Cell Communication and Cell Cycle

cell communication

key to maintaining homeostasis and surviving

receiving and processing information from outside the cell

cell signaling overview

involves a ligand, signaling molecule, and receptor protein

signal transduction pathway produces a cellular response

3 stages:

1. reception - ligand binds to receptor

2. transduction - signal is converted

3. response - cell process is altered

types of cell communication

1. direct signaling

2. local signaling

3. long distance signaling

direct signaling

through cell junctions (A) and plasmodesmata (P)

signaling substances and other material dissolved in cytoplasm pass freely through adjacent cells

ex: immune cells connect T cells

Local signaling

regulator/ligand travels very short distance through extracellular fluid

chemical message will cause response to target cell

paracrine and synaptic signaling

long distance signaling

plant & animal cells use hormones

P hormones travel through vascular tissue (xylem up, phloem down)

A hormones use endocrine signaling: hormones go to circulatory system

ex: insulin

ways of cell signaling

direct cell-to-cell signaling, paracrine signaling, endocrine signaling, synaptic signaling (and autocrine signaling, but first four are most important)

direct cell-to-cell signaling

direct physical contact of cells

basically direct signaling already mentioned

paracrine signaling

cell releases short lived signal to localized area that will induces changes in nearby cells

diffuse only over relatively short distances

endocrine signaling

cells far apart with a long lasting signal - hormone

goes throughout the organism

protein hormones are large, need to bind to receptors on the cell membrane

steroid hormones can pass through membrane and attach to integral protein receptors

synaptic signaling

specialized nerve cell, neuron, and target cell - called chemical synapse

release of neurotransmitters from neuron into synaptic gap to target the target cell

autocrine signaling

cell sends signal to itself by secreting something that binds to specific receptors on its own membrane

signal transduction pathway

ligand binds to receptor, sometimes changing its shape, activation of enzyme or binding of other molecules

starts signaling cascade and can amplify reactions that lead to cellular response resulting to change in cell’s behavior/characteristics

phosphorylation

signal transduction pathway may require activating proteins by phosphorylation

phosphorylation catalyzed by kinase enzymes

dephosphorylation catalyzed by phosphatases

secondary messengers

small water soluble molecules/ions, diffusion

calcium: common secondary messenger in cells example, muscle contractions

cyclic AMP (cAMP): involved in many signal cascade pathways - protein hormone activation w/ relay G-proteins to produce cAMP

types of receptors

1. g-protein:interact with G-proteins, initiating signaling cascades upon activation. They play key roles in physiological processes

2. tyrosine kinases: catalyzes transfer of phosphate groups from ATP to amino acid Tyrosine on a substrate protein

3. ion channel receptors: proteins that change shape to allow ions to flow across the cell membrane when a ligand binds, crucial for rapid cell signaling.

4. intra-cellular: cytoplasm or nucleus of target cell

nuclear and cytoplasmic responses

1. regulate protein synthesis - turn genes on or off

2. regulate activity of proteins - outside nucleus

fine-tuning response

signal amplification, specificity of signal, signaling efficiency, termination

cell division in prokaryotes

binary fission - simply elongate until they’re double the size and then just split

cell cycle for eukaryotes

phases G₁, S, G₂, and M

G₁ and G₂ are growth phases to prepare for DNA replication and mitosis

S is where DNA is replicated

M is mitosis, cell division phase

first three phases are called interphase, 90% of cycle

mitosis phases

prophase, metaphase, anaphase, telophase, cytokenesis

prophase

nucleus and nucleolus disappear, chromosomes appear as two identical sister chromatids

mitotic spindle forms, centrioles move to opposite sides of the cell (A)

metaphase

prometaphase - spindle fibers attach to sister chromatids and kinetochore

metaphase - sister chromatids line up at metaphase plate

anaphase

spindle fibers pull apart sister chromatids

telophase

cleavage furrow to seperate the dividing cells; nuclear envelope re-forms around each set of chromosomes

cytokinesis

daughter cells split apart

chromosome folding

tightly wound around nucleosomes, made by histones - called heterochromatin, considered turned off, cna’t build proteins

not tightly wound are euchromatin, considered turned on, can build proteins

chromosome

made of chromatin

telomeres at ends

centromere region has a lot of repeated sequences of proteins and base pairs

control of cell division

checkpoints

density-dependent inhibition: making sure cell is not growing too big - ex: cancer cells

growth factors: if absent, won’t grow

cyclins and protein kinases

cyclins and protein kinases

CDK complexes regulate cell cycle by activating target proteins through phosphorylation, ensuring proper progression through the checkpoints.

apoptosis

programmed cell death

negative and positive feedback

negative: hormone halts further stimuli

positive: hormone amplifies for more stimuli

homeostasis

insulin/glucagon: regulates blood sugar levels from liver and pancreas by adding/decreasing insulin and glucagon levels

calcitonin/PTH: if calcium levels too low, PTH released to raise calcium levels, while calcitonin reduces calcium levels when they are high