Lectures 23-26

7.6 The Dynamic Cytoskeleton

• three major types of cytoskeletal elements in eukaryotic cells

  • actin filaments: two coiled strands

    • maintain cell shape by resisting tension (pull)

    • move cells via muscle contractions or cell crawling

    • divide animal cells in two

    • move organelles and cytoplasms in plants, fungi, and animals

  • intermediate filaments

    • keratins, lamins, or others

    • maintain cell shape by resisting tension (pull)

    • anchor nucleus and other organelles

  • microtubules

    • alpha and beta-tubulin dimers

    • maintain cell shape by resisting compression (push)

    • move cells via flagella/cilia

    • move chromosomes during cell division

    • assist formation of cell plate during plant cell division

    • provide tracks for intracellular transport

Actin Filaments

• actin proteins are not symmetrical

  • the head-to-tail polymerization results in polar ends

  • the plus end grows faster than the minus end

• particularly abundant under the plasma membrane, crisscrossing networks help stiffen/define the shape

• myosin-motor protein

  • when myosin hydrolyzes ATP → ADP, it extends its “head” region, attaches it to actin, then contracts to pull itself along the actin filament

  • gradually pulls itself towards the plus end of actin filament

• cytokinesis — final stage in cell division

  • active filaments connected to plasma membrane, arranged in a ring

  • myosin causes filaments to slide past one another, reducing diameter of ring, pulling in membrane that fuses to produce 2 cells

• cytoplasmic streaming — directed flow of cytosol/organelles often seen within plant or fungal cells

  • movement occurs along actin filaments, powered by myosin

• cell-crawling — groups of actin filaments grow, causes bulges in plasma membrane that extend and move the cell

Intermediate Filaments

• different types of intermediate filaments use different types of protein subunits

• do not have distinct ends, only have a structural role

• nuclear lamines — intermediate filaments forming a dense mesh inside nuclear envelope that anchors chromosomes, defines the shape of the nucleus, stabilizes the envelope

  • by controlling interaction between lamins, cells will break down and reform the nucleus during cell divison

Microtubules

• assembled from subunits consisting of alpha-tubulin and beta-tubulin that exist as stable dimers

• tubulin dimers polymerize in polar head-to-tail via noncovalent bonds forming thin chains called protofilaments that interact with each other to form hollow tubes

• microtubules exhibit polarity — alpha-tubulin at minus end, beta-tubulin at plus end

  • plus end grows faster

• microtubules come from microtubule-organizing centers (MTOCs)

  • plus ends grow outward, radiating thru cell

  • most animal and fungal cells have one MTOC near nucleus (centrosome in animal cells)

    • animal centrosomes consist of 2 bundles of MTs (centrioles) surrounded by proteins that help growth of new MTs

      • centrioles each consist of 9 triplets of MTs in a circle

• MTs provide stability and are involved in movement

• MTs serve as tracks for vesicle transport

  • vesicles from rough ER → Golgi apparatus also require MT tracks

• motor proteins pull vesicles along the tracks

  • requires ATP

  • kinesis — generates vesicle movement towards the plus end

    • 2 large subunits each have 3 major functional regions: head section, tail with small polypeptides, and a stalk connecting head and tail

    • head binds to the microtubule while tail region binds to transport vesicle

    • each “step” hydrolyzes one ATP → ADP + Pi

Flagella and Cilia: Moving Entire Cell

• eukaryotic flagella project from cell surface for locomotion

  • flagella are surrounded by plasma membrane

  • consist of several microtubules made from tubulin dimers

  • undulate (whip back and forth) to move the cell

• prokaryotic flagella

  • consist of a single helical rod made of flagellin (in bacteria)

  • move cell by rotating like a propeller

  • not surrounded by plasma membrane

• eukaryotic cilia/flagella

  • cillum — short, hairlike projection found in some eukaryotic cells

  • most have an axoneme (9+2) arrangement of MTs

    • consist of 9 MT doublets surrounding 2 central MTs

    • 9 doublets originate from basal body

      • structurally identitcal to a centriole (9 MT triplets in a circle), MTOC for axoneme doublets

• beating of cilia requires ATP

• dynein — make up the arms between axoneme doublets

  • motor protein using ATP to move along MTs towards minus end

  • as dynein “walks”, the connections cause cilia/flagella to bend, forming a swimming motion

12.2 M Phase

• chromatin — DNA wrapped around globular histone proteins

Mitosis

• prophase — chromosomes condense, spindle apparatus forms

  • spindle apparatus — moves replicated chromosomes in early mitosis and pulls chromatids apart in late mitosis

  • at the start of prophase, the two centrosomes move to opposite sides of the nucleus to form the spindle apparatus

    • polar microtubules — extend from each spindle pole and overlap with one another

• prometaphase — once chromosomes have condensed, nuclear envelope disintegrates and cytoplasmic MTs attach to chromosomes at kinetochores

  • kinetochore — forms on a chromosome during M phase at the centrosome. contains motor proteins

    • each sister chromatid has its own kinetochore on opposite sides of each replicated chromosome

    • kinetochore microtubules — MTs attached to the kinetochores

  • kinesin/dynein motors attached to kinetochores “walk” the chromosomes up and down MTs, when they reach the plus end, kinetochore proteins secure their attachment

    • each chromosome will have its 2 kinetochores attached to MTs from opposite end of spindle apparatus

    • the chromosomes are pushed/pulled by MTs until they reach middle of spindle

• metaphase — all the chromosomes are lined up between the spindle poles on the metaphase plate

  • polar MTs extend from each spindle pole overlap, forming pole-to-pole connections

  • each chromosome is held by kinetochore MTs reaching out from opposite ends with same amount of tension

  • spindle poles held in place by astral MTs that extend from MTOCs and interact with proteins on plasma membrane

• anaphase — cohesins holding sister chromatids together are cleaved by an enzyme

  • each replicated chromosome is pulled apart, moved to opposite poles because of shrinking kinetochore MTs

  • two spindle poles are pulled/pushed further apart

    • pushed by motor proteins in overlapping polar MTs

    • pulled by different motors on plasma membrane which walk along astral MTs, dragging poles to opposite sides

• telophase — nuclear envelope reforms around each set of chromosomes, which begin to decondense

How Do Chromosomes Move During Anaphase

• mitotic spindle forces — when cells transition from metaphase → anaphase, plus ends of kinetochore MTs switch from adding tubulin dimers to removing them

• fibers that extend from kinetochore are attached to a ring that surrounds the MT

  • during anaphase, the plus end frays and disassembles, the fraying forces the ring and chromosome towards the minus

Cytokinesis

• in plant cells, polar MTs define and organize where the new plasma membrane and cell wall will form

  • golgi vesicles carry components for a new cell wall to middle of dividing cell, moving along polar MTs via motor proteins

  • in the middle, vesicles fuse and form the cell plate, which grows and fuses with existing plasma membrane, forming the two daughter cells

• in eukaryotic cells, cytokinesis starts with the cleavage furrow

  • ring of overlapping actin filaments contract just inside plasma membrane

    • caused by myosin motor proteins binding to actin, sliding them past one another, shrinking the ring

    • vesicles imported into this zone called the midbody, fuse with the plasma membrane

Bacterial Cell Replication

• binary fission — basically like mitosis

12.3 Control of the Cell Cycle

Cell-Cycle Regulatory Molecules

• M-phase promoting factor (MPF) — substance that initiates M phase; in cytoplasm

• cyclins — proteins that increase in concentration right before M phase, plummets in interphase

• MPF made of two subunits: cyclin protein and protein kinase (constant concentration)

  • protein kinase catalyzes phosphorylation from ATP → target protein

  • MPF phosphorylates proteins that trigger M phase

  • cycle-dependent kinase (Cdk) — protein kinase subunit in MPF is only functional attached to cyclin subunit

• MPF’s Cdk subunit is further regulated by 2 phosphorylation sites. one activates, one inhibits

  • both sides are phosphorylated after cyclin binds to Cdk subunit, allowing concentration of dimer to increase without starting M phase