Mitochondrial DNA (mtDNA) is a small, circular chromosome found inside mitochondria that carries genetic information separate from the nuclear DNA, replicates on its own, and is not wrapped around histones, much like a prokaryotic chromosome.
Mitochondrial DNA, or mtDNA, is the genetic material that lives inside your mitochondria instead of in the nucleus. It's a small, circular chromosome that is not associated with histones, the proteins eukaryotic cells use to condense their nuclear DNA (CED 6.1.A). That setup probably sounds familiar, because it's basically how prokaryotes organize their DNA. That resemblance is the whole point: mitochondria are thought to descend from free-living bacteria, so they kept the bacterial-style genome.
So a single eukaryotic cell carries two kinds of DNA at once. The nuclear DNA is linear, wrapped on histones, and inherited from both parents. The mtDNA is circular, histone-free, replicates autonomously, and you inherit it almost entirely from your mother. Same base-pairing rules apply to both, though. mtDNA is still built from nucleotides where adenine pairs with thymine and guanine pairs with cytosine (CED 6.1.B). The structure of the chromosome changes; the chemistry of the code does not.
This term lives in Unit 6 (Gene Expression and Regulation), Topic 6.1 DNA and RNA Structure, and it supports two learning objectives at once. CED 6.1.A asks you to describe the structures that pass hereditary information from one generation to the next, and mtDNA is the textbook example of an extra-chromosomal, circular eukaryotic genome that breaks the 'eukaryotes have linear histone-bound chromosomes' rule. CED 6.1.B covers why DNA works as hereditary material, and mtDNA shows that conserved base pairing holds no matter where the DNA sits. The big reason it shows up so often, though, is evolution. Because mtDNA mutates and is passed down a clean maternal line, it's a clock for tracking relatedness, which ties Unit 6 straight to phylogenetics in Unit 7.
Keep studying AP Biology Unit 6
Maternal Inheritance (Unit 6)
You get your mtDNA almost entirely from your mother's egg, because sperm contribute essentially no mitochondria. That's why a mitochondrial disorder shows a distinctive pedigree pattern: an affected mother can pass it to all her kids, but an affected father passes it to none.
Prokaryotic Cells & Endosymbiosis (Unit 6)
Circular DNA with no histones is the prokaryotic blueprint, and mitochondria carry exactly that. This is the structural fingerprint behind the endosymbiotic theory that mitochondria were once free-living bacteria.
Phylogenetic Trees (Unit 7)
Because mtDNA accumulates mutations along a maternal line, scientists compare mtDNA sequences across species to build evolutionary trees. The 2018 bear FRQ and 2019 primate FRQ both used mtDNA sequence data exactly this way.
Chloroplast DNA (Unit 6)
Chloroplasts also carry their own small circular, histone-free DNA for the same endosymbiotic reason. If you can explain mtDNA, you can explain cpDNA using the identical logic.
On multiple choice, the classic stem hands you a cell with nuclear DNA that is linear and histone-bound and mitochondrial DNA that is circular and histone-free, then asks you to justify a claim from those differences. The right move is to connect the circular, histone-free structure to the prokaryotic ancestry of mitochondria. Watch for the plasmid trap too, where a small autonomous circular molecule appears in a yeast nucleus, since that's a plasmid, not mtDNA. On free response, mtDNA almost always shows up as evolution data. The 2018 and 2019 FRQs gave mtDNA sequence comparisons and asked you to interpret a phylogenetic tree, and the 2021 FRQ used a glucose-metabolism disorder to test whether you can read a mitochondrial (maternal) inheritance pattern in a pedigree.
Both are small, circular, extra-chromosomal DNA, so they're easy to mix up. The difference: mtDNA sits inside the mitochondrion and encodes essential mitochondrial functions, while a plasmid is a separate circular molecule that can show up in the cytoplasm (or even the nucleus in some organisms) and replicates independently. If the question says the molecule is inside the mitochondria, it's mtDNA; if it's a free circle elsewhere replicating on its own, it's a plasmid.
Mitochondrial DNA is a small, circular chromosome inside mitochondria that is not wrapped around histones, unlike the linear nuclear chromosomes of eukaryotes.
Its prokaryotic-style structure (circular and histone-free) is the structural evidence behind the endosymbiotic theory that mitochondria descended from bacteria.
You inherit mtDNA almost entirely from your mother, which creates a maternal inheritance pattern in pedigrees.
mtDNA still follows normal base pairing, with adenine pairing to thymine and guanine pairing to cytosine, so the genetic code chemistry is unchanged.
On the FRQ, mtDNA sequence comparisons are used to build phylogenetic trees and measure evolutionary relatedness, linking Unit 6 to Unit 7.
Don't confuse mtDNA with a plasmid: mtDNA lives inside the mitochondrion, while a plasmid is a separate circular molecule elsewhere in the cell.
It's the small, circular, histone-free chromosome found inside mitochondria, separate from the nuclear DNA. It maps to Topic 6.1 (DNA and RNA Structure) and supports learning objectives 6.1.A and 6.1.B.
No. mtDNA is inherited almost exclusively from your mother, because the egg supplies essentially all the mitochondria and sperm contribute virtually none. That's why mitochondrial disorders pass from an affected mother to all her children but never from the father.
Nuclear DNA is linear and wrapped around histone proteins, while mtDNA is circular and has no histones. mtDNA also comes only from your mother, whereas nuclear DNA comes from both parents.
Because mitochondria are thought to have evolved from free-living bacteria through endosymbiosis, so they kept the prokaryotic-style circular, histone-free genome. This structural similarity is the evidence the AP exam wants you to connect to bacterial ancestry.
Because mtDNA is passed down a clean maternal line and accumulates mutations over time, comparing mtDNA sequences across species reveals how closely related they are. Released FRQs from 2018 and 2019 used mtDNA comparisons to build phylogenetic trees of bears and primates.