Classify the microtubules involved in mitosis

What is Microtubules?

Microtubules are cylindrical, hollow structures made of tubulin protein subunits, and they are a key component of the cytoskeleton in eukaryotic cells.  Microtubules have several essential functions, including:
  • Structural support: They help maintain the shape of the cell.
  •  Intracellular transport: They serve as tracks for the movement of organelles, vesicles, and other cellular components.
  •  Cell division: They form the mitotic and meiotic spindle apparatus, which segregates chromosomes during cell division.
  •  Cell motility: In structures like cilia and flagella, microtubules provide movement.

What is Cell Division?

Cell division is the process by which a parent cell divides into two or more daughter cells. It is ensures the continuity of life by allowing organisms to grow, repair damaged tissues, and reproduce. There are two primary types of cell division:
  1. Mitosis: 
    • This is the type of cell division that results in two genetically identical daughter cells. It occurs in somatic (non-reproductive) cells and is involved in growth, tissue repair, and asexual reproduction.
  2. Meiosis:
    • Meiosis is a specialized type of cell division that occurs in the formation of gametes (sperm and eggs) and reduces the chromosome number by half. It creates four haploid cells that are each genetically distinct from the parent cell. This process is essential for sexual reproduction as it contributes to genetic diversity through the separation of homologous chromosomes and crossing over.

Microtubules Involved in Mitosis

Microtubules are critical components of the cytoskeleton in eukaryotic cells, playing an essential role in a variety of cellular processes, including cell division. During mitosis, microtubules form the spindle apparatus, which is crucial for the accurate segregation of chromosomes into daughter cells. 

There are three primary types of microtubules involved in mitosis: 
  1. Kinetochore Microtubules
  2. Polar Microtubules
  3. Astral Microtubules.

Each type of microtubule kinetochore, polar, and astral has distinct but interconnected functions that contribute to the successful completion of mitosis. Understanding these microtubules and their roles provides valuable insights into the mechanisms of cell division and the potential consequences of errors in this fundamental process.

01. Kinetochore Microtubules

Kinetochore microtubules are specialized microtubules that attach to kinetochores, which are protein structures located at the centromere of chromosomes. As the cell prepares for mitosis, the spindle apparatus begins to form and originates from the centrosomes located at opposite poles of the cell. Kinetochore microtubules extend from the centrosomes toward the kinetochores of each chromosome and create a connection that is essential for chromosome movement.

During metaphase, chromosomes align along the metaphase plate and the equatorial plane of the cell. This alignment is crucial because it ensures that each daughter cell will receive an identical set of chromosomes. Once the chromosomes are properly aligned, the kinetochore microtubules exert forces to pull the sister chromatids apart. This process occurs during anaphase when the kinetochore microtubules shorten and create tension on the kinetochores while separating the sister chromatids. The movement of sister chromatids toward opposite poles is essential for accurate chromosome segregation. The proper functioning of kinetochore microtubules is critical; any errors in this process can lead to aneuploidy, which is characterized by an abnormal number of chromosomes in the daughter cells and is often linked to diseases such as cancer.

02. Polar Microtubules

Polar microtubules extend from the spindle poles toward the center of the cell and do not directly attach to the kinetochores of chromosomes. Instead, these microtubules overlap with one another at the spindle midzone. Polar microtubules play a significant role in maintaining the structure and stability of the spindle apparatus. Their overlapping nature helps to stabilize the spindle and ensures that the forces exerted during chromosome movement are balanced.

During anaphase, polar microtubules undergo elongation which pushes the spindle poles apart. This elongation is essential for facilitating the further separation of the daughter chromosomes and helps ensure that both sets of chromosomes are correctly positioned within the dividing cell. The interaction between polar microtubules and kinetochore microtubules is vital for the overall mechanics of mitosis. By providing structural integrity and facilitating the movement of the spindle poles, polar microtubules are crucial for the successful and equal distribution of chromosomes into the daughter cells.

03. Astral Microtubules

Astral microtubules radiate outward from the spindle poles toward the cell membrane. Their primary role is to anchor the spindle apparatus to the cell cortex which helps maintain the correct orientation of the spindle within the cell. This positioning is crucial for ensuring that cell division occurs symmetrically and that each daughter cell receives the appropriate complement of genetic material.

During cell division, astral microtubules are also involved in guiding the positioning of the cleavage furrow which forms during cytokinesis and the final stage of mitosis. The cleavage furrow is the indentation that eventually separates the two daughter cells. Proper positioning of the spindle and cleavage furrow is essential for ensuring that the resulting daughter cells are of equal size and possess the correct genetic content.

The dynamic nature of astral microtubules allows them to rapidly grow and shrink, enabling the spindle apparatus to adjust its position and orientation as needed during mitosis. The correct functioning of astral microtubules is essential for guiding the orientation of the spindle during cell division and ensuring a successful and symmetrical cytokinesis process.







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SAQ

1 Fill in the blanks: 
a) The basic unit of microtubules is …………………

b) α tubulin occurs at.............end, and β tubulin is at ..............of microtubules

c) GTP-GTP tubulin cap stabilises and promotes the .......................

d) The plus end-directed motor protein of microtubules is ………….

e) The hydrolysis of GTP from β-tubulin causes …………. of microtubules.

f) The cellular function of γ-tubulins is. …………………….

g) Colchicine binds to ……………

Answers:
a) α β tubulin heterodimers
b) plus, minus
c) polymerisation of microtubules
d) kinesin
e) Depolymerisation 
f) nucleate the growth of MTs 
g) free tubulin

SAQ 2

SAQ 3 

TERMINAL QUESTIONS

6. Draw the labelled diagram of microtubules.




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