Chap 9 Extranuclear Inheritance

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Chloroplast DNA (cpDNA) is double-stranded and fairly large, ranging from 100 to 225 kb in length,

The genes carried on the DNA encode products involved in photosynthesis and translation.

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Mitochondrial DNA (mtDNA) is also double-stranded but smaller than chloroplast DNA. Introns and gene repetitions are usually absent.

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Myoclonic Epilepsy and Ragged Red Fiber disease (MERRF) is a mitochondrial mutation with maternal inheritance, and exhibits ataxia (lack of muscular coordination), deafness, dementia, and epileptic seizures.

(a): The muscle fiber exhibits heteroplasmy, with mild proliferation of mutant mitochondria, with "ragged-red" fibers when stained with dye.

(b) Marked proliferation where most mitochondria have the mutant gene.

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The poky phenotype (slow growth) in the filamentous bread mold Neurospora crassa is a mutation in cytochrome proteins that disrupts aerobic respiration. This trait is maternally* inherited, suggesting cytoplasmic transmission via mitochondria.

* Neurospora mating involves fusion of conidia of opposite mating types. One of the cells contributes most of the cytoplasm and may be considered the "maternal" parent.

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In autogamy a Paramecium loses the genes from one micronucleus since only one of the 8 meiosis products survives.

A heterozygous cell will become a homozygote, and a population of heterozygotes will produce a 1:1 ratio of cells homozygous for each allele.

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Paramecia can undergo sexual exchange of DNA through conjugation where two mating cells exchange haploid micronuclei, resulting in new, recombinant micronuclei with identical genotypes.

Autogamy is a similar process involving a single cell.

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Killer Paramecia possess toxic kappa particles (symbiotic bacteria) and depend on a dominant nuclear K allele for their maintenance.
Both cells from a conjugation between homozygous strains are heterozygous.
They can become Killers only if, during conjugation, they received some cytoplasm containing kappa particles, otherwise they remain sensitive.
The Killer phenotype persists only if the kappa particles are supported by at least one dominant K allele, since kk cells are sensitive even if they inherit the cytoplasm from a Killer.

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Inheritance of petite phenotype in S. cerevisiae.
Segregational: Some mutants are caused by nuclear mutations, and exhibit Mendelian 1:1 segregation. Neutral: all offspring are wild-type, having inherited normal mtDNA from the wild-type parent, which are replicated in the offspring. Suppressive: all offspring are petite, exhibiting "dominant" behavior to suppress wild-type mitochondrial function.

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The petite phenotype (small colonies) in the unicellular yeast Saccharomyces cerevisiae is another mutation that causes abnormal aerobic respiration.

Most of these mutants have lost their mitochondrial DNA (mtDNA).

When mitochondrial function is lost, the yeast can grow anaerobically by fermenting glucose.

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Mitochondria and chloroplasts arose independently about 2 billion years ago by endosymbiosis when free-living prokaryotes were engulfed by primitive eukaryotic cells. The engulfed cells specialized in aerobic respiration and photosynthesis, respectively, and developed a mutually beneficial relationship with their host.

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The wild-type dominant allele yields brown eyes in the moth Ephestia kuehniella since it can synthesize a precursor pigment molecule, kynurenine. The a mutation interrupts synthesis of kynurenine and yields red eyes.

  • In an Aa x aa cross, the aa larvae exhibit wild-type brown eyes if the Aa parent is female. These aa larvae gradually develop red eyes as they grow into adults.
  • The Aa oocyte contains sufficient kynurenine in the cytoplasm to be distributed to the cells of young larvae, who start out life with brown eyes. As the pigment is diluted among many cells as the larva grows, red eyes emerge.

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    Shell coiling in the hermaphroditic snail Limnaea peregra may be right-handed (dextral) or left-handed (sinistral).
    The coiling depends on the genotype of egg donor parent, regardless of the phenotype of that parent.

  • If that parent is DD or Dd, the offspring will be dextral.
  • The dominant allele (D) produces a gene product that influences spindle orientation in the first cleavage division of the zygote, which in turn affects subsequent cell divisions to produce permanent coiling in that direction.

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    In humans, mtDNA encodes many parts needed for protein synthesis and cellular respiration; while nuclear DNA codes the rest (arrows entering the organelle). For example, replication is dependent on enzymes encoded by nuclear DNA.

    Mitochondrial ribosomes also differ from cytoplasmic ribosomes.

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    Ribosomes in mitochondria from different species exhibit various sedimentation coefficients, different from the cytoplasmic ribosome coefficient of 80S of all eukaryotes. This supports the endosymbiotic hypothesis of the origin of mitochondria.

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    The unicellular green alga Chlamydomonas has a single large chloroplast containing more than 50 copies of a circular double-stranded DNA. Its strR (streptomycin resistance) trait exhibits uniparental inheritance: the phenotype is transmitted only through the mt+ parent. Reciprocal crosses between strS and strR strains yield offspring which only express the genotype of the mt+ parent. After fertilization, the single chloroplasts of the two mating types fuse. The resulting chloroplast only retains DNA from the mt+ parent.

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    This four o'clock possess either white, green, or variegated leaves. Inheritance is uniparental: leaf color is determined only by the phenotype of the ovule source.

    For example, if the seeds were derived from ovules of plants with green leaves, all progeny plants bore only green leaves, regardless of the phenotype of the source of pollen.

    This inheritance is transmitted through the cytoplasm of the female parent, since the pollen is tiny and contributes little cytoplasm to the zygote.