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.
