Cell Cycle

M phase 2

Nucleus and cytoplasm divide

(mitosis and cytokinesis)

Interphase 4

Period between cell division (metabolic activity, growth repair)

• G1 phase

• s phase (dna replication)

• g2 phase

Many mature cells…

Do not divide (nerve, muscle, red blood cells)

Some cells only…

divide given a stimulis (Liver cells when damaged)

Which cells continously divide

hematopoietic and epithelial stem cells

Cells in G0

do not divide, and only stay metabolically active (mature)

Cell cycle control system

Delays later cell events until earlier events are completed

Start transition

G1 to S phase (is environment favourable for DNA replication - nutrients, signal molecules)

G2/M transition

Is all DNA replicated, is all dna damage repaired

Meta to Ana transition

Are all chromosomes attached properly to mitotic spindle

Molecular switches

Mechanism in cell cycle that controls progression

Kinases, cyclin cdk complex

Prophase

replicated chromosomes condense, and mitotic spindle assembly starts and requires duplicated centrosomes

Chromosome condensation occurs…

in prophase as well as sister chromatid (remember cohesins → condensins)

Microtubules during mitosis

Reassembled during prophase - requires duplication of centrosomes

Centrosome structure 3

Organized at right angles to each other

9 fibrils split into triplets

Contains y-tubulin ring complexes

Centrosome duplication is initiated and completed… they form..

In G1 and G2 - form poles of mitotic spindle

Mitotic spindle assembly starts in

prophase

Nuclear envelope breakdown occurs at…

boundary between prophase and prometaphase

Nuclear envelope breakdown is caused by…

phosphorylation of lamins and nuclear pore proteins - triggers disassembly into small membrane vesicles

Prometaphase key characteristics

• nuclear envelope is disassembled

• Mitotic spindle assembly can be completed

• Kinetochore microtubules attach to duplicated chromosomes

• chromosome movement begins

3 mitotic spindle parts

1. astral microtubules - help position mitotic spindle

2. non-kinetochore MT - cross linked - help stretch cell

3. kinetochore MT - attach to sister chromatid and pull

Kinetochore - chromatid pairing

1 kinetochore per chromatid

chromatid needs microtubules from both sides for equal tension to align at equator

metaphase

all chromosomes align at metaphase plate (equator)

To maintain metaphase spindle there is…

continuous addition of tubulin dimers (at plus end) and removal at minus end - no length change due to treadmilling effect

Anaphase

-Sister chromatid SEPARATED (by separase)

-cohesin complex cleaved

Anaphase a vs b

A - kinetochore MT shortened to pull (loses tubulin at both ends)

B - Spindle poles move outward (Pushed by non-kinetochore MT)

Proteins in anaphase B

Kinesins (sliding force) and dyneins (Pull by anchoring at cell membrane

Telophase (4)

• Chromosomes now separate

• Nuclear envelope reassembles (lamina, pores, dephosphorylation)

• Mitotic spindles gone

• Chromosomes decondense

Cytokinesis in animals

Contracticle ring (actin filament) assembled that squeezes to create cleavage at midpoint

Cell division in plants differences

No centrosome (different mechanism)

Cytokinesis very different due to cell wall

Cytokinesis in plants

Phragmoplasts form in telophase that coordinate formation of vesicles at cell plate that become new cell wall - becomes wall in G1

HOw were cyclin dependent protein kinases discovered

Yoshio masui injected cytoplasm from fertilized eggs into oocytes (contents of cytoplasm could induce maturation from prophase 1 to metaphase 2 due to presence of certain promotor factors)