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

The Chromosomes of
Organelles Outside the
Nucleus Exhibit NonMendelian Patterns of
Inheritance
Outline of Chapter 15
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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
Mitochondrial and chloroplasts are organelles of
energy conversion that carry their own DNA
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Chloroplasts – capture solar energy and
store it in carbohydrates
Mitochondria – release energy from
nutrients and convert it to ATP
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
Two stages by which mitochondria
convert food to energy
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Krebs cycle
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Metabolize pyruvate and fatty acids
Produce high-energy electron carriers NADH and
FADH2
Oxydative phosphorylation
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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
Chloroplasts are sites of
photosynthesis
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Capture,
conversion,
and storage of
solar energy in
bonds of
carbohydrates
Fig. 15.3
Photosynthesis takes place in two
parts
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Light trapping phase
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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
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Calvin cycle enzymes use ATP and NADPH to fix
atmospheric carbon dioxide into carbohydrates
Energy is stored in carbohydrate bonds
The genomes of mitochondria
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Location
mtDNA lies within matrix of the organelle in
structures called nucleoids
 mtDNA of most cells does not reside in single
location
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The size and gene content of mtDNA
vary from organism to organism
Unusually organized mtDNAs of
Trypanosoma, Leishmania, Crithidia
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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
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Human mtDNA carries closely
packed genes
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16.5 kb in length, or 0.3%
of total genome length
Carries 37 genes
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Compact gene
arrangement
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Fig. 15.5 a
13 encode polypeptide
subunits that make up
oxydative phosphorylation
apparatus
22 tRNA genes
2 genes for large and small
rRNAs
No introns
Genes abut or slightly
overlap
The larger yeast mtDNA contains
spacers and introns
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Four times longer
than human and
other animal
mtDNA
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Figure 15.5 b
Long intergenic
sequences called
spacers separate
genes accounting
for more than half
of DNA
Introns form
about 25% of yeast
genome
The 186 kb mtDNA of the liverwort carries
many more genes than animals and fungi
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12 electron
transport genes
16 ribosomal
protein genes
29 genes with
unknown
function
Fig. 15.5 c
Mitochondrial transcripts undergo RNA editing, a rare
variation on the basic theme of gene expression
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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
RNA editing in trypanosomes
Fig. 15.6
Translation in mitochondria shows
that the genetic code is not universal
The genomes of chloroplasts: the
liverwort, M. polymorpha
Mitochondrial and
chloroplast genomes
require cooperation
between organelle and
nuclear genomes
Fig. 15.8
Origin and evolution of organelle
genomes: molecular evidence
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Endosymbiont theory
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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
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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
Gene transfer occurs through an RNA
intermediate or movement of pieces of DNA
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Genes transfer between organelles and the
nucleus
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COXII gene
mtDNA genome in some plants
 Nuclear genome in other plants
 Nuclear copy lacks intron – suggests transferred by
RNA intermediate
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Movement among organelles
Plant mtDNAs carry fragments of cpDNA
 Nonfunctional copies of organelle DNA are found
around the nuclear genomes of eukaryotes
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mtDNA has high rate of mutation
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10 times higher than nuclear DNA
Provides a tool for studying evolutionary
relationships among closely related
organisms
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maternal lineage of humans trace back to a few
women who lived about 200,000 years ago
Maternal inheritance only in most species
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Maternal
inheritance of
Xenopus mtDNA
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Fig. 15.9
Purified mtDNA
from two species
Hybridization only
to probes from same
species
F1 hybrids retain
only mtDNA from
mother
Maternal inheritance of specific
genes in cpDNA
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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
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A mutation in human mtDNA generates a
maternally inherited neurodegenerative disease
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Fig. 15.10
Leber’s hereditary optic neurophathy
(LHON) leads to optic nerve degeneration
and blindness
Substitution in mtDNA at nucleotide 11,778
Cells can contain one type or a
mixture of organelle genomes
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Heterplasmic – cells contain a mixture of
organelle genomes
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Mitotic products may contain one type, a
mixture of types, or the second type
Homoplastic – cells contain one type of
organelle DNA
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Mitotic products contain same type, except for
rare mutation
Mitotic segregation produces an uneven distribution of
organelle genes in heteroplasmic cells
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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
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Some cells produce tissues with normal ATP
production and others with low production
 If low production cells are in optic nerve, LHON
results
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Experiments with
mutants of cpDNA
in Chlamydomonas
reinhardtii reveal
uniparental
inheritance of
chloroplasts
Fig. 15.11 b
A cross of C. reinhardtii
gametes illustrates lack
of segregation of cpDNA
at meiosis
Fig. 15.11 c
Mechanisms of unipartental
inheritance
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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
In yeast, mtDNA-encoded traits show a biparental
mode of inheritance and mitotic segregation
Fig. 15.13
Recombinant DNA techniques to
study genetics of organelles
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Gene gun – biolistic
transformation
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Fig. 15.14
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
How mutations in mtDNA affect
human health
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Individuals with
certain rare diseases of
the nervous system are
heteroplasmic
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MERRF, myoclonic
epilepsy and ragged red
fiber disease
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Fig. 15.15 a
Uncontrolled jerking,
muscle weakness,
deafness, heart problems,
kidney problems,
progressive dementia
Maternal inheritance of MRRF
Fig. 15.15 b
Proportion of
mutant mtDNA
and tissue in
which they reside
influence
phenotype
Fig. 15.16
Mitochondrial inheritance in
identical twins
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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
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mtDNA mutations and aging
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Hypothesis: Accumulation of mutations in
mtDNA over lifetime and biased replication of
deleted mtDNA result in age-related decline in
oxidative phosphorylation
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Evidence:
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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
Alzheimer’s individuals have abnormally low energy
metabolism