Video Transcript
In this video, we’ll learn how to
describe the structure of the nucleus in a eukaryotic cell. We’ll also explore the structure
and function of chromatin and chromosomes, so let’s dive into the exciting world of
the nucleus.
If you’ve ever looked at onion skin
cells under a microscope, then you’ve probably seen something a bit like this. Here, we can see all the individual
cells that make up the onion skin, and the little dot that we can see inside each
cell is the nucleus. But what actually is it? The nucleus is a membrane-bound
organelle which contains an organism’s genetic material, its DNA. Remember, an organelle is just a
subcellular structure that carries out a particular function. The nucleus of each human cell
contains a mass of two meters of DNA that is tightly packed and organized. If we took all the DNA strands from
all of your cells and put them together end to end, it would measure up to 6.2
billion kilometers. That’s more than 41 times the
distance between the Earth and the Sun.
Now, let’s have a closer look at
cells. All cells have at least a cell
membrane and some cytoplasm, but not all cells have a nucleus, for example,
bacteria. We can use this information to
divide living organisms into two groups. Cells that contain a nucleus and
other membrane-bound organelles are called eukaryotic cells, and cells without are
called prokaryotic cells. Because this video is all about the
nucleus, we’re going to be concentrating on eukaryotic cells. The cytoplasm of a eukaryotic cell
is a gelatinous liquid containing water, dissolved salts, and other organic
molecules. It’s also home to several
organelles and other structures, for example, the mitochondria, which release
energy; ribosomes, which are responsible for protein synthesis; and chloroplasts,
which are the site of photosynthesis in plants.
Eukaryotes are organisms like us
humans which are mostly made up of eukaryotic cells. In fact, every known multicellular
organism is eukaryotic, whether it be a plant, an animal, or a fungus. Though some eukaryotes are
single-celled, for example, amoeba and yeast. Now, let’s have a look at the
nucleus in a bit more detail.
The nucleus is a spherical
organelle which contains the genetic material of a eukaryotic cell. As you can see in this diagram,
it’s usually the largest organelle within an animal cell. If we’re talking about more than
one nucleus, we refer to them as nuclei. When we look at cells under a
microscope, we can stain the nuclei so they’re easier to identify. If we’re looking at onion skin
cells like the ones we saw at the beginning of the video, the only reason the nuclei
are visible is because they’ve been stained with iodine, which gives them an
orange-brown color. The nucleus has a complex interior
structure. It’s surrounded by a double
membrane known as the nuclear envelope, which is usually continuous with another
organelle called the endoplasmic reticulum.
The nuclear envelope is responsible
for separating the contents of the nucleus from the rest of the cell, thereby
protecting DNA from reactions that may be occurring in the cell cytoplasm. This is called
compartmentalization, and it occurs in all organelles which have membranes. The nuclear envelope contains tiny
openings called nuclear pores through which small substances can pass between the
nucleus in the cytoplasm but larger molecules cannot. For example, while DNA molecules
are too large to fit through the nuclear pores and so remain within the nucleus,
mRNA molecules are smaller. So they can move out through the
nuclear pores towards ribosomes in the cytoplasm to carry out protein synthesis.
Let’s use an analogy to better
understand the role of the nuclear envelope. Imagine the nucleus is a school
library containing large amounts of information in books or, in the case of the
nucleus, genetic material. The doors to the library represent
the nuclear pores found within the nuclear envelope. While small objects like books
might be taken in and out of the library through the doors, larger objects like the
bookcases might be more difficult to move in and out.
Within the nucleus is a gel-like
substance called the nucleoplasm. The chemical reactions that occur
in the nucleoplasm are separated, or compartmentalized, from the cytoplasm by the
nuclear envelope. Notice how the words cytoplasm and
nucleoplasm both have the same -plasm suffix. This is because the cytoplasm is
the liquid portion of the cell and the nucleoplasm is the liquid portion of the
nucleus. The nucleoplasm contains DNA in the
form of chromatin as well as many other substances including nucleotides and
enzymes, which are used in DNA replication. If you’re not sure what we mean by
the term chromatin, then don’t worry because we’ll be coming back to it a bit later
in the video.
The final structure we’re going to
look at, which is also found in the nucleoplasm, is the nucleolus. The nucleolus is where ribosomes
are produced from proteins and RNA. Because the nucleolus is so dense,
it’s usually easy to spot in a microscope image. Ribosomes consist of a large
subunit and a small subunit. These subunits exit the nucleus via
nuclear pores and assemble to form a ribosome when protein synthesis needs to be
carried out. As we can see in our cell diagram,
many ribosomes remain free in the cytoplasm, but some can also attach to the rough
endoplasmic reticulum. Molecules of mRNA, which also move
through the nuclear pores, will then bind to the ribosomes when specific proteins
need to be produced. This demonstrates the important
role that ribosomes play in protein synthesis.
Now that we’ve looked at the
structure of the nucleus, you might be wondering what makes it so special that
almost every cell of a complex organism requires one to function effectively. To answer this question, we need to
know about the precious genetic material the nucleus is there to protect, DNA.
DNA is a specialized molecule which
is usually double-stranded and has a helical shape. Each DNA molecule contains many
genes. You may recall that a gene is a
section of DNA which often codes for a specific protein essential for the cell’s
activities. We already mentioned that the DNA
of just one human cell spans two meters in length. So how does all this DNA fit inside
a cell which is so tiny that it cannot be seen by the naked eye? The answer is that it’s very well
packaged. Eukaryotic DNA is wrapped around
specialized proteins called histones, which in turn coil up to build a chromatin
fiber. The chromatin fiber further coils
up and condenses to form a chromosome, which is located in the nucleus. This process of DNA packaging is a
bit like folding up your clothes so that you can fit them all inside your suitcase
when you go on holiday.
Let’s find out a bit more about
chromosomes. A chromosome is a single molecule
of DNA and its associated proteins which contains genetic material in the form of
genes. We often think of chromosomes as
having an X shape, but it’s important to note that most of the time this is not
actually the case. This is what a single unduplicated
chromosome looks like.
During cell division, DNA
duplicates. The duplicated chromosome is still
called a chromosome, but now it consists of two sister chromatids, which are joined
at the centromere. This is why a duplicated chromosome
looks like an X when we study it under a microscope. Even though an unduplicated
chromosome looks like a chromatid, it would be incorrect to refer to it as such
because it only becomes a chromatid during cell division when it’s part of a
duplicated chromosome. Chromosomes change their structure
and appearance depending on what stage of the cell cycle a particular cell is
in.
A cell has a life cycle called the
cell cycle, which can be divided into a growing phase I, which is short for
interphase, and a division phase M, where a cell divides either by mitosis or
meiosis. Mitosis produces genetically
identical daughter cells. It’s how an organism grows and how
cells are replaced if they become damaged. Meiosis, on the other hand,
produces genetically unique sex cells, which are known as gametes. Gametes contain half the genetic
material of a parent cell and are needed for sexual reproduction. This is where one gamete fuses with
another gamete from the opposite biological sex, and the genetic material of the
parents is passed down to the offspring.
A typical cell spends most of its
life in the interphase stage, making preparations before undergoing cell
division. During the first part of
interphase, the cell grows. Next, the genetic material on each
chromosome of the cell is copied exactly by DNA replication. So the new daughter cells will have
exactly the same DNA as the original parent cell. Once DNA replication is complete,
the cell grows again and prepares for cell division by mitosis or meiosis. Here is a diagram of some typical
cells under a light microscope. To be able to copy the DNA during
interphase, it cannot be in the condensed form of a chromosome. So under the light microscope, we
would not be able to make out any specific structures. On the other hand, during the
mitosis or meiosis cell division phase, the chromosomes condense and are now
visible.
Remember, the DNA is duplicated in
this stage, so the chromosomes have an X shape because they each consist of two
chromatids. If we zoomed in on one of these
chromosomes, it would look like this, two sister chromatids held together by a
specialized DNA sequence called the centromere. It’s the centromere that allows the
cell to control the distribution of DNA during cell division so that both daughter
cells will receive the same amount of DNA.
In most multicellular eukaryotes,
the nucleus breaks down before cell division leaving only a group of specialized
fibers called spindle fibers. These spindle fibers attach to the
centromeres during cell division to pull an equal number of chromatids to each end
of the cell. Once the cell has divided into two,
a new nucleus will form around the chromosomes in each of the new daughter
cells.
Now we’ve learned all about the
structure of the nucleus and the function of chromatin and chromosomes, let’s have a
go at a couple of practice questions.
The diagram provided is a drawing
of a eukaryotic cell. Which structure is indicated by
Y?
This question presents us with a
diagram of a eukaryotic cell. Eukaryotic cells contain a nucleus
and other membrane-bound organelles. We can see that our mystery
structure Y is located inside a larger circular structure that’s surrounded by
membranous folds. This larger structure is the
nucleus, and the membranous folds make up the rough endoplasmic reticulum, the
smooth endoplasmic reticulum, and the Golgi apparatus. To work out what Y is, let’s review
the structures of the nucleus.
The nucleus houses all of the
genetic information required for protein production and other life processes. Genetic information is stored in a
long molecule called DNA, which is coiled up into chromatin so that it can fit
inside the nucleus. As we can see, the chromatin is
represented as long thin strands and is therefore not our mystery structure. The nucleus is surrounded by the
nuclear envelope, which contains nuclear pores that allow small molecules like mRNA
to move in and out of it. The nuclear envelope is directly in
contact with the rough endoplasmic reticulum and is therefore also not structure
Y.
The final component of the nucleus
is a large dense structure called the nucleolus. The nucleolus is the site of
ribosome synthesis. You may recall that ribosomes are
the organelles that are responsible for synthesizing proteins. We have therefore solved the
mystery and deduced that structure Y is the nucleolus.
Let’s try a second practice
question together.
How do substances pass between the
inside of the nucleus and the surrounding cell cytoplasm? (A) Through nuclear pores, (B)
through gaps in the nucleus wall, (C) through the nuclear stomata, or (D) via
nuclear transport tissues.
The nucleus is a membrane-bound
organelle which contains the majority of a eukaryotic cell’s genetic material in the
form of DNA. Its function is to protect DNA from
the reactions that occur in the cytoplasm, and it does this using its membrane,
which is called the nuclear envelope. However, some material must pass
between the nucleus and the cytoplasm in order to give the cell essential
instructions. So how does this happen? The nuclear envelope contains small
openings called nuclear pores, which some molecules can pass through. Chromosomes are unable to pass
through the nuclear pores because they’re too large. mRNA molecules, on the other
hand, are much smaller, and therefore they can pass through easily.
Thanks to nuclear pores, mRNA can
pass from the nucleus into the cytoplasm where it can meet a ribosome, and together
they can carry out protein synthesis. We have therefore determined that
the correct answer is (A): substances pass between the inside of the nucleus and the
surrounding cell cytoplasm through nuclear pores.
Let’s recap some of the key points
about the nucleus that we’ve covered in this video. The nucleus is a membrane-bound
organelle which contains an organism’s DNA. Eukaryotic cells contain a nucleus
and other membrane-bound organelles, whereas prokaryotic cells do not. The nucleus is surrounded by a
nuclear envelope containing nuclear pores. The nucleoplasm is the liquid
portion of the nucleus, which contains the nucleolus and DNA in the form of
chromatin. Cells have a life cycle called the
cell cycle. And finally, during cell division,
chromosomes are duplicated and consequently have an X-shaped structure consisting of
two sister chromatids.
