Kearns-Sayre syndrome (KSS), Pearson syndrome,and progressive external opthalmoplegia (PEO) arerare diseases in which up to 7.6 kb of the mitochondrial genome is deleted. KSS affects the central nervous system, skeletal muscle, and heart; patientsoften die in young adulthood. Pearson syndrome ischaracterized by severe anemia and pancreatic dysfunction. The condition is usually fatal during infancy, but the few survivors often develop thesymptoms of KSS. PEO patients have ptosis (drooping eyelids) and weakness in the limbs, but theyhave normal life spans.a. How can you explain the variation in tissues affected and severity of symptoms in patients withthese three conditions, given that they all bearlarge deletions of mtDNA? (Assume that the size of the deletion does not contribute to phenotypicdifferences.)b. Assuming that mtDNA begins its replication from asingle origin, what can you conclude from thesediseases about the location of this replication origin?c. Although these syndromes are due to mtDNA deletions, they are not usually maternally inherited butinstead arise as a new mutation in an individual.For example, mothers with PEO usually do nottransmit this trait to their offspring. Propose an explanation for this surprising finding.
Kearns-Sayre syndrome (KSS), Pearson syndrome,
and progressive external opthalmoplegia (PEO) are
rare diseases in which up to 7.6 kb of the mitochondrial genome is deleted. KSS affects the central nervous system, skeletal muscle, and heart; patients
often die in young adulthood. Pearson syndrome is
characterized by severe anemia and pancreatic dysfunction. The condition is usually fatal during infancy, but the few survivors often develop the
symptoms of KSS. PEO patients have ptosis (drooping eyelids) and weakness in the limbs, but they
have normal life spans.
a. How can you explain the variation in tissues affected and severity of symptoms in patients with
these three conditions, given that they all bear
large deletions of mtDNA? (Assume that the size of the deletion does not contribute to
differences.)
b. Assuming that mtDNA begins its replication from a
single origin, what can you conclude from these
diseases about the location of this replication origin?
c. Although these syndromes are due to mtDNA deletions, they are not usually maternally inherited but
instead arise as a new mutation in an individual.
For example, mothers with PEO usually do not
transmit this trait to their offspring. Propose an explanation for this surprising finding.
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