BackExtranuclear Inheritance: Mitochondrial and Chloroplast Genetics
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Extranuclear Inheritance
Introduction
Extranuclear inheritance refers to the transmission of genetic material that is not located in the nucleus of the cell. This type of inheritance involves genes found in organelles such as mitochondria and chloroplasts. These genes are inherited independently of nuclear chromosomes and often display non-Mendelian patterns of inheritance.
Key Point: Extranuclear genes are located in organelles, not in the nucleus.
Key Point: Inheritance patterns differ from classical Mendelian genetics.
Example: Mitochondrial diseases and leaf variegation in plants.
Varieties of Extranuclear Inheritance
Types of Extranuclear Inheritance
There are two main varieties of extranuclear inheritance:
Organelle heredity: DNA contained in mitochondria or chloroplasts determines certain phenotypic characteristics of the offspring.
Infectious heredity: Inherited phenotype is affected by the presence of a microorganism in the cytoplasm of the host cell.
Mitochondria
Structure and Function
Mitochondria are organelles within eukaryotic cells that act as "power plants," generating most of the cell's energy through cellular respiration. Each mitochondrion contains its own small, circular DNA (mtDNA), which exists in multiple copies per organelle.
Key Point: Mitochondria have their own genome, separate from nuclear DNA.
Key Point: mtDNA is inherited almost exclusively from the mother (maternal inheritance).
Example: Human mitochondrial diseases are passed from mother to all children.
Maternal Inheritance
Most extranuclear genes are inherited from the mother because the egg cell provides the majority of the cytoplasm (and thus organelles) to the zygote. Sperm contributes little or no cytoplasm, and its mitochondria are typically excluded or destroyed after fertilization.
Key Point: All children of an affected mother may inherit the trait.
Key Point: Sperm mitochondria are usually not transmitted to offspring.
Example: Mitochondrial myopathies, Leber's hereditary optic neuropathy.
Size of Mitochondrial Genomes in Various Organisms
The size of mitochondrial DNA varies among species:
Organism | Size (kb) |
|---|---|
Homo sapiens (human) | 16.6 |
Mus musculus (mouse) | 16.2 |
Xenopus laevis (frog) | 18.4 |
Drosophila melanogaster (fruit fly) | 18.4 |
Saccharomyces cerevisiae (yeast) | 75.0 |
Pisum sativum (pea) | 110.0 |
Arabidopsis thaliana (mustard plant) | 367.0 |
Mutations in Mitochondrial DNA
mtDNA is particularly vulnerable to mutations for several reasons:
Key Point: mtDNA lacks the structural protection provided by histone proteins.
Key Point: DNA repair mechanisms for mtDNA are limited.
Key Point: Mitochondria generate reactive oxygen species (ROS), which are highly mutagenic.
Example: Mutations in mtDNA can cause disorders such as mitochondrial encephalomyopathy.
Chloroplasts
Structure and Function
Chloroplasts are organelles found in plants and algae, responsible for photosynthesis. Like mitochondria, chloroplasts contain their own DNA (cpDNA), which is separate from nuclear DNA.
Key Point: Chloroplast DNA controls photosynthesis-related traits.
Key Point: Chloroplast inheritance is often maternal or biparental, depending on the species.
Example: Leaf variegation in Mirabilis jalapa (four o'clock plant).
Chloroplast DNA (cpDNA) and Inheritance
Chloroplasts are involved in extranuclear inheritance because they contain their own DNA, which is inherited separately from nuclear DNA. In many plants, cpDNA is inherited almost exclusively from the maternal line, as demonstrated by traits like leaf color in variegated plants.
Key Point: Cytoplasmic inheritance differs from Mendelian genetics.
Key Point: Chloroplast DNA is usually inherited from the egg cell.
Example: Variegated branches in Mirabilis jalapa depend on the chloroplasts present in the egg cell.
Non-Mendelian Inheritance Patterns
Characteristics of Extranuclear Inheritance
Extranuclear inheritance does not follow the typical dominant/recessive rules of Mendelian genetics. Traits may not segregate in Punnett square patterns and can skip generations or appear only in the maternal lineage.
Key Point: Inheritance is cytoplasmic, not chromosomal.
Key Point: Traits may show irregular patterns across generations.
Example: Mitochondrial diseases only passed through mothers, not fathers.
Summary
Extranuclear inheritance involves genes outside the nucleus, specifically in mitochondria and chloroplasts.
These genes are typically maternally inherited.
Extranuclear inheritance is crucial for understanding non-Mendelian genetics.
