Explain the role of microtubules in cell division

Microtubules play a crucial role in cell division, particularly during mitosis and meiosis, where they form a structure called the mitotic spindle. This structure is responsible for segregating chromosomes into the daughter cells.

The three major kinds of microtubules occur during mitosis are:

01. Kinetochore Microtubules

These are the microtubules that connect to a special structure on the chromosomes called the kinetochore. The kinetochore forms at the centromere, which is the central part of a chromosome where the two sister chromatids (identical copies of the chromosome) are joined.

Role in Mitosis:
  • Kinetochore microtubules pull the sister chromatids apart during anaphase (the stage of mitosis where the chromatids separate). They ensure that each new cell gets the correct number of chromosomes.

02. Polar (or Interpolar) Microtubules

These microtubules stretch from one side of the cell to the other, connecting the two centrosomes (the structures that organize microtubules). They do not connect to the chromosomes directly.

Role in Mitosis:
  • Polar microtubules push against each other as they grow, which helps to separate the centrosomes and create tension in the mitotic spindle. This action is important for keeping the structure of the spindle stable and ensuring that the chromosomes are aligned correctly in the middle of the cell during metaphase.

03. Astral Microtubules

These microtubules extend out from the centrosomes toward the edges of the cell, forming a sort of star shape (hence the name "astral"). They extend from the centrosomes to the cell membrane, helping to position the spindle apparatus and define the axis of cell division.

Role in Mitosis:
  • Astral Microtubules help position the spindle apparatus (the structure made of microtubules that organizes the chromosomes) inside the cell. They connect with the cell's outer layer (called the cortex) and help pull the spindle poles apart. This action defines the direction in which the cell will divide, ensuring that division occurs symmetrically and that the daughter cells are of equal size.

Summary of Their Functions:
  1. Kinetochore Microtubules: Pull chromosomes apart.
  2. Polar Microtubules: Push the spindle poles apart and maintain the structure.
  3. Astral Microtubules: Position the spindle apparatus and anchor it to the cell.

Role of Microtubules in Cell Division

1. Formation of the Spindle Apparatus:

The spindle apparatus is crucial for organizing and separating chromosomes during mitosis. The process begins with the centrosomes, which are the primary microtubule-organizing centers in the cell. Before mitosis, each centrosome duplicates and migrates to opposite poles of the cell. From each centrosome, microtubules extend outward, forming the spindle structure. As they grow, they establish a bipolar arrangement, creating the framework that will support chromosome movement. This spindle apparatus consists of three types of microtubules: kinetochore microtubules, polar microtubules, and astral microtubules. The dynamic nature of microtubules allows them to rapidly assemble and reorganize, ensuring the spindle apparatus is properly formed.

2. Attachment to Chromosomes:

Once the spindle apparatus is established, microtubules attach to the chromosomes. Each chromosome, during the early stages of mitosis, develops a specialized structure called the kinetochore at its centromere, which is the region where sister chromatids are held together. Kinetochore microtubules extend from the spindle apparatus and attach to these kinetochores. This attachment is vital for the movement of chromosomes during mitosis. The interaction between microtubules and kinetochores is strong, with multiple microtubules connecting to each kinetochore, providing a stable link for chromosome manipulation.

3. Alignment of Chromosomes:

During metaphase, the chromosomes must be aligned along the cell's equatorial plane, known as the metaphase plate. The spindle apparatus facilitates this alignment by exerting forces on the attached chromosomes through the kinetochore microtubules. The alignment is critical because it ensures that, during the next stage of mitosis, the sister chromatids will be separated evenly. The tension created by the attachment of microtubules to kinetochores helps stabilize the chromosomes in their aligned position. If any chromosome is improperly aligned, the cell has mechanisms in place to detect this and make adjustments to ensure proper alignment before proceeding to the next phase.

4. Segregation of Chromosomes:

Once the chromosomes are properly aligned, the cell transitions to anaphase, where sister chromatids are separated. Kinetochore microtubules begin to shorten, pulling the sister chromatids apart toward opposite poles of the cell. This process is driven by the depolymerization of microtubules at the kinetochore end, effectively "reeling in" the chromatids. Meanwhile, polar microtubules, which do not attach to kinetochores, elongate and push against each other, helping to further separate the spindle poles. This coordinated action ensures that each daughter cell receives an identical set of chromosomes.

5. Cytokinesis:

The final step in cell division is cytokinesis, where the cytoplasm divides to form two separate daughter cells. Microtubules play a role in organizing the contractile ring that is primarily composed of actin filaments and myosin proteins. The contractile ring forms at the cell's equator and as it contracts, it pinches the cell membrane, leading to the physical separation of the two daughter cells. Microtubules help position this contractile ring and facilitate the interaction between actin and myosin because they ensure that division is completed efficiently.



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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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