Cytoskeleton & Cell Division Flashcards
Functions of cytoskeleton
- Provide structural support for plasma membrane & cell organelles
- Intracellular movement/transport of substances
- Cell locomotion (amoeboid movement, embryonic development, cilia, flagella)
- Muscle contraction
What are the 3 main elements of cytoskeleton
- microfilaments
- intermediate filaments
- microtubules
Microfilament structure and function
distributed in 3-D network throughout cytoplasm
•Important in maintaining cell shape; facilitate shape changes during movement, wound contraction during healing & Mm contraction
what makes up microfilament
actin and myosin
actin structure
- Thin filament
- made of smaller G-actinsubunits
- 2 protofilaments twist together to form a double helix (F-actin). These combine to form larger actin filaments
G actin
G-actinis a small, globular protein monomer; polymerizes to form protofilaments
actin location
In microvilli& stereocilia, and beneath plasma membrane, actin links with another protein, filamin.
filamin
- Forms support meshwork called cell cortex(= terminal web)
- Prevents cell from deformation
myosin structure
- Thick
- Myosin molecules are long & rod-shaped, with globular heads
- Form thick, ropy “cables” with myosin heads sticking out sides
- Myosin heads form cross-bridges between adjacent filaments
- Ca & ATP required for movement of filaments for Mm contraction
what can myosin be classified as?
“motor protein” along with dynein and kinesin
intermediate filament structure
- Intermediate in size between microfilaments & microtubules
- Generally form large filaments that bind with intracellular structures
- Heterogenous group; molecular make-up varies between cells
intermediate filament function
- 1’ structural function
- Several different classes, each expressed in different cell types—1 cell may produce more than 1 type of filament
- Useful in immunohistochemistry & tumor ID
types of intermediate filaments (6)
- Cytokeratin
- vimentin
- desmin
- neurofilament proteins
- GFAP
- Lamin
Cytokeratin
characteristic of all epithelial cells; In epidermis of skin forms tonofibrils
(part of intermediate filament)
vimentin
found in mesodermal cells of mesenchymal origin (includes endothelial cells, muscle & neuroectodermalcells)
(part of intermediate filament)
desmin
unique to muscle cells; mesodermal origin
part of intermediate filament
neurofilament proteins
resent in nerve cells (neurons)
part of intermediate filament
GFAP
Glial Fibrillaryacidic Protein. Found in glial (support) cells of nervous system(astrocytes)
(part of intermediate filament)
Lamin
forms layer on inside of nuclear membrane.
part of intermediate filament
Microtubules Structure
- Larger than microfilaments or intermediate filaments
- Composed of 2 types of globular protein subunits—a& b tubulin—arranged in coiled, spiral pattern
- Polymerize to form hollow tubes—readily assemble & disassemble
Microtubles Function
- cell movement, maintenance of cell shape, & intracellular transport of substances
- Movement occurs via addition or subtraction of tubulin subunits from microtubules
- Stabilize microtubules along with capping proteins
Microtubule- associated proteins (MAP’s)
provide energy as ATPase
how do motor proteins interact with microtubules?
Motor proteins dynein& kynesinattach microtubules to organelles; allow movement in cytoplasm
Classic example of microtubules
cell spindle during cell division
what drugs inhibit polymerization of microtubules and cell division?
vincristine & vinblastin
Where can microtubules be found in?
- cilia & flagella
- centrioles
- basal bodies of cilia
- mitotic spindles
Axoneme Structure
•9 pairs of microtubules (peripheral doublets) containing dynein arms arranged in circle with central doublet in middle
(9 + 2 structure)
•Central doublet connects to peripheral doublets by radial spokes& peripheral doublets connect to each other by protein nexin
what is the structure of axoneme in cilia?
In cilia, axoneme grows from basal body, derived from modified centriole
Centrioles location
in region of cell called centrosome, or “cell center” near nucleus
Centrioles Function
cell divison. A pair of centrioles oriented at right angles to each other within centrosome
what is a pair of centrioles called?
diplosome
Centrioles Structure
Each centriole consists of 9 triplets of microtubules arranged in a cylinder
Acts as a nucleation center for microtubules
Mitotic spindle
Occurs during mitosis, centrioles divide.
Role: controls distribution of chromosomes in daughter cells
Cell division
Movement of chromosomes occurs via tubulin subunits & attachment proteins to chromatids at kinetochore(= centromere)
Mitosis
growth & replication of cell
What is interphase and its subphases?
“resting phase”, or time between divisions—occupies most of life of cell
subphases: (G1, S, G2)
G1
Longest of cell phases (hrs to days); during which cell growth, maturation, & differentiation occur
what specifically occurs during G1
hypertrophy—↑in cell size
what specifically occurs during mitosis?
hyperplasia—↑in cell #
Synthesis
Replication of DNA prior to division
what happens at the beginning of S phase?
chromosome # is 2n;at end = 4n
what happens prior to replication, in S phase?
each chromosome has a single chromatid with attached centromere
what is the result of replication of centrioles during synthesis?
2 diplosomes
G2
- Preparation for mitosis
* Synthesis of ATP & tubulin for mitotic spindle
during cell division, what is Mitosis characterized by?
karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division)=> 2 daughter cells
what happens to chromosomes during S & M phase?
chromosomes condense, become visible, look like coiled snakes
what happens to chromosomes during G1 & Go phase?
chromosomes exist in an unraveled mass
Go
-extended phase where some cells end up when they lose capacity for cell divison.
-may be permanent or reversible
(“o” outside cell cycle)
Facultative dividers
retain capacity for division with proper stimulus (e.g., reserve stem cells)
Terminally differentiated
cells lose the capacity to divide (e.g., neurons, cardiac myocytes)
Prophase
- Chromosomes visibly condensed
- Microfilaments & microtubules of cytoskeleton disaggregate
- Centrioles migrate to poles of cell, form spindle apparatus with interpolarmicrotubules between them
Prometaphase
- late prophase
- Nuclear membrane & nucleoli disappear at end of prophase
- Mitotic spindle attaches to chromosomes at kinetochore
Metaphase
chromosomes line up along equator (“metaphase plate”)
Anaphase
- centromeres split apart, chromosomes migrate to opposite ends of cell
- Pulled by microtubules connecting centriole & kinetochore
Telophase
- Nuclear envelope reassembles, nucleoli reappear
- Mitotic spindle disaggregates
- Plasma membrane forms cleavage furrow, cytokinesisbegins => 2 genetically identical daughter cells
Mitotic index
- Used to describe proportion of cells in a tissue in mitosis at any given time
- Important in tumors—estimated by counting # of mitotic figures / HPF (normally <1)
Meiosis
- Reduction division
- Sexual reproduction requires production of haploid gametes(eggs & sperm) via gametogenesis
Where does meiosis occur?
only in germ cells of gonads; in male = spermatogenesis; in female = oögenesis
what does meiosis involve?
- Involves chromosomal duplication followed by two consecutive cell divisions (=> haploid gametes)
- Fusion of gametes (fertilization) produces diploid zygote(fertilized egg)
Result of meiosis
2 daughter cells
what 2 ways do mitosis and meiosis differ?
- During prophase I, homologous pairs of chromosomes form tetrads, with exchange of chromatin via cx (chiasmataformation)
Results in hybrid chromatids different from parents.
Each chromosome pair has potential for cx during prophase I (23 pairs total in humans) - Centromeres do not split during anaphase I
Second meiotic division
- Brief interphase II (interkinesis) with no replication of DNA
- Nocrossing over during prophase II
- Anaphase II—centromeres split
- Telophase II—cytokinesis yields four haploid gametes, each genetically unique
Gametogensis in male and female
- In male 3 or 4 viable gametes (sperm) produced
* In female, cytoplasmic division unequal =>1 functional gamete (ovum) & 2 or 3 non-functional polar bodies
when does spermatogenesis occur in males
at puberty
when does oogenesis begin in females
during fetal development
what occurs during oogenesis
- Female germ cells enter prophase I ~ 5th month of gestation during fetal development & stop
- Remain in prophase Iuntil ovulation (12-50 years later)
what triggers apoptosis?
- Timing in fetal development; referred to as programmed cell death
- Some cells have finite, predetermined life spans (e.g., epithelial cells of skin or GI tract)
- Growth & regression of ovarian follicles
- Destruction of virus/tumor infected cells
- Clonal deletion in thymus; removal of t-cells that react to “self” molecules
Failure of clonal deletion may => autoimmune diseases
how does apoptosis happen?
- Begins with nuclear chromatin condensation & shrinkage of nucleus (pyknosis)
- Cell swells (as result of influx of water due to loss of ATP to maintain pumps in cell membrane)
- Lysosomes release catalytic enzymes into cytoplasm => autolysis
caspace cascade
final pathway initiated. caspaces are normally inactive enzymes
karyolysis
chromatin in nucleus begins to degenerate
Karyorhexis
nuclear material begins to fragment & nuclear membrane disintegrates
what is apoptotic bodies, and what does it lead to?
Fragmented nuclear debris.
Leads to dead, necroticcells, later phagocytosed by macrophages & neutrophils
Necrosis
refers to death of cells as a result of inflammation, traumatic injury, or pathology
