I was sitting down trying to figure out the best way to honor Mother’s Day with a blog post. Sure I could show cute pictures of animals and their babies (but I have done that before), but I decided I am going to do it my way….
At this juncture in human history almost every person knows about DNA, or by its formal name Deoxyribonucleic acid. DNA is the ingredients list for life (notice I did not say instruction manual, this is a common misconception, DNA merely lists the parts but does not explain how to put them together). We all know the story, half of your DNA comes from your father and half of your DNA comes from your mother and a brand new never before seen human arises from this six foot long molecule.
Some of you may be sophisticated enough to know that the DNA molecule is shaped in the form of a double helix. If you can’t visualise that, here is an image of DNA….
It is a little cartoony for my taste but you will see why I chose that specific one in a moment. So if you laid this double helix out flat it would look like a ladder. The rungs of the ladder are base pairs and they join in the middle of the rung through a hydrogen bond. There are four base pairs in DNA, four, for every living organism in the world there are only four different base pairs, Adenine, Thymine, Guanine and Cytosine, that means that whether you are a bacteria or a human your parts list is all written in the same language. As you read this the DNA in your body is dividing and replicating over and over again hundreds of millions a time per day. Every single one of your somatic cells (every cell but sperm and egg cells) has a strand of DNA residing in it. So why not the sperm and egg cell? The sperm and egg cannot have a complete set of DNA because if they did when they met they would have twice the amount they need to produce an offspring. So our bodies half the DNA in our reproductive cells in a process called meiosis so that when the sperm meets the egg they will form one new whole DNA strand…..aka you. This is important because this what gives sexually reproductive organisms so much variety, the constant recombination of DNA.
So there it is, your father gives half and your mother gives half.
Well not quite.
So lets jump back a minute, every eukaryotic cell, basically every organism besides bacteria, has a double helix shaped DNA molecule. So what does Bacteria have? Circular DNA. It is simple, easy to replicate, and limited in what it can do, but as you can tell bacteria are doing just fine. Bacteria do not recombine DNA, they simply continually make new copies of themselves, changes occur due to copying mistakes in the DNA.
For our purposes the words are not important, what is important is the shape, circular. So that is one of the major differences between us (humans) and bacteria, the shape of our DNA. However like I said earlier, both bacteria and human DNA are written in the same language, but again, not important for where I am going with this……which leads us to the last piece of the puzzle that this blog has become…..lets talk about mitochondria.
For those who do not know, mitochondria is a tiny organelle (this means a structure inside of a cell, like organs in your body) in our cells that produces a material called ATP. Why is it important? Because ATP is literally the reason you can move, breath, pump blood, fight disease etc. ATP is the battery of the cellular world. It fuels every cell in your body. Here is a little look at this wonderful little organelle.
Now this is where things get really interesting. In mitochondria there is something strange, hiding in this structure is DNA. It is unusual enough to find DNA inside an organelle, but it gets even stranger. Don’t forget all human DNA is in the shape of a double helix. Well the mitochondrial DNA (mtDNA) is not in the shape of a double helix. Surprisingly enough the shape of your mitochondrial DNA is in fact, circular! Why is it circular you ask? Great question. Let me show you a picture of mitochondrial DNA.
Does it look familiar? It should because as you can probably put two and two together it has the exact same shape as bacterial DNA, one of earth’s oldest living organisms. This is also why mitochondrial DNA is shaped differently than nucleic or regular DNA. The origins of mtDNA stem directly from bacteria DNA. Mitochondria were at one point their own cell before being modified. In fact there are similar genes between the two. Without getting into too much detail there are certain cellular processes, for instance ATP production and electron transport that are the same in every organism, so it makes sense that the gene to do this will be the same for every organism. For our purposes this is true. The genes on human mitochondrial DNA are very ancient and linked to electron transport chains and protein synthesis. Interestingly enough mitochondria does not recombine with your normal DNA. It does the same thing as bacteria DNA, makes copies of itself.
So we know how circular DNA got into mitochondria, but how did the mitochondria get into us? The short of it? Way back in the day eukaryotes being so much bigger than bacteria could easily ingest them into their cell and essentially trap them there. The mitochondria being such a wonderful producer of energy developed a synthesis with the larger cell. Now our cells have little mitochondria in them. Which as you now know, we desperately need to survive. So here we see evolution in action. But I know what you are thinking “thanks for the science lesson but what does this have to do with Mother’s Day?”
So now that you know about the mitochondria in your body and it’s circular DNA I want you to hold that thought while I explain reproduction to you. Now I know most of you might think they don’t need a lesson in reproduction, but I am not talking about the “Sometimes when a man and woman love each other they express that love physically…..” speech. No, I am going to explain to you exactly how this works.
As you know you get half of your DNA from each parent, and this half is contained in either a sperm or an egg depending on the sex. But how does this happen and why is it important? Let’s look at males first, because as in most instances, they are just simpler. A male produces sperm, which in essence is nothing more than a DNA transport vessel with a one time use. So how is sperm produced? Well when the cell begins to divide it does something with the DNA, it copies it. So now the cell has two strands of DNA. Then the cell splits into two cells with one DNA strand in each, then those two cells divide again making 4 sperm cells with four halves of DNA. From one cell we get four sperm. Here is a simple diagram.
Now, lets maintain a level of maturity here as I discuss the actually sperm cell. This cell is very unique, and highly modified to do one thing. Deliver the DNA. Much like a man, there are no frills here. Precious cargo in a container and bare bones, stripped down vehicle built to move. The sperm cannot be weighed down with nonessential structures. It is the cafe racer of the cell world. What this means is that the cell does not have a long lifespan because it does not have what it needs to carry out sustainable functions. It only has what it needs to survive for 24-48 hours.
However structurally it is pretty amazing, it truly is a feat of evolutionary engineering. Lets take a closer look at it. As you can see in the diagram the head is where all the importan material (DNA) is kept. The rest of this cell is made solely for the purpose of transport. Now let me quiz you for a second. Having just learned about mitochondria and how they produce ATP, why do you think we find a bunch of it clustered in the mid-piece?Hopefully you thought “Well mitochondria produce ATP, ATP is fuel, so ATP is going to power the flagellum (tail) of the sperm so it can propel itself.” If you said this then you are correct. However remember the sperm is on its own with only 24-48 hours supply of materials. After that it can no longer produce ATP and dies. Unless it reaches the egg and the head of the sperm is absorbed into the egg leaving out the mid-piece and tail and fertilization occurs.
Which leads us to females, and just like women, nothing is simple. But I am going to break it down for you in a way that will make it somewhat easy to understand. To begin with we need to understand how an ovum (egg) is produced for reproduction and how exactly this relates to the female cycle. Bear with me men, I know it is awkward but that is because it is mysterious to us, and thus can be used against us by women. So if you educate yourself on the workings of it you can actually use it to your advantage. I can’t tell you how many times a woman has said to me “You’re a man you don’t know anything about it.” To which I reply, “Do YOU know anything about it?” Side note: women do not appreciate this. Amazingly enough most women have no idea what is actually going on. Sure they can tell you the basics, but not the important details that actually leads to the effects they feel.
So lets educate.
A woman is born with every single egg she will ever produce in her lifetime. Let me say that again, at birth a female’s ovaries have in them ever single egg she will ever produce. That means she carries them with her throughout her whole life and like most things the longer they go unused the more damaged they can become from environmental pathogens (chemicals, disease, etc.).
So seems simple enough, all the eggs are there, much simpler than men right? Wrong. Because just like the vessel that transports these eggs a lot of preparation must occur to go out. Like the sperm the egg needs to make sure it has only half a strand of DNA so the egg in preparation to leave the ovary must undergo a change.
A normal human female goes through a 28 day cycle. The first 7 days is dedicated to shedding the unused materials that were prepped for pregnancy, this is the time when interactions with females are best avoided, and jokes should be kept to zero. Then on the 8th day they enter the follicular phase, this is the preparation of the uterus to receive a fertilized egg. This takes about another six to seven days and then the female enters the ovulation phase. This is what I want to focus on.
When the uterus is ready the female body releases FSH, which is the Follicle Stimulating Hormone. This triggers the final preparation of the egg before being released into the fallopian tubes. FSH is responsible for one of my favorite words in biology…..oogenesis. That is not a misspelled word, it is pronounced oh-oh-genesis. Oogenesis is the female equivalent to spermatogenesis. Like you learned, from one cell in a male they can produce four sperm cells. Not so, in a female one cell will only produce one egg cell. However it needs to go through the division process to get the DNA molecules down to a single half strand. The following diagram will make this a little easier. Unlike the spermatogenesis we get one egg cell and three polar bodies, which do not contain enough materials to make a functional cell.
The one egg cell needs as much as the material as possible because it is going to be receive nothing but DNA from the sperm so it is responsible for having all the life support needed to make sure once fertilization occurs the newly formed zygote can carry out cellular division. So all the important organelles needed to carry out cellular activity are contained within the egg cell including the ever important mitochondria.
Think about that for a minute. Now add that to what you have learned about the sperm, mitochondria, DNA, and fertilization.
Do you see where this is going? Remember my previous statement. “So there it is, your father gives half and your mother gives half…..well not quite.”
When the sperm fertilizes the egg, the only thing the egg takes in is the DNA. The rest of the materials, including the mitochondria in the mid-piece are spent and left outside. More importantly, just like bacterial DNA, mitochondrial DNA only recombines with itself……..mitochondrial DNA only recombines with itself……..mitochondrial DNA only recombines with itself….
Yes, that is exactly what I am saying. Every human being on earth is carrying with them the same little circle of DNA that came from their mother, and their mother, and their mother, and their mother, and their mother, and so one and so forth until we get back to her……Mitochondrial Eve. The mother we can all be traced back to, she is estimated to have lived more than 200,000 years ago, and for all this time her little circle of mitochondrial DNA has been passed from mother to child over and over and over again without any influence or recombination with the DNA of the father.
As I sat trying to figure out what I could write to make my mother feel special on a day when so many other children are doing the same thing I wanted to do something that would go beyond the normal praise. I wanted to do something to make her realize how truly important her role is as a mother.
So mom, while you are exceptional at being a mother to a rather unique son, I wanted you to realize that you have accomplished the most important task in life, something no man can ever do, you connected me to every human on earth and kept that 200,000 year-old unbroken molecular chain intact.