Nucleus and Chromosomes

Structure and Function of the Nucleus

The nucleus is the control center of eukaryotic cells, containing most of the cell's genetic material and directing cellular activities.

• Nuclear Envelope: Double membrane that encloses the nucleus and separates its contents from the cytoplasm.

• Pores: Protein-lined channels in the nuclear envelope that regulate the passage of substances in and out of the nucleus.

• Nuclear Lamina: A network of protein filaments lining the inner surface of the nuclear envelope, providing mechanical stability.

Organization of Genetic Material

• Chromosomes: DNA is organized into discrete units called chromosomes, each consisting of one long DNA molecule associated with proteins called histones.

• Chromatin: The complex of DNA and proteins that makes up chromosomes.

• Genetic Information: Chromosomes carry genetic information necessary for cell function and inheritance.

Nucleolus and Protein Synthesis

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• mRNA: Messenger RNA is synthesized in the nucleus and carries genetic instructions from DNA to the cytoplasm for protein synthesis.

Ribosomes

Types and Functions of Ribosomes

Ribosomes are molecular machines responsible for protein synthesis. They exist in two cytoplasmic regions:

• Free Ribosomes: Suspended in the cytosol; typically synthesize proteins that function within the cytosol.

• Bound Ribosomes: Attached to the endoplasmic reticulum (ER) or nuclear envelope; generally synthesize proteins destined for insertion into membranes, packaging within organelles (like lysosomes), or export from the cell (secretion).

• Structurally, free and bound ribosomes are identical and can switch roles as needed.

Endomembrane System

Components and Functions

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins.

• Includes: nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, plasma membrane, and various types of vesicles and vacuoles.

• Functions:

  • Synthesis of proteins

  • Transport of proteins

  • Metabolism

  • Movement of lipids

  • Detoxification of proteins

Endoplasmic Reticulum (ER)

Structure and Regions

The endoplasmic reticulum is a network of tubules and sacs that plays a central role in the synthesis and transport of biomolecules.

• ER Lumen: The internal compartment of the ER, separated from the cytosol by the ER membrane.

• Two distinct regions:

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.

  • Rough ER: Studded with ribosomes; involved in the synthesis of proteins, especially those destined for secretion or membrane insertion.

Smooth ER Functions

• Synthesizes lipids, including oils, steroids, and new membrane phospholipids.

• Metabolizes carbohydrates.

• Detoxifies drugs and poisons.

• Stores calcium ions.

• Enzymes in smooth ER are important for the synthesis of lipids and steroids (e.g., sex hormones).

Rough ER Functions

• Many cells secrete proteins produced by ribosomes attached to rough ER.

• Most secretory proteins are glycoproteins (proteins with carbohydrates covalently bonded to them).

• Carbohydrates are attached to proteins in the ER lumen.

• Rough ER also synthesizes membrane phospholipids and assembles them into the ER membrane.

Golgi Apparatus

Structure and Function

The Golgi apparatus acts as a warehouse for receiving, sorting, shipping, and even some manufacturing of cellular products.

• Products of the ER, like proteins, are modified and stored, then sent to other destinations.

• Extensive in cells specialized for secretion.

• Consists of a group of associated, flattened membrane sacs called cisternae.

• Membrane of each cisterna in a stack separates its internal space from the cytosol.

Cis and Trans Faces of the Golgi

• Cis Face: Located near the ER; receives vesicles containing ER products.

• Trans Face: On the opposite side; gives rise to vesicles that pinch off and travel to other sites.

• Before dispatch, the Golgi adds tags (such as molecular identification markers) to vesicles to ensure proper sorting and delivery.

Example Table: Golgi Apparatus Faces

Key Terms and Concepts

• Chromosome: A DNA molecule with associated proteins, carrying genetic information.

• Histone: Basic protein associated with DNA in chromosomes.

• Glycoprotein: Protein with covalently attached carbohydrate.

• Vesicle: Small, membrane-bound sac used for transport within cells.

• Cisternae: Flattened membrane sacs in the Golgi apparatus.

Summary

• The nucleus stores genetic information and directs protein synthesis.

• Ribosomes synthesize proteins, either for use in the cytosol or for export/insertion into membranes.

• The endomembrane system coordinates the synthesis, modification, and transport of proteins and lipids.

• The ER and Golgi apparatus are central to processing and shipping cellular products.