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Lesson Explainer: The Nucleus Biology

In this explainer, we will learn how to describe the structure of the nucleus in a eukaryotic cell and recall the structure and function of chromatin and chromosomes.

Each human cell contains a massive two metres of DNA that is tightly packed and organized. If all the DNA strands in all of your cells were put together end to end, it could measure up to 6.2billion kilometres, more than 41 times the distance between Earth and the Sun!

Although the term animals is not used usually to refer to humans, according to the modern system of classification, we humans are classified in kingdom Animalia, as our bodies are generally made of trillions of the same basic cell that is found in other animals, which is the animal cell. The animal cell is a type of eukaryotic cell. This means that our cells all contain a nucleus, with a short list of exceptions such as the mature red blood cell. Every known multicellular organism is eukaryotic, whether plant, animal, or fungus, though some eukaryotes are also single celled such as the amoeba or the fungus yeast.

Key Term: Eukaryotic

A eukaryotic cell is a cell that contains a membrane-bound nucleus and other membrane-bound organelles.

Eukaryotic cells differ slightly in the proportion of different organelles they possess because of their different functions. For example, take a look at Figure 1 below. A sperm cell requires plenty of energy to move toward the egg cell and so will require more mitochondria to provide this energy compared to some other cells. Red blood cells, for example, do not contain any mitochondria as they do not require the amount of energy a sperm does because they are moved with the help of other factors. Although the mature ones do not possess nuclei (plural of nucleus), red blood cells are actually an example of eukaryotic cells; they begin their life with nuclei and other organelles, but they lose them during maturation only to free up more space for a greater amount of the oxygen-carrying hemoglobin.

Figure 1: A diagram displaying the structure of sperm cells and mature red blood cells. Sperm cells require more mitochondria to release energy than red blood cells.

All eukaryotic cells will contain a cytoplasm, which is a gelatinous liquid containing water, dissolved salts, and other organic molecules. The cytoplasm is also home to several organelles and other structures, for example, mitochondria to release energy, ribosomes for protein synthesis, and chloroplasts in plant cells to carry out photosynthesis.

Key Term: Organelle

An organelle is a subcellular structure that carries out a specific function.

The organelle we will be focusing on in this explainer is the nucleus and what makes it so special that almost all complex organisms’ cells require it to function effectively. To understand this, we need to know about the precious material it functions to protect: DNA. Then, we will discover more about the complex structure of the nucleus itself.

Chromosomes are located in the nucleus, each consisting of a long strand of DNA. Each DNA strand contains many different genes, sections of DNA that code for specific proteins that are essential for the cell’s activities.

Meiosis is a type of cell division a eukaryotic cell undergoes to produce gametes (egg cells and sperm cells in humans). During meiosis, these chromosomes will first be duplicated in a process called DNA replication. Next, they will be split up and rearranged twice to result in the formation of gametes with half of the genetic information of the original parent cell. The genes located on chromosomes are heritable. This means that genes can be passed from parents to offspring when the egg cell and sperm cell fuse in fertilization.

Each human chromosome consists of one very long DNA molecule that, if unraveled, would measure an average of 4.8 cm. This is just one chromosome out of a full 46 in each normal body cell’s nucleus; therefore, there is a significant packaging problem!

To avoid this issue, chromatin is formed within the nucleus when DNA strands in eukaryotic organisms associate and coil around specific proteins called histones. This is helpful in tightly compacting the DNA. The chromatin coils up and condenses to form chromosomes. Before DNA replication, chromosomes can be seen to be made up of what looks like one chromatid, while if they were observed after replication, they would be made up of two chromatids.

Key Term: Chromatin

Chromatin is a complex formed when DNA associates with histone proteins.

Key Term: Chromatid

A chromatid is each of the two threadlike strands into which a replicated chromosome divides during cell division. Each chromatid contains a single molecule of DNA.

These words are easily confused and can take a bit of practice to use in the correct context, but the diagram in Figure 2 displays how they are all linked together.

Figure 2: This diagram shows the components of a replicated chromosome consisting of two chromatids. The chromosome was formed by condensed chromatin fibers consisting of DNA strands coiled around histone proteins.

The chromosome in the top left of this diagram consists of two chromatids, each a long strand of DNA coiled around histone proteins. This is how the chromosomes appear after DNA replication, but before they are separated in cell division. Chromosomes will be most likely to appear like this during metaphase of mitosis, before the two chromatids are pulled apart in anaphase (both metaphase and anaphase are later phases in mitosis that takes place after DNA replication).

Key Term: Chromosome

A chromosome is a long molecule of DNA and associated proteins that contains the genetic information of an organism in the form of genes.

Example 1: Defining the Function of Chromosomes

Chromosomes are formed from the nucleic acid DNA. What is the primary function of these chromosomes?

  1. To keep the contents of the nucleus and the cytoplasm separate
  2. To act as the site of cellular respiration
  3. To provide essential nutrition for the cell
  4. To carry genetic information and allow it to be passed from parent to offspring

Answer

Let’s approach this question through the process of elimination by using information about the function of chromosomes. Keep in mind that when a question asks you for the “primary function” of something, several of the answers may technically be correct. You are looking for the one that defines the function of the chromosomes best, so all answers must be carefully scrutinized.

Option A refers to keeping different components separate. This is the role of a membrane, and, in this specific case, is referring to the nuclear envelope. Chromosomes are not membranes, and therefore option A can be eliminated.

Option B refers to the site of respiration, which is actually the function of the mitochondria. This is therefore an incorrect function of a chromosome.

Option C is referring to another essential life process: obtaining nutrition. Some eukaryotic organisms can obtain nutrition from structures within their own cells. For example, plants are eukaryotic autotrophs that obtain their nutrition by converting light energy into chemical energy within their chloroplasts. However, as not all eukaryotes can do this, this cannot be the role of a structure common to all eukaryotic organisms such as the chromosome. Most animals (which are also eukaryotes) are heterotrophic and so obtain their nutrition from food. This food is broken down into its constituent parts by the digestive system to be absorbed into the bloodstream to carry these nutrients to body cells. There is no single structure that is found in all eukaryotic cells that is solely responsible for providing nutrition, especially not chromosomes.

This leaves us with option D, which defines the two main aspects required when defining the function of chromosomes: that they hold genetic information in the form of DNA and that this genetic information is heritable so can be passed from parent to offspring.

This makes the following our correct definition: to carry genetic information and allow it to be passed from parent to offspring.

Under the light microscope, chromosomes will only be visible in eukaryotic cells during cell division. There are two main types of eukaryotic cell division: meiosis, which we mentioned earlier, and mitosis. In contrast to meiosis that produces four cells with half the genetic material of the parent cell, mitosis yields two genetically identical daughter cells with the same full set of genetic material as the parent cell. This is because the main purpose of mitosis is for growth and to replace worn-out cells.

During the cell cycle, a typical cell spends most of its life in a stage known as interphase, making preparations before undergoing mitosis. During interphase, the genetic material on each chromosome in a cell is copied exactly so that the new daughter cell will have the same DNA as the original parent cell via DNA replication.

The structure of a chromosome following DNA replication, and right before its chromatids are separated during cell division, is shown below in Figure 3.

Figure 3: A diagram showing a chromosome composed of two sister chromatids, following DNA replication and chromatin condensing, in an early stage of mitosis. These chromatids will be pulled apart during the later stages of mitosis.

One chromosome in Figure 3 consists of two identical sister chromatids, joined at a centromere. The chromosome is visible as DNA has coiled around histone proteins to form chromatin, which itself condenses further until it forms tightly compacted chromosomes. The chromosomes will then be ready to divide in the process of mitosis to form two cells with identical DNA.

Key Term: Centromere

A centromere is the section of DNA that joins two sister chromatids of a replicated chromosome and to which certain fibers will attach during mitosis to pull these chromatids apart.

The nucleus is a spherical or oval organelle which contains the genetic material of a eukaryotic cell. It is often the largest organelle within an animal cell, as seen in Figure 4 below, and it is often stained when observed with a light microscope which makes it easy to identify in a photograph taken using a microscope, also called a “micrograph.”

Key Term: Nucleus

The nucleus is an organelle surrounded by a double membrane that contains genetic information in the form of DNA molecules.

Figure 4: A diagram showing a typical eukaryotic animal cell with the nucleus in purple in the center.

Let’s look at two examples of micrographs with visible nuclei in different eukaryotic cells.

The image below is a light microscope micrograph of some muscle fibers of the muscular tissue of the heart. Each of these specialized cells have a single oval-shaped nucleus visible in dark purplish blue.

Micrograph of some muscle fibers of the muscular tissue of the heart
Figure 5

The image below is a light microscope micrograph showing elongated, roughly rectangular cells from the inner membrane from a common onion (Allium cepa) that have been stained with iodine. Iodine makes each cell’s nucleus visible as a small orange/brown circle.

Cells from the inner membrane from a common onion
Figure 6

The nucleus has a complex interior structure, as shown in Figure 7 below.

Figure 7: A diagram showing the structure of the nucleus including nuclear envelope, nuclear pores, nucleoplasm, chromatin, and nucleolus.

The nucleus is surrounded by a double membrane known as the nuclear envelope, which is usually continuous with another organelle called the endoplasmic reticulum. The nuclear envelope effectively separates the contents of the nucleus from the rest of the cell, therefore protecting DNA from reactions that may be occurring in the cell’s cytoplasm. This is called compartmentalization, and it occurs in all organelles which have membranes.

Key Term: Nuclear Envelope

The nuclear envelope is a double membrane layer that separates the contents of the nucleus from the rest of the cell and protects the cell’s genetic material from the chemical reactions that take place outside the nucleus.

The nuclear envelope contains small openings called nuclear pores through which small substances can pass between the nucleus and cytoplasm, but larger molecules cannot. For example, while DNA is too large to fit through the nuclear pores and so remains within the nucleus, mRNA molecules are smaller, so they can move out through the nuclear pores and toward ribosomes in the cytoplasm to carry out the process of protein synthesis.

Key Term: Nuclear Pore

Nuclear pores are small gaps within the nuclear envelope that allow certain molecules to move into and out of the nucleus.

Let’s use an analogy to better understand the role of the nuclear envelope. Imagine the nucleus as a library within a school, 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. Just like books might be removed from or brought into the library through the doors, things like tables, chairs, and bookcases may be difficult to move as they are too large.

Example 2: Defining the Role of the Nuclear Envelope

The nucleus is surrounded by a double membrane known as the nuclear envelope. What is the function of this membrane?

  1. To prevent any substances from entering or leaving the nucleus
  2. To act as the site of RNA synthesis
  3. To allow the attachment of the nucleus to the cell membrane
  4. To separate the contents of the nucleus from the cytoplasm

Answer

Let’s approach this question through the process of elimination by using information about the function of the nuclear envelope and other cellular components.

We know the role of membranes is usually to control what enters and leaves a certain area or cell, but they would be no use if they blocked all substances from entering or leaving. The nuclear envelope is designed with nuclear pores to prevent certain larger substances from entering and leaving the nucleus, but smaller molecules like RNA do pass into and out of the nucleus, so option A is incorrect as it states that all substances are prevented from passing through the membrane.

The nucleolus is the area of the nucleus where ribosomal RNA is synthesized, which means that option B is also not describing the function of the nuclear envelope. It helps to look at the information that you are given in the question, where the nuclear envelope is described as a double membrane. Remember what you know about the function of a membrane, which is not to synthesize new substances but to compartmentalize chemical reactions occurring in different organelles.

Option C describes the nucleus attaching to the cell membrane as a function of the envelope, which does not occur. Though the nuclear envelope will be attached to the rough endoplasmic reticulum membrane, and all organelles will be connected to the cell membrane via the cytoskeleton, this is not a significant function of the nuclear envelope itself.

Option D however states that the function of the membrane is to separate the contents of the nucleus from the cytoplasm, which describes the key function of all membranes: separating the contents of something from something else outside it so that different chemical reactions can occur in each section. Though this does not mention the other function of the nuclear membrane of controlling what substances pass through it, this can be seen more as a function of the nuclear pores within the nuclear envelope and not the envelope itself.

This makes the following our best option for the function of the nuclear envelope: to separate the contents of the nucleus from the cytoplasm.

Within the nuclear envelope is a gel-like substance called the nucleoplasm. The suffix -plasm is similar to that of the cytoplasm for a reason: while cytoplasm is the liquid portion of the cell, nucleoplasm is the liquid portion of the nucleus. It contains the nucleolus, chromatin, and many different substances, such as nucleotides and enzymes, to be used in DNA replication. The chemical reactions that occur in the nucleoplasm are separated, or compartmentalized, from the cytoplasm by the nuclear envelope.

Key Term: Nucleoplasm

The nucleoplasm is a gel-like substance that contains chromatin and substances, such as nucleotides and enzymes, for DNA replication. It is separated from the cytoplasm by the nuclear envelope so that the different reactions that occur in each can be compartmentalized.

Example 3: Defining the Role of the Nucleoplasm

Which of the following best describes the nucleoplasm?

  1. The double membrane that surrounds the nucleus to separate its contents from the cell cytoplasm
  2. The viscous substance found within the nucleus that maintains the organelle’s cuboidal shape
  3. A gelatinous liquid found within the nucleus that contains chromatin
  4. The fatty layer that encloses the nucleolus to separate it from the nucleus
  5. An aqueous solution that is the site of all the chemical reactions that occur within a cell

Answer

Let’s approach this question through the process of elimination by using information about the function of the nucleoplasm and other nuclear components.

By looking at the suffix -plasm at the end of the word nucleoplasm, we can already tell that this will likely have a similar function to another -plasm word we know: cytoplasm. Therefore, we can deduce that the likely function of the nucleoplasm will be to contain dissolved substances and to be liquid but to be associated with the nucleus (the nucleo- in nucleoplasm referring to the nucleus) instead of the cell (the cyto- in cytoplasm referring to cell).

We know that the nucleoplasm therefore is not a membrane, and so we can eliminate options A and D, as the function of the liquid is not to separate structures from any others or to enclose the nucleolus itself. Both of these options refer more to the role of a membrane-like structure, which in A is the nuclear envelope but in D is not a real cellular structure at all.

Option B is not a probable answer for several reasons. The role of the nucleoplasm is not to maintain the shape of the nucleus, just as the role of the cytoplasm is not to maintain the shape of the cell. The option also states that the shape of the nucleus is cuboidal, whilst it is, in fact, generally spherical. If you spot an error in an answer, it is a useful way to eliminate one of your options.

Finally, option E is incorrect for the definition of nucleoplasm, as it instead gives us a slightly incorrect definition of the cytoplasm. Remember that the cytoplasm is the cell’s general site of chemical reactions, but not all reactions occurring in the nucleus will also occur in the rest of the cell too. This highlights the need for compartmentalization within different organelles, as different chemical reactions will be occurring within each of them. No one liquid section of the cell is responsible for all chemical reactions.

This means that option C describes the function of the nucleoplasm best, as the defining aspects of nucleoplasm are its consistency (liquid or gel) and that it contains chromatin, the substance formed when DNA associates with and coils around histone proteins. So, our correct answer is that the nucleoplasm is a gelatinous liquid found within the nucleus that contains chromatin.

The nucleolus is a dense area within the nucleus that usually makes it easy to spot in a micrograph. Ribosomes are formed in the nucleolus from proteins and RNA.

Key Term: Nucleolus

The nucleolus is an area within the nucleus composed of proteins, RNA, and DNA that is responsible for producing ribosomes that will be the site of protein synthesis.

As you can see in Figure 8 below, ribosomes consist of a large and a small subunit.

Figure 8: This diagram shows a ribosome carrying out the second stage of protein synthesis, by translating the mRNA molecule fit between its large and small subunits.

The large and small ribosome subunits exit the nucleus via a nuclear pore and assemble in the cytoplasm when protein synthesis needs to be carried out. Ribosomes can either attach to the rough endoplasmic reticulum or remain free in the cytoplasm. mRNA molecules, which also move through the nuclear pores, will then bind to the ribosomes when specific proteins need to be produced. Therefore, ribosomes play an important role in protein synthesis.

Example 4: Defining the Role of the Nucleolus

What is the role of the nucleolus in a eukaryotic cell?

  1. To wrap around chromatin when forming chromosomes
  2. To synthesize ribosomal RNA
  3. To provide a barrier between the nucleus and the cytoplasm
  4. To initiate and control cell division
  5. To act as the site of cellular respiration

Answer

Let’s approach this question through the process of elimination by using information about the function of the nucleolus and other cellular components.

The key function of the nucleolus is to produce ribosomes that then play an important role in protein synthesis.

One of the options suggests that the nucleolus wraps around chromatin. This cannot be a valid answer, as nothing wraps around chromatin. Instead, chromatin is the complex formed when DNA strands wrap around histone proteins to form chromosomes.

Another option suggests that the nucleolus provides a barrier between the nucleus and cytoplasm. This is referring to the role of the nuclear envelope. We know that the nucleolus is not a membrane or barrier itself, so the function of the nucleolus is not to separate the nucleus from the cytoplasm as it is located within the nucleus itself rather than on the boundary to the cytoplasm.

Let’s look at the option referring to initiating and controlling cell division.

DNA, which acts as a template to synthesize proteins needed for cell division to occur and to control it, is located within the nucleus. However, the sections of DNA that are present in the nucleolus are mainly those responsible for producing ribosomes and not for triggering cell division. In fact, during cell division, the nucleolus temporarily disappears. This is mainly because the nucleolus is a dense region of chromatin surrounded by ribosomal proteins and ribosomal RNA, and during cell division, this region, the nucleolus, disperses as a result of the condensation of its chromatin part into chromosomes. This makes this option incorrect, as ribosome production does not control mitosis, and the nucleolus will vanish during cell division so it could not possibly control it.

One of the options refers to the site of respiration, which is the function of the mitochondria. This is therefore an incorrect function of the nucleolus.

This makes the following the correct option describing the function of the nucleolus: to synthesize ribosomal RNA.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • The nucleus is a defining component of eukaryotic cells in which DNA (genetic information) is stored to be replicated; it carries the code to synthesize proteins to control cell activities and is used to form gametes which when fertilized will pass on genetic material to offspring.
  • The nucleus consists of a dense nucleolus within the nucleoplasm that also contains chromatin, a complex formed when DNA coils around histone proteins.
  • The nucleus is surrounded by a nuclear envelope, compartmentalizing the reactions within the nucleus, with nuclear pores to allow certain substances such as RNA to move into or out of the nucleus.

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