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12/26/2008
Chapter 19
The Genetics of Organelles
Mitochondria
Chloroplast
Figure 19.1 The biological energy wheel
Figure 19.2 Leaf variegation caused by the segregation
of different types of chloroplasts
Heteroplasmy: mixture of two
types of organelles within a cell
Homoplasmy: presence of single
type of organelle within a cell
Figure 19.3 Chloroplast sorting during mitosis
Maternal inheritance
Figure 19.4 Correns’ experiments on the inheritance of
leaf variegation in Mirabilis.
Non-Mendelian biparental inheritance
Figure 19.5 Baur’s experiments on the inheritance of leaf
variegation in Pelargonium.
Sm2,
high-level streptomycin resistance
Sm3,
low-level streptomycin resistance
ery,
erythromycin resistance
spc,
spectinomycin resistance
Figure 19.8 A genetic map of chloroplast DNA in
Chlamydomonas, based on the work of Sager and Ramanis.
Figure 19.9 Non-Mendelian
segregation of (a) neutral
and (b) suppressive petite
mutations in yeast.
Figure 19.10 Inheritance
of mitochondrial (mt)
DNA in crosses between
petite and wild-type
strains of yeast.
Figure 19.11 Mitochondrial DNA (yellow) in the
unicellular organism Euglena gracilis.
The genomes of mitochondria
1. Mitochondrial DNA (mtDNA) lies within the matrix, it appears
in highly condensed structure called nucleoids. The mtDNA
of most cells does not reside in a single location.
2. The number of mitochondria, nucleoids, and mtDNA molecules
are variable. The mechanisms are not yet understood.
3. Mitochondria can fuse with each other as well as divide.
4. In general, mitochondria double in size and then divide in half
in each cell generation.
5. The replication of mtDNA, and the division of the mitochondria
are independent of the nuclear DNA and cell division.
6. Which mtDNA undergo replication seems to be determined at
random.
7. The size, gene content and shape (circular or linear) of mtDNA
vary from organism to organism.
Figure 19.12 Intramolecular recombination in
the mtDNA of the Chinese cabbage, Brassica
campestris.
Human mtDNA
- 16,659 bp
-37 genes (2 rRNA, 22
tRNA, 13 polypeptides)
-Two large transcripts
-Polyadenylated mRNAs
are translated by
mitochondrial ribosomes
-Different codon usage
Figure 19.13 Map of human mtDNA showing the
pattern of transcription.
Translation in mitochondria shows that
the genetic code is not universal
Mitochondria have their
own translational
apparatus (rRNA, tRNA,
ribosome, genetic code)
No single mitochondrial
genetic code functions
in all organisms.
Genetics, from Genes to Genomes,
Hartwell et al., 2nd edition.
RNA Editing
Mitochondrial transcripts undergo RNA editing, a rare
variation on the basic theme of gene expression.
Precursor RNA  RNA editing  functional mRNA
RNA editing occurs in the mitochondria of the
following organisms:
-Trypanosomes (protozoan parasite),
add or delete uracils
-Some plants,
add or delete cytosines (mechanism not known)
-Some fungi
Fig. 11.22 Editing of the mitochondrial cytochrome b pre-mRNA
in the trypanosome Leishmania tarentolae. (p297)
Figure 19.14 Trans-splicing in wheat mitochondria
Hypothetical example of LHON pedigree
LHON (Leber’s hereditary optic neuropathy)
- A disease in which defects in the mitochondria’s
electron transport chain lead to optic nerve
degeneration and blindness.
- Mutation in the NADH dehydrogenase subunit 4 gene.
Genetics, from Genes to Genomes, Hartwell et al., 2nd edition.
Heteroplasmic cells:
Cells contain a mixture of organelle DNA.
Homoplasmic cells:
Cells carry only one type of organelle DNA.
Random partitioning of
organelle during cell
division is the basis of
the mitotic segregation.
Mutant mtDNA (or cpDNA)
Wild-type mtDNA (or cpDNA)
Maternal inheritance of the
mitochondrial disease MERRF
(myoclonic epilepsy and
ragged red fiber disease).
Symptoms:
Uncontrolled jerking, muscle weakness,
deafness, heart problems, kidney
problems, and progressive dementia.
Pedigree analysis:
-Maternal inheritance
-Variations in the severity
of symptoms
Genetics, from Genes to Genomes, Hartwell et al., 2nd edition.
Disease phenotypes reflect the ratio of mutant-to-wildtype mtDNAs and the reliance of cell type on
mitochondrial function
MERRF patient:
- Heteroplasmic
mitochondrial tRNA
mutation
- Random partitioning
- Different tissues are
affected differently
Genetics, from Genes to Genomes, Hartwell et al., 2nd edition.
•Unicellular, haploid
•Two different mating types:
“+” and “-”
Figure 19.6
Cells of the unicellular
alga Chlamydomonas
reinhardtii.
Uniparental inheritance
(phenotype from the “+” parent)
Figure 19.7 Sager’s
experiments showing
uniparental
inheritance of
streptomycin
resistance (stm-r) and
sensitivity (stm-s) in
Chlamydomonas.