Mitochondrial DNA

Mitochondrial DNA.

Almost every human cell is powered by mitochondria, 

bean shaped organelles that use oxygen to synthesise useable energy.

These structures evolved billions of years ago from free swimming bacteria.

They were engulfed by some of human’s earliest ancestors.

Because of this history, mitochondria still have their own ring shaped DNA.

This is completely separate from the 23 chromosomes that make up the human genome.

These chromosomes come from both parents.

However, nearly all humans inherit their mitochondrial DNA from their mother’s egg cell.

What happens to the mitochondrial DNA in the sperm cell?

Understanding this process is important for studying mitochondrial diseases,

which are genetic disorders when these power houses don’t function properly.

Scientists know that molecular processes breakdown the sperm’s mitochondria,

soon after fertilisation, in other animals.

But no one has been able to pin point when this elimination happens in humans.

Now scientists have discovered that it happens early.

In fact, human sperm’s few mitochondria contain virtually no DNA.

This mitochondria DNA elimination process might play a role in human infertility.

This could help science understand mitochondrial disease.

Using molecular biology and microscopy techniques, 

researchers examined human sperm cells across their developmental stages.

They found that mitochondria in the sperm’s precursor cells did contain DNA,

along with a protein called mitochondrial transcription factor A, or TFAM, 

that maintained and protects that DNA.

But when the sperm cells matured, a slight chemical change prevented TFAM. 

from entering their mitochondria.

Instead it entered the nucleus, where it could no longer prevent the mitochondrial DNA, 

from degrading.

If mitochondrial DNA sticks around in sperms mitochondria, 

it could become a source of infertility.

Studies have shown that people with decreased sperm counts and motility, 

have elevated amounts of mitochondrial DNA in those cells.

Studies in animals have found that paternal mitochondria DNA can be eliminated, 

after egg fertilisation.

It shows that multiple mechanisms make contribute to maternal mitochondrial inheritance, 

in different organisms.

There are probably yet other unidentified mechanisms that regulate mitochondrial DNA,

in different cells.

This might contribute to mitochondrial disease if disrupted.

We need to still discover these mechanisms.