Forensics 101: Tracing Lineage Through Maternal Mitochondrial DNA

Deoxyribonucleic Acid (DNA)Last week in a post on the potential discovery of Richard III’s remains, I wrote that scientists hope to confirm the identity of the remains based on DNA, specifically mitochondrial DNA. Over the next few weeks, I’m going to explain DNA identification, starting with the basics and then branching out into how it is used to name the dead.

DNA (deoxyribonucleic acid) is the code of life, the genetic information contained in every cell in the form of twenty-three pairs of chromosomes (except for sperm and eggs cells which have twenty-three single chromosomes). The information contained in this genomic DNA specifies everything about us, from how we look to which health problems we’ll have an increased likelihood of developing. But there is also another kind of DNA contained in our cells—mitochondrial DNA (mtDNA).

Mitochondria are the power houses in our cells, the organelles or cellular structures that are responsible for producing the chemical energy required for every cellular activity—from protein production to ion transport to cellular reproduction. But unlike other organelles, mitochondria have their own unique tiny DNA genome—mtDNA. Due to the process of human fertilization, these small bits of DNA are transmitted in family lineages only through the maternal line.

Electron micrograph of two mitochondria

An egg cell is a stripped down version of a typical cell in the body; essentially it only contains a nucleus carrying twenty-three single chromosomes plus several hundred mitochondria in the cytoplasm. A sperm cell is a small sack of DNA attached to a long tail for mobility, the base of which is packed with energy producing mitochondria to fuel the journey. At the moment of fertilization, the tiny sperm head fuses with the egg membrane and injects its DNA while the tail drops off and is lost. As a result, the only DNA inserted into the egg cell is the genomic DNA contained within the sperm head. All mitochondrial DNA that is then replicated as the fertilized zygote splits from one cell into approximately one hundred trillion cells comes solely from the mother. Fortunately, due to the nature of its sequences, mtDNA has a very low mutation rate. In other words, the same mtDNA is passed from grandmother to mother to child through the generations. Genetic testing of specific sequences of those mtDNA samples, even if separated by many generations, can definitively prove a family match.

Due to the number of copies of mitochondrial DNA in each cell (providing approximately five hundred identical copies of each gene versus two copies on the cell’s genomic DNA), forensic anthropologists often are more successful recovering mtDNA than genomic DNA from ancient and historical samples. So the combination of DNA yield and the consistency of the maternal line can provide identification, even for remains over five hundred years old.

Next week we’ll look more specifically at the kind of testing used to make a genetic match between samples providing conclusive DNA evidence for identification.

Photo credit: ynse and Louisa Howard