Extranuclear chromosomes & uniparental inheritance

1Genetics Lecture

Outline1. 2. What are organelles? Mitochondrial genome

3.4. 5. 6.

Chloroplast genomeEvolution of organelles Non-Mendelian inhertiance mtDNA disorders

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4 oclock flowerWhat will be the possible explanations of these test crossess result?P:

F1:

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Saccharomyces cerevisiae

GrandeLarge colony that can respire

PetiteSmall colony that cannot respire

Disappear in daughter colonies

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Why?????This is non-Mendelian inheritance

These traits are determined by genes that reside in chloroplast ( in 4 oclock ) and mitochondria ( in yeast)

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Mitochondria & chloroplastMitochondria Chloroplast

Sites of the krebs cycle and an electron transport chain. Starting with O2 & carbohydrate, fats => ATP

sites of photosynthesis. Starting with solar energy & CO2 => carbohydrate + O2

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Outline1. 2. What are organelles? Mitochondrial genome

3.4. 5. 6.

Chloroplast genomeEvolution of organelles Non-Mendelian inhertiance mtDNA disorders

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Mitochondrial genomes Location: in the matrix Nucleoids: high condensed structure of mtDNAs Cells contain a variable number of mitochondria & nucleoids Replication of mtDNA & division of the mitochondria occur throughout the cell cycle independent of the replication of nuclear DNA & of cell division mtDNAs undergo replication randomly Some molecules replicate many times Others do not replicate9

Mitochondrial genomes The size, gene content and shape of mtDNA vary from organism to organism.Mitochondrial DNA SizesOrganism Plasmodium Yeast Drosophila Pea human Size (kb) 6 75 18 110 16.5

The shape of mtDNAs varies Most species: circular Ciliated protozoans (Tetrahymena, Paramecium), alga Chlamydomonas, yeast Hansenula: linear10

TABLE Organism Yeast Marchantia Human

Comparison of Some Functions Encoded in mtDNA Oxidative Phosphorylation Genes 7 14 13 tRNAs 25 29 22 Genome Size (kb) 75 186.0 16.5

Kinetoplast

single-celled eukaryotic organisms (the genera Trypanosoma, Leishmania, and Crithidia) carry a single mitochrondria mtDNA exists in 1 place Minicircles 10- 25000 0.5- 2.5 kb in length play a role in RNA editing 50- 100 21- 31 kb in length contain most of genes

A large network Maxicircles

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Types of mitochondrial genomesAncestral mitochondria Derived mitochondria

Similar to eubacterial ancestors

Different from typical eubateria

Larger genome size More genes & few introns

Smaller genome size Fewer genes

Use universal codons

Use nonuniversal codons12

Human mtDNA Circular molecule 2 rRNAs 22 tRNAs

13 proteins

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Yeast mtDNA Circular molecule 2 rRNAs 25 tRNAs

16 proteins

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RNA editing Mitochondrial transcripts undergo RNA editing, a rare variation on the basic theme of gene expression RNA editing is the process that converts pre-mRNAs to mature mRNAs 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)15

Translation in mitochondria shows that the genetic code is not universalMitochondria have their own translational apparatus (rRNA, tRNA, ribosome, genetic code) No single mitochondrial genetic code functions in all organisms.

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Outline1. 2. What are organelles? Mitochondrial genome

3.4. 5. 6.

Chloroplast genomeEvolution of organelles Non-Mendelian inhertiance mtDNA disorders

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Chloroplast genomes Location: stroma Shape: circular Genome size is more uniform than the ones of mitochondria, 120 217kb (mostly 120-160kp) More genes than mtDNA. They are closely packed with few nucleotides between adjacent coding sequences Many chloroplast genes are organized into operon-like clusters19

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Outline1. 2. What are organelles? Mitochondrial genome

3.4. 5. 6.

Chloroplast genomeEvolution of organelles Non-Mendelian inhertiance mtDNA disorders

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The endosymbiotic theory Mitochondria & chloroplasts were once free-living bacteria that became internal inhabitants (endosymbionts) of early eukaryotic cells With time, the endosymbionts became an integral part of the eukaryotic host cells & evolved into present-day organelles Anaerobic eubacteria mitochondria Photosynthesizing eubacteria chloroplasts

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Evidences for the endosymbiotic theory Mitochondria & chloroplasts are similar in size & have their own DNA which replicates independently of the nuclear genome. mtDNA, cpDNA, bacterial DNA are not organized into nucleosomes by histones Use N-formyl methionine & tRNAfMet for translation Antibiotics inhibit protein synthesis in eubacteria & organelles, but do not affect protein synthesis in the cytoplasm Comparing mtDNA, cpDNA & bacterial rRNA genes suggest: Mitochondria derive from the -proteobacteria Chloroplasts derive from cyanobacteria25

Evolution of mtDNA mtDNA of vertebrates changes almost 5-10 times more rapidly than the nuclear DNA Reflect more errors in replication & less efficient repair mechanism Provide valuable tool for studying evolutionary relationships of organisms whose nuclear DNA are very similar

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Outline1. 2. What are organelles? Mitochondrial genome

3.4. 5. 6.

Chloroplast genomeEvolution of organelles Non-Mendelian inhertiance mtDNA disorders

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Uniparental inheritance In most species, progeny inherit organelle DNA from only one parent (usually the maternal)

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Random partition of organelles during cell division

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Why do most species use uniparental ? Difference in gamete size Cell degrade the organelles or the organelle DNA of male gametes. Early division of the zygote distribution all or most parental organelle genome to cells that are not to become a part of embryo. Details of fertilization prevent a paternal cell from contributing its organelles to the zygote, or zygote destroys the parental organelles after fertilizationOrganelle chromosomes exhibit non-Mendelian pattern of inheritance30

Biparental inheritanceInherit their organelle genomes from both parents

Geraniums (Pelargonium zonale)

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Outline1. 2. What are organelles? Mitochondrial genome

3.4. 5. 6.

Chloroplast genomeEvolution of organelles Non-Mendelian inhertiance mtDNA disorders

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Mutations in mitochondria can have very serious effects Mitochondrial mutations affects mostly on tissues that have high energy demands (brain, retinal, skeletal muscle, and cardiac muscle tissues).

Due to the maternal pattern of mitochondrialinheritance, almost diseases pass down through maternal line.

The proportion of mutant mtDNAs and the tissue inwhich they reside influence phenotype.33

Common features of mitochondrial DNA-associated diseases

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Lebers hereditary optic neuropathy (LHON)

Cause: mutation in the mtDNA gene encoding subunit 4

of NADH-CoQ reductase. LHON most commonlyassociated with a homoplasmic mitochondrial DNA mutation, although heteroplasmic transmission also

occursLHON is inherited only from the mother. Effect: degeneration of the optic nerve loss of vision35

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Background readingGenetics: From Genes to Genomes, 2nd Ed., Chapter 14, pp.489-517

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