Cell Signaling & the Cell Cycle

junctions in animal cells for cell signaling

gap junctions

junctions in plant cells for cell signaling

plasmodesmata

cell-cell recognition

allows cells to communicate via direct contact betw. membrane-bound cell surface molecules

Paracrine signaling

secreted molecules diffuse locally and trigger a response in neighboring cells

synaptic signaling

a special type of local signaling in animal nervous systems electric signals in a nerve cell triggers neurotransmitter secretion

synapse

a narrow space betw. nerves and target cells that chemical signals diffuse across to trigger a response during synaptic signaling

hormones

animal and plant chemicals used for long distance signaling

endocrine (hormonal) signaling

specialized cells release hormones that travel via the circulatory system (animals) or xylem/phloem (plants) to target cells that respond to them

Epinephrine (adrenaline) in cell signaling

glycogen -> releases glucose 1-phosphate -> converted to glucose 6-phosphate -> has phosphates taken and put into blood -> fight or flight reaction stimulates glycogen breakdown by activating glycogen phosphorylase enzymes W/O entering cells

stages of cell-to-cell signaling

reception, transduction, response

reception

the target cell detects a signaling molecule that binds to a receptor protein on the cell surface or inside the cell

only ______ target cells _______________ to signal molecules

specific ... detect and react

receptor proteins on/in target cells allow

signal detection and response

signal shapes are _________ to specific sites on the receptor

complementary

ligand

A molecule (the signaling molecule) that binds specifically to another molecule, usually a larger one.

ligand binding causes receptors

to undergo changes in shape = direct activation of the receptor, enabling it to interact w/ other molecules

most receptors are inside or are plasma membrane proteins (ligands are water-soluble + too big to pass thru)

3 types of transmembrane receptors

1. G protein-coupled receptors

2. ligand-gated ion channels

3. receptor tyrosine kinases

G protein-coupled receptor

a cell-surface transmembrane receptor that works with the help of a G protein, a protein that binds w/ energy-rich GTP

Ligand-gated ion channel

Type of membrane receptor that has a region that can act as a "gate" when the receptor changes shape (opens/closes, allowing/stopping ion diffusion)

intracellular receptors

steroids, thyroid hormones, and nitric oxide in cytoplasm or nucleus of targets.

Signals pass thru target cell membranes bc they're hydrophobic enough to cross

transcription factors

special proteins that control which genes are turned on/transcribed by RNA

second messengers

1. 1) Cyclic AMP

   2) Calcium ions

   Small, non-protein water soluble molecules or ions that send messages throughout the cells by diffusion.

cyclic AMP (cAMP)

Cyclic adenosine monophosphate

(w/ epinephrine) binding to a GPCR leads to activation of adenylyl cyclase via a G protein.

adenylyl cyclase (a cytoplasmic enzyme) converts ATP to cAMP, which broadcasts signals until an enzyme converts cAMP to AMP

usually activates kinase A, which phosphorylates various proteins.

transduction

the conversion of the signal to a form that can bring about a specific cellular response, done by a signal transduction pathway made up of relay molecules

transduction pathways are

multistep and have many proteins

proteins are activated/inactivated by

adding/removing phosphates or others small messenger ions/molecules

the original signaling molecule is

NOT physically passed along the pathway

the original signaling molecule almost NEVER

enters the cell changing shapes from phosphorylation transduces the signal through the pathway

protein kinase

An enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating (activating/inactivating) the protein. Acts as relay molecules in pathways

phosphorylation cascade

a pathway where a cascade of protein phosphorylations transmit a signal, each bringing w/ it a shape change from being activated or inactivated.

protein phosphatases

phosphorylation cascade enzymes that rapidly remove phosphate groups (dephosphorylation) from proteins, inactivating them

TURNS OFF the signal transduction pathway when the initial signal is not present

makes kinases available for REUSE

response

transduced signal triggers a specific cellular response, usually in the nucleus or cytoplasm

the final activated molecule in a pathway may function as

a transcription factor

cell cycle

the life of a cell from the time it is formed during division of a parent cell until its own division into 2 daughter cells

genome

all of an organism's genetic material/DNA

prokaryotes: usually one DNA strand

eukaryotes: many large DNA molecules

chromatin

the entire complex of DNA (carries genes for inherited traits) and proteins (maintain structure + control gene activity) that is the building material of chromosomes

somatic cells

all body cells except the reproductive cells. diploid cells that have 46 chromosomes (2 sets of 23 from each human parent)

gametes

reproductive cells (sperm and eggs). haploid cells that have 23 chromosomes (1 set of 23 in humans)

after DNA replication, chromosomes

condense (chromatin fibers fold up so they're shorter and thicker)

sister chromatids

joined copies of the original chromosome. Each duplicated chromosome has two. has identical DNA attached by cohesin protein complexes

centromere

a region of chromosomal DNA where the chromatid is attached most closely to its sister chromatid by proteins bound to centrosomic DNA

mitosis

the division of genetic material and the nucleus

meiosis

a variation of cell division that produces gametes, yielding daughter cells w/ only one set of chromosomes

happens in special ovary or testes cells

fertilization returns the number of chromosomes back to normal by

fusing two gametes together

functions of cell division

1. reproduction

2. growth and development

3. tissue renewal

MPF activity during the cell cycle

peaks during mitosis, then dips significantly during G1, S, and rises again late G2

Cyclin during the cell cycle

peaks during mitosis (not as high as MPF), then sharply drops to a constant rate during G1 and accumulates during S and G2

Cyclin binds with Cdk to

make MPF and pass the G2 checkpoint/start mitosis

mitosis is stopped and G1 starts when

cyclin in MPF degrades during anaphase and Cdk is recycled, repeating the cycle

G2 of Interphase

centrosome is duplicated + microtubules of the spindle are organized, each region w/ 2 centrioles

Prophase

chromatin fibers condense into chromosomes

nucleoli disappear; centrosomes move away from each other by lengthening microtubules

mitotic spindle begins to form, made up of centrosomes, microtubules, and asters

duplicated chromosomes appear as 2 identical sister chromatids, joined at the centromere

mitotic spindle

begins to form in the cytoplasm during prophase. consists of proteins and microtubules from the cytoskeleton.

microtubules elongate (polymerize) by adding tubulin subunits + shorten (depolymerize) by losing subunits

centrosome

A structure present in the cytoplasm of animal cells that functions as a microtubule-organizing center and is important during cell division. A centrosome has two centrioles. Duplicates and each go to either end of the spindle

aster

A radial array of short microtubules that extends from each centrosome toward the plasma membrane in an animal cell undergoing mitosis.

kinetochore

A structure of proteins attached to the centromere that links each sister chromatid to the mitotic spindle. When "captured," chromosomes move toward the poles on opposite ends but have equal forces pulling them = stuck in middle

metaphase plate

An imaginary structure located at a plane midway between the two poles of a cell in metaphase on which the centromeres of all the duplicated chromosomes are located.

Prometaphase

chromatids have a kinetochore at the centromere and centrosome microtubules invade the nuclear area, attaching to the kinetochores, pulling them back and forth

metaphase

centrosomes are at opposite ends of the cell chromosomes are at the metaphase plate (middle)

anaphase

cohesins cleaved (chromatids -> full chromosomes) separated chromosomes go to opposite ends of the cell as kinetochore microtubules shorten cell elongates

telophase

2 nuclei and envelopes form, nucleoli reappear, chromosomes are less condensed, and spindle depolymerizes

cytokinesis (late telophase)

the division of the cytoplasm (actual cell division) formation of the cleavage furrow -> cell pinches into 2

cleavage furrow

The first sign of cleavage in an animal cell; a shallow groove in the cell surface near the old metaphase plate. Cytoplasmic-side actin microfilament ring interacts with myosin, making it contract and deepen the furrow until the cell splits

plant cells do not have

a cleavage furrow

cytokinesis in plant cells

vesicles from the Golgi coalesce, producing a cell plate in the middle of the cell

cell plate enlarges until it fuses with the plasma membrane and cell walls form inbetween = 2 daughter cells

mitotic (m) phase

mitosis and cytokinesis

interphase

longest phase of the cell cycle (includes G1, S, and G2)

G1 phase

"first gap"/subphase of interphase. cell grows

S phase

"synthesis" subphase of interphase. DNA is replicated and more cell growth

G2 phase

"second gap"/subphase of interphase. more growth and prep for mitosis

all 3 subphases

help the cell grow by producing proteins and cytoplasmic organelles

some cells

do not or rarely divide, staying in G1 to do their jobs

binary fission

A form of asexual reproduction in single-celled organisms by which one cell doubles in size and divides into two identical cells (eukaryotic single-celled organisms still perform mitosis)

origin of replication

where circular DNA of bacterial chromosomes begin to replicate, beginning cell division.

replication produces 2 origins and 1 goes to the other end of the cell.

possibly anchored by proteins instead of spindles

during binary fission, the cell divides by

pinching the plasma membrane inward

mitosis might have come from a simpler prokaryotic cell because

some binary fission proteins are related to mitosis proteins

nuclear envelopes and larger genomes are also

part of ancestral processes of binary fission, somehow evolving into mitosis

a cell that is fused with another further along the cell cycle will

jump to that stage in the cell cycle because of the cytoplasmic signals in the second cell

cell cycle control system

A cyclically operating set of molecules in the eukaryotic cell that both triggers and coordinates key events in the cell cycle, regulated by internal and external signals

checkpoint

A control point in the cell cycle where stop and go-ahead signals transmitted by signal transduction pathways can regulate the cycle.

G1 "restriction" checkpoint

determines whether the cell pauses and enters G0 (no go-ahead signal) or whether cell passes the G1 checkpoint and continues the process for cell division (gets go-ahead signal)

G0 phase

A nondividing state in which a cell has left the cell cycle. some cells can be "called back" to the cell cycle by external cues

M phase checkpoint

mitosis checkpoint to make sure chromosomes are attached to the spindle at the metaphase plate, ready to initiate anaphase. Delayed until ready, then go-ahead signal activates separase -> sister chromatids separate

growth factor

A protein secreted by certain body cells that stimulates other cells to divide

ex:) PDGF from blood platelets binds w/ fibroblast receptors, triggering a signal transduction pathway that tells cells to pass G1 and divide

density-dependent inhibition

a phenomenon in which crowded cells stop dividing because of an external physical factor. They divide until there is one layer and fill in any gaps if cells are removed. Caused by cell-surface proteins sending signals that inhibit division

anchorage dependence

cells must be attached to a surface to divide. Usually signaled to the cell cycle control system via pathways involving plasma membrane proteins and the cytoskeleton

cancer cells DO NOT

-have density-dependent inhibition or anchorage dependence

-follow signals regulating the cell cycle

-stop growing when growth factors are depleted

-stop at normal checkpoints (stops randomly)

cancer cells may make

growth factors themselves or have an abnormality in the signaling pathway that conveys growth factors even if its not there

gene mutations can also cause

abnormalities in cell cycle control systems, altering protein functions = poor regulation

cancer cells can divide

indefinitely if supplied w/ nutrients

transformation

when cells acquire the ability to divide indefinitely, causing them to behave like cancer cells

cancer cells avoid

apoptosis (programmed cell death) and the immune system's defenses

benign tumor

A mass of abnormal cells with specific genetic and cellular changes such that the cells are not capable of surviving at a new site and generally remain at the site of the tumor's origin.

malignant tumor

cells whose genetic changes enable them to spread to new tissues and impair organ functions, becoming cancerous

also considered "transformed" bc cells divide indefinitely

some cancers can separate and enter

blood and lymph vessels to go to other parts of the body

metastasis

The spread of cancer cells to locations distant from their original site.

treating localized tumors

radiation (damages DNA more than regular DNA and it can repair the damage)

treating cancer

chemotherapy (administering drugs that are toxic to dividing cells thru the circulatory system) makes you sick bc of the effects on healthy cells too

proto-oncogenes

normal versions of genes that code for proteins that stimulate normal cell growth and division

oncogenes

cancer-causing genes that are closely related to proto-oncogenes

tumor-suppressor genes

A gene whose protein product inhibits cell division, thereby preventing the uncontrolled cell growth that contributes to cancer.