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Chapter 19

The Genetics of Organelles

12/26/2008

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Figure 19.1 The biological energy wheel

Mitochondria

Chloroplast

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Figure 19.2 Leaf variegation caused by the segregation

of different types of chloroplasts

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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

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Figure 19.4 Correns experiments on the inheritance of

leaf variegation in Mirabil is.

Maternal inheritance

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Figure 19.5 Baurs experiments on the inheritance of leaf

variegation in Pelargonium.

Non-Mendelian biparental inheritance

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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

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Figure 19.9 Non-Mendelian

segregation of (a) neutral

and (b) suppressive petitemutations in yeast.

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Figure 19.10 Inheritance

of mitochondrial (mt)

DNA in crosses between

petite and wild-typestrains of yeast.

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Figure 19.11 Mitochondrial DNA (yellow) in the

unicellular organism Euglena gracilis.

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The genomes of mitochondria

1. Mitochondrial DNA (mtDNA) lies within the matrix, it appears

in highly condensed structure called nucleoids. The mtDNAof 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.

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Figure 19.12 Intramolecular recombination in

the mtDNA of the Chinese cabbage, Brassica

campestr is.

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Figure 19.13 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 bymitochondrial ribosomes

-Different codon usage

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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.

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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 thefollowing organisms:

-Trypanosomes (protozoan parasite),

add or delete uracils

-Some plants,

add or delete cytosines (mechanism not known)

-Some fungi

RNA Editing

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Fig. 11.22 Editing of the mitochondrial cytochrome b pre-mRNA

in the trypanosome Leishmania tarentolae. (p297)

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Figure 19.14 Trans-splicing in wheat mitochondria

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Hypothetical example of LHON pedigree

LHON (Lebers hereditary optic neuropathy)

- A disease in which defects in the mitochondriaselectron 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.

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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 ofthe mitotic segregation.

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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.

Di h fl h i f ild

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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.

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Figure 19.6

Cells of the unicellular

alga Chlamydomonas

reinhardt i i.

Unicellular, haploid

Two different mating types:+ and -

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Figure 19.7Sagers

experiments showing

uniparentalinheritance of

streptomycin

resistance (stm-r) and

sensitivity (stm-s) in

Chlamydomonas.

Uniparental inheritance

(phenotype from the + parent)