Cell Communication, Signaling Pathways, and Cell Cycle in Biology

cell-to-cell communication

critical for function and survival of cells, responsible for growth and development of multicellular organisms

how do cells communicate

direct contact, local signaling, long-distance signaling

direct contact

communication through cell junctions, signaling substances and other material dissolved in the cytoplasm can pass freely between adjacent cells (animal cells - gap junctions, plant cells - plasmodesmata)

local regulators

a secreting cell will release chemical messages (local regulators/ligands) that travel a short distance through the extracellular fluid; chemical messages cause a response in a target cell

types of local regulation

paracrine and synaptic

paracrine signaling

secretory cells release local regulators (ex growth factors) via exocytosis to an adjacent cell

synaptic signaling

occurs in animal nervous systems, neurons secrete neurotransmitters which diffuse across the synaptic cleft - space between the nerve cell and target cell

long distance signaling

animals and plants use hormones for long distance signaling; plants release hormones that travel in vascular tissue (xylem and phloem) or through air to reach target tissues, animals use endocrine signaling

endocrine signaling in animals

specialized cells release hormones into the circulatory system where they reach target cells

stages of cell-to-cell messaging

reception, transduction, response

reception

ligand binds to a receptor; receptor is a macromolecule that binds to ligand and is highly specific, receptor then activates a conformational change, allowing it to interact with other molecules and initiate a transduction signal

transduction

signal is converted from extracellular to intracellular to bring a cellular response, requires a sequence of changes in a series of molecules known as a signal transduction pathway, signal is amplified via second messengers (small, non-protein molecules and ions that help relay the message and amplify response ex cyclic AMP or CAMP)

response

cell process is altered

all receptors have...

an area that interacts with the ligand and an area that transmits a signal to another protein

location of receptors

plasma membrane or intracellular

plasma membrane receptors

most common type, binds to ligands that are large and polar

intracellular receptors

cytoplasm or nucleus of target cell, hydrophobic molecules that can pass through plasma membrane such as steroid or thyroid hormones

signal transduction pathway

regulates protein activity through phosphorylation by kinase (relays signal inside cell) or dephosphorylation by phosphatase (shuts off pathway)

how to signal transduction pathways influence cell response in environment

result in changes in gene expression and cell function, which can alter the phenotype or result in cell death; mutations to receptor proteins or any component of pathway will change signal transduction

G protein coupled receptor (GPCR)

largest category of cell surface receptors, important in animal sensory systems, binds to a G protein that can bind to GTP, which is an energy molecule similar to ATP; GPCR, enzyme, and G protein are inactive until ligand binding to GPCR on extracellular side and causes cytoplasmic side to change shape, GDP becomes GTP

ion channels

located in plasma membrane, important in the nervous system, receptors act as a gate for ions; when ligand binds to receptor, the gate open or closes allowing the diffusion of specific ions, initiating a series of events that lead to a cellular response

set points

values for various physiological conditions that the body tries to maintain, has a normal range for which it can fluctuate

homeostasis

the state of relatively stable internal conditions; organisms detect and respond to a stimulus, and homeostasis is maintained through feedback loops

stimulus

a variable that causes a response

receptor/sensor

sensory organs that detect a stimulus; information sent to control center

effector

muscle or gland that will respond

response

changes the effect of the stimulus

negative feedback

reduces the effect of the stimulus

positive feedback

increases the effect of the stimulus

reasons why the body cannot regulate homeostasis

genetic disorders, drug or alcohol abuse, intolerable conditions

disease

when the body is unable to maintain homeostasis (cancer-cannot regulate cell growth, diabetes- cannot regulate blood glucose levels)

cell division allows for

the reproduction of cells, growth of cells, and tissue repair

cell cycle

the life of cell from its formation until it divides

DNA organization

cells must organize and package their DNA before division; DNA associates with and wraps around proteins known as histones to form nucleosomes

after DNA replication, chromatin condenses to form

chromosomes, allowing for easier division

chromosomes copies join together to form

sister chromatids

centromere

the region on each sister chromatid where they are most closely attached

kinetochore

proteins attached to the centromere that link each sister chromatid to the mitotic spindle

genome

all of a cell's genetic information (prokaryotes- singular, circular DNA, eukaryotes- one or more linear chromosomes)

homologous chromosomes

two chromosomes (one from mom and one from dad) that are the same length, have the same centromere position, and carry genes controlling the same character

somatic cells

body cells, diploid (2n): two sets of chromosomes, one set from each parent, divide by mitosis

gametes

reproductive cells, haploid (n): one set of chromosomes, divide by meiosis

n=

23, 2n=46

the cell cycle consists of alternating phases of

interphase and mitosis

interphase

longest portion of cell cycle; G1: first gap phase- cell grows and carries out normal functions, S: synthesis phase- DNA replication and chromosome duplication occurs, G2: second gap phase- final growth and preparation for mitosis

m phase

mitosis

: nucleus divided, cytokinesis: cytoplasm divides; results in 2 identical diploid daughter cells

phases of mitosis

prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis

prophase

chromatin condenses, nucleoli disappear, sister chromatids appear, mitotic spindle begins to form, centromeres move away from eachother

prometaphase

nuclear envelope fragments, microtubules enter nuclear area and some attach to kinetochores

metaphase

centrosomes are at opposite poles, chromosomes line up at metaphase plate, microtubules are attached to each kinetochore

anaphase

sister chromatids separate and move to opposite ends of the cell due to the microtubules shortening, cell elongates

telophase and cytokinesis

two daughter nuclei form, nucleoli reappear, chromosomes become less condensed, cytokinesis occurs (animals: cleavage furrow appears due to contractile ring of actin filaments, plants: vesicles produced by the golgi travel to the middle of the cell and form a plate

checkpoints in the cell cycle

regulate cell cycle, cells receive stop/go signals

G1 checkpoint

most important, checks for cell size, growth factors, and DNA damage; go- completes whole cell cycle, stop- enters a nondividing state known as G0

G0

some cells stay in phase forever, come called back into cycle

G2

checks for completion of DNA replication and DNA damage; go- proceeds to mitosis, stop- cycle stops and if damage cannot be repaired then cell will undergo apoptosis

M (spindle) checkpoint

checks for microtubule attachment to chromosomes at the kinetochores at metaphase; go-proceeds to anaphase and complete mitosis, stop- pauses mitosis to allow for spindles to finish attaching to chromosomes

regulation of cell cycle involves internal control system that consists of

proteins called cyclins and enzymes known as cyclin-dependent kinases CDKs; concentration remains constant through each phase of cell cycle, active only when specific cyclin is present

each cyclin-CDK complex has a

specific regulatory affect

active CDK complexes do what to target proteins

phosphorylate them

growth factors

hormones released by cells that stimulate cell growth; signal transduction pathway initiated which activates CDKs leading to progression through cell cycle

contact or density inhibition

cell surface receptors recognize contact with other cells, initiating signal transduction pathway that stops the cell cycle in G1 phase

anchorage dependence

cells rely on attachment to other cells or to extracellular matrix to divide

normal cells become cancerous through

DNA mutations (changes)

normal cells

follow checkpoints, divide 20-50 times in culture, undergo apoptosis when there are significant errors

cancer cells

don't follow checkpoints, divide infinitely when in culture, considered to be immortal, evade apoptosis and continue dividing with errors

uncontrollable growth of cancer cells can lead to a

tumor

tumor

mass of tissue formed by abnormal cells

benign

cells are abnormal but not considered to be cancerous yet and cells remain only at tumor site and are unable to spread elsewhere in the body

malignant

mass of cancerous cells that lose their anchorage dependency and can leave the tumor site

metastasis

when cells separate from tumor and spread elsewhere in the body