PCB 3023 Final Part I

Fall 2026. Dr Daughdrill

What are the three parts of the cytoskeleton

Intermediate filaments, Microtubules, and Actin filaments

Describe the size of Intermediate filaments, function, and where they are found'

10nm in diameter (larger than actin, smaller than myosin)

Function is to provide cell to withstand mechanical stress (have high tensile strength)

Found in the cytoplasm (anchored to plasma membrane at cell-cell junctions called desmosomes, found in nucleus (form the nuclear lamina)

Desrcibe the structure of one unit of the intermediate filament

8 tetramers

Tetramere: 2 staggered coiled-coil dimers

Dimer: 2 eclipsed alpha helical monomers

Describe the three types of cytoplasmic intermediate filaments

Ropelike

Keratin filaments: In epithelial cells

Vimentin filaments: In connective tissue, muscle cells, and glial (non-neuronal cells in the nervous system that do not conduct electrical impulses. They support the neurons ) cells

Neuro filaments: In nerve cells

Decribe the nuclear intermediate filaments

2D meshwork

Nuclear lamins: In all animal cells

What are nuclear lamins and what do they do?

Nuclear lamins are the proteins that create intermediate filaments that form the nuclear lamina. Those filaments stengthen the inner nuclear membrane

What happens to the nuclear lamina during cell division? How does it do this?

It disassembles and reassembles during cell divisoin

Phosphorylation of the lamins = weakens interactions between lamin tetramers, lamins disassemble

Dephosphorylation of the lamins = strengthens interactions between lamin tetreamers, allows for lamins to reassemble

What are microtubules and what do they do?

Microtubules are formed from tubulin subunits that create tracks in the cell. These tracks can be used by motor proteins to transport organelles, vesicles, etc.

What is an example of stable and dynamic microtubules?

Stable: Cilia and flagella

Dynamic: Mitotic spindle during cell division

Describe the structure of one unit of a microtubule

13 parallel protofilaments forming a cylindrical structure

Protofilament: Dimer of alpha and beta tubulins

Dimer: (+) Beta tubulin and (-) Alpha tubulin

What leads to polymerization and depolymerization of a microtubule protofilament? Mention the GTP cap

Note that the Alpha and Beta tubulins are constantly hydrolyzing the GTP on Beta tubulin. The rate of the addition of a GTP bound Alpha-Beta tubulin dimer relative to the rate of the Alpha-Beta tubulin dimer getting its GTP hydrolyzed determines if the microtubule protofilament is growing or shrinking

Polymerization: GTP bound to Beta tubulin

Depolymerization: GDP bound to Beta tubulin

The GTP cap determines if the microtubule will grow or shrink. If there is no cap, that means the rate of GTP hydrolysis is faster than the rate of adding GTP bound Alpha-Beta tubulin dimers; the microtubule will rapidly shrink

What is the structure of a centrosome? What is the Alpha-Beta tubulin dimer and nucleation site complex called?

A centrosome is a centriole pair, surrounded by a gel-like protein matrix that has nucleation sites, with microtubules sprouting out from the nucleation sites

Alpha-Beta tubulin dimer and nucleation site complex is called the Gamma-tubulin ring complex. The Alpha-Beta tubulin’s negative (Alpha) end is embedded here with growth occurring at the postive (Beta) end

Do pen model for motor protein moving

Pen model for motor protein moving answer

Motor proteins do what to the Golgi and ER?

Contract Golgi and stretch ER

What is the basic strucutre of flagella/cilia? How does this make the flagella/cilia move?

They are made up of 9 pairs of microtubules plus associated dyneins surrounding 2 single microtubules

It makes the flagella/cilia move by one half of the ring structure using its dynein to pull its adjacent microtubule pair. The other half stays relaxed. This gies the oscillating movement of flagella and whiplike motion of cilia

Describe Actin filament structure, size, function, and where they are found

Actin filaments are made up of a twisted chain with a (+) and (-) end

Thin and flexible, 7nm in diameter

Function is to provide movement for cell. Play large role in muscle contractions

Found in the cytoplasm concentrated in a dense network just under the membrane called to cell cortex

How does polymerization of Actin filaments differ depending on the concentration of free actin in the cell?

High concentration of free actin: Actin filament can polymerize at both (-) and (+) end

Low concentration of free actin: Actin filament undergoes treadmilling

Describe treadmilling of Actin filaments. What is the result to the length of the actin filament

Actin monomers with ADP leaves from the (-) end, Actin monomers with ATP joing from the (+) end

This process keeps the Actin filment are about the same length

What proteins promote Actin polymerization? What protein prevents Actin polymerization?

Promotes: Actin-related proteins (ARP’s) and Formins

Prevents: Thymosin

Describe the structure of Myosin I. What direction does it move towards?

Myosin I has a general head domain and a varying tail domain. The tail binds to cargo and the head moves along the actin filament in the (+) direction

Note that Myosin I can also be anchored to the cell membrane and move actin filaments in the cell cortex. This is essential for reshaping the cell

Do hand model for Myosin I moving along actin

Hand model for Myosin I moving along actin

What is the structure of Myosin II? What is the structure of a Myosin II filament? What is the function of Myosin II?

Myosin II is a dimer with two heads and a coiled-coil tail. These tails interact with eachother to form the Myosin II filament with the heads protruding. The function of Myosin II is to slide oppositely oriented actin filaments passed each other. This is what causes a muscle contraction

What is the structure of a skeltal muscle fiber. Create a flow chart of descending complexity

Skeletal muscle fiber > Myofibrils > sarcomeres (contractile unit of a myofibril) > Myosin II and actin filament arranged in specific structure

How do you trigger a contraction?

An electrical signal from an action potential travels through the T tubules which triggers release of Ca2+ from the Sarcoplasmic reticulum. The Ca2+ is released into the cytoplasm surrounding the myofibrils. The Ca2+ binds to Troponin (associated with the end of Tropomyosin) which causes a conformation change in the Tropomyosin. This change causes the Tropomyosin to no longer inhibit the Myosin heads so that they can now slide the Actin filaments past eachother, causing a muscle contraction.

How does Tropomyosin inhibit Myosin?

It blocks the binding site of the Myosin head and Actin filament.