Regardless of our relationship with our biological mother, we cannot run away from the fact that we inherited our mitochondrial DNA from that human. So, before I scream #HappyMothersDay , let’s first talk a bit about mitochondria and their contribution to life.

A thread… 1/

2/

We are introduced to mitochondria as ‘the powerhouse of the cell.’ They are responsible for supplying our cells with the bulk of energy it needs (in the form of ATP) through cellular respiration. But this is just one of the many cool things about them!

3/

Many of us probably remember the ‘Endosymbiotic Theory,’ which formulates that organelles such as mitochondria and plastids originated from formerly free-living prokaryotes taken one inside the other that co-evolved through mutual benefit.

4/

Before I go on, big shout out to the 1ry modern proponent of this theory, Dr. Lynn Margulis (1938-2011) who was often mocked for her theories and her landmark paper “On the Origin of Mitosing Cells” (1967) was rejected ‘about 15 times.’ #WomenInSTEM

5/

Now, we simply cannot survive without mitochondria. Some cells contain 1-2 while others 1000s. For example, cells that need a lot of energy such as muscle cells have many more mitochondria than a non-muscle cell. Here a pic I took circa 2014 showing this.

6/

These compartments are a signature of eukaryotic cells. Nonetheless, biology is full of exceptions. For example, single-celled eukaryotes of the Monocercomonoides genus lacks mitochondria; and our red blood cells lose them during their development.

7/

Did y’all know that they (1) are dynamic, (2) can change shape, (3) divide through fission (similar to bacteria) to create new mitochondria, and (4) can even fuse with one another to complement each other?! Look at the green mitochondria dividing and fusing with a red one!

8/

Why is this important? Because cells have quality control mechanisms to deal with damaged or unwanted mitochondria. In some cells, 1 in 5 daughter mitochondria is naturally eliminated by a process known as ‘mitophagy’ where the cell ‘eats’ its own defective mitochondria.

9/

Back to mitochondrial DNA (mtDNA). Each mitochondrion contains several copies of a single (usually) circular DNA organized as nucleoids (unlike the nuclear linear DNA wrapped around in histones and condensed as chromosomes in the nucleus.)

10/

Now to the fun part: how is it that most animals, including humans, end up with strictly maternal-derived mitochondrial DNA and not paternal or from both?

P.S. Yes, exceptions do exist for biparental inheritance (e.g. some plants and fungi)... it’s biology get over it.

11/

In mammals, paternal mitochondria (tightly packed in the midpiece of the sperm) generally does enter the egg’s cytoplasm after fertilization BUT these are selectively destroyed or ‘diluted out’ and never transmitted to the offspring.

12/

Of course, there is not a single way for this to happen and different mechanisms have been reported in different species. Different mechanisms to destroy the sperm’s mitochondria prior to and/or after fertilization have been reported for different species.

13/

Uniparental inheritance of mtDNA is almost universal, but the ‘why’ remains poorly understood. Some claim that it may reduce the spread of harmful mtDNA mutations or that is simply a byproduct of differences in gamete sizes (e.g. egg >>> larger than sperm)

14/

But I would personally argue against the latter given than in some species the mtDNA from one parent is degraded before the mitochondria itself and uniparental mtDNA inheritance is also seen in some species with gametes of the same size. Evolution is wonderful.

15/

TL;DR

Mitochondria are amazing. They move, divide, fuse, and have their own DNA that is maternally inherited bc sperm DNA is usually destroyed shortly after fertilization.

Plus, they are gorgeous and I have had the pleasure of imaging them in the past. #HappyMothersDay !