slides/AP videos 4.1-4.7

Cell Communication

cells do this via chemical signals

ex) cells of diff mating types locate each other via secreted factors specific to each type

direct contact

cells of multicellular organisms often maintain physical contact with other cells or make physical contact with other cells during certain activities

come unicellular organisms live in colonies and are in physical contact with other organisms in that colony

cells can send chemical chemical signals directly to adjacent cells → wall modifications

• plant cells have plasmodesma

• animal cells have gap junctions

target cell

the cell receiving the chemical signal

Local Signaling

Short-distance communication where cells use local regulators to communicate with nearby cells (often used to communicate with cells of the same type)

ex) direct contact, cell-to-cell recognition

Long-Distance Signaling

Communication over long distances, often using hormones in plants and animals → the target cell is not in the same area as the cell emitting the signal (often used to communicate to cells of another type)

Signal Transduction Pathway

a series of steps converting a signal on the cell’s surface into a specific cellular response

reception, transduction, response

three stages of signal transduction

reception

detection of a signal molecule coming from outside the cell

occurs when a signal molecules, or ligand, binds to a __ protein, altering the _’s shape

ligands

signaling molecules that are highly specific to particular receptors

membrane receptors

receptor proteins on the cell surface

→ have polar ligands

intracellular/cytosolic receptors

receptor proteins inside the cell; are found in the cytosol or nucleus of target cells

→ have nonpolar ligands (small/hydrophobic)

nonpolar ligands

small or hydrophobic chemical messengers can readily cross the membrane and activate receptors

ex) steroid and thyroid hormones of animals

transduction

converting signals to a form that can bring about a cell response

response

specific cellular response to the signal molecule

Receptor Proteins

Proteins on the cell surface or inside the cell that bind to specific ligands

G-Protein-Coupled Receptor (GPCR)

Membrane receptor that activates G-proteins upon ligand binding

ligand binds to the extracellular domain, slightly altering the receptors shape

1. G-protein is activated by the GPCR and released, as it displaces its GDP with a GTP molecule

   • The active G-protein may begin transduction

   • One GPCR can activate dozens of G-proteins

2. G-protein dephosphorylates its own GTP, forming GDP and  inactivating itself

3. When ligand concentrations drop, the ligand dissociates from the receptor, deactivating the GPCR

ligand-gated ion channel receptor

a receptor that acts as a gate when the receptor changes shape

→ when a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca 2+, through a channel in the receptor

multi-step pathways

can be used to amplify signals: a few molecules can produce a large cellular response

can provide more opportunities for coordination and regulation of the cellular response

signal-transduction pathways

many ___ include protein modification and phosphorylation cascades *(the signal is transmitted by a cascade of protein phosphorylation)

• regulate protein synthesis by turning on/off genes in the nucleus

• regulate activity of proteins in the cytoplasm

• cascades of molecular interactions relay signals from receptors to target molecules

• phosphorylation cascade: enhance and amplify signal

protein kinases

transfer phosphates from ATP to protein, a process called phosphorylation

Second Messengers

small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion (molecules that relay and amplify the intracellular signal)

→ participate in pathways initiated by GPCRs

Cyclic AMP

a common second messenger

amplifying, specificity, efficiency, termination

A cell’s response can be fine-tuned in the following ways:

• 1 the signal (and thus the response)

• 2 of the response

• Overall 3 of response, enhanced by scaffolding proteins

• 3 of the signal

Apoptosis

Programmed cell death triggered by various signals like DNA damage or protein misfolding → prevents enzymes from leaking out of a dying cell and damaging neighboring cells

(Components of the cell are chopped up and packaged into vesicles that are digested by scavenger cells)

triggered by:

• An extracellular death-signaling ligand 

  • can be the response of a signal transduction

• DNA damage in the nucleus

• Protein misfolding in the endoplasmic reticulum

phenotype

manifested in an organisms appearance

→ signal transduction may result in changes in gene expression and cell function

• signaling pathways can target gene expression and alter the amount and/or type of a particular protein produced in a cell

  • changes in protein type and/or amount can result in a ___ change

mutations

(alterations) _______ in any domain of the receptor protein or in any component of the signaling pathway may effect the downstream components by altering the subsequent transduction of the signal

• changes in protein structure can result in change in function

• one disruption in a pathway can affect the downstream reactions

chemicals

(alterations) ____ that interfere with any component of the signaling pathway may activate or inhibit the pathway

homeostasis

the constant set of internal conditions of an organism (maintenance of a stable internal environment)

feedback mechanisms

processes used to maintain homeostasis by increasing or decreasing a cellular response to an event

→ organisms use ___ ____ to maintain their internal environments and respond to environmental changes (both internal and external)

negative feedback

maintain homeostasis for a particular cell condition by regulating physiological processes (can return the system back to its target set point if disrupted)

*operate at the cellular and molecular levels

positive feedback

a stimulus that causes an organism to migrate away from its homeostatic level, amplifying the response

• the variable initiating the response is moved farther away from the initial set point, disrupting homeostasis

• amplification occurs when the stimulus is further activated, which, in turn, initiates an additional response that produces the system change

Cell Cycle

the life of a cell from formation to its own division (a highly regulated series of events for the growth and reproduction of cells)

1. interphase

   1. G1

   2. S

   3. G2

2. m-phase

interphase

(cell cycle) growth and preparation

3 sequential stages:

1. G1 - cell growth

2. S - copies of DNA are made

3. G2 - the cytoplasmic components are doubled in preparation for division

m-phase (mitotic phase)

(cell cycle) after interphase

• mitosis - division of the nucleus

• cytokinesis - division of the cytoplasm

Genome

All DNA in a cell, packaged into chromosomes in eukaryotes.

→ Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

Mitosis

Process ensuring the transfer of a complete genome from a parent cell to two genetically identical daughter cells

• plays a role in cell growth, tissue repair, and asexual reproduction

• alternates with interphase in the cell cycle

• sequential stages:

  • prophase

  • metaphase

  • anaphase

  • telophase

• followed by cytokinesis

  • cytokinesis ensures equal distribution of cytoplasm to both daughter cells

Cytokinesis

Division of the cytoplasm following mitosis, ensuring equal distribution to daughter cells.

chromatin

Eukaryotic chromosomes consist of _______, a complex of DNA and protein that condenses during cell division

somatic cells

non-reproductive cells

are diploid (2n) → have two sets of chromosomes

gametes

reproductive cells (i.e. sperm, eggs)

are haploid (n) → one set of chromosomes

centromere

the narrow “waist” of the duplicated chromosome, where two chromatids are most closely attached

sister chromatids

joined copies of the original chromosome

each duplicated chromosome has two, which separate during cell division

chromosomes

separated chromatids

prophase

(mitosis) step 1

• nuclear envelope begins to disappear

• DNA coils into visible chromosomes

• fibers begin to move double chromosomes toward the center of the cell

prometaphase

(mitosis) step 1.5

• fibers begin to move double chromosomes toward the center of the cell

*sometimes considered its own phase, sometimes grouped with prophase

metaphase

(mitosis) step 2

• fibers align double chromosomes across the center of the cell

anaphase

(mitosis) step 3

• fibers separate double chromosomes into single chromosomes (chromatids)

• chromosomes separate at the centromere

• single chromosomes (chromatids) migrate to opposite sides of the cell

telophase

(mitosis) step 4

• nuclear envelope reappears and establishes two separate nuclei

• each nucleus contains a complete genome

• chromosomes will begin to uncoil

cytokinesis overlaps → separating the cell into two daughter cells, each containing identical genomes

Cell Cycle Control System

Directs sequential events of the cell cycle, similar to a clock

→ regulated by both internal and external controls

G1 Checkpoint

End of G1 phase where cell size, nutrients, growth factors, and DNA damage are checked

*seems to be the most important\

if passes, the cell will most likely also complete S, G2, and m-phase

G0 Phase

Nondividing state cells enter if they do not receive a go-ahead signal at the G1 checkpoint

G2 checkpoint

end of G2 where DNA damage and DNA replication are checked

m-spindle checkpoint

fiber attachment to chromosome check

Cyclins

Group of proteins associated with specific cell cycle phases, fluctuating concentrations, promoting or inhibiting cell cycle progression

used to activate CDKs → ___ are specific to the CDK they activate

Cyclin-Dependent Kinases (Cdks)

Enzymes requiring cyclin binding for activation, fluctuating activity during the cell cycle

→ phosphorylate substrates, promotes certain cell cycle activities

MPF (Maturation-Promoting Factor)

a cyclin-Cdk complex that triggers a cell’s passage past the G₂ checkpoint into the M phase

Growth Factors

Ligands initiating cell division

(an external factor that may influence cell cycle)

Anchorage Dependence

Requirement for cells to be bound to a substratum to divide

(an external factor that may influence cell cycle)

Density-Dependent Inhibition

Causes cells to stop dividing once a space is filled

Cancer Cells

Cells with unregulated cell cycle, not responding to normal control mechanisms, leading to tumor formation (masses of abnormal cells within otherwise normal tissue)

• may not need growth factors to grow and divide

  • may make their own growth factor

  • may convey a growth factor’s signal without the presence of the growth factor

  • may have an abnormal cell cycle control system

benign tumor

when abnormal cells remain only at the original site

malignant tumor

abnormal cells that invade surrounding tissues and may metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors