Organelle DNA - Online Tutor, Practice Problems & Exam Prep

Mitochondria and chloroplasts possess their own DNA, known as mtDNA and cpDNA, respectively, due to the endosymbiotic theory, which suggests they evolved from free-living bacteria. Inheritance of this organelle DNA occurs through uniparental inheritance, primarily from the mother. Mutations in this DNA can lead to serious medical conditions, such as MERRF, which causes deafness and seizures. Cytoplasmic segregation can result in variegation in plants, where healthy and mutant organelles segregate into different daughter cells, affecting the phenotype.

Organelle DNA Characteristics

Hi. In this video, I'm going to be talking about organelle DNA. So, mitochondria and chloroplasts are two organelles, and they both contain their own DNA. And so sometimes you'll see these written as mtDNA, and this is for the mitochondria. And sometimes you'll see cpDNA written for the chloroplasts. Now cells are classified by the DNA sources they contain, not they contains. So, heteroplasmic cells contain DNA in the nucleus and from organelle sources, like the mitochondria or the chloroplasts. Homoplasmic cells contain only DNA from one source. So it could be from the nucleus, most likely, though. It's prokaryotic cells, and they, instead, contain DNA just from their nucleoid. Now the endosymbiont theory explains partly why certain organelles like mitochondria and chloroplasts would have their own DNA. And so this explains how they evolved. So, it states that mitochondria and chloroplasts were once free-living bacteria with their own DNA, their own replication system. They were just prokaryotic cells just, like, living by themselves, and eventually, they were engulfed. Either they entered, or the eukaryotic cell just sort of ate them a little, but either way, they somehow got inside a eukaryotic cell. And once they were in there, they weren't degraded. They weren't eaten. They just sort of stayed. They stayed around, kept evolving, and so today, the mitochondria and chloroplasts could never be free-living. Mitochondria don't live without a cell, but they do still have remnants of that ability of a prokaryotic cell or bacteria because they have their own DNA. And so, this kind of explains how these two random organelles have DNA in them. Now mutations in the organelle DNA can cause some serious medical defects, so one that you might hear is this long word that is summarized in MERF, and it causes deafness, seizures, and other issues, and it's a mutation in the mitochondria in humans. So here's an example of the endosymbiont theory. You can see here that here's a cell, and here's a bacterium that is being engulfed by the cell. Eventually, it just, like, stays inside and evolves into the mitochondria, which is how we did today. And because of this pathway, this is why mitochondria and chloroplasts have DNA while other organelles do not. Now, the DNA typically found in organelles is small and circular, which is to be expected because it's very similar to prokaryotic DNA, which is where they originally came from. So human mitochondrial DNA has certain properties. It has a heavy chain, which has more guanine nucleotides in it, and a light chain, which has more cytosine nucleotides in it, and it's called heavy and light based on actual weight. And the codon code, which if you remember, that's the three nucleotides that encode for an amino acid, codon code, is pretty much universal across all organisms, with the exception of the mitochondria and chloroplasts. So, for instance, there are only a few cases of this. It's mostly universal, but it's really not entirely. So AGA is three nucleotides, it's a codon, and normally codes for arginine. But in fruit fly or Drosophila mitochondria, it codes for serine instead. So the codon code is not universal in mitochondria and chloroplasts. It's mostly but not 100%. That's kind of an overview of mitochondria and chloroplast DNA.

Let's now turn the page.

Organelle Inheritance

Okay. Because the mitochondria and chloroplasts have DNA that's separate from the nucleus, the inheritance of the organelle DNA is different from nuclear DNA. In nuclear DNA, each daughter cell gets half; it is replicated, and each daughter cell receives half. However, mitochondrial and chloroplast DNA instead undergo uniparental inheritance, whereby progeny inherit DNA solely from one parent. Thus, my mitochondrial DNA, and your mitochondrial DNA come maternally. My mitochondrial DNA is the exact same mitochondrial DNA my mother has, and I got none of it from my father. The same applies to all of you. This DNA is something you only get from your mother, in the case of mitochondrial DNA in humans.

Thus, if you have a mutant female and a wild-type male mate, all the progeny will be mutant. This is because if there is a mutation in the mitochondria, it will be passed to all offspring. Conversely, a wild-type female and a mutant male mating results in all progeny being wild type because that DNA is only received from one parent, specifically, in humans, from the mother.

Now, there is another term you need to know about called cytoplasmic segregation, which is when two organelles apportion themselves or divide into different daughter cells. For instance, if you have two mitochondria types, one mutant and one normal, all the mutant mitochondria will go into one daughter cell, and all the wild-type mitochondria will go into a separate daughter cell. This is actually unusual because it's not that cells just have one mitochondrion; they can have hundreds and sometimes even thousands of mitochondria and chloroplasts. They divide independently, in a process known as cytoplasmic segregation.

This process results in what is called variegation in plants. Variegation essentially means having multiple colors. I'll provide an example shortly. Another instance of this you may read about or hear about is called the "Poky" Neospora fungi, where the Poky mutants grow slower due to mutations in their mitochondria. Through divisions, it has been identified that the mutation is inherited through the mother and it involves cytoplasmic segregation.

Due to cytoplasmic segregation, an affected female, who may have both mutant and wild-type mitochondria, can sometimes produce healthy offspring because the wild-type mitochondria actually segregate into a gamete, which then goes on to produce the healthy offspring. However, this is very rare. Uniparental inheritance typically means that if, for example, your mother has a mutation in her mitochondria, that will be passed to all offspring. The only exception occurs if she possesses a combination of mutant and wild-type mitochondria, where sometimes the wild-type mitochondria can enter the gametes used to produce offspring. This is how a female with mutations in her mitochondria could potentially produce healthy offspring if she possesses both healthy and mutant mitochondria.

Here's an example of variegation: you can see that most of this plant is green but there are spots that are white. The white regions occur because there are mutations in the chloroplasts, preventing the proper production of chlorophyll, which makes the plant green. Given that this mutation is in the chloroplasts, when the plant reproduces and grows, if it undergoes cytoplasmic segregation, the healthy chloroplasts create the green parts. The mutated chloroplasts, on the other hand, segregate into their own daughter cells and produce the white regions. This exemplifies cytoplasmic segregation, as this single organism hosts some mutant chloroplasts and some healthy ones, where the healthy ones create the green parts of the plant and the mutated ones are isolated into their own daughter cells, creating the white parts of the plant.

With that, let's now move on.

Here’s what students ask on this topic:

What is the endosymbiotic theory and how does it explain the presence of DNA in mitochondria and chloroplasts?

The endosymbiotic theory posits that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells. These bacteria formed a symbiotic relationship with their host cells, providing them with additional capabilities, such as ATP production in mitochondria and photosynthesis in chloroplasts. Over time, these bacteria evolved into organelles, retaining some of their original DNA. This explains why mitochondria and chloroplasts have their own DNA, separate from the nuclear DNA. The DNA in these organelles is typically small and circular, resembling the DNA of their bacterial ancestors.

How is mitochondrial DNA inherited differently from nuclear DNA?

Mitochondrial DNA (mtDNA) is inherited through uniparental inheritance, specifically from the mother in humans. Unlike nuclear DNA, which is inherited from both parents, mtDNA is passed down solely from the mother. This means that all offspring inherit their mitochondrial DNA from their mother's egg cell, with no contribution from the father's sperm. This type of inheritance results in all progeny having the same mtDNA as their mother. This is different from nuclear DNA, where each daughter cell receives half of its DNA from each parent.

What are some medical conditions caused by mutations in mitochondrial DNA?

Mutations in mitochondrial DNA can lead to several serious medical conditions. One example is MERRF (Myoclonic Epilepsy with Ragged Red Fibers), which causes symptoms such as deafness, seizures, and muscle weakness. These conditions arise because mitochondria are crucial for energy production in cells, and mutations can impair their function. Since mitochondrial DNA is inherited maternally, these conditions are passed from mother to offspring. Other examples of mitochondrial diseases include Leber's Hereditary Optic Neuropathy (LHON) and mitochondrial myopathy.

What is cytoplasmic segregation and how does it affect plant variegation?

Cytoplasmic segregation is the process by which organelles, such as mitochondria or chloroplasts, apportion themselves into different daughter cells during cell division. In plants, this can lead to variegation, where different parts of the plant exhibit different colors. For example, if a plant has both healthy and mutant chloroplasts, cytoplasmic segregation can result in some cells containing only healthy chloroplasts (producing green tissue) and others containing only mutant chloroplasts (producing white tissue). This segregation leads to a variegated appearance, with patches of green and white on the plant.

Why is the codon code not entirely universal in mitochondria and chloroplasts?

While the codon code is nearly universal across all organisms, there are exceptions in mitochondria and chloroplasts. These organelles have slight variations in their genetic code. For instance, the codon AGA typically codes for arginine in most organisms, but in the mitochondria of fruit flies (Drosophila), it codes for serine. These differences are remnants of the organelles' prokaryotic origins and their evolutionary divergence from the universal genetic code. Such variations can affect protein synthesis and are important considerations in genetic studies involving these organelles.