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Cell Division in Eukaryotes

Cell division is very important for eukaryotes.

In unicellular eukaryotes, cell division is necessary in order for the organism to reproduce.

Microscope image of single-celled algae. The image is of a few algal cells on a blue background. Each cell appears roughly circular in shape.

Unicellular algae viewed through a microscope. Like all unicellular eukaryotes, they use cell division to reproduce. Image: CSIRO on Wikimedia Commons (CC BY 3.0 - https://creativecommons.org/licenses/by/3.0/deed.en)

In multicellular eukaryotes, cell division is involved in a wide range of processes, including:

  • Reproduction (both sexual and asexual)
  • Growth and development
  • Maintenance and repair of tissues
Photograph of bladder wrack seaweed in water.

Bladder wrack (shown above) is a type of seaweed. Seaweeds are multicellular algae. Like all multicellular eukaryotes, they use cell division for reproduction, growth and development, and the maintenance and repair of tissues.

Some animals, including humans, many other mammals and most birds, stop growing once they reach a certain life stage (adulthood in humans). Once these animals reach this mature stage, cell division is mainly used for maintenance and repair of tissues, as well as for reproduction.

Photograph of a kingfisher sitting on a branch.

Most birds, such as this kingfisher, stop growing once they reach a certain size. In mature birds of these species, cell division is mainly used for repair and maintenance of tissues. The same is true for many mammals, including humans.

However, most multicellular organisms actually continue growing throughout their entire lifetime. This is true for many animals as well as most plants, multicellular fungi and multicellular algae. So in these organisms, growth continues to be an important reason for cell division through their lives.

Photograph of a sand lizard on a rock. The lizard's body is mostly bright green, but with some darker patches/areas.

Most lizards continue growing throughout life. The same is also true for many other animals and most plants and multicellular fungi and algae. In these organisms, growth continues to be an important reason for cell division throughout life.

Cell division in eukaryotes involves either mitosis or meiosis

There are actually two different kinds of cell division in eukaryotes. One involves a process called mitosis and the other involves a process called meiosis.

Meiosis is a process that is used to produce sex cells for sexual reproduction in many (but not all) eukaryotes. Mitosis is the process used in cell division in all other situations.

On this page, we will focus on cell division involving mitosis. Cell division involving meiosis will be covered later in the course.

In eukaryotes, the cell cycle has three phases

In eukaryotes, the cell cycle takes place in three phases called:

  • Interphase
  • Mitosis
  • Cytokinesis

Interphase happens first, then mitosis, then cytokinesis. The cell cycle then repeats.

Diagram of the cell cycle in eukaryotes. The cell cycle is represented by a ring containing the words "The Cell Cycle". The ring is made up of three curved arrows. The first arrow represents interphase. It starts at the of the ring and goes clockwise about two thirds of the way around. The next arrow represents mitosis. It start at the end of the first arrow and goes around almost back to the top. The final arrow represents cytokinesis. It goes from the end of the second arrow back around to the top. This illustrates the fact that after cytokinesis the cell cycle starts again with interphase.

In eukaryotes, the cell cycle has three phases: interphase, mitosis and cytokinesis.

Interphase involves cell growth and the replication of sub-cellular structures and DNA

In interphase, the following things happen:

  • The cell grows.
  • Sub-cellular structures (such as ribosomes and mitochondria) are replicated.
  • The DNA is replicated.

(These processes do not necessarily happen in the order listed above. In fact, some of them may happen at the same time as each other.)

The nucleus is not replicated during interphase. This happens later, during mitosis.

Diagram of interphase. On the left is a drawing of a cell before interphase. On the right is a drawing of a cell after interphase. The cell on the right is bigger and has twice as many of every sub-cellular structure apart from the nucleus. In the cell of the left, the nucleus contains three single chromosomes. In the nucleus of the cell on the right, there are three double chromosomes (a double chromosome is made of two copies of a chromosome joined together).

During interphase the cell grows, sub-cellular structures are replicated, and the DNA is replicated.

The DNA in eukaryotes is in the form of chromosomes, which are located in the nucleus.

At the start of interphase, each chromosome is just a single DNA molecule.

During interphase, each of these DNA molecules is replicated to form two identical copies. However, the two copies remain joined together. This structure, made of the two copies joined together, is still called a chromosome.

Therefore, although the amount of DNA in the nucleus doubles when the DNA is replicated in interphase, the number of chromosomes does not change.

Diagram showing what happens to the chromosomes during DNA replication. On the left, there is a drawing of a nucleus before DNA replication. If contains three chromosomes. Each of them is just a single line. A label pointing to one of them explains, "This is a chromosome. Before DNA replication, it is a single DNA molecule". An arrow from the nucleus points to the right to a drawing of the same nucleus but after DNA replication. The arrow is labelled, "DNA Replication (during interphase)". The nucleus on the right also contains three chromosomes, but each one is now an X shape. Labels explain, "This is a chromosome after DNA replication. It is made up of two copies of the original chromosome joined together.", and "Although the amount of DNA in the nucleus has doubled, the number of chromosomes has not changed.".

During DNA replication, each chromosome is copied. The structure that is formed is still called a chromosome, but now it is made up of two copies of the original chromosome joined together. The nucleus shown here contains three chromosomes. The actual number varies greatly between species.

When viewed through a microscope, a chromosome that is made up of two copies joined together looks like an X.

Microscope image of all of the chromosomes from a human body cell. Each chromosome appears as a dark purple X shape.

Microscope image of the chromosomes from the nucleus of a human body cell. Each chromosome appears as an X shape.

Mitosis involves the separation of the chromosomes and the division of the nucleus

As explained above, at the end of interphase each chromosome is made up of two identical copies joined together.

In mitosis, these two copies are pulled apart and go to opposite sides of the nucleus.

Once they are separated, each of the two copies is called a chromosome. This means that the word 'chromosome' can refer to a single copy on its own or two identical copies joined together.

Once the chromosomes have reached opposite sides of the nucleus, the nucleus itself divides to form two nuclei.

Diagram of what happens to the nucleus during mitosis. On the left is a diagram of the nucleus near the beginning of mitosis. It contains three chromosomes. Each one is made up of two DNA molecules joined together. An arrow pointing to the right points to a diagram of a nucleus in which the two DNA molecules of each chromosome have been pulled apart and are moving towards opposite ends of the nucleus. Each of them is now a chromosome. Another arrow points to a diagram of a nucleus in which the chromosomes have reached the two ends of the nucleus. A final arrow points to two smaller nuclei, each containing half of the chromosomes. The nucleus has divided in two.

At the start of mitosis, each chromosome is made up of two identical copies of an original chromosome. During mitosis, these two copies are separated and pulled to opposite ends of the cell. The nucleus then divides in two.

So, at the end of mitosis, the cell contains two nuclei, and each of these nuclei has the same amount of DNA as the cell had at the beginning of interphase (before the DNA was replicated).

Diagram of mitosis. On the left is a cell at the start of mitosis. On the right is a cell at the end of mitosis. An arrow labelled "Mitosis" points from the first cell to the second. The first cell is the same as a cell at the end of interphase. It is large and contains many sub-cellular structures. It has one nucleus and each chromosome in that nucleus is an X shape made up of two DNA molecules joined together. The second cell is the same as the first, except it has two nuclei and each chromosome is just a single DNA molecule. Text on the diagram explains, "The two DNA molecules in each chromosome are pulled apart and become separate chromosomes. They move to opposite sides of the nucleus and the nucleus divides".

During mitosis, the chromosomes separate and the nucleus divides.

Cytokinesis involves the cell splitting in two

In cytokinesis, the cell itself splits in two. The cell membrane splits to form two cells. The cytoplasm is divided between these two new cells. Each of the cells receives one of the two nuclei.

Diagram of cytokinesis. On the left is a large cell containing two nuclei. This is a cell that has just completed mitosis. From this cell there is an arrow labelled "Cytokinesis" which points to two smaller cells, indicating that the cell has divided in two. Each of the two new cells contains one nucleus. Text on the image explains, "The cell splits in two. Each cell receives one of the nuclei".

During cytokinesis the cell splits in two. Each cell receives one of the nuclei.

Cell division is now complete.

The two cells produced are genetically identical to the original cell and to each other (note that this is only true for cell division involving mitosis, not cell division involving meiosis).

The cell cycle can now repeat.

Diagram of the entire cell cycle, with drawings of the cell at each stage. At the top of the diagram is a cell at the beginning of interphase. An arrow leading away from this cell is labelled "Interphase. - Cell growth. - Replication of sub-cellular structures". This arrow points to a cell at the end of interphase (bigger, more sub-cellular structures, DNA replicated). An arrow then points away from this cell with the label, "Mitosis. The two DNA molecules in each chromosome are pulled apart and become separate chromosomes. They move to opposite sides of the nucleus and the nucleus divides.". This arrow points to a cell at the end of mitosis (two nuclei). Then an arrow points away from this cell, labelled "Cytokinesis. The cell splits in two. Each cell receives one of the nuclei.". This arrow splits. One branch points back to the original cell. The other points to an identical cell.

The entire cell cycle.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/7

What is cell division used for in eukaryotes?

2/7

What are the two different types of cell division in eukaryotes?

3/7

For cell division involving mitosis, what are the three phases of the cell cycle?

4/7

What happens during interphase?

5/7

What are the chromosomes like before and after DNA replication?

6/7

What happens during mitosis?

7/7

What happens in cytokinesis?

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