Structure and Expression

Mitochondrial DNA and Extranuclear Inheritance

9

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Ed. Chap 17

• The genomes are composed of circular DNA that is not extensively complexed with histone-like proteins.

• The genomes are organized with functionally related genes close together and often expressed as a single unit.

• Within eukaryotic cells, mitochondria contain their own ribosomes (mitoribosomes), which synthesize a handful of proteins, all essential for the biogenesis of the oxidative phosphorylation system.

• The ribosome particles on which protein synthesis takes place have major subunits whose size is similar in organelles and in prokaryotes but differs from the size of the subunits in cytoplasmic ribosomes in eukaryotic cells.

• Chloroplast DNA in most plants ranges in size from 120 to 160 kb.

• mitochondrial genome in mammals is 16.5 kb, in Drosophila melanogaster 18.5 kb plants 100kb

• mitochondria 40 genes chloroplast 100genes

1.The UGA codon is not a stop codon but encodes tryptophan instead.

2.The AGA and AGG codons are stop codons rather than arginine codons.

3.AUA and AUU encode methionine rather than isoleucine.

mtDNA RFLP using HaeIII digestion

Total mito size 16.5 kb other bands not shown.

• maternal extranuclear inheritance

• maternal effect: egg cytoplasm can influence early embryo

• heteroplasmy – most humans are not heteroplasmic the ratios can change

• Heteroplasmy is the presence of more than one mtDNA type in an individual

• mitochondrial DNA tRNA Lys mutation is pleiotropic because all genes

that have lys affected.

Rf dominant nuclear gene: restorer of fertility

Cytoplasmic Inheritance

in Plants

Green color depends on the presence of chloroplasts, and pollen contains no chloroplasts.

Vegetative segregation of chloroplasts into daughter cells is somewhat irregular and subject to chance variation.

The cells of white tissue contain mutant chloroplasts that lack chlorophyll.

Respiration-Defective Mitochondrial Mutants in Yeast

suppressive mutant

here nuclear gene causes mito phenotype

• Cytoplasmic petites (mitochondrial)

• segregational petites (nuclear)

• defective in anaerobic respiration

- Complex nuclear genome

- Simple mitochondrial genome

In nuclear genome, genes are distributed to 24 different chromosomes.

Mitochondrial genome is circular;

Code for 37 genes;

Code for mitochondrial RNA and proteins.

But most of the proteins are encoded by nuclear genome.

They are transported from cytoplasm to the mitochondria.

Human genome:

Mitochondrial genome:

Sequence published in 1981 by Fred Sanger in Cambridge;

16.6 kb in length, 44% GC

Two strands have different base compositions;

Heavy strand (H-strand) à rich in guanines Light strand (L-strand) à rich in cytosines

In each cell, there are many mitochondria, each with 1-2 copies of mitochondrial DNA à may be 1000s of mit DNA.

Mit genome is maternally inherited.

Males do not transmit their mit to subsequent generations.

During cell division via mitosis, the multiple mit are distributed randomly to daughter cells.

tRNA^Val

NADH dehydrogenase

ATP synthase cytochrome c oxidase MITOMAP database at http://www.mitomap.org

• unidirectional replication and bidirectional transcription.

• Start at specific origins in H- and L-strands.

• Replication of H-strand is repeated producing a third strand known as 7-S DNA.

• 7-S DNA pairs with L-strand producing a triple stranded region.

• 7S DNA contains many control sequences – promoters.

• It also known as CR/D-loop region (control region/Displacement loop)

• Origin of replication of H-strand lies in CR/D-loop.

• Origin of replication of L-strand is in between two tRNA genes.

• H-strand replication uses L-strand as a template,

• Displaces H-strand and allows L-strand of origin to be accessed.

• Replication of L-strand is in opposite direction and use H-strand as a template.

Mitochondrial genes:

Encodes 37 genes;

• 28 encoded by H-strand

• 9 encoded by L-strand

Transcription starts from a common promoter in CR/D-loop and continues in opposite directions for two different strands.

24 genes produce a non-coding RNA;

• 22 mit t-RNA genes

• 2 rRNA genes

à 16S rRNA à large subunit of mit ribosome à 12S rRNA à small subunit of mit ribosome All other proteins that form the mit are

encoded by the nuclear genome and

synthesized in the cytoplasm.

Mitochondrial genes

• Mit DNA is extremely compact à 1 gene/450 bp

• Genes lack introns

• Genes encoding 6th and 8th subunits of mit ATP synthase show overlap

• Coding sequences are seperated by 1-2 bases.

• Some genes lack termination codons

• UAA codons are introduced to the transcripts by post-transcriptional enzymes.

• Mit DNA is not associated with histones

• It is not known whether mit DNA undergoes recombination

• In general, each human cell contains several hundred to 1,000 mitochondria, and each mitochondrion has 2 to 10 copies of mtDNA. The mitochondrial genome is more

vulnerable to oxidative damage and undergoes a higher rate of mutation compared with

nDNA

Mitochondrial genetic code:

• It produces a very small number of proteins, thus, able to drift by mutation, and slightly different from universal genetic code.

à Not so strongly selected against

• 4 stop codons à UAA, UAG, AGA, AGG (AGA and AGG specify Arg in nuclear genome) UGA stop codon of nuclear genome codes for tryptophan in mit.

AUA specifies Methionine not İsoleucine.

• 60 codons specify amino acids.

22 tRNA à 8 tRNA each recognize 4 codons that differ at the third nucleotide

à14 tRNA each recognize 2 codons that share a purine or pyrimidine at the third nucleotide

(8X4) + (14x2) = 60 codons

Horizontal Gene Transfer

Advantages of loosing mito genes?

escape from oxidative phosphorylation

INFERRING FEMALE AND MALE HISTORIES USING MITOCHONDRIAL DNA AND THE Y CHROMOSOME uniparental inheritance: mitochondrial DNA (mtDNA) is passed only from mothers to their children,

and the male-specific region of the Y chromosome (MSY) is passed only from fathers to their sons.

mtDNA and MSY are each a single genetic locus.

No crossing over so diversity that we observe in populations results only from mutations

sequence from 320 male humans

out of Africa Expansion

population size larger for mtDNA: polygyny 1M x >2F selective

male breeding!