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
Figure 15.2
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
Figure 15.3
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
Tables 15.1 and 15.2

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 maxicircles 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 genes 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
Figure 15.5 a
Fig 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
Figure 15.5 b

13. 12 electron transport genes 16 ribosomal protein genes
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
Figure 15.5 c
Fig 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
Figure 15.6
Fig. 15.6

16. Translation in mitochondria shows that the genetic code is not universal
Table 15.3

17. The genomes of chloroplasts: the liverwort, M. polymorpha
Figure 15.7

18. Mitochondrial and chloroplast genomes require cooperation between organelle and nuclear genomes
Figure 15.8
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. Genes transfer between organelles and the nucleus
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
Figure 15.9
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. Substitution in mtDNA at nucleotide 11,778
A mutation in human mtDNA generates a maternally inherited neurodegenerative disease
Figure 15.10
Leber’s hereditary optic neurophathy (LHON) leads to optic nerve degeneration and blindness
Substitution in mtDNA at nucleotide 11,778
Fig

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. Women with heteroplasmic LHON mutation
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
Figure b
Fig b

28. A cross of C. reinhardtii gametes illustrates lack of segregation of cpDNA at meiosis
Figure c
Fig c

29. 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
Figure 15.13
Fig

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
Figure 15.14
Fig

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
Figure a
Fig a

33. Maternal inheritance of MRRF
Figure b
Fig b

34. Proportion of mutant mtDNA and tissue in which they reside influence phenotype
Figure 15.16
Fig

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
Alzheimer’s individuals have abnormally low energy metabolism