Mitochondrial DNA - for Dummies.

[Vexer]

All I know is that cellular mitochondria has it’s own DNA. Which confuses my simple understanding of a few things.

 

This rather blurs my idea of what a species is. I thought, ‘one species, one DNA’

 

Does this mean humans are colony animals? Presumably all creatures have the same duality?

 

 

 

 

The Eve study examined mitochondrial DNA (mtDNA), which is passed only by mothers to their offspring.

 

How can DNA ‘pass’ a different (mito) DNA? Or, how does the mtDNA get into a new human then?

 

 

(You’ll be ignorant about how ignorant I am, so err on the side of simplistic concision).

[MALDI-TOFTOF]

First of all you have to realise that mitochondria and other organelles in fact originated from endocytoses of bacteria in other cells early in evolution. This is in fact the origin of eukaryote cells. These bacteria had their own dna ofcourse, but have lost a lot of it during evolution, nut not everything. Thus they remain with some imortant genes for the organelle function.

 

Second, what you think as: one species, one dna reflects on nuclear dna. But even then it's not entirely correct. Within a species the gene content is alike, but a lot of differences exist within the genes.

 

Thirdly, when you look at conception, you see that the spermatozoïd is very small, and carries only dna, from the father. But after conception, there is a need for proteïns to start cell division. That is why the egg cel is much bigger, because next to its nuclear dna it contains cytoplasm with the Endoplasmic reticulum, ribosomes, etc. But also mitochondria, so the dna of the mitochondria is passed through the egg cell, or in other words: by the mother

[Mr Skeptic]

All I know is that cellular mitochondria has it’s own DNA. Which confuses my simple understanding of a few things.

 

This rather blurs my idea of what a species is. I thought, ‘one species, one DNA’

 

Well, then you might want to check out bacteria. They have little bits of DNA separate from their "main" DNA, and can even exchange it with each other.

 

 

Does this mean humans are colony animals? Presumably all creatures have the same duality?

 

Very little. We are a colony of cells, as are every multicellular creature, but the cells are genetically identical.

 

The Eve study examined mitochondrial DNA (mtDNA), which is passed only by mothers to their offspring.

 

How can DNA ‘pass’ a different (mito) DNA? Or, how does the mtDNA get into a new human then?

 

The mother's egg contains mitochondria, which are passed on to the mother's offspring. The sperm's mitochondria does not enter the egg. So all your mitochondria came from your mother, who got it from her mother, etc.

[Vexer]

So, Mr Skeptic, I now understand that normal sexual reproduction in fact passes two sets of DNA as a matter of course. I never knew that.

 

(Maldi/... said):

 

That is why the egg cell is much bigger, because next to its nuclear dna it contains... also mitochondria, so the dna of the mitochondria is passed through the egg cell, or in other words: by the mother.

 

uhuh. Hm.

[CharonY]

Actually I have to complicate matters by adding that occasionally paternal transmission of mt DNA in animal, including humans, occurs.

For instance: Schwartz M, Vissing J (2002). Paternal inheritance of mitochondrial DNA. N Engl J Med 22: 576–580

[Vexer]

Wait a sec, my simple understanding is:

 

The sperm joins with the ova and their DNA entwines. (Sounds almost romantic…).

 

This creates the first cell of the new person. Which divides and etc.

 

Where is the mDNA in this? I don’t see the transfer? I just see the one DNA.

 

 

(Yes, I’m dim)

[SkepticLance]

Vexer

 

The two DNA's entwining, as you put it, is correct, though simplified. However, every living cell also needs mitochondria, and these pass only from the mother's side. In actual fact, the mitochondria reproduce quite separately. They divide from time to time, just like the bacteria they derived from. Their rate of division is finely adjusted to the needs of the cells they live in, but not directly linked to the parent cell rate of division. They divide enough to supply all the new cells with new mitochondria.

 

When an egg is made inside mother's body, it retains a certain number of mitochondria, but only superior ones. There is a process of removal of mitochondria that have suffered harmful mutations, and the egg retains only pristine specimens. The sperm is too small for this, and normally (with rare exceptions) does not keep any mitochondria.

 

 

When sperm and egg meet and fertilise, making a zygote, the zygote starts to divide to form the early embryo. The mitochondria also divide to make lots more mitochondria, so that all the daughter cells have enough.

[r_bo_99]

Here's a link to an interesting article talking about the role of mutations in mtDNA on metastasis. http://sciencenow.sciencemag.org/cgi/content/full/2008/403/1

 

Randy (BeBoBio)

[Vexer]

“These pass”

 

“…every living cell also needs mitochondria, and these pass only from the mother's side.”

 

How do they “pass”?

[Sayonara]

“These pass”

“…every living cell also needs mitochondria, and these pass only from the mother's side.”

How do they “pass”?

Vexer, if you are intent on not using the quote function, and only quote a snippet from all the posts that have been made since you last replied, you really need to indicate who you are talking to.

 

In simple terms, the mitochondria divide according to the energy needs of the parent cell, rather than in response to the parent cell's cell cycle. When the parent cell divides, each cell copy receives an (essentially random) number of mitochondrial copies.

 

So in the case of human reproduction, you have the male gamete (sperm) which under normal circumstances have no mitochondrial DNA in them, and the female gamete (the egg) which contains mitochondria originating from the female parent's cells at the time that the egg was produced by her. Once fertilised, the egg becomes the zygote and begins to divide. Mitochondria divide during this process as they would in any other cell system.

 

The main idea to take away is that mitochondrial genes are not inherited by the same means as the cell's nuclear genes. You can think of mDNA as a kind of genetic passenger, but this is really only applicable to cell division within an organism as opposed to gamete fusion during reproduction. IOW, mDNA is not 'passed and recombined' during fertilisation as is the parental DNA - it passes directly from mother to child by means of cellular division.

[Ophiolite]

“…every living cell also needs mitochondria, and these pass only from the mother's side.”

How do they “pass”?

Vexer, this has been answered by more than one person implicitly and explicitly. However, you asked for it 'simple'.

 

The mitochondria are inside the mother's egg.

[Vexer]

How do they get there?

[SkepticLance]

Vexer

 

Mitochondria are in every cell in the body. When a cell divides, both daughter cells inherit mitochondria, which also divide to increase their numbers, roughly at the same rate as the cells divide. When an egg cell is formed, it inherits those mitochondria, with the difference that it destroys those mitochondria that are imperfect. Sperm cells are too small. They contain nucleus and almost nothing else. No room for mitochondria.

 

If you are asking how mitochondria originally got into cells, then we are talking about a stage in evolution - probably about a billion years ago. The theory is that the first eucaryote cells accepted into their mass some bacteria. Those bacteria were probably parasites at first, but evolved into symbiotes. The bacteria evolved into mitochondria, which perform the function of converting adenosine diphosphate into adenosine triphosphate (ADP to ATP) - thus storing energy for the cell.

[MedGen]

Vexer

Sperm cells are too small. They contain nucleus and almost nothing else. No room for mitochondria.

 

That strictly is not true, as given in the example of parental inheritance of mtDNA published in the New England Journal of Medicine, sperm cells do contain mitochondria. However, they are localised to the area of the cell near the flagella, seeing as this is the site of highest ATP requirement in sperm cells. During gamete fusion it is the nuclear DNA that is transfered to the egg, which additionally contains all of the requirements for the first few days of cell division (note not strictly the same as mitosis, one of the other reasons egg cells are so large).

 

This is exemplified in the above journal article where the paternal mtDNA was located solely in the patients skeletal muscle IIRC. The importance of localisation during cell division is underestimated, particularly when pertaining to stem cell differentiation.

[SkepticLance]

To MedGen

 

GRRRRR

You spoiled my explanation. You are right, but Vexer asked for a very simple explanation, which I was trying to give. Your add on is correct, but adds complexity, which I was trying to avoid.

[MedGen]

To MedGen

 

GRRRRR

You spoiled my explanation. You are right, but Vexer asked for a very simple explanation, which I was trying to give. Your add on is correct, but adds complexity, which I was trying to avoid.

 

Terribly sorry, the explanations so far have been very simple, I thought perhaps that Vexer was at least at the level of the interested layman, re-reading this thread and I think I regret that assessment.

[Vexer]

I do thank you for your time. Thank you.

 

 

Hm. "Inherits..."

 

 

So the Mitochondria are in the 'egg' and a just go along for the ride, once 'fertilisation' occurs.

 

It still kinda violates my idea of one DNA. So the 'first' cell of a new animal has the 'regular' DNA, and... (within the cell, as 'passengers') the mDNA? They're there as part of the female 'egg'.

 

But when the cell divides... I don't see how the mDNA can be 'carried' to the next generation.

 

 

(MedGen... you should regret someth8ng else)

 

Unless, the cell divides, and the mitos are in the 'body' of that cell. Even though the 'main' DNA...

 

...ok.... So the cell does the DNA 'entwining' ... splits, and you say... each new cell has remanent mitos within the cell, unrelated to the 'main' DNA..

 

 

That's weird.

 

When the cell duplicates... it still doesn't 'know' about mDNA... the mitos just split (as passengers) on each split?

 

That's what you were saying, all along.

[Ophiolite]

But when the cell divides... I don't see how the mDNA can be 'carried' to the next generation..

I am having real difficulty in understanding what it is you are not understanding. The egg contains mDNA just like other cells in the body. When cells divide mitochondria are present in both halves.

The fertilised egg is the next generation. So the mechanism of 'carrying' should be apparent. Any questions?

[MedGen]

Mitochondria themselves replicate autonomously, they are not dependent on the cells replication/division. It's one of the reasons that certain cell types are paced full of mitochondria, whilst others have only the required amount. This also tends to be reliant on the ATP requirements of the cells they are contained within.

 

Cells are not "aware" in the sense that you or I are, however they are capable of signalling chemically and electrochemically to each other, and even within the cell there are significant signalling pathways that can elicit various responses.

 

Nuclear DNA is the what is often referred to as the genome, all though this is strictly a misnomer because mtDNA should be included, even though it is fully independent. Mitochondria however not fully independent (hence the semi-autonomous replication) because they rely on some of the nuclear DNA genes to function properly.

[Mr Skeptic]

Vexer, you will probably be better off not considering mtDNA as DNA, but rather a part of mitochondria. The mitochondria are somewhat self-contained, and contain some ribosomes and DNA to make some of their own proteins. The mtDNA is not contained in the nucleus, but rather in the mitochondria. (However, some mitochondrial proteins are encoded in the genome and made by the cell). So mitochondria are somewhat independent but still require proteins from the cell.

[Ophiolite]

Oops. :embarass:The correct abbreviation for mitochondrial DNA is mtDNA, not mDNA, which I inadvertently duplicated from Vexer's post. MedGen has it right. (mDNA is too close to mRNA for comfort.)

[Vexer]

Ok. Thanks all. My dumbness must be annoying. I'm sorry.

 

 

 

That's what I said, Mr. Skeptic, are humans actually colony animals?