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Cell Division

Author: Sophia
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what's covered
In this lesson, you will learn about the process of cell division and the structures that carry out cell division in prokaryotic and eukaryotic cells. Cell division is critical in the reproduction of single-celled organisms. It also allows multicellular organisms to grow and heal from injuries. Some organisms use cell division to produce reproductive cells such as eggs and sperm. Understanding cell division is essential in understanding how living organisms reproduce and grow but has important applied value as well. For example, problems with regulation of cell division can lead to cancer and understanding normal cell regulation can help in developing treatments. Specifically, this lesson will cover the following:

Table of Contents

1. Prokaryotic Cell Division

As explained by the cell theory, cells arise from other cells. Cell division is the process by which this happens.

Prokaryotic cells have cell cycles during which they prepare for and then undergo cell division. Prior to cell division, they grow and increase the number of cell components. Most prokaryotic cells actually divide using a process you were introduced to in the lesson on spontaneous generation, called binary fission. In this process, their single circular chromosome is replicated and then the cell grows apart before forming new outer layers (including cell membranes and cell walls) between the two halves.

“Binary” means two and “fission” means splitting in half, so binary fission is a very general term for division into two parts that can describe more than one type of division. In this course, the term refers to prokaryotic cell division unless otherwise specified.

step by step
  1. DNA replication is used to produce two copies of the single, circular, prokaryotic chromosome. After DNA replication, each of the daughter chromosomes is separately attached to the cell membrane.
  2. During cell elongation, the cell grows in length. As the plasma membrane elongates, the two chromosomes move apart.
  3. The protein FtsZ (a prokaryotic cytoskeletal element) forms a contractile Z ring attached by FtsZ-binding proteins to the cytoplasmic membrane on each side of the cell. This ring defines the division plane between the daughter cells.
  4. Next, a structure called a divisome is formed as more proteins are added to the Z ring.
  5. The divisome activates to produce a peptidoglycan cell wall and build a septum (central dividing wall).
  6. The septum separates the two halves of the cell into daughter cells. For this to occur, the outer cell structures must be remodeled to form complete cell membranes, cell walls, and outer membranes (if present) to produce two entirely separate cells.
  7. To build new cell walls, enzymes are needed that break apart peptidoglycan to produce free ends to which new peptidoglycan subunits can be added.

The image below shows a micrograph of a rod-shaped bacterial cell undergoing binary fission (a) and the steps of binary fission (b). It shows how DNA replication is followed by cell elongation, formation of the division septum, and separation of one cell into two complete daughter cells.

Part (a) is a micrograph of two rod-shaped cells attached together. Part (b) shows the steps of binary fission. The process begins with a circular cell containing a single, circular chromosome. The cell replicates its DNA and elongates. Then, as the cell continues to elongate, each loop of DNA moves in an opposite direction and the cell then starts to constrict in the center. This results in two cells each containing a loop of DNA.

The image below shows how the cleavage furrow is formed by a ring of FtsZ that forms a septum that divides the single cell into two cells.

Details of the process of binary fission. A cell is shown that has constricted in the center. The indentation in the center of the dividing cell is labeled cleavage furrow. In the narrow center of the cell where it has constricted, there is a vertical circle of spheres labeled FtsZ ring that connects the plasma membrane from the top and bottom halves of the cell. Next, an illustration shows that the invaginations of the plasma membrane have joined to form a vertical septum with a vertical line of spheres from the FtsZ ring in its center. Finally, the two halves of the cell separate to form two adjacent spheres that each have a single circular chromosome and multiple individual spheres from the FtsZ ring.

2. Eukaryotic Cell Division

Division of eukaryotic cells involves some complexities not experienced by prokaryotic cells. In eukaryotic cells, DNA is enclosed in a nucleus and cannot move freely until the nucleus has broken down. Additionally, eukaryotic cells have multiple chromosomes whereas prokaryotic cells usually have a single circular chromosome. This means that the chromosomes must be carefully separated so that each daughter cell has a copy of each necessary chromosome. Eukaryotic cells have specialized processes of cell division that address these challenges.

2a. The Cell Cycle

The eukaryotic cell cycle describes the stages of the process by which a cell prepares to divide. As shown in the image below, most of the cell cycle is interphase. This is the period of time in which the cell is not dividing. During interphase, the cell begins in the G1 phase. This is the time during which the cell is growing, performing its normal functions, and preparing itself for subsequent stages. During the next phase, the S phase, the cell copies its DNA in preparation for cell division. Next, the cell enters the G2 phase and continues to grow and replicate organelles in preparation for division. After G2, the cell enters the mitotic phase. During this phase, the cell divides to produce daughter cells. The first part of this phase is mitosis, during which karyokinesis (division of the nucleus into two nuclei) occurs. Next, cytokinesis (the splitting of one cell into two) occurs. After cytokinesis, the newly produced daughter cells enter the G1 phase.

Although the term mitosis is generally used to refer to eukaryotic cell division that produces two identical daughter cells through a specific series of steps, it most specifically refers to karyokinesis. It is possible for mitosis to occur without cytokinesis to produce a multinucleate cell.

This cycle is carefully regulated so that the cell does not divide when it is not ready or when division is inappropriate. When regulation fails, the cell divides at inappropriate times. This leads to cancer.

Diagram of the cell cycle showing interphase stages G1 cell growth, S DNA synthesis, and G2 cell growth, followed by the mitotic phase with mitosis and cytokinesis leading to formation of two daughter cells.

terms to know
Interphase
The part of the cell cycle during which the cell is not dividing. This includes the G1, S, and G2 phases of the cell cycle.
Mitosis
The division of one cell to produce identical daughter cells. Mitosis specifically describes karyokinesis, but usually involves cytokinesis as well.
Karyokinesis
The division of the nucleus into two nuclei.
Cytokinesis
The splitting of one cell into two.

2b. Mitosis

Mitosis involves a series of steps that divide one cell into two identical daughter cells. Most often, a cell with two sets of chromosomes (diploid) divides to produce two identical diploid daughter cells. However, mitosis can also produce identical daughter cells from parent cells with different numbers of sets of chromosomes.

step by step
The Stages of Mitosis
Prophase Chromosomes condense and become clearly visible. The nuclear envelope breaks down, freeing the chromosomes to move throughout the cell. The nucleolus becomes invisible. Additionally, cytoskeletal spindle fibers emerge to assemble a spindle apparatus that moves chromosomes as needed.
Prometaphase The chromosomes continue to condense. The mitotic spindle microtubules (a type of cytoskeletal element discussed later in this lesson) attach to structures called kinetochores on the chromosomes. In animal cells, structures called centrosomes anchor the spindle fibers and begin to move toward opposite poles of the cell. In other cells, the spindle fibers are anchored differently but the process is similar.
Metaphase The mitotic spindle is fully developed and the chromosomes are lined up on an imaginary line across the center of the cell (the metaphase plate). Once each chromosome has been duplicated prior to mitosis, each duplicated chromosome consists of two sister chromatids joined together at a centromere. In metaphase, the two sister chromatids in a pair are each attached to a spindle fiber originating from an opposite pole.
Anaphase Cohesin proteins binding the sister chromatids together break down and the former sister chromatids (now daughter chromosomes) are pulled toward opposite poles by the spindle fibers. Other spindle fibers lengthen to elongate the cell.
Telophase Chromosomes arrive at opposite poles and begin to decondense. Nuclear envelope material surrounds each set of chromosomes, forming new nuclei. Additionally, the mitotic spindle breaks down.
Cytokinesis One cell splits into two. In animal cells, a cleavage furrow forms that pinches the cells apart. This furrow is formed by actin microfilaments (a type of cytoskeletal element), not FtsZ. In plant cells, a cell plate forms that separates the daughter cells and gives rise to a new cell wall.

A diagram shows the events that take place during the stages of mitosis and cytokinesis as described in the text. Each stage is accompanied by a micrograph illustrating the locations of the chromosomes and spindle fibers.

term to know
Diploid
Having two sets of chromosomes.







2c. Meiosis





Unlike mitosis, which produces identical daughter cells, meiosis produces varied daughter cells with half the number of chromosomes of the parent cell (in other words, they are haploid). This contributes to genetic variation in organisms that reproduce sexually. 



As shown in the image below, mitosis and meiosis differ in several important ways. 







EXAMPLE

Mitosis has one division and meiosis has two divisions. Mitosis produces two identical daughter cells that are also identical to the parent cell. If the parent cell is diploid, then both daughter cells are diploid. Meiosis produces four different daughter cells with half the number of chromosomes of the parent cell. If the parent cell is diploid, then the daughter cells are haploid.







The two divisions of meiosis are called meiosis I and meiosis II. The stages of each division have the same names as the stages of mitosis with I or II added.



Genetic variation is also increased by a process called crossing over that takes place in meiosis but very rarely in mitosis. In sexually reproducing organisms, a diploid individual has received one set of chromosomes from one parent and a second set of chromosomes from the other parent. If there are 46 chromosomes in an organism, then that means that there are 23 sets of two chromosomes each. During prophase I of meiosis, pairs of homologous chromosomes (meaning different chromosomes of the same type, one inherited from each parent) come into physical contact and exchange genetic material. This is called crossing over and produces new genetic combinations.







EXAMPLE

In humans, there are 23 chromosomes in a set. Each haploid egg and sperm cell contains 23 chromosomes. When an egg and sperm fuse, the resulting zygote (fertilized egg) has 46 chromosomes and is diploid.







Part (a) shows two sets of chromosomes. The left-hand set is labeled female and the right-hand set is labeled male. Each set consists of twenty three pairs of chromosomes arranged roughly from largest pair to smallest pair. Each pair is numbered, ranging from 1 to 22, except for the last pair. In the female karyotype, the final pair consists of two relatively long chromosomes labeled as XX. In the male karyotype, the final pair consists of one relatively long chromosome labeled X and a much shorter chromosome labeled Y. Part (b) shows a micrograph of a male karyotype consisting of stained chromosomes. The chromosomes vary by having darker and lighter regions. Some of the chromosomes are somewhat curved. The final pair of chromosomes is labeled X/Y and shows a large X chromosome next to a small Y chromosome. 



Another important difference between mitosis and meiosis occurs in metaphase I. In mitosis and in metaphase II of meiosis, chromosomes line up on the metaphase plate and then the sister chromatids separate in to move toward opposite sides of the cell. In metaphase I of meiosis, pairs of homologous chromosomes line up on the metaphase plate and then move toward opposite poles.



This illustration compares meiosis and mitosis. In meiosis, there are two rounds of cell division, whereas there is only one round of cell division in mitosis. In both mitosis and meiosis, DNA synthesis occurs during S phase. Synapsis of homologous chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Crossover of chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Homologous pairs of chromosomes line up at the metaphase plate during metaphase I of meiosis, but not during mitosis. Sister chromatids line up at the metaphase plate during metaphase II of meiosis and metaphase of mitosis. The result of meiosis is four haploid daughter cells, and the result of mitosis is two diploid daughter cells.
















terms to know






Meiosis
A type of eukaryotic cell division that produces varied daughter cells with half the number of chromosomes of the parent cell.
Haploid
Having one set of chromosomes.
Zygote
A fertilized egg.





















make the connection





If you're taking the Microbiology Lab course simultaneously with this lecture, it's a good time to try the Activities in Unit 2 of the Lab course. Good luck!


 














summary






In this lesson, you learned about cell division. Depending on the type of organism, cell division is important in reproduction, growth, and repair. You learned about prokaryotic cell division, including the steps of binary fission. You also learned about ways in which eukaryotic cell division differs, including details of the cell cycle. Finally, you learned the steps of mitosis and meiosis as well as similarities and differences between these processes. As you learn more about different types of microbes, the information in this lesson will be valuable. For example, understanding how chromosomes move during meiosis is necessary to fully understand inheritance patterns, because genes for specific traits are physically located on chromosomes and move according to the patterns that you have just learned.














Source: THIS TUTORIAL HAS BEEN ADAPTED FROM OPENSTAX "MICROBIOLOGY." ACCESS FOR FREE AT openstax.org/details/books/microbiology. LICENSE: CC ATTRIBUTION 4.0 INTERNATIONAL. Accessed by August 2022.














REFERENCES


Karyotype. (2022, October 21). In Wikipedia. https://en.wikipedia.org/wiki/Karyotype



Merriam-Webster. (n.d.). Binary. In Merriam-Webster.com dictionary. Retrieved August 13, 2022, from
www.merriam-webster.com/dictionary/binary



Merriam-Webster. (n.d.). Fission. In Merriam-Webster.com dictionary. Retrieved August 13, 2022, from 
www.merriam-webster.com/dictionary/fission



Parker, N., Schneegurt, M., Thi Tu, A.-H., Lister, P., & Forster, B. (2016). Microbiology. OpenStax. Access for free at 
openstax.org/books/microbiology/pages/1-introduction



Wikipedia: Karyotype. (2022, October 21). In Wikipedia. https://en.wikipedia.org/wiki/Karyotype



















Terms to Know









Cytokinesis





The splitting of one cell into two.





Diploid





Having two sets of chromosomes.





Haploid





Having one set of chromosomes.





Interphase





The part of the cell cycle during which the cell is not dividing. This includes the G1, S, and G2 phases of the cell cycle.





Karyokinesis





The division of the nucleus into two nuclei.





Mitosis





The division of one cell to produce identical daughter cells. Mitosis specifically describes karyokinesis, but usually involves cytokinesis as well.
































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