Unit 4. Cell Communication & the Cell Cycle

Hormones

Chemical signals

Juxtacrine Signaling

signaling by direct contact

Plasmodesmata

used in direct contact signaling for plants

Gap junctions

used in direct contact signaling by animals

Autocrine Signaling

A cell produces the ligand for its own receptor

Paracrine signaling

ligands are received by nearby cells

endocrine signaling

ligands travel through the bloodstream

Reception

the ligand interacts with its specific receptor

transduction

the signal is converted to an intracellular message

response

a change occurs within the cell

Enzyme-Linked Protein Receptors

Ligand activates receptor

Activation initiates signal transduction

Ex. - Receptor Tyrosine Kinase

Ion-Channel Linked Receptors

Ligand binding opens channel (conformational change)

G Protein-Coupled Receptors

Ligand binding activates the G protein (attached to receptor)

Activated G protein activates other proteins (signal transduction)

Phosphorylation

activates relay proteins

Phosphorylation cascade

one enzyme phosphorylates another in a chain reaction; signal is amplified

Negative feedback loop

stops the signal

Positive feedback loop

amplify the signal until the process is complete

The Cell Cycle

the series of events that occur between cell divisions

Interphase

longest phase

G1

growth phase

G0

cell cycle is stopped

S phase

DNA replication

G2

last stage before mitosis

Mitosis

Nuclear division that results in two identical nuclei

Prophase

Nucleolus & nuclear membrane disappear

Chromatin condenses to duplicated chromosomes

Centrosomes move to opposite poles

Spindle apparatus forms

Metaphase

Spindle fibers align chromosomes at the equator of the cell (metaphase or equatorial plate)

Anaphase

Sister chromatids are separated:

Motor proteins shorten kinetochore microtubules & pull chromatids to either pole

Polar microtubules lengthen and push

Telophase

Spindle disappears

Chromosomes unwind into chromatin

Nucleolus & nuclear membrane reform

Cytoplasm begins to divide

Cytokinesis

division of the cytoplasm

Tumor suppressors

negative regulators of the cell cycle

Cancer

uncontrolled cell division of cells with damaged DNA