lec 10- The cytoskeleton Flashcards
what are the three types of cytoskeletal elements?
-Microtubules (move vesicles)
-microfilaments/actin filaments (help with motility)
-intermediate filaments (structural support)
what type of polarity do epithelial cells have?
apical-basal polarity
which cytoskeletal component is the largest?
microtubules
what are the two types of microtubules and what do they do?
cytoplasmic microtubules- found in cytosol and maintain nerve cells, move vesicles, formation of spindles, maintaining or altering cell shape
Axonemal microtubules- include the organized and stable microtubules found in the structure specialized for movement such as cilia, flagella, basal bodies to which cilia and flagella attach
what is the axoneme?
the central shaft of a cilium or flagellum, is a highly ordered bundle of MT’s
what is the structure and composition of microtubules (MT)?
MT are straight, hollow cylinders of varied length that consist of usually 13 longitudinal arrays of polymers called protofilaments
what makes protofilaments?
alpha-beta-heterodimers
how are a-tubulin and b-tubulin binded together to form a heterodimer?
by non-covalent bonds
what do MAPs (microtubule associated proteins) do?
have one domain attached to microtubule while other protrudes out as a tail, they stablilize the microtubule and help it take shape
how is MAP activity controlled?
MAP activity is controlled by the the addition or removal of the phosphate groups from amino acid residues
which MAP is responsible for alzheimers disease when it has high levels of phosphorylation?
tau
which type of binding sites do a-tubulin and b-tubulin have?
a-tubulin: has only a GTP binding site
b-tubulin: has both GTP and GDP
why do the ends of protofilaments have charges resulting in a polarity?
it is due to the dimers in the MT being oriented the same way
which tubulin when exposed results in minus and plus end?
alpha exposed makes the end negative, while beta makes it positive
in what patter do microtubules extend outwards to support the cells shape?
a radial array, resulting in round flattened shape
can microtubules also play a key role in maintaining the internal organization of cells?
yes
which drugs depolarize or disassemble microtubules?
colchicine and nocodazole
what happens to the golgi when the disassembling drugs are used on the microtubules?
disperses the golgi into seperate stacks scattered throughout the cytoplasm
which motor proteins that bind to microtubules move vesicles anterograde (forward) and retrograde (backward)?
kinesin and dynein
what does myosin do?
it is a motor protein that attaches to actin filaments and helps move them
what do molecular motor proteins convert?
convert energy from ATP to mechanical energy
what is the chemical cycle steps that helps motor proteins move microtubules/actin filaments?
binding of ATP, hydrolysis of ATP, release of ADP and Pi, binding of new ATP
do motor proteins have momentum?
no, once the energy is used up they come to an immediate stop
what movement mechanism does kinesin use?
“hand over hand” mechanism
can kinesin and dynein be binded simultaneously by an organelle?
yes, they can both move across the same microtubule, one above while the other below
can motor proteins regulate the dispersal of melanosomes?
yes, dynein aggregates it while kinesin disperses it
what determines the microtubules dynamic instability?
the growth of a microtubule is if there is a large concentration of GTP bound free tubulin, low concentration means shrinking of microtubule
where do microtubules form and from which side do they start growing?
form from microtubule organizing centers and they start growing from the negative side
what is a centrosome?
a microtubule organizing center
what are the stages in the cell cycle?
G1- cell grows and carries out normal metabolism, organelles duplicate
S- DNA replication and chromosome duplication
G2- cell grows to prepare for mitosis
M- mitosis: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis
how long does the M phase and the interphase (G1, S, G2) last for?
M phase- an hour or so
Interphase- days, weeks, or longer
what are the steps of Mitosis?
prophase- chromosomes condense and spindles are formed, nuclear envelope is dispersed
prometaphase- microtubules connect to kinetochores of chromosomes, chromosomes move to spindle equator
metaphase-chromosomes are alligned in the middle of microtubules from one side to the other, both poles
anaphase- centromeres split, and chromatids seperate, spindles move apart
telophase- chromotids cluster at opposite ends of spindle, daughter cell formed by cytokinesis
what is centrosomes made of?
-yTURCS (tubulin ring complex), where nucleation occurs
-centrosome matrix
-centrioles (short cylinder of modified microtubules)
from which end to what does the microtubule grow?
”-“ end to “+” end
what is the yTURC made of?
y-tubulin
what are the three classes of microtubules in a spindle?
kinetochore microtubules- are connected to chromosomes
Astral microtubules- project toward the cell cortex and interact with it thereby orienting the spindle of division
polar microtubules- interact with microtubules from the opposite pole of the cell
what are the three components of the nuclear envelope?
nuclear pores, nuclear lamina (composed of intermediate filaments), and nuclear membranes
what happens to the nuclear envelope in prophase?
the three parts of the nuclear envelope is disassembled in different processes:
-the membrane is disrupted by dynein making wholes tearing it apart
-phosphorylation of human lamin causes depolymerization of the intermediate filaments in the nuclear lamina
what happens in prometaphase?
-mitotic spindle is formed and chromosomes are moved by microtubules into the center of the cell, a single kinetochore is attached to microtubules from both spindle poles
-the metaphase plate is generated by a tug of war between two centrosomes
-microtubule plus end can polymerize or depolymerize
-microtubule negative end can only depolymerize
what is a kinetochore?
a complex of proteins that associate with the centromere off a chromosome, “+” end of microtubules attach to the kinetochores
what happens in metaphase?
metaphase occurs when chromosomes are aligned at metaphase plate
what happens in anaphase?
-tubulin subunits are lost from both ends of kinetichore microtubules, and lost from minus end of polar microtubules, then added to plus end of polar microtubules
-motor proteins push polar microtubules apart
what happens in telophase?
spindle is disassembled, nuclear envelopes are reassembled, chromosomes become dispersed, cytoplasm is split between both daughter cells in a process called cytokinesis
what happens in cytokinesis?
-a signal is released from the spindle midzone
-a belt-like bundle of actin filament (ring) forms just below the plasma membrane in early anaphase
-cleavage progresses and ring tightens around cytoplasm
-contraction of ring is generated by interaction between actin and myosin motor protein
