In this video, we’ll learn more
about the DNA of prokaryotes. We’ll discuss the circular nature
of their chromosome, how it’s compacted, and see how this compares to
eukaryotes. Then we’ll learn more about
extrachromosomal DNA in the form of plasmids and some of the functions they might
have. Finally, we’ll discuss the origins
of the mitochondria and chloroplast and see how they may have evolved from
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
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 the 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
could see ribosomes which help with protein production and the prokaryote’s
chromosomal DNA. This DNA often exists as a single
circular chromosome, which is not contained inside the nucleus like the eukaryotic
cell but is 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 and it looks like a big bundle of it. 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 order to fit it inside the nucleus. In fact, if we were to take all the
DNA in a single human cell, a eukaryote, 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 is packaged using specialized proteins called
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, which 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 our 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 100 nucleotides long. 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 an
environment 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 with it. Disease symptoms typically include
diarrhea, vomiting, fever, and in some cases death. E. coli 0157 has a special plasmid
called p0157. This plasmid contains dozens of
virulence factors involved in and infecting humans, some of which helped 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 the cell, from one to several 100 copies. When the prokaryote undergoes cell
division, the plasmids it contains are carried over as well. As we have 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
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 a 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 that way. Next, one of the strands of 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, the recipient cell 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 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
So then we treat the 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 this is if we
run 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 the mitochondrion and the chloroplast, known as
the endosymbiotic theory.
The endosymbiotic theory is the
theory that the mitochondrion and chloroplast evolved from a prokaryotic
ancestor. Early eukaryotes, which lacked
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’ll 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 the 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 a
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.
Now let’s take a moment to go over
the key points that we covered 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 a 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, while eukaryotic DNA is. Prokaryotes typically carry
extrachromosomal DNA on plasmids, while eukaryotes do not. Finally, mitochondria and
chloroplasts may have evolved from prokaryotes.