cell biology test 5

constitutive genes

genes that are continuously expressed regardless of environmental conditions “always on”

regulated genes

genes whose expression is determined by the needs of the cell “turned off”; allows cells to adapt to nutrient availability, stress, growth, and more

operon

a prokaryotic group of genes under the control of a single promotor and operator

promotor

where RNA polymerase binds

operator

regulatory site for repressors or activators

repression eukaryotes

bind to silencer sequences, block activator proteins, recruit histone deacetylases (condensing chromatin)

repression prokaryotes 

bind to the operator region to block RNA polymerase from binding; transcription of genes turn off

enhancers

regulatory DNA sequences in eukaryotes that increase transcription of specific genes by stabilizing RNA polymerase II when activator proteins bind

induction

the process in which a gene is activated in response to a regulatory molecule such as inducer or activator protein to initiate transcription of a gene

induction eukaryotes

enhancers activated by activator proteins and recruit transcription machinery

induction prokaryotes

binds repressor which releases operator allowing RNA polymerase to transcribe gene

lac operon

found in E. coli, enables bacterium to metabolize lactose when glucose is scarce

lacZ

involves beta galactosidase breaks lactose into glucose + galactose OR lactose into allolactose the inducer of the operon  

lacY

permease enzyme that is a membrane transport protein which allows lactose into the cell

lacA

enzyme thiogalactoside transacetylase transfers acetyl groups to certain lactose analogs and plays a role in detoxification possibly

when lactose is absent

repressor binds to operator which blocks RNA polymerase meaning no transcription

when lactose is present

lactose converted to allolactase the inducer binds to repressor causing the repressor to change shapes and release the operator allowing transcription

cAMP

small signaling molecule in cells; made by the enzyme adenylate cyclase which is inhibited by glucose

when glucose is absent 

adenylate cyclase is active and cAMP levels are high; cAMP binds to CAP and together bind to CAP sites on an operon, effectively turning on the nearby genes

when glucose is present

enzyme inhibited and cAMP is low; CAP doesn’t bind making weak transcription

trp operon

found in E. coli and other bacteria; encodes enzymes for tryptophan biosynthesis which allows cells to make tryptophan when its not available in the environment

chorismate

precursor molecules used in the biosynthesis of the amino acid tryptophan

when tryptophan is present

it binds to the trp repressor activating it so it can bind to the operator and turn off transcription of the trp operon

eukaryotic chromatin structure

euchromatin, histone modification, DNA methylation

eukaryotic transcriptional control

promotors and enhancers, transcription factors

eukaryotic RNA processing 

alternative splicing, RNA editing (5’capping and 3’tails)

eukaryotic mRNA stability and transport

UTR elements and miRNAs

eukaryotic translational and post-translational control

protein modifications

UTRs (untranslated regions of mRNA)

located at the 5’ and 3’ end of mature mRNA; do not encode a protein but contain regulatory sequences

dicer

a specialized RNAse III endonuclease that cuts double-stranded RNA (dsRNA) into small fragments creating miRNAs and small interfering RNAs (siRNAs)

RISC complex

an assembly of miRNAs, siRNAs, argonaute proteins (AGO), and accessory proteins

steps of RISC complex

1. the guide RNA finds complementary bases

2. AGO binds RNA and executes silencing (or even cleaving of mRNA)

3. accessory proteins assemble, stabilize, and recruit RISC

sensory input

the nervous system collects information from the environment via sensory receptors

integration

the brain and spinal cord process and interpret the input and decide if an action should be taken

motor output

the nervous system sends signals to muscles or glands in response to decision about input

central nervous system (CNS)

composed of brain and spinal cord; receives sensory info and coordinates responses

peripheral nervous system (PNS)

all other neural tissue outside of the CNS connecting it to organs, muscles, and glands; divided into somatic (voluntary) and autonomic (involuntary) branches

afferent (sensory)

signals carry info from receptors and arrive at the CNS

efferent (motor)

signals exit the CNS and travel to the PNS

soma

the cell body; contains nucleus; the control center of processing signals

dendrites

receive signals and send them towards the soma

axon

conducts electrical impulses away from the soma (to muscles or other neurons)

axon terminals

contain synaptic vesicles filled with neurotransmitters to be transmitted to the next cell

myelin sheath

protective fatty layer wrapping around axon; aids in speed of signal transmission and protects axon

nodes of Ranvier

small gaps between the myelin sheath along the axon; help with saltatory conduction

saltatory spread

jumping along myelinated axons

neuroglia

supporting cells of the nervous system (outnumber neurons 10:1); do not conduct impulses; provide structure, support, supply nutrients

astrocytes

provide structural support by anchoring neurons to capillaries; regulate chemical balance; help form blood-brain barrier; most abundant of the glial cells (in the CNS)

oligodendrocytes

produce myelin sheaths (in CNS)

microglia

specialized immune cells of the CNS that act as phagocytes responding to injury and inflammation/fighting pathogens

ependymal cells

lining the ventricles of CNS; produce and circulate cerebrospinal fluid; form a barrier between cerebrospinal fluid and nervous tissue

schwann cells

form myelin sheath around peripheral axons; aid in axon regeneration after injury (in PNS)

satellite cells

surround neuron cell bodies in ganglia; regulate microenvironment by controlling exchange of nutrients, ions, and signaling molecules; provide structural support (in PNS)

resting membrane potential

the electrical charge difference across the plasma membrane of a resting neuron (typically -70 mV); essential for generating action potentials (nerve impulses)

depolarization 

triggered by opening of voltage-gated Na+ channels; membrane potential becomes less negative

action potential

a rapid, temporary change in cell’s membrane potential that, once it reaches threshold, is propagated along the length of the neuron’s axon

threshold potential

critical voltage (around -55 mV) needed to trigger an action potential; if reached Na+ channels open all together and full spike occurs

refractory period 

the short time after an action potential when a neuron cannot fire again or its ability is limited 

absolute refractory period

no new action potential possible; Na+ channels are open or inactivate (ensures each action potential is separate)

relative refractory period 

a new action potential can occur if stimulus is stronger than usual (during K+ efflux and hyperpolarization; neuron more negative so it needs a push to reach threshold)

voltage-gated ion channels

open/close in response to changes in membrane potential

ligand-gated ion channels

open when a specific chemical (ligand) binds

steps of transmission of nerve impulse along neuron

1. a stimulus disturbs the membrane of the neuron at the dendrite opening Na+ channels at the site

2. Na+ flows inward through open channels and inside of neuron becomes less negative

3. local depolarization triggers voltage-gated Na+ channels causing more Na+ to rushed in making the inside of the neuron more positive (now +30 mV) causing action potential peak

4. Na+ channels inactivate shortly after opening, K+ channels now open and K+ flows out of neuron (repolarization)

5. Na/K pumps restore original gradients and the neuron cannot fire again or needs stronger stimulus

axon hillock

specialized part of the neuron where the soma transitions into the axon; where action potential begins before propagating down the axon

electrical synapses 

direct, physical connection between two neurons that allows ions to flow instantly from one cell to another through gap junctions 

chemical synapses 

junction where one neuron communicates with another cell by releasing neurotransmitters across a small gap call the synaptic cleft 

neurotransmitters

chemical messengers of the nervous system; released from the presynaptic neuron into the synaptic cleft and bind to receptors on the postsynaptic cell

acetylcholine

muscle contraction, learning, memory

biogenic amines

help regulate mood + more (dopamine, serotonin, histamine)

neuropeptides 

send slow long-lasting messages for controlling pain, stress, or mood

temporal summation

occurs when multiple signals arrive at a neuron in rapid succession from the same presynaptic input and their effects “add together” increasing the chance of reaching threshold

spatial summation

occurs when multiple presynaptic neurons release neurotransmitters at the same time onto a single postsynaptic neuron; postsynaptic neuron adds up these inputs across space

tranquilizers 

psychoactive drugs that act on the CNS to reduce anxiety, fear, and agitation

minor tranquilizers

used as antianxiety agents; increase GABA activity to inhibit more creating a calming effect

major tranquilizers

block dopamine receptors to treat bipolar disorder, schizophrenia and more

myasthenia gravis (MG)

rare, chronic autoimmune disease that causes muscle weakness due to the immune system blocking communication between nerves and muscles; antibodies block or destroy acetylcholine receptors

multiple sclerosis (MS)

chronic autoimmune disease where the immune system targets myelin forming scar tissue which blocks nerve signals and over time damages the nerve fibers themselves

endocrine signals

chemical messages (hormones) released by glands into the bloodstream to regulate distant organs

paracrine signals

type of local cell communication involving a release of signaling molecules called paracrine factors into the extracellular space affecting neighboring cells

autocrine signals

occurs when a cell secretes molecules that bind to receptors on its own surface altering its own behavior/eliciting a response

2nd messengers

intracellular molecules that relay signals/messages from receptors on the cell surface further inside to help amplify and spread the message (cAMP, Ca2+, cGMP)

hydrophilic messengers

molecules which tend to be polar or charged which cannot easily cross the lipid bilayer of the cell; bind to cell surface receptors and trigger second messenger pathways; response fast but short lived

hydrophobic messengers

lipid-soluble molecules that easily diffuse through the cell membrane; bind to intracellular receptors and often act as transcription factors to alter gene expression; response is slow but long lasting

down regulation

the process by which cells decrease their sensitivity to a signal, usually by reducing the number or activity of receptors; cell could be pulled into cell and degraded or recycled, make fewer new receptors, use chemical modification to desensitize the receptors

cancers caused by defective growth factor

non-small cell lung cancer, breast cancers, glioblastoma, colorectal cancer, bladder cancer, ovarian cancer, prostate cancer

achondroplasia

most common genetic cause of dwarfism resulting from a mutation in the FGFR3 gene; acts as a stop to cartilage proliferation in growth plates, and the mutation makes the receptor overactive constantly inhibiting growth and impairing bone elongation

growth factors

naturally occurring proteins or steroid hormones that bind to specific receptors on cell surfaces to trigger intracellular signaling cascades related to growth

G protein-coupled receptors (GPCR)

a large family of cell-surface receptors that detect signals outside the cell and activate internal pathways via G proteins

steps of GPCR

1. a ligand (1st messenger) binds to the GPCR

2. the receptor changes shape and activated a nearby G protein

3. the G protein swaps GDP for GTP to switch on

4. the G protein splits into alpha and beta subunits which move along the membrane'

5. either subunit may bind to target proteins called effectors which produce second messengers to magnify the original signal

6. the cell will change activity based on the signal spread by the second messenger

inositol triphosphate (IP3) and diacylglycerol (DAG)

produced when GPCR + G protein activates phospholipase C enzyme in plasma membrane by splitting PIP2 into IP3 and DAG

functions of inositol triphosphate (IP3) and diacylglycerol (DAG)

activate platelets, cause smooth muscle contraction, cause insulin secretion, cause antibody production, open calcium channels

cyclic AMP (cAMP)

produced when GPCR + G protein activates adenylyl cyclase in plasma membrane and converts ATP to cAMP often activating protein kinase A (PKA) for the above functions 

functions of cAMP

glycogen breakdown, strengthened heart contraction, inhibits smooth muscle contraction, causes kidney cells to excrete salt and water

cholera

caused by Vibrio cholerae, a digestive system infection which secretes cholera toxin; toxin modifies G protein to be in a constantly on state by preventing GTP hydrolysis leading to the continuous production of cAMP; this excess of cAMP in intestinal cells drives CFTR chloride channels to pump Cl- into the intestinal lumen causing Na+ to follow passively to balance the charge and water follows osmotically

protein kinase associated receptors

trigger intracellular signaling by activating protein kinases

receptor tyrosine kinases (RTKs)

single-pass transmembrane proteins with extracellular ligand-binding domain and intracellular tyrosine kinase domain

phosphotyrosines

serve as docking site for other proteins that will be phosphorylated and activated; activating downstream signaling protein processes like cell growth; in RTKs

receptors associated with kinases

these receptors lack their own kinase domain, but recruit cytoplasmic protein kinases to do the phosphorylation for them