Cell Communication

Unit 3 Notes

Intercellular Signaling—Local

• Cell Junctions—signaling substances in cytosol pass freely between cellso Gap Junctions in Animalso Plasmodesmata in Plants

• Cell-Cell Interaction—interaction between molecules that portrude from cell surface (animals)o Important in embryonic

development and immune responses

Intercellular Signaling—Local

• Local Regulators—cell secretes a local regulator molecule which acts on specific nearby target cellso Paracrine Signaling—regulators (i.e. growth factors) are released into

extracellular fluid)o Synaptic Signaling—neurotransmitters are released into synapse

Intercellular Signaling—Long

Distance• Hormones—molecules used in plants and animals

for long-distance signalingo Animals = Endocrine Signaling—hormones move through circulatory

systemo Plants = Plant Growth Regulators—move through vessels or (more

commonly) diffuse through cells or air

Intercellular Signaling—Long

Distance• Nervous System in Animals—uses a combination

of electrical and chemical signals to send a message

Intracellular Signaling• Three Major Parts—

o Receptiono Transductiono Response

Reception• Signaling molecule binds to a receptor protein,

causing it to change shapeo Ligand—molecule that specifically binds to another

molecule (messenger molecule)

• Two Major Types of Receptorso Plasma Membrane Receptorso Intracellular Receptors

Reception• Plasma Membrane Receptors—transmembrane

proteins transmit information into the cell by changing shape or aggregating

o G-Protein-Coupled Receptor—works with the help of a G protein.

o Receptor Tyrosine Kinases—attach phosphates to tyrosines (amino acid)…leading to activation of proteins

o Ligand-Gated Ion Channels—ligand binds to gated channel protein and opens gate to let in ions, i.e., Na+, Ca2+

Reception

G-Protein-Coupled Receptors

ReceptionTyrosine-Kinase Receptors

Reception

Ion Channel Receptors

Reception• Intracellular Receptors—chemical messengers

(small & hydrophobic) enter cell and bind to a receptor in the cytoplasm or nucleus

o Testosterone—small steroid hormone • Activates receptor protein in cytoplasm of target cell

by binding to it• Activated receptor protein (with attached

testosterone) enters nucleus and turns on specific genes that control male sex characteristics = transcription factor (proteins that control which genes are on and off)

Reception

Intracellular Receptors

Transduction• Cascades of molecular interactions relay signal

from receptors to target molecules• Information is relayed by shape changes of

proteins

• Ex: Protein Phosphorylation & Dephosphorylationo Protein Kinases—enzymes that transfer phosphate

groups from ATP to a protein (usually activating the protein)

o Protein Phosphatases—enzymes that remove phosphate groups from proteins (usually deactivating protein and so turn off pathway)

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Transduction• Second Messengers—non-protein, water-soluble

molecules or ions that can pass signal to proteins (1st Messengers are the original ligands)

• Involved in pathways started by G-protein-linked receptors and receptor tyrosine kinases

• Most common second messengers:o Cyclic AMP (cAMP)o Ca2+

Transduction• Cyclic AMP (cAMP)

o Adenylyl cyclase converts ATP to cAMPo cAMP activates a protein kinase

Transduction• Ca2+--increase in Ca2+ leads to many responses in

plants and animalso [Ca2+] in blood and extracellular fluid is often 10,000x greater

than in cello Ca2+ is actively transported out of the cell and into ERo Another molecule, inositol trisphosphate (IP3) stimulates

release of Ca2+

Response• Cell Signaling leads to regulation of transcription

of cytoplasmic pathways

o Response may be the regulation of protein synthesis by turning specific genes on or off

o Response may be the regulation of a protein’s activity

ResponseNuclear

ResponseCytoplasmic

Response

Fine-Tuning Responses

• Signal Amplificationo Number of activated products increases at each catalytic

step of a cascade because enzymes are active long enough to catalyze many reactions

o Consequence = small number of ligands can lead to large response

• Specificity of Cell Signaling and Coordination of Responseo Specific types of cells have specific receptors, relay

proteins, and/or proteins needed for a response to occur

Fine-Tuning Responses

Signal is unique to different types of

cells.

Fine-Tuning Responses

• Scaffolding Proteins & Signaling Complexeso Scaffolding Proteins—large relay proteins that other relay

proteins attach to simultaneouslyo Same proteins can be involved in multiple pathways

• Termination of the Signalo Proteins must be inactivated

Cell CycleLife of a cell from origin through division.

Cell Cycle PhasesMitosis and Interphase

Mitosis (M Phase)• Mitosis—Division of the Nucleus• Cytokinesis—Division of the cytoplasm• Mitosis + Cytokinesis = ~10% of Cell Cycle• Mitosis Phases

o Prophaseo Metaphaseo Anaphaseo Telophase

Mitosis (M Phase)• Mitosis allows cell to

go from 4n 2n• Produces somatic cells

(2n)• Does not produce

gametes (1n)

Interphase• Interphase—phase in which the cell grows,

metabolizes, and copies DNA• Interphase = ~90% of Cell Cycle• Split up into 3 smaller phases

o Gap 1 Phase (G1)—Takes up ~35% of Interphase• Growth• Cell is 2n

o Synthesis Phase (S)—Takes up ~35% of Interphase• DNA Replication occurs• Cell goes from 2n 4n

o Gap 2 Phase (G2)—Takes up ~ 30% of Interphase• More Growth—Particularly molecules for division• Cell is 4n

Interphase

Phases of MitosisObjective 11

Animal Cell

Phases of Mitosis Summary

• Prophaseo Chromatin condenses into chromosomes (made of sister chromatids

attached at centromere)o Microtubules formo Centrioles/Centrosomes move to poles

Phases of Mitosis Summary

• Metaphaseo Microtubules attach to the kinetochores of each sister chromatido Chromosomes line up along the metaphase plate

Phases of Mitosis Summary

• Anaphaseo Sister Chromatids split and move to poles

Phases of Mitosis Summary

• Telophaseo New nuclei formo Microtubules degradeo Cytokinesis occurs

Phases of Mitosis

Plant Cell

Reproduction in Prokaryotic Cells

DNA, Binary Fission & BuddingObjective 14

Bacterial DNA• One loop of DNA attached to Cell Membrane

o Still highly folded to fit into cello Only one set of genes (not one from “mom” and one from “dad” like in

eukaryotes)

• May contain 1 or more Plasmids o Tiny loops of extra DNA that are able to move from 1 bacteria to

anothero Allows for recombination = advantage!

Binary Fission• Asexual Reproduction of most Prokaryotes• Basic Steps:

o DNA Replication—unzips to copyo Cell Pinches

• New cells should be cloneso No genetic recombinationo Only variation through mutations

• Rate = divides as fast as every 20 minutes

Budding• Some prokaryotes reproduce in this manner—

Asexual Reproduction• Basic Steps

o Cell Develops a bulge or budo DNA copieso Bud Breaks Off

Listeria monocytogenes

Control of Cell Cycle

Checkpoints & Regulatory Proteins/ConditionsObjective 12

Check Points• Between G1 and S

o Go ahead signal from the environment is needed (i.e. growth factors from other cells)

o Then…there are checks for enough mass and the condition of the DNAo If there is no signal…cell goes to G0 (non-dividing state)

• Most cells in G0 never divide (i.e. nerve/muscle) or they only divide if there is an injury

• Between G2 and Mo Checks for mass and correct DNA replicationo If all okay…cell commits to divide

• Note: Cancer often occurs because cell is quickly pushed from G1 S without proper checks

Regulatory Molecules/Conditions

• Regulatory Proteins/Enzymeso Cyclin-Dependent Kinase (Cdk)—enzymes needed to drive the cell

cycle• Cyclin-Dependent Kinase (enzyme that activates or deactivates

other molecules by phosphorylation) only works when activated by cyclin—a protein that rises and falls in the cell cycle

o Example: Maturation Promoting Factor (MPF)—• Cyclin increase in concentration in G2 and bind with a specific Cdk

to form MPF• MPF signals the start of Mitosis• At end of mitosis, enzymes break down cyclin…so no MPF…and no

more dividing (Cdk concentration remains the same)

Regulatory Molecules/Conditions

Regulatory Molecules/Conditions

• Other internal signalso Kinetochores must be attached before anaphase can occur—

unattached kinetochores send a signal to stop sister chromatids from splitting

Regulatory Molecules/Conditions

• External Signalso Growth Factors—proteins released by certain cells that stimulates other

cells to divideo Density-Dependent Inhibition—Crowded cells stop dividing because

there aren’t enough growth factors and nutrients for it to divideo Anchorage-Dependence—If cells aren’t attached to the extracellular

matrix, they do not get growth factors—so don’t divide

Cancer and the Cell Cycle

Objective 13—Define cancer and explain how aberrations in the cell cycle can lead to tumor formation.

Cancer • Complex collection of diseases that can arise in

almost any tissue in the body.

• All cancers arise as a result of the loss of cell cycle control.

Cytotoxic T Cell Attacking a Cancer Cell

Cancer Cell Characteristics

• Uncontrolled growth

• Lack of response to stop signals

• Immortality

• Ability to divide infinitely

• Recruits food supplies (angiogenesis)

• Random migration

Cancer Cell Characteristics

Benign versus Malignant

Benign (not cancer) tumor cells grow

only locally and cannot spread by invasion or metastasis

Benign (not cancer) tumor cells grow only locally and cannot spread by invasion or metastasis

Malignant (cancer) cells invade neighboring tissues, enter blood vessels, and metastasize to different sites

Cancer-Related Genes• Stability Genes—code for proteins that keep

genetic alterations to a minimumo If mutated, mutation rates increase

• Oncogenes—code for proteins that promote cell divisiono If mutated to be overly active = cancer

• Tumor Suppressor Genes—code for proteins that inhibit cell from progressing from G1So If mutated to be underactive = cancero Example TP53 codes for p53 protein = transcription factor that

normally inhibits cell growth and stimulates cell death when induced by cellular stress

Check Points