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The Chromosomes of
Organelles Outside the
Nucleus Exhibit NonMendelian Patterns of
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
A comprehensive example of mutations in
mitochondrial DNA that affect human
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
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
Krebs cycle
Metabolize pyruvate and fatty acids
Produce high-energy electron carriers NADH and
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
Chloroplasts are sites of
and storage of
solar energy in
bonds of
Fig. 15.3
Photosynthesis takes place in two
Light trapping phase
Solar energy is trapped and boosts electrons in
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
The genomes of mitochondria
mtDNA lies within matrix of the organelle in
structures called nucleoids
 mtDNA of most cells does not reside in single
The size and gene content of mtDNA
vary from organism to organism
Unusually organized mtDNAs of
Trypanosoma, Leishmania, Crithidia
Protozoan parasites with single
mitochondrial called kinetoplast
mtDNA exists in one place within
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
Human mtDNA carries closely
packed genes
16.5 kb in length, or 0.3%
of total genome length
Carries 37 genes
Compact gene
Fig. 15.5 a
13 encode polypeptide
subunits that make up
oxydative phosphorylation
22 tRNA genes
2 genes for large and small
No introns
Genes abut or slightly
The larger yeast mtDNA contains
spacers and introns
Four times longer
than human and
other animal
Figure 15.5 b
Long intergenic
sequences called
spacers separate
genes accounting
for more than half
of DNA
Introns form
about 25% of yeast
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
Fig. 15.5 c
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
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
Also found in mitochondria of some plants and
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
Endosymbiont theory
1970s, Lynn Margulis
Mitochondria and chloroplasts orginated more than a billion years
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
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
Genes transfer between organelles and the
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
mtDNA has high rate of mutation
10 times higher than nuclear DNA
Provides a tool for studying evolutionary
relationships among closely related
maternal lineage of humans trace back to a few
women who lived about 200,000 years ago
Maternal inheritance only in most species
inheritance of
Xenopus mtDNA
Fig. 15.9
Purified mtDNA
from two species
Hybridization only
to probes from same
F1 hybrids retain
only mtDNA from
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
A mutation in human mtDNA generates a
maternally inherited neurodegenerative disease
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
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
Mitotic segregation produces an uneven distribution of
organelle genes in heteroplasmic cells
Women with heteroplasmic LHON
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
Experiments with
mutants of cpDNA
in Chlamydomonas
reinhardtii reveal
inheritance of
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
Differences in gamete size
Degredation of organelles in male gametes of some
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
Gene gun – biolistic
Fig. 15.14
Small (1mm) metal beads
with DNA are shot at
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
How mutations in mtDNA affect
human health
Individuals with
certain rare diseases of
the nervous system are
MERRF, myoclonic
epilepsy and ragged red
fiber disease
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
Fig. 15.16
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
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
Deleterious mtDNA mutations early in life diminish ATP
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