BIOL2021 - Chapter 17 - Cell cycle

What is the correct order of mitosis?

1. Prophase

2. Prometaphase

3. Metaphase

4. Anaphase

5. Telophase

What happens in prophase of the cell cycle?

• Replicated chromosomes condense into sister chromatids

• Mitotic spindles assemble between centrosomes

What happens in prometaphase of the cell cycle?

• Nuclear envelope breaks down

• Chromosomes attach to spindles at their kinetochores

What happens in metaphase of the cell cycle?

• Chromosomes align at the equator

• Kinetochores attach chromatids to opposite poles of the spindle

What happens in anaphase of the cell cycle?

• Sister chromatids separate towards poles and form daughter chromosomes

• Kinetochore microtubules get shorter

What happens in telophase of the cell cycle?

• Daughter chromosomes arrive at poles and decondense

• Nuclear envelope reassembles

• contraction ring forms

What happens during cytokinesis in the cell cycle?

• Cytoplasm is divided into two by the contractile ring

• Cell divides into 2 daughter cells

Which does a cell spend more time doing; growing and increasing protein mass and organelles, or undergoing cell division?

Growing and increasing protein mass and organelles

When does cell growth occur?

During interphase

What happens during S phase?

• Chromosome (DNA) replication

• Replication DNA held together as sister chromatids

• Centrosome replication in animal cells

What happens during M phase (M for mitosis)?

• Mitosis

• Cytokinesis

What are the 4 phases of the eukaryotic cell cycle?

1. G1 (gap 1) phase

2. S phase

3. G2 (gap 2) phase

4. M phase

What happens during gap phases (G1 and G2)?

• Growth

• creating proteins and organelles required for next stage

• Monitoring readiness

Which phases are included in Interphase?

• G1

• S

• G2

What phase does a cell enter of conditions/signals are not suitable for cell division?

• G0 phase/quiescence (not dividing or preparing to divide)

What is a post differentiated state?

• A state a cell can be in

• a G0 phase where the cell cannot re-enter the cell cycle

What are senescence cells?

• Type of G0 cell

• Will not re-enter the cell cycle

• Caused by stress

What system triggers the major events of the cell cycle?

The cell-cycle control system

When and what are the main checkpoints in the cell cycle control system

• G2/M transition; all DNA is replicated and favorable environment

• Begin Anaphase and cytokinesis; Are chromosomes attached to spindle

• Start cell cycle/S phase; Favorable environment

What are the “master regulators” of the cell-cycle control system?

Cyclin-dependent kinases (Cdks)

What are Cdks regulated by?

• Cyclins (major regulators)

• Phosphorylation (G2/M transitions)

How do Cdk protein levels vary through the cell cycle?

They stay constant

How do cyclin protein levels vary during the cell cycle (in general)?

Rise and fall by synthesis and degration

What are the 3 main classes of cyclins? What do they form?

• G1/S-cyclins; G1/S-Cdk complex

• S-cyclin; S-Cdk complex

• M-Cdk; M-Cdk complex

• G1-cyclin;

How do G1-cyclin levels vary in the cell cycle? What do they control?

• Rise gradually during G1 phase and fall during M phase

• Regulate G1/S-Cdk activity

True/False; All eukaryoticncell have G1/S cyclin, S-cyclin, M-cyclin, and G1-cyclin

False; only most have G1-cyclin

How do cyclins work?

Bind and activate kinases that phosphorylate proteins related to cell cycle progress

What do G1/S-cyclin do? How does its concentration vary in the cell cycle?

• Forms G1/S-Cdk complex

• Activates Cdk in late G1 and helps trigger progression through the start

• Help commit to cell-cycle entry

• Levels rise before the start in G1 phase, and fall in S phase

What do S-cyclin do? How does its concentration vary in the cell cycle?

• Forms S-Cdk complex

• Binds Cdk soon after progression through the start of the cell cycle

• Helps stimulate chromosome duplication (S phase) and some early mitosis activities

• Levels rise after the start in G1 phase and remain elevated until mitosis

• Contributes to the control of some early mitotic events

What do M-cyclins do? How does its concentration vary in the cell cycle?

• Forms M-Cdk complex

• Activates Cdk that stimulate entry into mitosis at G/M transition

• Levels rise in G2 phase and fall mid mitosis (just before metaphase-anaphase transition)

What is CAK and what does it do?

• Cdk-activating kinase (CAK)

• Phosphorylates Cdk active site

• Induces full activity

What part of a Cdk does cyclin bind/attach to?

• The T-loop

What is required for Cdk to be fully active?

• Cyclin binding

• Phosphorylation by CAK

What does Wee1 do to a Cdk complex?

• Adds 2 inhibitory phosphatases to the roof of its active site

• Inactivates it

What phosphatase removed inhibitory phosphates from a Cdk complex?

• Cdc25 phosphatase

What are CKIs and what do they do?

• Cdk inhibitor proteins

• suppressed Cdk activity

What is p27 and what does it do?

• a CKI

• Binds to both Cdk and cyclin

• Changes conformation of its active site

What is the Met-Ana transition triggered by?

Proteolysis (protein degradation) by Anaphase-promoting complex or cyclosome (APC/C)

What are Anaphase-promoting complex and Cyclosome (APC/C) and what do they do?

• Ubiquitin ligase

• Initiates anaphase and destruction of M and S cyclins

• Remains active into G1

What controls levels of cyclin?

Transcriptional regulators

What Cdk activity happens during the start of the cell cycle in G1?

• G1-Cdk stimulates transcription of G1/S and S-cyclin genes

• G1/S-Cdk activity inactivates APC/C (phosphorylation) and CKIs that inhibit S-Cdk, initiates centriole duplication in animal cells at the start of S phase

• S-Cdk activity rises

What Cdks and stage of the cell cycle is blocked by DNA damage?

• G1/S-Cdk and S-Cdk are inhibited, which blocks the start (in G1)

• M-Cdk is inhibted,which blocks the transition into M

What is DNA replication and Histone synthesis initiated by and how?

S-Cdk; phosphorylates proteins that activate helicase and recruit DNA synthesis machinery, which increases histone synthesis

How many times does S-Cdk initiate DNA replication in the cell cycle?

1

What does DNA replication start with?

DNA helicase; Unwinds double-stranded DNA

How is it ensured that chromosome duplication only occurs once per cycle?

Initiation phase of DNA replication is divided into 2 distinct steps

What are prereplicative complexes?

A pair of inactive DNA helicases loaded on the replication origins from late mitosis to early G1

What happens when DNA helicases are activated by S-Cdk?

• DNA winding

• Initiation of DNA synthesis

What do DNA helicases move out of the replication origins with? How does the ensure replication only happens once?

They move with the replication forks, meaning the origins cannot be reused until new replicative complexes form at the end of mitosis

What are sister chromatids are held together by?

Cohesin

What is cohesin composed of?

• two Smc molecules (Smc3 and Smc1) held together at their hinge

• An Scc1 bound at the ATPase domain of the Smc, with an Scc3 bound to it

When does M-Cdk become active?

At the G2/M transition

What fully activates M-Cdk at the onset of mitosis?

Dephosphorylation of the inhibitory phosphate by Cdc25

What Cdk and stage does unreplicated DNA inhibit?

Inhibited M-Cdk, which blocks the transition into M

What early mitotic events does M-Cdk activate?

• Chromosome condensation

• Nuclear envelope breakdown

• Separation of centrosomes and setting up mitotic spindle

How does M-Cdk help chromosome condensation and nuclear envelope breakdown?

1. Phosphorylates condensins

2. Phosphorylates nuclear pore complex (NPC) subunits, causing disassembly

3. Phosphorylates nuclear Lamins, causing disassembly

How does M-Cdk drive formation of the mitotic spindle?

By phosphorylating:

• Microtubule associated proteins and other proteins to create shorter, more dynamic microtubules

• Centrosome proteins; more gamma-tubulin complexes, more microtubule polymerization

• Motor proteins; centrosomes move apart, spindle sets up

What is the bipolar spindle set up by?

• Microtubules

• Motor proteins

What happens during the formation of the mitotic spindle?

• Kinetochore microtubules + ends attach to sister chromatids

• Astral microtubules radiate out from spindle pole and contact the cell cortex (actin filaments under plasma membrane)

• Interpolar microtubules interdigitate at the equator and cross link with other microtubules

When does spindle assembly begin?

Early mitosis

What do kinesin-5 motors do and what are they activated by?

• Phosphorylated by M-Cdk and other protein kinases

• Drive centrosome separation

What do microtubule-dependent motor proteins govern?

Spindle assembly and function

What is Kinesin-5 and what does it do?

• Motor protein with 2 motor domains that move toward + end

• Pushes poles of the cell apart

What is Kinesin-14 and what does it do?

• Single domain motor protein

• Interacts with antiparallel microtubules

• Pulls poles together by moving towards - end

What are Kinesin-4 and Kinesin-10 and what do they do?

• Type of chromokinesins

• Move towards + end

• Associated with chromosome

• Push chromosome away from pole

What role does Dynein have in spindle assembly and function?

• Link astral microtubules with actin cytoskeleton at the cell cortex

• Pulls spindle poles towards cell cortex and away from each other, by moving towards - end

What does APC/C trigger?

• Sister chromatid separation (anaphse)

• Completion of mitosis

What is APC/C activated by?

M-Cdk or Cdc20

What does APC/C activation by Cdc20 trigger?

• Destruction (ubiquitination) of securin (releases separase) and cyclins

• Leads to sister chromatid separation and completion of mitosis

What molecules does APC/C ubiquitinate?

• Securin

• S-cyclins

• M-cyclins

What is the key regulator of the M-A transition?

APC/C

Which of the following is most likely to be inhibited at the M-A checkpoint if chromosomes are improperly attached?

APC/C complex

What would be likely to happen during mitosis to a mammalian cell that would normally be dividing if you injected it with high levels of a mutant securin protein that could not be ubiquitlated, keeping levels of active securin high at all times?

The chromosomes would not be able to separate in anaphase

What does Cdk inactivation allow for, before the completion of mitosis?

Allows phosphatases to dephosphorylated

What happens during cytokinesis?

• Rho-A GEF activates Rho-A at cell cortex

This activates

• formin; stimulates actin filament polymerization

• Rho-activated kinases, which activate myosin II

What allows for cells to escape from G1 or G0?

External signals

• Unicellular organism; Nutrient

• Multicellular organism; Mitogen

What is a mitogen?

An external signal that can allow cells to escape from G1 or G0, to stimulate cell division, in multicellular organism

What stimulates cells to grow?

growth factors

What signals cells to survive/suppress apoptosis?

Survival factors

What happens in the Mitogen pathway?

1. Mitogen interacts with a mitogen receptor

2. Mitogen receptor interacts with Ras

3. Ras interacts with MAP kinase

4. Map kinase causes activation of transcription regulatory protein

5. Causes gene expression

6. Transcription regulatory protein is synthesized

What does Myc activate?

Transcription of G1-cyclin, which activates G1-Cdk, leading to transcription of other genes involved in cell growth and cell division, including G1/S cyclin.

What are E2Fs?

• Gene regulatory proteins

• Bind promotors to stimulate transcription of G1/S cyclin, S-cyclin, DNA synthesis proteins, and other proteins required for S-phase

What are E2F proteins regulated by?

Rb proteins, which are inactivated by phosphorylation by G1/Cdk

Why do activated APC/C and CKI levels need to be regulated? What are they regulated by?

• APC/C will ubiquitylate S-cyclin

• CKI will inhibit Cdks

• Both are phosphorylated by G1/S Cdk

What happens if problems are detected at checkpoints in the cell cycle control system?

• Transition is delayed

• Inhibitory mechanisms act on cyclin complexes

What happens of DNA damage is detected by protein kinases?

• p53 is phosphorylated, removing Mdm2

• active p53 bids to regulatory region of p21 gene, transcribing it

• CKI is produced

What happens to Mdm2 and p53 in cells with undamaged DNA?

• Mdm2 tags p53

• p53 is ubiquitinated and degraded by proteasomes

What is p21?

A CKI (Cyclin-dependent kinase inhibitor proteins) for G1/S-Cdk and SdK

Why are p53 mutations commonly found in cancers?

• p53 stops the cell cycle until DNA damage is repaired

• p53 failure means that damaged DNA will be replicated

• extending damaged DNA into other cells increases the number of mutated cells and can lead to cancer

How many cancers is p53 mutations found in?

Half of all cancers

True/False; Many pathways stimulate cell division and cell growth

True