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Ravela Da Cruz

Mitosis Study Guide

What is mitosis? A cell cycle phase where the chromosome containing the nucleus divides between two cells. Therefore, two daughter cells are produced from a mother cell.

Introduction:

What does yeast used in bread and frogs have in common? They all grow, but how? They contain cells that divide continuously to produce new cells which would be completely genetically identical to the mother cell. This is the purpose of mitosis or the primary mechanism of growth because cells tend to generally wear out and, therefore, reproduce new ones and grow bigger.

Brilliantly, Walter Flemming was among the first cytologists to describe chromosome behavior in animal cell division and how precisely mitosis occurs.

Definition and explanation of mitosis

Mitosis is the process of cell division where one mother cell would divide to produce two daughter cells. They would be genetically identical. In this stage of cell division, the DNA would be split into two, having an equal number of chromosomes. Most of the cell division happening in your body undergoes mitosis, where it replaces the worn-out cells.

What is the goal behind this cell division? It gives rise to two daughter cells that would have full identical sets of chromosomes; otherwise, it could lead to genetic abnormalities. Cells with too many chromosomes could even lead to the growth of cancerous malignant cells. Therefore, for a healthy set of daughter cells, chromosome duplication is done in carefully organized phases of mitosis.

What is the purpose of mitosis?

The purpose of mitosis could be brought down to three main reasons:

Mitosis Source

  • Growth Alongside Development: After the completion of meiosis, as a gamete is produced, it then fuses with another one to form an embryo. This embryo would then grow through mitosis and will continue and become more complex. As old cells wear out and new cells take their place, the original set of chromosomes is preserved.

  • Results In Cell Replacement: When an old cell is damaged, new cells are created to replace them to ensure consistent functioning. Examples would be broken bones.

  • Helps In Asexual Reproduction: Single-celled organisms, alongside some multicellular organisms, resort to different stages of mitosis to reproduce. This would include fragmentation (planaria), budding (sea anemones).

Different stages of mitosis in order

__Phases of mitosis in order __

Mitosis mainly occurs in four crucial stages known as prophase, metaphase, anaphase, and telophase. In some places, you would notice the prophase stage being divided into early prophase and prometaphase. Cytokinesis is the process of cell division to create two new cells, which happen in anaphase or telophase.

Mitosis is the process of cell division

MItosis Phases Source

Here, the cell has already entered the interphase stage where it has copied the DNA sequence, and the nucleus now contains chromosomes with sister chromatids. The chromosomes are not always precisely visible as they are still in their decondensed form.

  • In the first stage of prophase (early prophase), the cell would start breaking down for initiation of chromosome division. The chromosome also starts condensing, which makes it easier to separate. The mitotic spindle structure made of microtubules forms. Its primary job would be to organize the sets of chromosomes and help them move during mitosis. The spindle is observed to grow between the centrosomes. The part of the nucleus, nucleolus, where the ribosomes are created, disappears.

  • In the prometaphase phase, the chromosomes are captured by the mitotic spindle and are organized. The chromosomes then become even more condensed, and this stage results in the breakdown of the nuclear envelope, releasing the sets of chromosomes. The mitotic spindle develops more, and the microtubules then start capturing the chromosomes.

Note: Microtubules can assist in binding the chromosomes at the kinetochore ( this is a patch of protein found on each sister chromatid centrosome). Being able to bind chromosomes, the microtubules are often termed kinetochore microtubules. If they cannot bind to the kinetochore, the microtubules bind with other microtubules to stabilize the spindle.

  • All the chromosomes have been captured by the spindle and are lined at the center of the cell, ready to separate in the metaphase. The chromosome swiftly lines up at the metaphase plate (this is a term used in explaining the plane where the chromosomes line up). During this stage, the kinetochores must be attached to the microtubules.

Before moving towards other steps of mitosis (next stage anaphase), the cell ensures that the chromosomes are precisely lined up with their kinetochores attached to the microtubules. This is termed as the spindle checkpoint to guarantee that the sister chromatids are divided evenly when in the next stage, two new daughter cells are formed.

  • In the anaphase stage, here the sister chromatids are separated by breaking the ‘protein’ glue and settle at the opposite ends of the cell. The microtubules, which are not attached to the chromosomes, elongate, pushing apart, separating the pole and elongating it. Motor proteins regulate the whole process. During mitosis, these motor proteins carry the chromosomes as they walk.

  • In the telophase stage, the cell is almost done separating and starts re-establishing as a normal structure through cytokinesis. The mitotic spindle breaks apart, and two nuclei form for two sister cells. The nucleoli alongside the nuclear membrane reappear. In this stage, the chromosomes decondense and get back to their original ‘stringy’ form.

Cytokinesis, which is the formation of two new sister cells, often overlaps with that of the final mitosis stage.

Conclusion:

  • Mitosis is the formation of two new sister cells.
  • Occurs in four major stages - prophase, metaphase, anaphase, telophase.
  • Equal chromosome distribution between two daughter cells
  • Helps with growth and cell replacement
  • Helps in maintaining genome purity.

FAQs:

1. What is mitosis and meiosis? What are the stages of mitosis?

Mitosis is a cell division process where two new sister cells are formed from a mother cell. It consists of four main stages:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

Meiosis is the process where one single cell divides twice to form four cells containing half of the genetic information.

2. What is the importance of mitosis?

Mitosis is invloved in growth and development, cell replacement, and asexual reproduction.

3. How does mitosis help growth?

It helps in dividing a single cell into two having the same genetic material to help organisms grow in size and for repairing damaged tissues.

__4. Do cells divide all the time? __

Skin cells divide rapidly and more often, and therefore, we end up losing around 50 million cells every day; however, brain and nerve cells divide less often.

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Sources:

  1. Mitosis and Cell Division. https://www.nature.com/scitable/topicpage/mitosis-and-cell-division-205/. Accessed 8 Dec, 2021.
  2. Mitosis. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/2.34/primary/lesson/mitosis-and-cytokinesis-bio/. Accessed 8 Dec, 2021.
  3. The Stages of Mitosis and Cell Division. https://www.thoughtco.com/stages-of-mitosis-373534. Accessed 8 Dec, 2021.
  4. What Is Mitosis?. https://www.livescience.com/52512-mitosis.html. Accessed 8 Dec, 2021.