Cytoskeleton and Microtubules Overview

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Questions and Answers

What characterizes constitutive heterochromatin?

It is easily transcribed.

It varies significantly between different cell types.

It contains active gene sequences.

It mainly contains repetitive sequences. (correct)

Which type of chromatin is most abundant in active cells like neurons?

Euchromatin (correct)

Facultative heterochromatin

Constitutive heterochromatin

Barr body

What does a high ratio of heterochromatin to euchromatin indicate about a cell?

The cell is highly transcriptionally active.

The cell has low protein synthesis activity. (correct)

The cell has high metabolic and biosynthetic activity.

The cell is engaged in extensive cell division.

What is the primary function of the nucleolus in cells?

Assembly of ribosomal sub-units Signup and view all the answers

Where is facultative heterochromatin typically found in female cells?

In the Barr body Signup and view all the answers

Which of the following drugs disrupts the activity of the mitotic spindle and can be used in chemotherapy?

Paclitaxel Signup and view all the answers

What is the primary function of actin filaments in cells?

Transport vesicles and organelles Signup and view all the answers

What process allows actin monomers to be added and dissociate from the actin filament?

Treadmilling Signup and view all the answers

What is the primary function of lipid droplets in cells?

Store lipids Signup and view all the answers

Which type of filament provides mechanical stability and is intermediate in size?

Intermediate filaments Signup and view all the answers

Which component of the nucleus is responsible for stabilization?

Nuclear lamina Signup and view all the answers

Which protein is associated with the formation of the nuclear lamina?

Lamin Signup and view all the answers

Which of the following proteins is primarily found in epithelial cells?

Keratins Signup and view all the answers

What is the structure that connects the two layers of the nuclear envelope?

Nuclear pores Signup and view all the answers

Which type of chromatin is typically less condensed and associated with active gene transcription?

Euchromatin Signup and view all the answers

What is the function of myosin motors in relation to actin filaments?

Transporting cargo Signup and view all the answers

Which of the following statements is true regarding inclusions in cells?

They have little to no metabolic activity. Signup and view all the answers

What primarily characterizes the granules of hemosiderin?

Aggregates of denatured ferritin with iron Signup and view all the answers

How are nucleosomes formed?

By wrapping DNA around histone proteins Signup and view all the answers

What is melanin's primary role in skin cells?

UV protection Signup and view all the answers

What structure is formed by two identical chromatids connected by cohesin proteins?

Chromosome Signup and view all the answers

What is the primary function of microtubules in the cell?

Determine cell shape and aid in movement of organelles Signup and view all the answers

Which component is primarily responsible for the polymerization of microtubules?

Microtubule organizing centers (MTOCs) Signup and view all the answers

What is the structural composition of centrioles?

Nine organized microtubule triplets Signup and view all the answers

What role do kinesins play in microtubule transport?

Carry material away from the MTOC towards the plus end Signup and view all the answers

In which cellular process do microtubules play a critical role during cell division?

Chromosome movements by the mitotic spindle Signup and view all the answers

What is the outer diameter of microtubules?

25 nm Signup and view all the answers

Which statement about dyneins in intracellular transport is accurate?

They move material towards the nucleus Signup and view all the answers

What is the main biological role of microtubules in neurons?

Facilitate axoplasmic transport Signup and view all the answers

Signup and view all the answers

Study Notes

Cytoskeleton, Intermediate Filaments, Cytoplasmic Inclusions, Nucleus

The cytoskeleton is a complex array of microtubules, microfilaments (actin filaments), and intermediate filaments.
It is only visible with an electron microscope.
These structures determine cell shape and are vital for organelle and vesicle movement within cells.
They are also involved in whole cell movement.

Microtubules

Composed of heterodimers of αβ-tubulin.
Microtubules have a hollow tube structure with 13 parallel protofilaments.
Diameter: 25nm, Wall thickness: 5nm
The a and β tubulin subunits (54 kDa).
Are dynamic (growing and shrinking).
Stable in axonemes and dynamic in cytoplasm.
Primarily located in radiating arrays from centrosomes, in cilia, and flagella.
Crucial for cell shape, track for organelle and chromosome movement, cilia and flagella movement.
Assembly is directed by microtubule-organizing centers (MTOCs).
Ca and Mg ions, and microtubule-associated proteins (MAPs) are involved in microtubule function.

Microfilaments (Actin Filaments)

Abundant in all cells.
Composed of actin subunits.
Diameter: 5-7 nm.
More flexible than microtubules.
Polarized polymers (have + and - ends).
Dynamic: monomers rapidly add and subtract from ends. This process is called "treadmilling".
Primarily located beneath the cell membrane, in microvilli, in cell extensions.
Myosin motors use ATP to transport cargo along actin filaments.
Crucial for cell contraction. Organelle and vesicle trafficking. Cytoplasmic streaming, cytokinesis (cell division), endocytosis, and muscle contraction.

Intermediate Filaments

Intermediate in size between microtubules and microfilaments.
More stable than other filaments and provide mechanical support to cells.
Types of intermediate filaments:
- Keratins: acidic and basic forms in epithelial cells; form large bundles (tonofibrils), and attach to junctions between cells. Crucial to skin protection. Structure of feathers, beaks, horns, and reptilian scales.
- Vimentin: common in mesenchymal cells (connective tissue).
- Neurofilaments: in neurons; three distinct sizes, forming heterodimers.
- Lamins: found in the nucleus, forming a structural framework called the nuclear lamina.
- GFAP: common in astrocytes, which support nerve cells.

Cytoplasmic Inclusions

Accumulated metabolites and substances, lacking membrane bound activity.
Types of inclusions include lipid droplets, glycogen granules, melanin, lipofuscin, and hemosiderin.
- Lipid droplets: storage of lipids (adipocytes).
- Glycogen granules: carbohydrate storage (hepatocytes).
- Melanin: protection from UV radiation (skin).
- Lipofuscin: age pigment, complex material in stable and non-dividing cells (neurons, cardiac muscle)
- Hemosiderin: iron storage in specialized phagocytic cells, results from red blood cell destruction

Nucleus

Contains DNA (genetic information for proteins).
Components: nuclear envelope, chromatin, and nucleolus.
Nuclear envelope: selectively permeable double membrane barrier between nucleus and cytoplasm.
Lamina: inside the inner membrane for structural support; a network of proteins (like lamins).
Nuclear pores: complex structures that bridge the two layers to permit passage of materials in and out.
Proteins like importins and exportins work with GTP energy in transport.

Chromatin

DNA plus associated proteins.
Condenses during mitosis, forming chromosomes.
Types of chromatin include euchromatin (gene-rich) and heterochromatin (gene-poor).
Euchromatin is more prevalent in active cells (neurons).
Heterochromatin is seen near nuclear lamina.
The ratio of heterochromatin to euchromatin is an indicator of cell activity.

Nucleolus

Cell structure in the nucleus involved in ribosome subunit assembly.
Highly basophilic (staining strongly with basic dyes) due to high rRNA concentration.
rRNA synthesis, processing, and assembly into ribosomal subunits.
Involved in protein synthesis, and transport of ribosomes.

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Description

This quiz explores the structures and functions of the cytoskeleton, focusing on intermediate filaments and microtubules. Learn about their composition, visibility under an electron microscope, and role in cell shape and movement. Test your knowledge on the dynamic nature of microtubules and their importance in cellular organization.