Different DNA can be just as helpful

GRAHAM WILLIAMS, a lecturer in forensic science at the University of Huddersfield, looks at a different type of DNA.

EARLIER this year, workers on a building site in Miller Street, Manchester, uncovered a skeleton.

This was initially treated as non-suspicious until the post-mortem examination revealed a broken neck, collar bone and jaw.

One of the first questions asked would be “Whose skeleton is this?”

In terms of identification, the use of fingerprints can be ruled out due to the extensive decomposition.

DNA profiling is still a possibility, but this depends on the extent of decomposition. As long as muscle or organ tissue is still present, then a standard DNA profile can be obtained. It can also be possible to obtain a standard DNA profile from the bone marrow or the teeth.

If the decomposition has progressed even further then it may no longer be possible to get a standard DNA profile. However, it may still be possible to carry out mitochondrial DNA analysis.

In our body and cells, we have two different sets of DNA. The first one is DNA from the nucleus of the cell and the second is DNA from the mitochondria.

It is nuclear DNA (or nDNA) that is analysed to produce a standard DNA profile. Mitochondria are found in the main body of the cell, the cytoplasm. These structures are the cell’s sources of energy. The amount of mitochondria varies from cell to cell depending on the cell function. For example, muscle cells would have a large number of mitochondria as more energy is required.

One fascinating bit of information about the mitochondria is that it used to exist as its own independent life form.

Millions of years ago, when our ancestors were no more than single celled organisms, we formed a symbiotic relationship with the mitochondria. This worked so well that soon we could not exist without each other.

Over time, as we evolved, the mitochondria lost some of its DNA until it became the structure we are familiar with now.

Though mitochondrial DNA (mtDNA) analysis is used in forensics, it does have its disadvantages compared with nDNA analysis.

The main disadvantage is that mtDNA is inherited through the maternal line only.

This differs from nDNA in that half of your nDNA comes from your mother and half from your father; whereas all of your mtDNA comes from your mother and none of it comes from your father.

This means that large swathes of the country and indeed the world will have indistinguishable mtDNA.

So, why do we use mtDNA analysis in forensic science?

Firstly, it can conclusively eliminate someone as being the source of the mtDNA. If the mtDNA sequences differ, then that person could not have contributed to the DNA tested.
However, if the sequences match, then that person cannot be excluded as being the source of the DNA, but cannot be included either.

However, it can provide information. Since the mtDNA is inherited through the maternal line, it can indicate which maternal line a person belongs to. This does not mean we can tell which family they came from, but it can provide a probability as to their geographical origin.

For example, a European’s mtDNA might be different to mtDNA from an African and an Asian.

One advantage of mtDNA analysis is the abundance of genetic material when compared with nDNA.

Each cell contains no more than one nucleus. Each nucleus only contains two copies of nDNA.

On the other hand, each cell can contain between tens and hundreds of mitochondria. Each mitochondrion can hold between hundreds and thousands of copies of mtDNA. Which means that each single cell can hold between hundreds and tens of thousands copies of mtDNA. This means that it takes much longer for all of the mtDNA to fully decay than it would do for nDNA.

The mtDNA analysis has been used to provide information about a large number of cases; for example it was utilised in the “Torso in the Thames” case where a small boy’s body was found in the river. Analysis of mtDNA was used to determine the geographical origin of the body.

The mtDNA analysis was used to determine whether remains recovered in 1979 were that of Tsar Nicholas II, Tsarina Alexandra and their children, who were executed by Bolsheviks in 1918.

This was done by comparing the mtDNA extracted from the bones and teeth of the remains with the mtDNA from known surviving relatives — in this case, Prince Philip, the Duke of Edinburgh, whose grandmother was Tsarina Alexandra’s sister, and the Duke of Fife, whose great-grandmother was Tsar Nicholas’s aunt.