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Lecture 18

Lecture 18 - Chapter 17 - Mitosis - March 21.docx

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York University
BIOL 2021
Patricia Lakin- Thomas

March 21, 2013 Chapter 17- Mitosis Mitotic Spindle  Figure 17 -28  Intrapolar muctitubules o Antiparallel overlap to interact  Kinetochoremicrotubules attach sister chromatids to opposide poles  Astral: radiate outward from centrosomes to cell cortex Formation of the Mitotic Spindle  G1/S cdk  centrosome duplication o Beginning of S phase  Beginning of M-phase o M-cdk phosphorylates o MAPs, catastrophe factors  microtubules become shorter and more dynamic o M-cdk can other kinases phosphorylate centrosome, get more gamma tubulin ring complexes, more microtubules o M-cdk phosphorylates motor proteins  Centrosomes move apart  Motor Proteins in the Spindle o Figure 17-30 o Dyenin tries to walk to minus end, pulling microtubules to the cortex  Attach to actin at the cortex o Kinesin 5 , walking to the plus ends, has two heads pushing poles apart – ONE motor head o Kinesin 14 – push them together, walking to the minus ends o Note: Pushing and pulling happen at the same time for pure regulation, ensuring that the cell doesn’t get too big or too small o Kinesin 4 and 10 – plus end movement, carry chromosomes to + end, centre of the cell o Forming spindle before anaphase  Attachment of the Chromosomes o Attached at the kinetochore o Protein structure (heterochromatin) at the centromere (part of chromosome that has hereochromatin and where kinetochore is built and where chromosomes will pull apart to separate) o Fig 17-36 o 2 kinetochores per chromosome – one per chromatid o Microtubules capture chromosomes o Kinetochore microtubules connect to chromatids from opposite ploes  Form a metaphase plate  Anaphase! o Activation of APC (Anaphase promoting complex) o When this is activated - Fig 17-43** o Activation Pathway:  Cdc 20 activates APC  Cdc20 gene transcription  more cdc20 protein  Phosphorylate APC by M-cdk help Cdc20 bind  APC  Cause destruction of securin  Get active separase  Active separase cleaves cohesin – allows sister chromatins to separate  Normally, cohesin holds chormatids together at end of S-phase Fig 17- 24C  APC also targets all other S-cyclins and M-cyclins, marking them for destruction (cdks have been inactivate)  M-cdk causes its own inactivation and destruction – negative feedback  Phosphotases dephosphorylate cdk substrates  Summary on Moodle o Cell Cycle checkpoints – stop the cycle if something’s wrong (default is continue)  Notes: Often times the errors can be fixed  Spindle assembly checkpoints  Spindle attachment checkpoint monitor tension at kinetochore  Sends a negative signal to inhibit APC o Anaphase A and B  Figure 17-46
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