Mitochondrial DNA (mtDNA) Facts, Maternity, Genealogy, Forensics

Apr 11, 2010
Donald Reinhardt

MItochondrial DNA H1 and H2 Control DNA Regions - FBI, U.S.

Mitochondrial DNA in the egg is passed directly to all daughters and sons. mtDNA is useful for forensics, maternity, female lineages, and ancestry studies.

Mitochondrial DNA, mtDNA, is unique to eukaryotes. Mitochondrial DNA is much smaller, but more resilient, than the cell's nuclear DNA . mtDNA is important in criminology, maternity, lineage and ancestry investigations.

Mitochondria, Oxidative Organelles, Facts and Properties

Mitochondria are thread-like structures found in eukaryotic organisms such as plants, animals, fungi and protistans. Eukaryotes are organisms with membrane-bound nuclei and organelles such as mitochondria and chloroplasts.

Mitochondria are oxidative, energy-generating (ATP-producing), cytoplasmic organelles that are filled with enzymes, RNA and circular DNA (mtDNA) macromolecules.

Facts and Properties of Mitochondrial DNA, mtDNA

mtDNA contains base pairs, that when analyzed are valuable for maternity, ancestry, lineage studies, and forensics. Y chromosome and STR autosomal studies also are useful.

Significant facts about mitochondrial DNAs (mtDNAs) are:

circular DNAs, of 2 to 3 copies in each mitochondrion;
extractable as dozens to hundreds of copies per cell;
of similar DNA composition in similar cells within the same organism, but differ between different individuals of the same species, and among different species or organisms;
able to crossover (recombine) mtDNAs within each human mitochondria, but recombinants are not produced because the mtDNAs are all the same;
heavy chain is guanine-rich, and lighter chain is cytosine-rich;
light and heavy chains can be unwound from one another, and then separated by centrifugation;
numbered as 16,569 base pairs (bp), compared to the 3.1 billion bp of the 23 human chromosome pairs;
of 2 Hypervariable Regions (control regions, or displacement loop D-regions), termed HV-1 and HV-2; both HVs are non-coding regions — they do not specify a specific endproduct;
HV- 1 has 342 bp (16,024 to 16,365), HV-2 has 268 bp (73 to 340) – a combined total of 610 bp;
both HV-1 and HV-2 regions are valuable for forensics, ancestry and maternal investigations;

mtDNA Facts and Uses for Ancestry, Lineage, Paternity and Forensics

When a human sperm unites with an egg, the sperm tail with its many mitochondria typically is excluded from entering the egg; only the sperm head (containing the haploid (n) nucleus and some mitochondria) passes across the membrane into the egg's cytoplasm. The egg has its own haploid (n) nucleus, and many more mitochondria than a sperm. The haploid nuclei of the male and female cells fuse to form a diploid (2n) cell.

Shortly after fertilization, the male mitochondria are chemically marked and are soon destroyed. Therefore, only the female egg mitochondria survive, and these same female mitochondria are present in both immediate male and female offspring. It is true to say that mothers express their love to all their diploid (2n) offspring with a haploid (n) nucleus and a full bouquet of mitochondria, and, in contrast, the male contribution is only the haploid (n) number of chromosomes.

mtDNAs have these properties and characteristics:

identification and connection to the maternal female lineages, with the ability to extensively trace backwards over decades, or even centuries;
ability to survive degradation and to be preserved within hair shafts, bone and teeth, thus being invaluable for forensic evidence — nuclear DNA frequently degrades, mtDNA often does not;
have valuable HV-1 and HV-2 non-coding regions for bp analyses that permit inclusion or exclusion of suspects in comparisons made with crime scene evidence;
mt DNA coding SNP sites also are being developed for multiplex analysis.

mtDNA – Forensic, Maternity, Ancestry, Lineage Matches and How They are Made

mtDNA FBI forensic analyses occur as follows:

primary visual analysis, to assure the specimen is acceptable for processing;
sample preparation, cleaning and removal of extraneous and interfering debris;
DNA extraction, chemical separation of DNA from other biochemicals — after treatment and centrifugation hydrophilic, DNA supernatant is obtained.
polymerase chain reaction (PCR) amplification, primers, nucleotides and thermal cycling with Taq polymerase enhance the HV-1 and HV-2 regions;
postamplification quantification of the DNA, determines the amount of DNA available;
automated DNA sequencing, determines the linear order and type of base pairs;
data analysis, comparison mtRNA derived from a placenta is the reference standard known as the Anderson sequence, or the updated, revised Cambridge sequence (rCRS);

For the final report only differences between the rCRS sample and the evidentiary and suspect samples are reported. In an example, given by Koblinsky et al, the evidence and suspect mtDNAs match each other and differ from the control, and a report might read in part: 109C, 222G for the evidence, and 109C and 222G for the suspect. This is an example of an "inclusion". If the 2 specimens had differed, then the report is an "exclusion".

Similar mtDNA analyses for maternity and lineage matches can also be made. The story and value of DNA continues to amaze.

Sources

Kobilinsky, L., L. Levine, and H. Margolis-Nunno. 2007. Forensic DNA Analysis. Chelsea House of Infobase Publishing, New York, N.Y.

Lodish, H. et al. 2000. Molecular Cell Biology. Fourth Ed., W. H. Freeman and Co., New York, N.Y.