Textbook Question
What is the endosymbiotic theory, and why is this theory relevant to the study of extranuclear DNA in eukaryotic organelles?
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Ch. 9 - Extranuclear Inheritance
Chapter 9, Problem 20
Payne, B. A. et al. (2013) present evidence that a low level of heteroplasmic mtDNA exists in all tested healthy individuals.
What genetic conditions within a given mitochondrion are likely to contribute to such a variable pool of mitochondria?
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Understand that heteroplasmy refers to the presence of more than one type of mitochondrial DNA (mtDNA) within a cell or individual.
Recognize that mutations in mtDNA can lead to heteroplasmy, as mitochondria have their own DNA separate from nuclear DNA.
Consider that during cell division, mitochondria are distributed randomly to daughter cells, which can lead to varying levels of mutated mtDNA in different cells.
Acknowledge that the bottleneck effect during oogenesis (egg cell formation) can result in a wide range of mtDNA variants being passed to offspring.
Realize that selective replication of certain mtDNA types within cells can also contribute to the variability in the mitochondrial pool.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Heteroplasmy
Heteroplasmy refers to the presence of more than one type of mitochondrial DNA (mtDNA) within a single cell or individual. This condition can arise from mutations in mtDNA, leading to a mixture of normal and mutated genomes. The level of heteroplasmy can vary among tissues and individuals, influencing mitochondrial function and potentially contributing to various genetic conditions.
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Organelle DNA Characteristics
Mitochondrial Inheritance
Mitochondrial inheritance is a unique form of genetic transmission where mtDNA is passed down maternally. Unlike nuclear DNA, which is inherited from both parents, mtDNA is inherited exclusively from the mother. This mode of inheritance can lead to the expression of mitochondrial diseases that may vary in severity based on the proportion of mutated mtDNA present in the offspring.
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Mitochondrial Dynamics
Mitochondrial dynamics encompass the processes of mitochondrial fusion and fission, which are crucial for maintaining mitochondrial function and health. These processes allow mitochondria to adapt to cellular energy demands and stress. Variations in these dynamics can lead to a heterogeneous population of mitochondria within a cell, contributing to the observed heteroplasmy and influencing the overall health of the cell.
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Related Practice
Textbook Question
Mutations in mitochondrial DNA appear to be responsible for a number of neurological disorders, including myoclonic epilepsy and ragged-red fiber disease, Leber's hereditary optic neuropathy, and Kearns-Sayre syndrome. In each case, the disease phenotype is expressed when the ratio of mutant to wild-type mitochondria exceeds a threshold peculiar to each disease, but usually in the 60 to 95 percent range.
Given that these are debilitating conditions, why has no cure been developed? Can you suggest a general approach that might be used to treat, or perhaps even cure, these disorders?
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Textbook Question
Mutations in mitochondrial DNA appear to be responsible for a number of neurological disorders, including myoclonic epilepsy and ragged-red fiber disease, Leber's hereditary optic neuropathy, and Kearns-Sayre syndrome. In each case, the disease phenotype is expressed when the ratio of mutant to wild-type mitochondria exceeds a threshold peculiar to each disease, but usually in the 60 to 95 percent range.
Compared with the vast number of mitochondria in an embryo, the number of mitochondria in an ovum is relatively small. Might such an ooplasmic mitochondrial bottleneck present an opportunity for therapy or cure? Explain.
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Textbook Question
Payne, B. A. et al. (2013) present evidence that a low level of heteroplasmic mtDNA exists in all tested healthy individuals.
What are two likely sources of such heteroplasmy?
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Textbook Question
As mentioned in Section 9.3, mtDNA accumulates mutations at a rate approximately ten times faster than nuclear DNA. Thus geneticists can use mtDNA variations as a 'molecular clock' to study genetic variation and the movement of ancestral human populations from Africa to different areas of the world more than 125,000 years ago. Propose an explanation for how an analysis of mtDNA can be used to construct family trees of human evolution.
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Textbook Question
Because offspring inherit the mitochondrial genome only from the mother, evolutionarily the mitochondrial genome in males encounters a dead end. The mitochondrial genome in males has no significant impact on the genetic information of future generations. Scientists have proposed that this can result in an accumulation of mutations that have a negative impact on genetic fitness of males but not females. Experiments with Drosophila support this possibility. What experimental data or evidence would you want to evaluate or consider to determine if an accumulation of mtDNA mutations negatively impacts the fitness of males of any species?
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