tRNA genes
• 2 genes for large and small rRNAs
• Compact gene arrangement
• No introns
• Genes abut or slightly overlap

Fig. 15.5 a
12
The larger yeast mtDNA contains spacers and
introns

• Four times longer than human and other animal
  mtDNA
• Long intergenic sequences called spacers separate
  genes accounting for more than half of DNA
• Introns form about 25 of yeast genome

Figure 15.5 b
13
The 186 kb mtDNA of the liverwort carries many
more genes than animals and fungi

• 12 electron transport genes
• 16 ribosomal protein genes
• 29 genes with unknown function

Fig. 15.5 c
14
Mitochondrial transcripts undergo RNA editing, a
rare variation on the basic theme of gene
expression

• Discovered in trypanosomes
• Sequence of maxicircle DNA reveals only short,
  recognizable gene fragments instead of whole
  genes
• RNAs in kinetoplast are same short fragments and
  full length RNAs
• kDNA encodes a precursor for each mRNA
• RNA editing conversion of pre-mRNA to mature
  mRNA
• Also found in mitochondria of some plants and
  fungi

15
RNA editing in trypanosomes
Fig. 15.6
16
Translation in mitochondria shows that the
genetic code is not universal
17
The genomes of chloroplasts the liverwort, M.
polymorpha
18
Mitochondrial and chloroplast genomes require
cooperation between organelle and nuclear genomes
Fig. 15.8
19
Origin and evolution of organelle genomes
molecular evidence

• Endosymbiont theory
• 1970s, Lynn Margulis
• Mitochondria and chloroplasts orginated more than
  a billion years ago
• Ancient precursors of eukaryotic cells engulfed
  bacteria and established symbiotic relationship
• Molecular evidence
• Both chloroplasts and mitochondria have own DNA
• mtDNA and cpDNA are not organized into
  nucleosomes by histones, similar to bacteria
• Mitochondrial genomes use N-formyl methionine and
  tRNAfmet in translation
• Inhibitors of bacterial translation have same
  effect on mitochondrial translation, but not
  eukaryotic cytoplasmic protein synthesis

20
Gene transfer occurs through an RNA intermediate
or movement of pieces of DNA

• Genes transfer between organelles and the nucleus
• COXII gene
• mtDNA genome in some plants
• Nuclear genome in other plants
• Nuclear copy lacks intron suggests transferred
  by RNA intermediate
• Movement among organelles
• Plant mtDNAs carry fragments of cpDNA
• Nonfunctional copies of organelle DNA are found
  around the nuclear genomes of eukaryotes

21
mtDNA has high rate of mutation

• 10 times higher than nuclear DNA
• Provides a tool for studying evolutionary
  relationships among closely related organisms
• maternal lineage of humans trace back to a few
  women who lived about 200,000 years ago

22
Maternal inheritance only in most species

• Maternal inheritance of Xenopus mtDNA
• Purified mtDNA from two species
• Hybridization only to probes from same species
• F1 hybrids retain only mtDNA from mother

Fig. 15.9
23
Maternal inheritance of specific genes in cpDNA

• Interspecific crosses tracing biochemically
  detectable species specific differences in
  chloroplast proteins
• Isolated Rubisco proteins in tobacco plants in
  which interspecific differences could be seen
• Progeny of controlled crosses contained version
  of Rubisco protein from maternal parent only

24
A mutation in human mtDNA generates a maternally
inherited neurodegenerative disease
Fig. 15.10

• Lebers hereditary optic neurophathy (LHON) leads
  to optic nerve degeneration and blindness
• Substitution in mtDNA at nucleotide 11,778

25
Cells can contain one type or a mixture of
organelle genomes

• Heterplasmic cells contain a mixture of
  organelle genomes
• Mitotic products may contain one type, a mixture
  of types, or the second type
• Homoplastic cells contain one type of organelle
  DNA
• Mitotic products contain same type, except for
  rare mutation

26
Mitotic segregation produces an uneven
distribution of organelle genes in heteroplasmic
cells

• Women with heteroplasmic LHON mutation
• Some ova may carry few mitochondria with LHON
  mutation and large number of wild-type
• Other ova may carry mainly mitochondrial with
  LHON mutation and few wild-type
• Consequence of heteroplasmy after fertilization
• Some cells produce tissues with normal ATP
  production and others with low production
• If low production cells are in optic nerve, LHON
  results

27
Experiments with mutants of cpDNA in
Chlamydomonas reinhardtii reveal uniparental
inheritance of chloroplasts
Fig. 15.11 b
28
A cross of C. reinhardtii gametes illustrates
lack of segregation of cpDNA at meiosis
Fig. 15.11 c
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Mechanisms of unipartental inheritance

• Differences in gamete size
• Degredation of organelles in male gametes of some
  organisms
• In some plants paternal organelle genomes are
  distributed to cells that are destined to not
  become part of the embryo during early
  development
• In some organisms, the zygote destroys paternal
  organelle after fertilization
• Other organisms, paternal organelles excluded
  from female gamete

30
In yeast, mtDNA-encoded traits show a biparental
mode of inheritance and mitotic segregation
Fig. 15.13
31
Recombinant DNA techniques to study genetics of
organelles

• Gene gun biolistic transformation
• Small (1mm) metal beads with DNA are shot at
  cells
• Rarely, DNA passes through cell wall and enters
  nucleus
• Used to transform cells
• E.g., GFP constructs can be used as selectable
  markers to identify transformants

Fig. 15.14
32
How mutations in mtDNA affect human health

• Individuals with certain rare diseases of the
  nervous system are heteroplasmic
• MERRF, myoclonic epilepsy and ragged red fiber
  disease
• Uncontrolled jerking, muscle weakness, deafness,
  heart problems, kidney problems, progressive
  dementia

Fig. 15.15 a
33
Maternal inheritance of MRRF
Fig. 15.15 b
34
Proportion of mutant mtDNA and tissue in which
they reside influence phenotype
Fig. 15.16
35
Mitochondrial inheritance in identical twins

• Mitochondrial genomes not same in twins but
  nuclear genomes are identical
• Symptoms of neurodegenerative diseases or other
  mutations may manifest in one twin, but not other
• In heteroplasmic mother, chance of phenotype
  depends on both partitioning of mutant mtDNA
  after fertilization, and tissue that receive
  mutation during development

36
mtDNA mutations and aging

• Hypothesis Accumulation of mutations in mtDNA
  over lifetime and biased replication of deleted
  mtDNA result in age-related decline in oxidative
  phosphorylation
• Evidence
• Deleterious mtDNA mutations early in life
  diminish ATP production
• Decreases in cytochrome c oxidase in hearts from
  autopsies (gene encoded in mtDNA)
• Rate of deletions increases with age
• Alzheimers individuals have abnormally low
  energy metabolism