BIOLOGY 2B03 Lecture Notes - Lecture 20: Spindle Apparatus, Sister Chromatids, G1 Phase
13 Jun 2018
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Cell Cycle Regulation: Cyclin-Dependent Kinases and E3 Ligases
Cell Cycle Phases
cycle divided into 4 events: G1, S-phase, G2, mitosis •
during G1, cell actively growing and engaged in gene •
expression/synthesis of proteins
during S-phase, cell replicates its genome - now a single •
chromosome contains 2 identical sister chromatids
during G2 phase, the cell prepares to enter mitosis •
cell cycle shown here is for proliferating stem cells - most cells •
leave this cycle and enter a quiescent state where division does
not occur
this is called the G0 stage ◦
G0 Stage
cells exit cell cycle and enter G0 •
cells can either go through G stages and mitosis to produce daughter cells or cease division and enter G0 •
can stay here for short period before entering cell cycle again ◦
cells can then begin process of differentiating into •
specific type of cell (neuron, RBC, muscle cell,
etc.)
can't divide any longer once it does this ◦
balance between division/G0 important because •
if all stem cells enter G0, then body can't
regenerate lost tissues
if divide too much without differentiating, ◦
body forms tumour
M Phase
M phase further divided into phases: •
interphase (G1-S-G2) - cell prepares for mitosis. Chromosomes replicated in S-phase and centrosomes ◦
duplicated in G1/S
prophase - chromosomes condense and mitotic spindles assemble as duplicated centrosomes separate to ◦
opposite sides of cell. Nuclear envelope dissolution and endomembranes breakdown
prometaphase - chromosomes fully condensed and attach to centrosomes. Kinetochores assemble at ◦
centromeres to mediate association with plus-ends of spindle microtubules
metaphase - spindle microtubules attach to every chromosome (bipolar attachment). Chromosomes ◦
aggregate in middle of mitotic spindle from pulling towards pole
anaphase - sister chromatids pulled to opposite poles of spindles after bipolar attachment ◦
telophase - after sister chromatid separation, cell reverses all changes from prophase. Chromosomes ◦
decondense and spindle disassembles and nuclear envelope and endomembrane systems reassemble
cytokinesis - two cells are separated with exact replicate of nucleus - cell membranes pinched off between ◦
both cells
Regulation of Cell Cycle
sequence of mitosis events must occur in order - ensured by two proteins: •
Cyclin-dependent kinases (CDKs): heterodimeric protein complexes - its kinase activity is regulated with ◦
cyclin. An activated kinase will initiate many cell processes via phosphorylation of target proteins
E3 ubiquitin ligases: required to regulate cell cycle events by targeting specific proteins for degradation in ◦
proteasome - cyclins can be degraded to turn off kinases of cell cycle inhibitors are degraded when
checkpoints passed in cycle
Cyclin-CDK Kinases
four major classes of cyclin-CDK kinases activated in different phases of cycle: •
G1 cyclin-CDK ◦
G1/S phase cyclin-CDK ◦
S phase cyclin-CDK ◦
mitotic cyclin-CDK ◦
all have same structure and activity but differ in •
their target proteins and timing
name of CDK leads to activation of next ◦
phase
i.e. G1 cyclin-CDK active in G1 and ‣
leads to activation of S phase
phosphorylate a collection of proteins needed for •
cellular changes occurring in prophase
E3 Ubiquitin Ligases
three different E3-ligase complexes that regulate •
different steps in cycle:
SCF complex: releases cell from G1 and ◦
allows transition into S-phase
Anaphase Promoting Complex (APC): has ◦
different target proteins depending on association with alternate accessory proteins called CSC20 and Cdh1
APC-Cdc20 regulates transition from metaphase into anaphase ‣
APC-Cdh1 mediates exit from mitosis ‣
G1 Cyclin-CDK and SCF E3 Ligase
targets of phosphorylation •
G1-cyclin-CDK major targets: ◦
phosphorylating APC-Cdh1, signals end of mitosis ‣
targets transcription factors for phosphorylation which activates them and leads to expression of S-‣
phase proteins such as nucleotide syntheses, replication factors and DNAp
phosphorylate S-phase inhibitors which bind to and prevent activation of S-phase Cyclin-CDK ‣
phosphorylation of inhibitor makes it target for ubiquitination and degradation •
regulated degradation •
SCF ◦
S-phase inhibitors is target for ubiquitination by SCF - allows S-phase to begin ‣
G1/S and S-phase Cyclin-CDK Complex
targets of phosphorylation •
G1/S-phase cyclin-CDK ◦
targets transcription factors that regulate expression of genes coding for mitosis ‣
also targets proteins for centrosome replication ‣
S-phase cyclin-CDK ◦
necessary for activation/assembly of pre-replication complex at sites of origins of replication ‣
phosphorylation at origins of replication ensure they only 'fire' once per cell cycle ‣
must only be one replication complex per origin - more than one results in too many copies of ‣
segments of DNA molecule
phosphorylation of M-phase CDK inhibits activation until cell prepared to enter mitosis •
M-Phase Cyclin-CDK Complex
targets of phosphorylation •
M-phase cyclin targets: ◦
Document Summary
Cell cycle regulation: cyclin-dependent kinases and e3 ligases. M phase further divided into phases: interphase (g1-s-g2) - cell prepares for mitosis. Chromosomes replicated in s-phase and centrosomes duplicated in g1/s prophase - chromosomes condense and mitotic spindles assemble as duplicated centrosomes separate to opposite sides of cell. Nuclear envelope dissolution and endomembranes breakdown prometaphase - chromosomes fully condensed and attach to centrosomes. Kinetochores assemble at centromeres to mediate association with plus-ends of spindle microtubules metaphase - spindle microtubules attach to every chromosome (bipolar attachment). Chromosomes aggregate in middle of mitotic spindle from pulling towards pole anaphase - sister chromatids pulled to opposite poles of spindles after bipolar attachment telophase - after sister chromatid separation, cell reverses all changes from prophase. Chromosomes decondense and spindle disassembles and nuclear envelope and endomembrane systems reassemble cytokinesis - two cells are separated with exact replicate of nucleus - cell membranes pinched off between both cells.
