Module 3.5 The Cytoskeleton

Overview of the Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, determines cell shape, and enables cellular movement. Its components can rapidly assemble and disassemble to meet the changing needs of the cell.

• Maintains cell shape and size by forming an internal framework.

• Provides strength and structural integrity, anchoring organelles and supporting plasma and nuclear membranes.

• Facilitates cellular movement through interactions with motor proteins.

• Specialized functions in different cell types, such as phagocytosis in macrophages and contraction in muscle cells.

Types of Cytoskeletal Filaments

The cytoskeleton is composed of three main types of protein filaments, each built from smaller protein subunits. These filaments allow for rapid assembly and disassembly, enabling the cell to adapt its structure as needed.

• Actin filaments (microfilaments)

• Intermediate filaments

• Microtubules

Actin Filaments (Microfilaments)

Actin filaments are the thinnest cytoskeletal filaments, composed of two intertwining strands of actin subunits.

• Provide structural support, bear tension, and maintain the cell's shape.

• Involved in cellular motion when combined with the motor protein myosin.

• Diameter: approximately 5–9 nm.

• Example: Form the core of microvilli, which increase surface area for absorption.

Intermediate Filaments

Intermediate filaments are ropelike structures made of various fibrous proteins, such as keratin. They are stronger and more permanent than actin filaments.

• Form much of the framework of the cell and anchor organelles in place.

• Help organelles and the nucleus maintain their shape.

• Enable cells and tissues to withstand mechanical stresses.

• Diameter: approximately 10 nm.

Microtubules

Microtubules are the largest cytoskeletal filaments, consisting of hollow tubes made from tubulin subunits.

• Can be rapidly assembled or disassembled to allow for changes in cell size and shape.

• Maintain the internal architecture of the cell and keep organelles in place.

• Serve as tracks for motor proteins (dynein and kinesin) to transport vesicles and organelles.

• Form the core of cilia and flagella.

• Diameter: approximately 25 nm.

Comparison of Cytoskeletal Filaments

The following table summarizes the main properties and functions of the three types of cytoskeletal filaments:

Module 3.6 The Nucleus

Overview of the Nucleus

The nucleus is the control center of the cell, directing cellular activities and determining the types and rates of protein production. It houses the cell's genetic material and is essential for gene expression and cell division.

• Contains DNA, which encodes nearly every protein in the body.

• Genes within DNA are transcribed by various types of RNA to produce proteins.

Structure of the Nucleus

The nucleus consists of three main structures:

• Nuclear envelope: A double phospholipid bilayer that surrounds the nucleoplasm.

• Chromatin: DNA and associated proteins in a loose structural arrangement.

• Nucleolus (plural: nucleoli): Region responsible for ribosomal RNA synthesis and ribosome assembly.

Nuclear Envelope

• Double membrane structure, similar to the mitochondrial membrane.

• Outer membrane is studded with ribosomes and continuous with the endoplasmic reticulum.

• Inner membrane is supported by a network of intermediate filaments called the nuclear lamina.

• Nuclear pores are large protein complexes that connect the nucleoplasm with the cytoplasm, allowing substances to move between these compartments.

Chromatin and Chromosomes

• Chromatin is a complex of one long DNA molecule and associated proteins, which organize and compact the DNA to conserve space.

• Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around a group of histone proteins ("beads on a string" structure).

• During cell division, chromatin condenses into chromosomes.

• Human cells contain 46 chromosomes (23 pairs: one maternal, one paternal).

• Sister chromatids are identical copies of a chromosome, joined at a region called the centromere, and are produced in preparation for cell division.

Nucleolus

• The nucleolus is a nuclear region responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomes.

• One or more nucleoli may be present in the nucleoplasm.

Key Terms and Definitions

• Cytoskeleton: A network of protein filaments that provides structural support and enables movement within the cell.

• Actin filament (microfilament): The thinnest cytoskeletal filament, involved in cell shape and movement.

• Intermediate filament: A strong, ropelike filament that provides mechanical support.

• Microtubule: A hollow tube that maintains cell shape and serves as a track for intracellular transport.

• Nucleus: The organelle that contains the cell's genetic material and controls cellular activities.

• Chromatin: The complex of DNA and proteins that forms chromosomes within the nucleus.

• Nucleolus: The site of ribosomal RNA synthesis and ribosome assembly within the nucleus.