Cell Communication & the Cell Cycle

Chemical signaling

main form of cell communication

Ligand

• chemicals

• interact with receptor proteins on the cell surface or in the cell

Juxtacrine signaling

• cell-to-cell signaling

• direct contact

• small molecules & ions move through chanells

Plasmodesmata

juxtacrine signaling in plants

• allow for shared cytoplasm

Gap junction

juxtacrine signaling in animals

• formed by proteins

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

Internal receptor

• cytoplasm or nucleus

• small, hydrophobic ligands

• regulate transcription (usually)

Cell surface receptor

• transmembrane proteins

• shape change initiates signal transduction

Enzyme linked protein receptor

• ligand activated receptor

• activation initiates signal transduction

ex: receptor tyrosine kinase

Ion channel linked receptor

ligand binding opens channel (conformational change)

G protein coupled receptor

• ligand binding activates the G protein (attached to receptor)

• activated G protein activates other proteins (signal transduction)

Kinase

add phosphates from ATP to another protein

Phosphorylation cascade

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

Phosphatase

remove phosphates

Second messenger

• assist with transduction

• relay & amplify the signal in the cell

ex: molecules (cAMP, DAG, IP₃), ions (Ca²⁺)

Feedback

response that either stimulates or inhibits the signal that produced it

Negative feedback loop

stop the signal

• return to a baseline or set level

• homeostasis: blood sugar, body temperature, etc.

Positive feedback loop

amplify the signal until the process is complete

• no return to baseline or set level

• achieve a process: birthing a child, producing milk, forming a blood clot, ripening fruit

Cell cycle

series of events that occur between cell divisions

Interphase

• longest phase

• grow & prep for mitosis

G1 phase

• growth, make more organelles

• metabolically active (normal cell functions)

• “Decision”: divide or enter G₀

G1/S checkpoint

Is the cell ready to divide?

• Check:

  • DNA OK?

  • Cell size?

  • Enough space?

  • Growth factors?

  • Sufficient ATP/resources?

G0

stopping the cell cycle

• normal functions

• non-proliferative cells stay here

• exit with proper signals (ex: growth hormones)

S phase

• DNA replication (chromatin)

• cell is committed to division

G2 phase

• synthesize proteins needed for mitosis (like microtubules)

• make lots of ATP

• two centrosomes present

G2/M checkpoint

Is the cell ready for mitosis?

• Check:

  • DNA OK?

  • Replication complete?

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

Chromosome

has one chromatid

Replicated chromosome

has two identical chromatids called sister chromatids

Sister chromatid

two identical chromatids

Centromere

links the sister chromatids together

Metaphase

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

M checkpoint

Is the cell ready for anaphase?

• Check:

  • Are the chromosomes properly lined up?

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

Positive regulator

advance the cell cycle

Mitogen

growth factors that promote cell division

Cyclin dependent kinase

kept at constant levels

Cyclin

made as needed & broken down after use

• made in response to growth factors

Tumor suppressor

• negative regulators of the cell cycle

• these proteins inhibit cyclin/CDK pairs, stopping the cell cycle

Cancer

uncontrolled cell division of cells with damaged DNA

p53

“Guardian of the Genome”

an important tumor suppressor