Lectures 1.4-1.5

Question 1

How does ion diffusion across a semi-permeable cell membrane lead to a membrane potential?

Answer 1

ion channels facilitate diffusion of ions across the lipid bilayer. A diffusion potential arises when diffusion of ions leads to an unequal number of charges on the inside versus the outside of the cell.

Question 2

equilibrium potential

Answer 2

the membrane potential where the net flow through any open channels is 0

Question 3

Goldman-Hodgkin-Katz equation

Answer 3

describes the membrane potential for biological membranes

Question 4

properties of action potentials

Answer 4

rapid, transient, self-propagating electrical excitation in the plasma membrane of an excitable cell.

• All-or-none

Question 5

what ion currents contribute to depolariation and repolarization phases of an action potential ?

Answer 5

Na+ flows through voltage-gated channels causing an inward positive current (depolarization) and K+ flows through voltage-gated channels causing an outward positive current (repolarization)

Question 6

what is the function of voltage gated ion channels during an action potential?

Answer 6

an inward current depolarizes membrane potential to threshold. this depolarization triggers voltage-gated Na+ channels to open, as Na+ floods into the cell, it is further depolarized and causes more voltage-sensitive Na+ channels to open.

Question 7

how do axon diameter and myelination affect action potential conduction velocity?

Answer 7

• increased axon diameter increases conduction velocity by decreasing internal resistance
• myelination increases conduction velocity

Question 8

how do refractory periods contribute to action potential conductance?

Answer 8

they limit the rate of action potential firing and help to ensure unidirectional propagation

Question 9

how do refractory periods arise?

Answer 9

inactivation gates of the Na+ channels are closed and no action potential can occur until the channel is in the resting closed conformation with the inactivation gates open

Question 10

local potential

Answer 10

small change in the resting membrane potential of a neuron at one point in the cell

Question 11

depolarization

Answer 11

an instance where the membrane potential becomes more positive than the resting potential

Question 12

diffusion potential

Answer 12

the potential difference generated across a membrane because of a concentration difference of an ion. It can be generated only if the membranes is permeable to the ion. The size of the diffusion potential depends on the size of the concentration gradient

Question 13

membrane repolarization

Answer 13

the period where the membrane potential returns back to resting potential

Question 14

threshold potential

Answer 14

• the membrane potential at which the action potential is inevitable.
• when a local potential leads to depolarization above the threshold potential, an action potential arises

Question 15

axon hilock

Answer 15

the section of axon adjacent to the cell body that has a high density of voltage-gated Na+ channels

Question 16

voltage-gated K+ channels

Answer 16

depolarization causes K+ channels to open slowly and increases K+ conductance

• repolarization is caused by outward K+ current

Question 17

internode

Answer 17

flattened plasma membrane that is wrapped around the axon to form a segment of biochemically specialized sheath

Question 18

nodes of ranier

Answer 18

regions of axon between internodes of myelin (location of most membrane channels)

Question 19

saltatory conduction

Answer 19

when the action potential jumps from node to node

• action potentials travel faster as it is not passing through the entire axon
• metabolic energy is conserved

Question 20

Multiple Sclerosis

Answer 20

demyelinating disease that affects the central nervous system

Question 21

relative refractory period

Answer 21

onset is at the end of the absolute refractory period and continues until membrane potential returns to resting levels

• an action potential can be elicited only by very large inward currents
• need more inward current than normal because the K+ conductance is higher than at rest and the membrane potential is closer to the K+ equilibrium than it is to the resting membrane potential

Question 22

absolute refractory period

Answer 22

the period during which another action potential cannot be elicited no matter how large the stimulus

• comprises the entire duration of the action potential
• due to the closed inactivation gates of the Na+ channels

Question 23

synapse

Answer 23

a junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter.

• Chemical synapses can be excitatory (open cation channels that cause Na+ influx/depolarization) or inhibitory (open ligand gated Cl- or K+ channels to suppress firing and make it harder to depolarize the membrane)

Question 24

neuromuscular junction

Answer 24

chemical synapse formed by the contact between a motor neuron and a muscle fiber

• depolarization of nerve terminal opens Ca2+ channels triggering release of neurotransmitter acetylcholine into synaptic cleft

Question 25

neuromuscular junction

Answer 25

chemical synapse formed by the contact between a motor neuron and a muscle fiber - depolarization of nerve terminal opens Ca2+ channels triggering release of neurotransmitter acetylcholine into synaptic cleft - acetylcholine binds to receptors in muscle cell plasma membraneopening Na+ channels and allows Na+ influx - depolarization causes action potential - depolarization causes Ca2+ channels to open and Ca2+ releases into sarcoplasmic reticulum causing muscle to contract.

Question 26

Neurotransmitter

Answer 26

a chemical substance that is released at the end of a nerve fiber by the arrival of a nerve impulse and, by diffusing across the synapse or junction, causes the transfer of the impulse to another nerve fiber, a muscle fiber, or some other structure.

Question 27

acetylcholine receptor

Answer 27

binds to acetylcholine on in the muscle cell plasma membrane and causes cation selective channels to open and allows Na+ influx into the muscle cell

Question 28

acetylcholine esterase

Answer 28

enzyme that hydrolyzes ACh and decreases ACh concentration in cleft to terminate the signal from the presynaptic cell

Question 29

voltage-gated calcium channels

Answer 29

opens due to depolarization of muscle plasma membrane. Releases Ca2+ into the sarcoplasmic reticulum thus causing muscle to contract

Question 30

Stages of eukaryotic cell

Answer 30

G1 (Gap 1), S (DNA Synthesis), G2 (Gap 2), M (Mitosis) G0 (nondividing/terminally differentiated cells) most cells in the body

Question 31

properties of cyclin-dependent kinase

Answer 31

- regulates the cell cycle - Cdk activity is needed for progression of cell cycle - two components: Cdks and cyclin - Cdk is only active when bound o a cyclin molecule

Question 32

what are the four mechanisms by which cdks is activated?

Answer 32

- only active when bound to a cyclin molecule - Cdk and cyclin synthesis regulated by cellular growth factors - regulated by phosphorylation (kinases and phosphotases) - other regulatory proteins (inhibitors) bind to Cdks and inhibit activity

Question 33

what happens at cellular checkpoints?

Answer 33

the cell cycle is stopped if an essential processe is not complete

Question 34

G1/S checkpoint

Answer 34

- DNA damage activates p53 (a transcription factor that regulates genes such as p21) - p21 concentration in the cell increases - p21 binds and inhibits cdk2/cyclinE - Rb is not phosphorylated and E2F is not activated so cells do not enter S phase

Question 35

How do cdks and checkpoint proteins regulate entry into S phase?

Answer 35

- Regulated through phosphorylation of Rb - active CDK2-Cycling E phosphorylates Rb - Reduces binding to E2F - E2F activates synthesis of proteins required for DNA synthesis (S phase)

Question 36

name the steps in DNA damage checkpoint activation

Answer 36

essentially make a kinase cascade: - sensor proteins bind to DNA damage and recruite transducer kinases - transducer kinases amplify signal by phosphorylating effector kinases - transducer and effector kinases phosphorylate protein effectors which modulate cell processes and stop the cell cycle

Question 37

differences between DNA and RNA molecular makeup

Answer 37

``` chemical difference: 2' H vs. 2' OH structural difference: RNA is base labile Named different ("deoxy" before DNA names) ```

Question 38

base pairing

Answer 38

- G-C, A-T; (RNA = A-U) - hydrogen bonds: 3 in G-C and 2 in A-T - base pairs have similar geometry - base pairing causes DNA to form antiparallel helix

Question 39

Hoogsteen pairing

Answer 39

- T bonds with AT - C+ bonds wih GC - Most stable at low pH when C is protonated - Allows formation of 3 and 4 strand DNA

Question 40

L=T+W

Answer 40

Linking number = Twist + Writhe

Question 41

A form

Answer 41

RNA-DNA duplexes

Question 42

Z form

Answer 42

left-handed helix that forms from methylation and negative supercoiling - forms from alernative purine/pyrimidine sequences

Question 43

Hairpins

Answer 43

forms from palindromes | - usually in ssDNA and RNA

Question 44

Cruciforms

Answer 44

formed from palindromes | - usually in dsDNA

Question 45

What allows specific DNA sequences to be identified in the lab (molecular diagnostics)?

Answer 45

Denatured DNA can anneal to form double stranded DNA

Question 46

Linking Number

Answer 46

number of times the two strands are linked together - sum of twist and supercoils - cannot change w/o breaking one or both strands of DNa

Question 47

topoisomers

Answer 47

DNA that differs only in topology (L)

Question 48

topoisomerases

Answer 48

enzymes that interconvert topoisomers

Question 49

twist number

Answer 49

number of watson-crick turns in a DNA molecule - only depends on number of turns in double helix - b-form= twist is +1 for every 10.4 bp - most DNA is B form so T= bp length of DNA / 10.4

Question 50

Writhing Number

Answer 50

supercoiling of the double helix - dependent on the path through space - number of times the double helix crosses itself in 3D - DNa with no supercoils is called relaxed DNA (lowest energy form)

Question 51

Writhing Number

Answer 51

supercoiling of the double helix - dependent on the path through space - number of times the double helix crosses itself in 3D - DNa with no supercoils is called relaxed DNA (lowest energy form)

Question 52

Why is controlling DNA topology important?

Answer 52

controlling topology is essential for maintaining and expressing genetic information. It affects DNA behavior.