Cell bio exam 3

CYTOSKELETON

Direct movement of things inside the cell

Muscle contraction

Cell division

Cytoplasmic streaming

Provide structure and shape to the cell

Allow the cell to move the membrane

THREE TYPES OF CYTOSKELETON

Intermediate filaments (cytoplasm and nucleus)

Microtubules (radiate from a primary location to reach many part of the cells)

Actin (microfilaments)(throughout the cells but concentrated along cell periphery and in processes)

Intermediate

INTERMEDIATE FILAMENTS FORM LONG ROPE LIKE STRANDS OF STACKED TWISTED PROTEIN

INTERMEDIATE FILAMENTS PROTECT FROM MECHANICAL STRESS Nucleus is surrounded and supported by the nuclear lamina

Intermediate filaments called lamin for lamina

ACTIN ASSEMBLY IS DRIVEN BY TREAD MILLING

Monomers assemble into a strand

ATP bound actin binds to growing strand actin hydrolyzes ATP to ADP reducing binding affinity for other actin high monomer concentration drives assembly at both ends ADP bound actin disassembles

MANY CELL SHAPES ARE DRIVEN BY ACTIN

ACTIN IS USED TO SUPPORT CELL MOVEMENT

Actin assembly pushes the cell membrane into long protrusions

Lamellipodia

Actin filament formation drives the membrane forward by interaction proteins on the membrane

Actin filaments interact with adhesion molecules to attach membrane to substrate

MICROTUBULES

MICROTUBULES Have directionality plus and minus end

Tubulin isoforms assemble to form filament

alpha and beta dimers form filament

gamma forms stable base

MICROTUBULES ASSEMBLY IS GOVERNED BY DYNAMIC INSTABILITY

GTP bound alpha/beta dimers assemble onto growing + end of microtubules

alpha/beta dimers naturally hydrolyze GTP to GDP

GDP bound alpha/beta dimers interaction is not as strong as GTP bound dimers

Microtubules collapse is called catastrophe

CYTOSKELETAL ELEMENTS INTERACT WITH EACH OTHER

HOLDING THE NUCLEUS IN PLACE

CELL DIVISION

Microtubules = mitotic spindle

Actin + myosin =

Physically split the cell

Form contractile ring

Tighten like a belt

ACTIN + MICROTUBULES IN CELL POLARITY

Microtubules:

Deliver materials (vesicles)

Define direction

Actin:

Forms cortex (outer structure)

Controls shape + tension

Adhesion proteins:

Anchor cells to each other

Cell division feet

feet

ACTIN AND MICROTUBULES INTERACT IN CELL POLARITY feet

ACTIN AND MICROTUBULES INTERACT IN CELL POLARITY feeet

PROTEINS CAN STABILIZE MICROTUBULES TO PREVENT CATASTROPHE

And Actin FIbers

Stabile microtubules are important in transport and cell division

Microtubules are rigid, hollow protein rods, roughly 25 nm in diameter, that form a key component of the eukaryotic cytoskeleton

Tau stabilizes microtubules

over phosphorylation can lead to neurodegeneration

Other molecules will cleave microtubules

These interactions can be facilitated or inhibited by post translation modification of tubulin dimer

Tubulin is building block of Microtubules

ACTIN FIBERS CAN TAKE MANY SHAPES

Actin filaments are thin and flexible

Important in cell movement and muscle contraction Lots of proteins regulate actin structure and polymerization

ROLE OF ACTIN MODIFYING PROTEINS IN CELL MOVEMENT

Cofilin cuts old filaments → creates new ends (“barbed ends”) + ends

ATP- F actin adds to those ends → filament grows

Arp2/3 makes branches → creates a web network

This pushes the membrane forward (protrusion)

Meanwhile, old filaments behind get broken down

CELL SIGNALING CAN ALTER ACTIN STRUCTURE

Structure is determined by interaction with regulatory proteins

What interactions occur are determined by cell signaling pathways

DRUGS AFFECT FILAMENTS

Binds and stabilizes microtubules

Binds tubulin dimers and prevents their polymerization

TABLE 17–2 DRUGS THAT AFFECT FILAMENTS

Actin-specific Drugs — Action

Binds and stabilizes filaments

Caps filament plus ends, preventing polymerization there

Binds actin monomers and prevents their polymerization

HOW DOES PACLITAXEL WORK TO TREAT CANCER?

Paclitaxel (Taxol) works by freezing microtubules in place

Normally: microtubules constantly grow and shrink

This is required for mitosis (cell division)

During anaphase, microtubules depolymerize, acting as the pulling force to move chromosomes to opposite poles.

Paclitaxel:

stabilizes microtubules (they can’t break down)

spindle fibers get stuck

Result: Cells get stuck in mitosis → can’t divide → die

MOTOR PROTEINS CAN CARRY CARGO IN TWO DIRECTIONS ALONG MICROTUBULES

Microtubules are like train tracks

Motor proteins are like delivery trucks

Kinesin → goes to + end (usually toward cell edge)

Dynein → goes to – end (usually toward nucleus)

Globular Head = walks on microtubule (uses ATP)

Tail = holds cargo

Adaptor protein = connects cargo to motor protein

ATP HYDROLYSIS AND BINDING ALTERS PROTEIN SHAPE Hydrolyzing ATP provide energy to lift leg

P binds to other leg to lock it

Energy generated from ATP hydrolysis makes reversal energetically unlikely

Binding of ATP swings leg forward.

Compare to the BCR-ABL arm that swings in and out without ATP - low energy needed for shift

THIS IS HOW THE MOTOR PROTEINS " walk"

walking Feet

feet

MOTORS PROTEINS,

MICROTUBULES ARE USED TO TRANSPORT CARGO AROUND THE CELL

Dynein and Kinesin are ATP binding motor proteins

Dynein moves to the minus end

Kinesin moves to the plus end

Assembly and stabilization of microtubules drives movement of chromosomes captured by the ends of microtubules

MICROTUBULES ARE USED TO TRANSPORT CARGO AROUND THE CELL

Microtubules are laid out like train tracks

Protein filled vesicles travel along microtubules

MICROTUBULES ARE USED TO TRANSPORT CARGO VESICLES AROUND THE CELL using these proteins

Vesicle movement feet

feet

MYOSIN IS A MOTOR PROTEIN FOR ACTIN

Myosin-I is a simple motor protein with 1 head

Move vesicles along a fixed actin filament

Myosin can be fixed and move an actin filament

toward the + end of actin

MYOSIN-II HAS TWO HEADS AND CAN FORM FILAMENTS

2 heads (instead of 1)

Forms thick filaments

Used for muscle contraction

FIXED MYOSIN-II MOVES ACTIN FILAMENTS

Sliding filament model

Myosin is fixed in place

Actin filaments slide past it

Myosin pulls actin toward the center

MYOSIN AND ACTIN FORM UNITS CALLED THE SARCOMERE

The sarcomere is a contractile unit

Myosin remains in place and actin is pulled together

Cellular contraction is the Z-discs pulling together

Look at diagram

THis is muscle contraction

myosin II

feet

Sarcomere feet

feet

MUSCLE CELLS CONTAIN MANY MYOFIBERS

MUSCLE CELLS CONTAIN MANY MYOFIBERS

Skeletal muscle cells are large multinucleate cells

Form by fusion of smaller cells

Myofibers are many sarcomeres laid end to end

Connection occurs at the Z-disc

MYOSIN AND ACTIN DYNAMICS

MYOSIN AND ACTIN DYNAMICS

ATP binding releases the myosin from actin

ATP hydrolysis shifts the myosin arm into a new conformation

ADP bound myosin binds to actin

Dissociation of ADP from myosin leads to the "power stroke" where myosin moves actin along

next step feet

feet

Step 3

feet

4 feet

feet

MYOSIN BINDING IS FACILITATED BY CALCIUM

Tropmyosin blocks myosin binding sites during low cytosolic calcium

Myosin binding sites are exposed with high cytosolic calcium levels (mediated by calcium )

CALCIUM IN THE CELL

Calcium levels in the cytosol are VERY low Small changes in Ca+ levels make a big difference

even a tiny increase = HUGE signal

It acts like a trigger molecule

CALICUM IN MUSCLE CONTRACTION

1. Ca2+ is released from Sarcoplasmic reticulum

2. Ca2+ activates myosin/ troposmyosin contractile mechanism . Ca²⁺ binds troponin Tropomyosin moves OUT

Myosin can bind actin

3. ATP is used to quickly remove Ca2+ from cytosol

Contraction stops → muscle relaxes

CALICUM IN MUSCLE CONTRACTION feet

feet

CILIA

Organelle made of membrane, cytosekelton, and motor proteins

9+2 arrangement :

9 outer doublets

2 central microtubules

Has dynein arms → movement - motile

9+0 arrangement - non-motile

Used for motion, environmental sensing, and other

No central pair

No proper dynein-driven motion

Mostly sensory (like antenna)

A cilium = tiny hair made of microtubules + motor proteins

MOVEMENT

Unattached microtubles will slide

Attached (has linking proteins) and microtubles will bend

Movement feet

feet

MOVEMENT

Dynein causes microtubules to flex

Dynein walks → causes sliding

One microtubule tries to slide past the other

If nothing stopped it → everything would just slide apart

But they are CONNECTED (links)

So instead of sliding freely...

Sliding gets converted into BENDING

Flagellum

Spiral / propeller motion

Example: sperm

Cilia

Back-and-forth beating

Like rowing

Same structure (microtubules + dynein) BUT different coordination patterns

TRANSPORT

This is how cilia are built and maintained

Cilia can’t make proteins inside themselves

So they need a delivery system

ANTEROGRADE TRANSPORT (→)

Base → Tip (outward) Motor: Kinesin-2

BUILDS the cilium

Carries:

Tubulin (microtubule building blocks)

Structural proteins

Membrane proteins

RETROGRADE TRANSPORT (←)

Tip → Base (inward) Motor: Dynein 2

CLEANS + RECYCLES

Carries:

Broken proteins

Waste

Used parts

CASE SUMMARY

Case Summary

Julia (31) & Robert (32) trying to conceive for 22 months

Regular menstrual cycle (29 days)

Prior pregnancy = ectopic → left fallopian tube removed

Robert: normal erectile function, no testicular injury

Robert has asthma

Female ovulation = normal

Male = likely normal

BIG RED FLAGS:

Ectopic pregnancy

Embryo implanted outside uterus (usually fallopian tube)

Required removal of left fallopian tube

Fertilization happened BUT embryo got stuck in tube

Something wrong with transport

Missing fallopian tube

Microtubules Microfilaments And intermediate filaments

Microtubules

Structure:

9+2 in cilia

Function:

Cilia/flagella movement

Vesicle transport

Mitotic spindle

Fertility:

Move sperm (flagella)

Move egg/embryo (cilia)

Defect → infertility

Microfilaments (Actin)

Structure:

Thin actin filaments

Polar

Function:

Cell movement

Muscle contraction

Vesicle transport (myosin)

Fertility:

Acrosome reaction

Sperm function

Reproductive tract contraction

Intermediate Filaments

Structure:

Rope-like fibers

Non-polar

Function:

Structural support

Resist stress

Fertility:

Minor role

Tissue integrity

Table of sperm and defects

Microtubules

present in sperm ? YES

Defect = No sperm movement (flagella fails)

Microfilaments

YES

defect= Fertilization issues

Intermediate filaments

YES

defect = Weak structure

Microtubules (cilia)

Cilia line fallopian tubes and they Beat to move embryo

TEST RESULTS

Robert’s sperm was fine

Focus shifts to female

Julia has chronic cough her whole life

Lungs:

Cilia clear mucus → mucus builds → cough

Fallopian tubes:

Cilia move embryo → embryo stuck → ectopic

SAME PROBLEM

Cilia Structure

Normal = 9+2

Julia = 9+0 (missing center pair)

Without central pair , No coordinated beating

Diagnosis

Primary Ciliary Dyskinesia (PCD)

Genetic disorder

Cilia don’t move

“Primary ciliary dyskinesia results from defective microtubule structure (9+0 instead of 9+2), causing loss of coordinated ciliary movement, which leads to impaired mucus clearance and failure of embryo transport, resulting in chronic respiratory issues and infertility.”

INTRACELLULAR SIGNALING PATHWAYS ARE A SERIES OF EVENTS

1.Transmit signal into cell

2. Amplify signal (Production of second messenger)

3. Integrate other signals (Fan in processes)

4. Distribute signals (Fan out processes:)

5. Output

TRANSMIT SIGNAL INTO CELL

Direct permeability

signal molecule (first messenger) enters cell

gated channels

signal is transduced

Transmembrane receptors

signal is transduced

Feedforward feet

self explanatory

FEEDBACK DISINHIBITION feet

same thing

SIGNAL TRANSDUCERS

G-protein coupled receptors

Enzyme linked receptors

Ion channel receptors

Nuclear receptors

HOW DO G-PROTEINS WORK

Signal molecule activates GPCR

GPCR acts as a GEF for alpha subunit (GTPase) of trimeric G-protein

Parts of complex have different activities

Alpha subunit is activated

Beta/gamma subunit separate and become active

Alpha and beta/gamma alter effector proteins

GPCR can activate more G-proteins

DIAGRAM 1

the Nitric Oxide (NO) → cGMP → Smooth Muscle Relaxation pathway

STEP:

1. Ca²⁺ increases in endothelial/neuron cell

2. Ca²⁺ binds calmodulin (CaM)

3. CaM activates nitric oxide synthase (NOS)

4. Arginine converted to → Nitric Oxide (NO)

STEP 2

NO enters smooth muscle cell

Guanylyl cyclase inactive → active

NO diffuses into smooth muscle cell

NO activates guanylyl cyclase (GC)

GC converts GTP → cGMP

cGMP activates PKG (protein kinase G)

PKG = enzyme that phosphorylates proteins

Effects\

Lower cytosolic Ca²⁺

Desensitization to Ca²⁺

↓ Ca²⁺ release from ER

↑ Ca²⁺ sequestration (stored)

↑ Ca²⁺ export out of cell

↓ Ca²⁺ entry

RESULT:

LOW Ca²⁺ inside cell

Low Ca²⁺ →

Myosin cannot contract

Smooth muscle RELAXES

Diagram 2

1. NO → GC

NO activates guanylyl cyclase

2. GTP → cGMP

GC converts GTP → cGMP

PDE5 (phosphodiesterase) converts cGMP → GMP

Turns OFF the signal

3. cGMP → PKG

cGMP activates PKG

4. PKG TARGETS (VERY IMPORTANT)

PKG TARGETS (VERY IMPORTANT)

A. Phospholamban (PLB)

PKG phosphorylates PLB

→ Ca²⁺ pumped into ER

↓ cytosolic Ca²⁺

B. IP3 receptor

PKG inhibits Ca²⁺ release

Less Ca²⁺ from ER

C. Blocks Ca²⁺ channels (membrane)

↓ Ca²⁺ entry

D. Ca²⁺ export pumps

↑ Ca²⁺ pumped OUT

E. Myosin light chain phosphatase

Activated → dephosphorylates myosin

prevents contraction

F. PKG activates K⁺ channels

“PKG activates potassium channels, causing hyperpolarization of the membrane, which closes voltage-gated calcium channels and reduces intracellular calcium levels."

PKG causes:

↓ intracellular Ca²⁺

↓ Ca²⁺ sensitivity

↑ myosin relaxation

RESULT = SMOOTH MUSCLE RELAXATION

BC Ca2+ binds tropomyosin which inhibits myosin from binding actin

CASE SETUP (DOG ED)

Dog has ED

Signalling pathway controls this

NO → cGMP → PKG → ↓ Ca²⁺ → relaxation

Smooth muscle relaxation

BLOOD FLOW IN CASE STUDY

Normal contracted muscle

reduced blood flow, no erection

cGMP pathway activated relaxed muscle increased blood flow, erection present

RECEPTOR LOCATION

Intracellular receptor

If pathway doesn’t work:

Not enough NO

Not enough cGMP

Too much PDE5 (breaks cGMP)

Result:

Muscle stays contracted

Blood can’t flow

cGMP BASICS

Guanylyl cyclase creates 2nd messenger cGMP

NO signals guanalyl cyclase to make cGMP very important molecule in many cellular functions

Guanylyl cyclase creates cGMP from GTP

cGMP activates down stream targets

cGMP activates:

PKG (main one)

Ion channels

Other proteins

Somewhere in this pathway is broken:

NO production

Guanylyl cyclase

cGMP production

PKG

Ca²⁺ regulation

PDE breaks down cGMP

VIAGRA

Dog is perscribed Viagra

Viagra = PDE5 inhibitor

Effect:

Blocks cGMP breakdown

Keeps cGMP HIGH

If Viagra works →

Problem is NOT upstream (NO/GC still works)

Problem is likely cGMP breakdown

GROUP THE PATHWAY INTO PARTS

START OF PATHWAY:

Nitric Oxide (NO)

Guanylyl cyclase (GC)

cGMP

Ca²⁺ CONTROL ZONE From PKG

Ca²⁺ channels (entry)

Ca²⁺ pumps (export)

ER storage (phospholamban)

IP3 receptor (release)

K⁺ channels (indirect effect)

MYOSIN CONTROL

Myosin light chain phosphatase

INHIBITION

PDE Breaks cGMP → stops signal

PKG - A "HUB" IN A PATHWAY

Protein kinase G (PKG) aka cGMP dependent protein kinase

Serine/threonine kinase Consensus amino acid binding sequence: R R “X” S/T “X”

What general role does this play?

Fan out or distribution

Already

Ca is keeping muscle contracted

cGMP being degraded by PDE5

1. NO increases cGMP production and activates PKG

2. PKG inhibits Ca release from ER (less Ca)

3. PKG inhibits Ca influx from extracellular environment (less Ca)

4. PKG inhibits myosin complex (no contract)

5. PKG inhibits PDE5 (more cGMP)

6. PKG activates Lamban (Ca sequestration)

sequestration

Sequestration = storing Ca²⁺ somewhere else so it's not free in the cytosol (usually ER)

ACTIVE TRANSPORT

Pumps work against concentration gradient

Requires energy input ATP or establish gradient

DIFFERENT TYPES OF PUMPS

Coupled pump

ATP driven pump

Light driven pump

CA+ PUMPS STORE CA+ IN THE SACROPLASMIC RETICULUM (SR) OR ENDOPLASMIC (ER)

Muscle release Ca+ from SR to contract muscles cells

Ca+ pump recovers that Ca+ back to the SR

1. Cytosolic Ca+ binds to pump

2. ATP is used to phosphorylate pump (aspartic acid on pump) and cause conformational shift

3. Shift eliminates Ca+ binding sites and Ca+ is released into SR.

RELAXATION

BLOOD FLOW CONNECTION

NORMAL:

Low blood flow

cGMP ACTIVE:

Relaxed

Increased blood flow

active transport feet

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3 types feet

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3 TYPES OF SIGNAL TRANSDUCERS

G-protein coupled receptors (GPCRs)

Enzyme-linked receptors

Ion channel receptors

Ras GTPase PATHWAY RECEPTOR TYROSINE KINASES/Enzyme linked

Receptor Tyrosine Kinases

phosphorylation occurs on tyrosine

Ligand (signaling molecule) binds and induces dimerization of RTKs

Cross phosphorylation occurs

Phosphorylation sites are binding sites for other proteins

10-20 different molecules

Ligand induces dimerization and phosphorylation

Adaptor protein binds to RTK

Ras-GEF binds to adaptor protein

localizes GEF to membrane

Ras-GEF facilitates Ras activation

Ras-GEF = turns Ras ON (GDP → GTP)

GEF

Guanine Nucleotide Exchange Factors (GEFs) are proteins that activate monomeric GTPases (like Ras) by swapping bound GDP (inactive) for GTP (active), triggering cellular signaling pathways.

Monomeric refers to single-subunit GTPases, or "small G-proteins," which operate as molecular switches rather than heterotrimeric complexes.

RAS GTPASE, GEFS AND GAPS

Ras is a monomeric GTPase

trimeric GTPase proteins are part of the GPCR pathway Most RTKs GTPases are monomeric

Ras-GAP:

turns Ras OFF (hydrolyzes GTP → GDP)

Ras activated by by many different factors

Ras has basal level of activity

Activate GEFS to activate Ras

GAPs and natural hydrolytic activity inactivate

30% of cancers contain mutant Ras that is constitutively active

Plants don’t have an equivalent family of GTPase principles are the same

Feet of this bs

feet

MAP Kinase Cascade

MAP kinase kinase kinase (MAPKKK) -

phosphorylate and activate MAP kinase kinases

Signal Amplification

Feedback regulation

Ras TURNS ON MAPKKK

MAP kinase kinase (MAPKK) - phosphorylate and activate MAP kinases

MAP kinase (MAPK) - phosphorylate effector molecules

Results in Changes in cellular function

Signal Amplification

Feedback regulation

Two ways

Fast

protein X, Y → immediate activity changes

Slow

transcription factors → gene expression

Used by many processes in the cell

. NGF - neuronal cell growth

PGDF - wound healing

Highly mutated in cancer

HOW DOES THE CELL TURN THIS OFF?

Tyrosine phosphotases Remove signal molecule Feedback regulation in MAPK cascade Internalization of receptor and degradation by lysosome using endocytosis

Whole thing feet

feet

Theoretical pathway order RTK → F → Ras → C → MAPK → cell division

RTK → F → Ras → C → MAPK → cell division

WITH CONSTITUITIVE Ras

Mutation stops signaling

Protein is REQUIRED

Overactive upstream rescues THEN

Protein is UPSTREAM

Overactive upstream does NOT rescue THEN

Protein is DOWNSTREAM

Constitutively active upstream proteins bypass upstream defects but not downstream defects.

More info

CELLS CAN RESPOND DIFFERENTLY TO THE SAME SIGNAL

Acetylcholine induces a different response in salivary cells and skeletal muscle cells

same ligand can activate different kinds of Gproteins depending on the cell

MULTIPLE SIGNALING PATHWAYS CAN BE STRUNG TOGETHER

CELLS INTEGRATE MULTIPLE SIGNALS

G-PROTEINS AND GTPases WORK EVERYWHERE for MOSTLY GROWTH

GTPases (like Ras) are used in MANY pathways

Cellular signaling pathways are interconnected networks that integrate multiple signals to produce context-dependent response

G-protein coupled receptors Pathway Step 1

G-PROTEIN COUPLED RECEPTORS (STRUCTURE)

7 transmembrane spanning helices

Family of proteins includes Rhodopsin (light activated photoreceptor)

signal binding happens deep inside pocket created by helices

controls cAMP and Ca+ among other second messengers

Step 1: No signal

GPCR = inactive

G protein = GDP-bound (OFF)

STEP 2: Signal binds

ligand binds GPCR

GPCR changes shape

Step 2

AFTER SIGNAL BINDS

GPCR becomes active

All subunits of G protein are inactive and complexed

THEN

GPCR facilitates GTP exchange in alpha

GTP bound alpha is active and separate from beta/gamma

G-PROTEIN SUBUNITS ACTIVATE OTHER PROTEINS

G-proteins are specific for a set of receptors or targets

3 protein subunits alpha, beta, gamma subunit separate and become active

Alpha works alone beta/gamma are complexed

Alpha and beta/gamma bind to proteins to alter effector activity - often regulate 2nd messenger

Gi - inhibitory and Gs excitatory/activating

Alpha hydrolyzes GTP into GDP and beta/gamma rejoin inactive complex - occurs in ~ 1sec

GDP for alpha = off

GTP = on

Step 3 of this

After activation:

α (with GTP) goes and activates enzymes (like adenylyl cyclase)

βγ can ALSO activates (like ion channels)

PKA - PROTEIN KINASE A

Alpha subunit activates adenylyl cyclase

Gs type g-protein because it stimulates adenylyl cyclase

Adenylyl cyclase → makes cAMP

cAMP activates PKA by releasing it from a regulatory protein

cAMP similar to cGMP, highly regulated second messenger

PKA prolific protein kinase

PKA → phosphorylates proteins → cell response

serine/threonine kinase

cAMP is degraded by a phosphodiesterase (PDE) (just like cGMP) keeping cellular concentrations low

PKA effector response

SLOW EFFECTOR RESPONSE

Activated PKA can be transported to the nucleus

Transcription is regulated in response to PKA phosphorylation of target proteins

Slow Response

HOWEVER

G proteins can effect ion channels to have immediate effect

Beta/gamma interact with interacts with K+ channel and allow K+ to flow across membrane

Gi G-protein inhibits muscle contraction in the heart by disrupting voltage gradient

K+ feet

feet

WHAT WOULD HAPPEN IF

you drink caffeine?

cholera bacteria invade the body and produce cholera toxin?

activate a Gi subunit of g-protein that inhibits adenylyl cyclase?

You drink caffeine?

Caffeine inhibits the PDE that degrades cAMP to AMP

PKA remains active for a longer period of time

More effector activity

cholera bacteria invade the body and produce cholera toxin?

Toxin causes a constitutively active alpha subunit

Adenylyl cyclase stays active for a longer period of time making more cAMP

PKA stays active for a longer = increased effector activity

activate a Gi subunit of g-protein that inhibits adenylyl cyclase.

Adenylyl cyclase is prevented from activating

all downstream processes slow or stop

GS ACTIVATES , GI inhibits

Cell division

CELLS DIVIDE AND FORM 2 DAUGHTER CELLS

Cell cycle varies in time

DAUGHTER CELLS ALSO INHERIT GENE EXPRESSION PATTERNS AND DNA MODIFICATION STATES

CELL CYCLE Not drawn to scale

M phase much shorter

G1 much longer

CHECK POINTS CONTROL WHEN A CELL ENTERS A DIFFERENT PHASE

Think of these as "if/then" statements

If DNA is fully replicated cell cycle can proceed

The cycle:

G1

S → copy DNA

G2

M → divide

M phase :

1. Prophase

Chromosomes condense and become visible as sister chromatids. The mitotic spindle begins forming as centrosomes move apart.

2. Prometaphase

The nuclear envelope breaks down. Spindle microtubules attach to chromosomes at kinetochores and begin moving them.

3. Metaphase

Chromosomes align at the center of the cell (metaphase plate). Each chromatid is attached to opposite spindle poles.

4. Anaphase

Sister chromatids separate and are pulled toward opposite poles. This ensures each daughter cell gets identical chromosomes.

5. Telophase

Chromosomes arrive at the poles and decondense. New nuclear envelopes form around each set, creating two nuclei.

6. Cytokinesis

The cytoplasm divides via a contractile ring. This produces two fully separate daughter cells.

CDKS

CDKS INiTIATE SPECIFIC STEP IN THE CELL CYCLE

Cyclin dependent kinase :Regulatory protein, cyclin, must bind to CDK for CDK to become active

CDKs phosphorylate specific proteins to initiate the next phase

Amount of CDK changes through cycle

SPECIFIC CDKS FUNCTION AT EACH PHASE

G1 cyclin/CDK

S cyclin/CDK

M cyclin/CDK

CDK inhibition

CDK INHIBITION

Different ways to inhibit CDKs

Ubiquitination by APC/C --> degradation in proteasome

Dephosphotylation

Binding of inhibitor (like p27)

P53 IS A TUMOR SUPRESSOR

Tumor suppressor - prevents tumors by stopping the cell cycle if something is wrong

p53 gets phosphorylates by protein kinase and binds to p21 gene and activates it.

p21 inhibits G1/S CDK

SELECTED EVENTS FROM PHASES OF THE CELL CYCLE

PHOSPHORYLATION CONTROLS NUCLEAR MEMBRANE BREAKDOWN

membrane breaks apart

MICROTUBULES BIND TO THE KINETOCHORE

ANAPHASE REGULATION

ANAPHASE A: chromosomes are pulled poleward

APC/C complex:

ubiquitinates securin

securin gets destroyed

separase becomes active then cuts cohesin (the glue holding chromatids)

chromosomes separate

CELL SIZE

BACTERIAL CELL SIZE SEEMS TO DEPENDENT ON NUTRIENT AVAILABILITY

Ratio of DNA/RNA/protein to cell size remains constant Timing of cell division vs growth determines size of daughter cell

If cell divides EARLY → small cells If it grows LONGER → bigger cells

EXTRACELLULAR SIGNALS CONTROL GROWTH IN LARGER ORGANISMS

Saturation level of growth factors can control cells size

Paracrine

nearby cells send signals

Local