Chromosomes carry the genetic code that determines the characteristics of a living thing. They are fascinating due to the varied factors they determine, the sometimes negative effects they can have and their complexity. Equally interesting are the stories of their discoveries. This series will explore the history of specific chromosomes and their impact on science.
In humans, mitochondrial DNA (mDNA) is regarded as the smallest chromosome, coding for 37 genes and containing approximately 16,600 base pairs. Taryn Cain continues this series by looking at mitochondria.
Let me take you back a bit. 1.4 million years back, in fact. A small bacterium was ingested by a large single-celled organism and, rather than being digested, the smaller body was instead left to its own devices. As time went on, the bacterium began transferring some of its genetic material over to the larger cell, until eventually it was as if the two had never been independent. The small bacterium involved itself with the inner workings of the cell, while the larger organism took over the world. That small bacterium was a mitochondrion, while the larger body became all multi-cellular life on earth. This evolutionary theory is known as symbiogenesis.
Mitochondria are small, single celled organisms that are distantly related to bacteria, but now reside in every cell in your body. Each cell can carry thousands of the tiny mitochondria, which means each human body holds around ten million billion of them. They produce a chemical called adenosine triphosphate (ATP) which allows us to store and use energy for all sorts of cellular activity. This has earned these structures the nickname of “the powerhouse of the cell”. Every time you work out at the gym, go hiking up a mountain or carry home bags of shopping, it’s your mitochondria doing the real work.
Mitochondria are only found in the cells of eukaryotes (organisms whose cells contain a nucleus). Humans are eukaryotes, along with all other animals, plants and fungi. When the mitochondrial genome was sequenced in 1981, it was found to have circular DNA like bacteria: 16,569 base pairs and 37 genes, all of which are for producing ATP or proteins. mDNA mutates at a greater rate than DNA. This in part led to the theory that mitochondria may have enabled life to diversify enough to move all over the globe.
Due to their integral role in our existence, it is probably not all that surprising that mitochondria are believed to be involved in more than just producing energy. They may also have a hand in your memory, cell division, oxygen transportation and calcium production, among other things.
Of course the hard-working mitochondria have a dark side. As any part of your body may go wrong, so might they. Whether you are born with it or it occurs as a part of ageing, mitochondria have been implicated in Alzheimer’s, heart disease, autism, diabetes, obesity, cancer and more.
The discovery of mitochondria was like a difficult jigsaw puzzle, one that different people add a different piece to over the course of a century.
- Albert von Kölliker, an anatomist, first saw them in the 1850s.
- Richard Altmann, a pathologist, noted their similarity to bacteria in the 1890s.
- Carl Benda, a physician, named them mitochondria in 1898.
- Kingsbury and Warburg first independently linked them to cell activity in 1913.
- Ivan Wallin, a biologist, thought mitochondria may have originated from free-living bacteria in 1927.
- Lynn Margulis, a biologist, showed that mitochondria and cells were living together in a mutually beneficial relationship in the 1960s.
- In the 1970s, it was discovered that mitochondria are related to a bacteria that causes typhus in humans today.
Mitochondria have been in the news for the past few decades for two reasons. The first is the 1987 discovery of Mitochondrial Eve: the woman who passed on her mDNA to all people living on earth today (in most species only women can pass on mDNA). The male equivalent is Y-chromosomal Adam.
The second reason mitochondria have been in the media is due to a procedure called Ooplasmic transfer, which you may know as ‘three-parent babies’. Like all cells, the sperm and egg need mitochondria to function. The egg carries thousands of mitochondria, whereas the sperm carries a mere hundred or so (their only job is to fuel the sperm’s journey to the egg). When the paternal mirochondria enter the egg, they are marked for destruction inside the embryo.
Only two or three species allow the mixing of mitochondria and only one, a saltwater mussel, allows it regularly. Normally this allows species to avoid genetic competition and make reproduction more efficient, but if the maternal mitochondria is damaged in some way there is no backup. A child could still be conceived, but its life is likely to be short and fraught with illness. As yet there is no cure for mitochondrial disease; however, there is a preventative measure which, just like IVF before it, has proved to be controversial.
Ooplasmic transfer involves taking an egg cell from an infertile woman and injecting it with the gel-like cytoplasm from another woman’s egg. This will allow the egg to fertilise and develop as normal, possibly because it also transfers healthy mitochondria. While it is true that mitochondria are heavily involved in the day to day activities of the cell, they cannot change the DNA of the cell, so will not affect the way the child looks, nor the traits it inherits. The controversy comes as the total cell DNA will not only come from three donors, but could also be passed on to future generations if the resulting child is a girl.
From its discovery in the 19th century, the future of mitochondria looks bright. Not only can it be used to save children, it is helping us understand ageing and has enabled us to map ancient human migration, along with other medical and anthropological possibilities. Due to the large numbers of mitochondria in each cell and its durability in teeth and bones, mitochondria have also proved a very useful tool in forensics.
Taryn is a Visitor Experience Assistant at Wellcome Collection.