Cells - cytoskeleton-dependent processes (mammalian biology)

How is motility provided in neurons?

• Axonal transport keeps the cells alive

• Neurons have to cope with long distance transportation - average organelle (1um) needs ~21,000 years to diffuse 30cm

• Synapsis has to communicate with the cell body in order to keep the neuron alive

What are the cargos used during axonal transport?

• Organelles (mitochondria, lysosomes, endosomes)

• Synaptic vesicles

• mRNA

• Proteins

What is the structure of muscles on a cellular level?

• Muscles consist of sarcomeres

• Thick filaments consist of myosin II

• Thin filament consists of F-actin and associated proteins

• There is no interaction between the myosin and actin in a relaxed muscle

How is muscle contraction controlled?

• Relaxed muscles have no interaction between myosin heads and actin filaments

• Stimulus from the neuron spreads over the plasma membrane

• Depolarisation of the membrane spreads calcium from the ER into the cytoplasm

• Binding of calcium to the troponin complex releases the myosin binding sites on actin

• Myosin binds actin and walks towards the Z-disc - contraction

• Calcium is removed by calcium pumps, myosin releases the actin filament and slides back - relaxation

What is the general structure and function of a flagellum?

• One or few per cell

• Function in cell locomotion

• Propeller-like motion

• Beats 10-40 times per second

What is the general structure and function of cellular cilia?

• Usually many per cell

• Function in fluid and particle transport

• Back and forth motion

• Beats 12-20 times per second

What is the structure of a flagellum/cilium?

• Basal body anchors the cilium/flagellum at the cell

• Axoneme is the core of the cilium/flagellum and is made from microtubules

• The basal body is made of centrioles

How does transport support the formation and function of cilia?

• ‘Rafts’ travel along the axoneme

• Kinesin and dynein drive the bidirectional transport

How is the dynein organised within a motile cilium/flagellum?

• Rings of dynein line the motile cilium/flagellum

• One has an outer arm and the other has an inner arm

• Radial spokes point towards the centre of the cilium

• Axonemal dynein is variable in its molecular structure: outer arm has 3 heads, inner arm has 1/2 heads

• Flagellar dynein bridges between adjacent microtubular pairs

• Dynein slides microtubules against each other - motor activity against the bridges causes bending

How are non-motile cilia important in the body?

• Many endothelial cells have one primary cilium

• Detect signals that govern cell proliferation

• Sense flow and bending - triggers various regulation pathways

• Primary cilia are essential for developmental processes

Which cells produce non-motile cilia?

• Inner ear

• Kidney

• Bile duct

• Pancreas

• Bone/cartilage

• Eye

What is the difference in function between motile and non-motile cilia?

• Motile cilia generate flow and clean surfaces

• Non-motile cilia sense environmental cues

How does the cytoskeleton support cell migration?

• Motility is an important feature of many animal cells

• F-actin helps to move cells along their surface

• Cell motility helps wounds heal and organ development

What are the different phases of mitosis?

• Prophase

• Metaphase

• Anaphase

• Telophase

How does the cytoskeleton support prophase of mitosis?

• Chromosomes condense

• Nuclear envelope breaks down

• Spindles are formed (microtubules)

How does the cytoskeleton support metaphase of mitosis?

• Microtubules make contact with the chromosomes

• Chromosomes are positioned in one plane

How does the cytoskeleton support anaphase of mitosis?

• Occurs in two sections: anaphase A and anaphase B

• Anaphase A - microtubules and motors pull on the chromosomes, chromatids move to the poles

• Anaphase B - rapid elongation of the spindle fibres, formation of a contractile ring

How does the cytoskeleton support telophase of mitosis?

• Cell centre contracts and separates (cytokinesis)

• Chromosomes decondense

• Nuclear envelope is reformed

How is the mitotic spindle organised?

• Two centrosomes at opposite sides of the cell

• Microtubules projecting out from the cell are astral microtubules

• Microtubules down the central plane and the outside of the cell are the polar microtubules

• Microtubules that make contact with the chromosomes in the metaphase plane are kinetochor microtubules

How does microtubule organisation and molecular motors help organise the chromosomes in mitosis?

• Microtubule polymerisation and depolymerisation at kinetochors oscillate the chromosomes to position them in the middle of the spindle

• Molecular motors elongate the spindle, supporting chromosome segregation in spindle function

• Two major motor-driven activities: sliding of polar microtubules against each other, pulling on astral microtubules

• Depolymerisation of kinetochor microtubules pull the chromosomes to the spindle poles

How can chromosomes have force-driven motility in mitosis?

• Oscillation of chromosomes in prometaphase - mediated by microtubule dynamics

• Movement of chromatids to the spindle poles in anaphase A - driven by motor proteins and microtubule dynamics

• Elongation of the spindle in anaphase B - driven by motor proteins

How does a mitotic checkpoint control the progression of the cell cycle?

• Metaphase checkpoint ensures all chromosomes are connected to the kinetochor microtubules

• If affirmative: will progress with mitosis

• If negative: will not progress with mitosis

What are the structures involved in cytokinesis?

• Telophase nucleus - decondensing chromosomes, new nuclear envelope is formed

• Midbody - contains microtubules and proteins involved in separation of both cells

• Cleavage furrow - formed by the contractile actin/myosin ring

• Centrosome - organising the new microtubule array

How does the contractile ring affect the location of cytokinesis?

• Localises at the area of constriction

• Ring forms near the cortex at the end of anaphase

• Ring contains myosin, actin, regulators and actin-binding proteins

• Myosin II and actin make the contractile ring

• Inhibition of myosin II disturbs the organisation of the actomyosin ring