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Non-nuclear Inheritance

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In eukaryotic cells, mitochondria and plastids, a category of plant cell organelles that includes chloroplasts, reproduce independently in the cytoplasm and they each have their own set of genes that directs their reproduction and function. Mitochondrial and plastid DNA takes the form of small, circular DNA molecules and is referred to as non-nuclear or cytoplasmic DNA. Each organelle carries many identical copies of its DNA, in contrast to the nucleus, where there are typically only two copies of each chromosome.

Further, during cell division, the organelles are randomly segregated into new daughter cells, so the non-nuclear DNA that is passed on does not follow a set pattern. During reproduction in animals and most plants, the cytoplasm in the zygote comes from the egg. Thus, while both parents contribute equally to the nuclear DNA, the non-nuclear DNA is passed down only from the mother. This means that certain traits, such as metabolic disorders resulting from mutations in the mitochondria, can be traced in a matrilineal fashion.

Additionally, some physical traits in plants, such as leaf color, are controlled by non-nuclear genes and are also passed down uniparentally.

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Most DNA resides in the nucleus of a cell. However, some organelles in the cell cytoplasm⁠—such as chloroplasts and mitochondria⁠—also have their own DNA. These organelles replicate their DNA independently of the nuclear DNA of the cell in which they reside. Non-nuclear inheritance describes the inheritance of genes from structures other than the nucleus.

Mitochondria are present in both plants and animal cells. They are regarded as the “powerhouses” of eukaryotic cells because they break down glucose to form energy that fuels cellular activity. Mitochondrial DNA consists of about 37 genes, and many of them contribute to this process, called oxidative phosphorylation.

Chloroplasts are found in plants and algae and are the sites of photosynthesis. Photosynthesis allows these organisms to produce glucose from sunlight. Chloroplast DNA consists of about 100 genes, many of which are involved in photosynthesis.

Unlike chromosomal DNA in the nucleus, chloroplast and mitochondrial DNA do not abide by the Mendelian assumption that half an organism’s genetic material comes from each parent. This is because sperm cells do not generally contribute mitochondrial or chloroplast DNA to zygotes during fertilization.

While a sperm cell primarily contributes one haploid set of nuclear chromosomes to the zygote, an egg cell contributes its organelles in addition to its nuclear chromosomes. Zygotes (and chloroplasts in plant cells) typically receive mitochondria and chloroplasts solely from the egg cell; this is called maternal inheritance. Maternal inheritance is a type of non-nuclear, or extranuclear, inheritance.

Why do mitochondria and chloroplasts have their own DNA? The prevailing explanation is the endosymbiotic theory. The endosymbiotic theory states that mitochondria and chloroplasts were once independent prokaryotes. At some point, they joined host eukaryotic cells and entered a symbiotic relationship—one that benefits both parties.

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