Which stage of mitosis includes the formation of kinetochores?

A. Anaphase.

B. Interphase.

C. Telophase.

D. Metaphase.

E. Prophase.

Diagram of a kinetochore protein
Diagram of a kinetochore protein

The correct answer is E. Prophase.

Mitosis and meiosis are the two types of cell division that occur in cells. Of the two types, mitosis is the form of division that occurs in body cells for growth, repair, and replacement of cells.

Cells undergo a cycle of stages which occur in a specific sequence. The first three stages are together called interphase and consists of G1, S, and G2. At the end of G2, the cell then enters the M stage which is mitosis.

The mitotic stage includes stages that occur in the following order: prophase, metaphase, anaphase, telophase, and cytokinesis.

It is during prophase that kinetochores are formed at the centromere of the chromosome. These structures are important since this enables microtubules of the spindle to attach to the chromatids.

During metaphase chromosomes line up in the middle of the cell with each chromatid attached to a spindle that extends to the opposite poles of the cell.

Anaphase is the stage when these chromatids are pulled apart by the spindle fibers. The microtubules making up the fibers of the spindle contract causing the separation of the chromatids.

Once the chromatids are at the opposite sides of the cell, the next stage, telophase occurs. Finally, the cytoplasm divides by cytokinesis and we end up with two new daughter cells formed from one parent cell.

Mitosis

There are two types of cell division that occur in eukaryotic cells: mitosis and meiosis.  Meiosis is the type of division that occurs in the sex cells or gametes, while mitosis occurs in the body cells or autosomes.

The main goal of mitotic divisions is to repair and replace cells that are worn out and to bring about the growth of tissues when needed.

Mitosis is one part what is known as the cell cycle. Each cell has a cycle consisting of four stages: G1, S, G2 and M phase. This last phase is the mitotic stage of the cycle which can only occur after G1, S, and G2 have successfully progressed.

Mitotic divisions produce cells which are usually genetically identical to the parent cell that divides. The chromosome number also remains the same, which means in the case of humans that each daughter cell produced during mitosis will have 46 total chromosomes just like in the parent cell.

Prophase to metaphase

The first stage of mitosis which occurs is known as prophase and is the time when chromatin condenses such that chromosomes become visible as individual structures. During this stage of division, the nucleolus also disintegrates and nuclear envelope breaks down.

In order for chromosomes to move apart during a later stage, they need to attach to the spindle apparatus. This structure consists of spindle fibers which are actually made of protein microtubules, and the structure is formed at the centrosomes of the cell.

Each chromosome in the cell has a particular region usually near the center of the chromosome that is called a centromere.  It is in this area of the chromosome that the kinetochore now forms.

The kinetochore is actually a rather complex structure that in fact consists of more than 50 proteins which assemble in a specific fashion.

Microtubules from opposite ends of the cell reach out and attach to these structures. This is known as an amphitelic attachment of microtubule fibers of the spindle to the kinetochores of the chromosome.

It is important to realize that each chromosome consists of two chromatids, each with a kinetochore to which a fiber attaches.

During the next stage of mitosis, metaphase, the chromosomes align at the center of the cell on the metaphase plate.

The chromatids are attached to spindle fibers that extend the length of the cell. The chromosomes are lined up in one line at the middle of the cell in preparation for the next stage of mitosis.

Anaphase to telophase

Anaphase is the third stage of mitosis and it involves contraction of the microtubules that make up the spindle apparatus.  As these proteins contract, it causes the fibers attached to each chromatid to actually shorten.

The result of this is that the chromatids of each chromosome are pulled apart and begin to move to the opposite poles of the cell. Once these chromatids have reached the ends of the cell, the next stage of the mitotic division is ready to begin.

Telophase involves these chromosomes now becoming chromatin and at the same time as this is happening the nucleoli begin to reform at each pole of the cell.

A nuclear envelope also reforms around what is to become a nucleus. At the end of this stage, there will be a fully formed nucleus at each pole of the cell.

Cytokinesis

The final stage of the mitotic division is the division of the cytoplasm which is known as cytokinesis. In animal cells, the cytoplasm pinches in at the central region of the cell. This process continues until two cells are eventually formed.

Plant cells have a cell wall so the process of cytokinesis is thus more complicated and involves the deposition of a cell plate in the middle of the cell. Vesicles formed in the Golgi body of the plant cell are used to help form this plate which becomes the cell wall.

It is important to understand that the daughter cells formed at the end of mitosis are genetically the same as the parent cell and have the same chromosome number.

After mitosis has ended the cell will reenter the G1 stage of the cell cycle, and then will continue through the other stages until ready to divide by mitosis again.

References

  1. Editors of Encyclopedia Britannica (2018). Cell cycle. Retrieved from Encyclopedia Britannica.
  2. CE Walczak, R Heald (2008).Mechanisms of mitotic spindle assembly and function.  International review of cytology.
  3. Y Yamagishi, T Sakuno, Y Goto, Y Watanabe (2014). Kinetochore composition and its function: lessons from yeasts.  FEMS microbiology reviews.
  4. TD Pollard, WC Earnshaw, J Lippincott-Schwartz, G Johnson (2017). Cell Biology, 3rd  USA: New York, Elsevier Publishers.
  5. RH Raven, RF Evert, SE Eichhorn (1987). Biology of plants, 4th edition. New York: USA, Worth Publishers.

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