Lesson Video: DNA in Prokaryotes Biology • Third Year of Secondary School

Video Transcript

In this video, we’ll learn more about the DNA of prokaryotes. We’ll discuss the circular nature of the chromosome, how it’s compacted, and see how this compares to eukaryotes. Then, we’ll learn more about extra chromosomal DNA in the form of plasmids and some of the functions they might have. Finally, we’ll discuss the origins of the mitochondria and chloroplasts and see how they may have evolved from prokaryotic cells.

Prokaryotes are single-celled organisms that are mainly distinguished from eukaryotes by the absence of a nucleus to contain their DNA. Prokaryotes are usually 10 to 100 times smaller in size than eukaryotes. But for now, we’ll represent them with a similar size to compare them more easily. Prokaryotes also lack membrane-bound organelles that we can find in eukaryotes, such as the mitochondrion.

Before we go over prokaryotic DNA, let’s review some of the details of the anatomy of a prokaryotic cell. Many prokaryotes have three separate layers that separate the inside of the cell from the exterior environment. The capsule is a sticky outermost layer that helps prokaryotes adhere to surfaces. The next layer is the cell wall, which helps protect the cell’s interior and maintains the cell’s shape. Then there’s the plasma membrane, which contains the interior components of the cell. On the outside of the cell are pili, singular pilus, which are short hairlike structures that can help with movement and are also involved in transferring DNA to other prokaryotes as we’ll see, and the flagellum, or flagella, which aid in movement.

In the interior of the cell, we can see ribosomes, which help with protein production, and the prokaryotes’ chromosomal DNA. This DNA often exists as a single circular chromosome, which is not contained inside a nucleus like eukaryotic cells. But it’s rather concentrated in a region called the nucleoid. This chromosome contains the majority of the genetic information for the cell, while additional DNA that isn’t part of the chromosome, called plasmid DNA, makes up the rest of it. As we’ll see, plasmids are smaller circular DNA molecules that contain certain genes that benefit the prokaryote.

Now let’s look at some of the characteristics of this prokaryotic DNA and compare it to eukaryotic DNA. As mentioned, eukaryotes have a nucleus surrounding their DNA, whereas prokaryotes do not. Instead, their DNA is concentrated in an area of the cell called the nucleoid. In terms of the shape of their chromosomes, with this diagram, it’s hard to tell what the shape is because there’s so much DNA. But if there was a much smaller quantity of it, we would see that the prokaryote usually has a single circular chromosome, which we can see here compared to eukaryotes, which typically have multiple linear chromosomes, which we can see here.

Prokaryotes generally have one copy of their chromosome and are therefore usually haploid, whereas eukaryotes typically have multiple copies and can be diploid with two copies or more. In both prokaryotes and eukaryotes, their DNA is highly packed. In fact, if we were to take all the DNA in a single human cell, a eukaryote, and line it all up and stretch it out, it would be about two meters in length. If we were to unravel eukaryotic DNA, we would see that it’s packaged using specialized proteins called histones.

In prokaryotes, their circular DNA is twisted or coiled many times to package it. In the prokaryote E. coli, a single cell contains about 1.4 millimeters of DNA. This is about 1000 times the size of the cell itself. To package the DNA this much, the DNA must undergo extensive coiling, called supercoiling. Imagine taking an elastic band, which represents a chromosomal DNA, and twisting it to form coils. This can be twisted again and again and then twisting it further so the coils begin to fold over each other to form a more compacted structure. After repeating this many times, a condensed ball of supercoiled DNA is the result. There are proteins involved in this process, but they aren’t histones like in eukaryotes.

The final difference that we’ll cover is that prokaryotes contain extrachromosomal DNA called plasmid DNA. This is generally not found in eukaryotes.

Now that we’ve seen some of the major differences between prokaryotic and eukaryotic DNA, let’s take a closer look at these plasmids. As we can see here, a plasmid is a piece of circular DNA. You’ll recall that the length of DNA is measured in nucleotides or base pairs. Generally, plasmids can range in size from over 100,000 nucleotides to just over a few hundred nucleotides. One of the largest plasmids discovered is 1.8 million nucleotides in size. And for some organisms, this would be larger than their chromosomal DNA. Plasmids often contain different genes that can be beneficial for the prokaryote by allowing it to grow in environments that would otherwise kill it. Some encode resistance to antibiotics or resistance to toxic heavy metal ions, for example.

Other groups of plasmid-encoded genes, called virulence factors, can help the prokaryote infect its host. A good example of this is found in the disease-causing E. coli 0157 strain. This is the strain of E. coli you often hear about in the news when beef or lettuce is contaminated. Disease symptoms typically include diarrhea, vomiting, fever, and in some cases death. E. coli 0157 gets its name from a special plasmid it has, called p0157. This plasmid contains dozens of virulence factors involved in infecting humans, some of which help the bacteria stick to intestinal cells so they can colonize the intestine. Others are involved in avoiding detection or evading the immune system or in tolerating the acidic environment of the stomach. This way, the plasmid helps E. coli adapt to its host and survive in this otherwise dangerous environment.

So how many plasmids can a prokaryote have? Some prokaryotes can have as many as 20 different plasmids, each with a different set of genes. Each of these plasmids can replicate independently of the chromosomal DNA. This way, many copies of a single plasmid might exist in a cell from one to several hundred copies. When the prokaryote undergoes cell division, the plasmids it contains are carried over as well. As we’ve seen, plasmids can be very useful for prokaryotes. In order to share the benefits, plasmids can be transferred to other prokaryotes in a process called conjugation. Let’s look at this in some detail.

So let’s say our prokaryote here has a plasmid that gives resistance to some antibiotic, like penicillin. Penicillin is an antibiotic that can disrupt the synthesis of the prokaryotic cell wall. This causes it to weaken, and eventually the cell dies due to lysis. Our penicillin resistance gene produces an enzyme that can cleave penicillin and prevent this from happening. So prokaryotes with this plasmid are resistant to penicillin. So, in this case, this prokaryote is our donor and is resistant to the antibiotic penicillin. And suppose we have another prokaryote that is not resistant to penicillin and is therefore sensitive. This is our recipient cell.

During conjugation, a specialized pilus, called the sex pilus, is extended from the donor cell to the recipient cell to bring it in closer. Now that the cells are in contact, a bridge forms between them. Although this plasmid is shown as a single loop, it’s actually double stranded. So let’s draw it this way. Next, one of the strands of plasmid DNA is cut and then transferred into the recipient cell. During this process, the second strand of DNA is synthesized, but for simplicity we’ll keep it looking this way. Once the plasmid is transferred over to the recipient cell, it now carries the plasmid and the antibiotic resistance gene and is therefore resistant to penicillin.

You may have heard of antibiotic resistance in bacteria and how it’s becoming a global problem. This is because antibiotics are overprescribed and easy to access, which has led to their misuse by the public. So suppose we have an infection with a population of bacteria. But one of them carries a plasmid that gives antibiotic resistance to penicillin. By treating this infection with penicillin, all of these bacterial cells will be killed, except for the one that’s resistant to penicillin. This bacterial cell can then multiply to create more penicillin-resistant bacteria. Or it could transfer the plasmid to other bacterial cells by conjugation.

Because all of these bacteria are now resistant to penicillin, treatment of this infection with penicillin won’t work. So then we treat this infection with another antibiotic, like amoxicillin, for example. And this will kill most of the cells once again, except for the one with amoxicillin resistance. And now this bacterium might have resistance to both antibiotics. By misusing antibiotics, this is how strains of bacteria can emerge that are resistant to them. The real fear in all of this is if we ran out of antibiotics to treat infections and what was once a simple infection can turn out to be deadly. Certainly, the future of humankind and prokaryotes looks bleak unless we address this issue.

Now, let’s go back in time to discuss the prokaryotic origins of mitochondria and chloroplasts, known as the endosymbiotic theory. The endosymbiotic theory is the theory that the mitochondrion and chloroplast are all from a prokaryotic ancestor. Early eukaryotes, which lack mitochondria or chloroplasts, may have ingested a prokaryote that was capable of aerobic respiration or photosynthesis. This may have led to a mutual endosymbiotic relationship where both cells benefited.

Symbiosis is a type of close and long-term biological interaction between two different organisms. And endo- refers to how one of these organisms is living inside the other. The eukaryote may have benefited from the energy released by the aerobic prokaryote, while the prokaryote may have benefited by receiving nutrients from the eukaryote. Over billions of years, this aerobic prokaryote may have evolved into the mitochondrion that we know today. A similar process may have occurred with photosynthetic prokaryotes leading to the chloroplast.

There’s several lines of evidence for this, including that mitochondria and chloroplasts both contain their own DNA that is circular and lacks histones, much like prokaryotes. Mitochondria and chloroplasts also have their own ribosomes, the organelles involved in producing proteins, that are very similar to prokaryotic ribosomes. Mitochondria and chloroplasts are also similar in size compared to prokaryotes. This evidence and more is why we believe that mitochondria and chloroplasts have evolved from prokaryotes. Now, let’s try out a practice question to apply what we’ve learned.

Where in a prokaryotic cell is the chromosomal DNA contained?

You recall that prokaryotes are single-celled organisms that are often smaller in size compared to eukaryotes and are distinguished by their lack of a nucleus or membrane-bound organelles. Let’s make this prokaryotic cell a bit bigger so we can review its anatomy to answer this question.

Many prokaryotes have three layers that separate the cell’s interior from the cell’s exterior. The capsule is the sticky outermost layer that some prokaryotes use to stick to surfaces. The cell wall is the middle layer and helps give the cell its shape and protects the inside of the cell. And finally, there’s the plasma membrane, which helps to keep the cell’s interior components in place. On the outside of the cell is the hairlike structure called the pilus, or pili as plural, which can help the prokaryote with movement. Also helping with movement is a tail-like structure called the flagellum, or flagella as plural.

If we turn our attention now to the interior of the cell, we have the cytoplasm, which is the liquid that fills the inside of the cell. The ribosome is involved in protein production. And the plasmid is a circular piece of extrachromosomal DNA that’s separate from the chromosome. Finally, we have the prokaryotic chromosome, which contains the chromosomal DNA that this question is asking us about. As mentioned, this is not contained inside a nucleus. Instead, it’s concentrated in an area of the cell called the nucleoid. Therefore, in a prokaryotic cell, the chromosomal DNA is contained in the nucleoid.

Now let’s take a moment to go over the key points in this video. Prokaryotic DNA exists as either chromosomal DNA or plasmid DNA, both of which are circular in structure. There are many differences between prokaryotic and eukaryotic DNA. Prokaryotic DNA is not contained in a nucleus, but rather in an area called the nucleoid, whereas eukaryotic DNA is contained in the nucleus. Prokaryotes typically have a single circular chromosome, while eukaryotes typically have multiple linear chromosomes. Prokaryotes are typically haploid, while eukaryotes are typically diploid or higher. Prokaryotic DNA is not packaged using histones, whereas eukaryotic DNA is. Prokaryotes typically carry extrachromosomal DNA on plasmids, while eukaryotes do not. Finally, mitochondria and chloroplasts may have evolved from prokaryotes, as stated by the endosymbiotic theory.