Mitotic spindle

Mitotic spindle, cellular and molecular biology, microtubules

1. MITOTIC SPINDLE Seminar - 01 Presented by: Jaineel Dharod M.Pharm Sem-1

2. Cell Centrosomes Microtubule

3. INTRODUCTION The spindle apparatus (or mitotic spindle ) refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell.

4. Search and capture model In 1986, Marc Kirschner and Tim Mitchison proposed that microtubules use their dynamic properties of growth and shrinkage at their plus ends to probe the three dimensional space of the cell. Plus ends that encounter kinetochores or sites of polarity become captured and no longer display growth or shrinkage. In contrast to normal dynamic microtubules, which have a half-life of 5–10 minutes, the captured microtubules can last for hours.

5. Self organization model Self-organization of molecular components provides a variety of possible spatial structures . This model proposes that microtubules are nucleated acentrosomally near chromosomes and spontaneously assemble into anti-parallel bundles and adopt a spindle-like structure

6. Characteristics

7. Astral microtubules. Polar microtubules. Kinetochore microtubules. TYPES & FUNCTION OF MICROTUBULES

8. Astral Microtubules Astral microtubules develop in the actin skeleton and interact with the cell cortex to aid in spindle orientation . They are organized into radial arrays around the centrosomes. The turn-over rate of this population of microtubules is higher than any other population. The role of astral microtubules is assisted by dyneins specific to this role. These dyneins have their light chains (static portion) attached to the cell membrane , and their globular parts (dynamic portions) attached to the microtubules . The globular chains attempt to move towards the centrosome, but as they are bound to the cell membrane, this results in pulling the centrosomes towards the membrane , thus assisting cytokinesis.

9. Polar microtubules interdigitate at the spindle midzone and push the spindle poles apart via motor proteins. POLAR Microtubules kinesin

10. KINETOCHORE Microtubules Kinetochore microtubules directly connect to the kinetochores. Each chromosome has two chromatids, and each chromatid has a kinetochore. The two kinetochores associated with a region of the chromosome called the centromere.

11. REGULATION OF SPINDLE ASSEMBLY Spindle assembly is largely regulated by phosphorylation events catalyzed by mitotic kinases . Cyclin dependent kinase complexes (CDKs) are activated by mitotic cyclins, whose translation increases during mitosis. CDK1 (also called CDC2 ) is considered the main mitotic kinase in mammalian cells and is activated by Cyclin B1 . Aurora kinases are required for proper spindle assembly and separation. Aurora A associates with centrosomes and is believed to regulate mitotic entry . Aurora B is a member of the chromosomal passenger complex and mediates chromosome- microtubule attachment and sister chromatid cohesion . Polo-like kinase, also known as PLK , especially PLK1 has important roles in the spindle maintenance by regulating microtubule dynamics.

12. REFERENCES Principles of anatomy and physiology by Gerard J Tortora/ Bryan Derrickson, pg-92 https:// www.nature.com/scitable/content/types-of-microtubules-involved-in-mitosis-14752887 https://en.wikipedia.org/wiki/Microtubule https:// en.wikipedia.org/wiki/Spindle_apparatus https://en.wikipedia.org/wiki/Tubulin#Eukaryotic