Chapter 19 The Genetics of Organelles 12/26/2008

Figure 19.1 The biological energy wheel Mitochondria Chloroplast

Figure 19.2 Leaf variegation caused by the segregation of different types of chloroplasts

Figure 19.3 Chloroplast sorting during mitosis Heteroplasmy: mixture of two types of organelles within a cell Homoplasmy: presence of single type of organelle within a cell

Figure 19.4 Correns’ experiments on the inheritance of leaf variegation in Mirabilis. Maternal inheritance

Figure 19.5 Baur’s experiments on the inheritance of leaf variegation in Pelargonium. Non-Mendelian biparental inheritance

Figure 19.8 A genetic map of chloroplast DNA in Chlamydomonas, based on the work of Sager and Ramanis. Sm2, high-level streptomycin resistance Sm3, low-level streptomycin resistance ery, erythromycin resistance spc, spectinomycin resistance

Figure 19.9 Non-Mendelian segregation of (a) neutral and (b) suppressive petite mutations in yeast.

Figure Inheritance of mitochondrial (mt) DNA in crosses between petite and wild-type strains of yeast.

Figure 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 Intramolecular recombination in the mtDNA of the Chinese cabbage, Brassica campestris.

Figure Map of human mtDNA showing the pattern of transcription. 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

Translation in mitochondria shows that the genetic code is not universal No single mitochondrial genetic code functions in all organisms. Mitochondria have their own translational apparatus (rRNA, tRNA, ribosome, genetic code) Genetics, from Genes to Genomes, Hartwell et al., 2nd edition.

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 RNA Editing

Fig Editing of the mitochondrial cytochrome b pre-mRNA in the trypanosome Leishmania tarentolae. (p297)

Figure 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. Mutant mtDNA (or cpDNA) Wild-type mtDNA (or cpDNA) Random partitioning of organelle during cell division is the basis of the mitotic segregation.

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-wild- type 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.

Figure 19.6 Cells of the unicellular alga Chlamydomonas reinhardtii. Unicellular, haploid Two different mating types: “+” and “-”

Figure 19.7 Sager’s experiments showing uniparental inheritance of streptomycin resistance (stm-r) and sensitivity (stm-s) in Chlamydomonas. Uniparental inheritance ( phenotype from the “+” parent )