CELS191 Lecture 9: Nucleus

Course Overview

  • Course Title: CELS191 Cell Structure and Diversity
  • Lecture: Lecture 9 - Nucleus
  • Instructor: Professor David Orlovich
  • Core Focus: Understanding the nucleus in cell biology

Lecture Objectives

  • Describe the basic structure and function of the nucleus.
  • Outline the structure of the nuclear pore complex and its role in nucleo-cytoplasmic exchange.
  • Explain the organization of DNA within the nucleus, including the differences between euchromatin and heterochromatin.

Key Concepts

  • Endomembrane System: The origin of proteins for cellular functions originates from genes in the nucleus which are transcribed into mRNA.
  • Ribosomes: Composed of rRNAs and proteins, found bound to the rough ER or free in the cytoplasm, playing a pivotal role in protein synthesis.

Structure of the Nucleus

  • General Characteristics:
    • Prominent organelle, typically 5–10 µm in diameter.
    • Usually contains one nucleus per cell.
    • Acts as the genetic control center and repository of hereditary information.
  • Components:
    • Nuclear Envelope: Double membrane comprising the outer and inner membranes with perinuclear space in between.
      • The outer membrane is continuous with the endoplasmic reticulum (ER).
    • Nuclear Pores: Composed of nucleoporins, these control the exchange of substances between the nucleus and cytoplasm. Approximately 1000 nuclear pores exist per cell, enabling selective transport.
    • Nucleolus: Non-membrane bound, specializes in synthesizing ribosomal RNA and assembling ribosomal subunits.

Function of the Nucleus

  • Nucleo-Cytoplasmic Exchange:
    • Molecules such as mRNA, tRNA, and ribosomal subunits exit the nucleus for protein synthesis.
    • Building materials, signals, and energy move in to regulate gene expression.

DNA Organization within the Nucleus

  • DNA Packing:
    • Each human cell contains about 2.5 m of DNA, necessitating efficient packing.
    • Histone Proteins: Proteins (H2–H4) enable the formation of nucleosomes, creating a 10-nm fiber.
    • Higher Order Structures:
      • 30-nm fibers form when nucleosomes coil further, resulting in 300-nm fibers during cell division.
  • Chromosomes: DNA is organized into distinct chromosome structures, which can be visually represented in karyotypes.

Euchromatin vs Heterochromatin

  • Euchromatin:
    • Less densely packed.
    • Contains actively used genes that need to be readily accessible for transcription.
  • Heterochromatin:
    • More densely packed and often contains inactive genes.

Summary of Key Definitions

  • Nuclear Envelope: Double membrane structure surrounding the nucleus.
  • Nuclear Pore Complex: Protein complexes that facilitate transport across the nuclear envelope.
  • Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.
  • Euchromatin: Less compact DNA, actively participates in transcription.
  • Heterochromatin: Tightly packed DNA, generally inactive.

Revision Questions

  • What is the nuclear envelope made of?
  • What lines the inner surface of the nuclear envelope?
  • What moves in and out of the nucleus via nuclear pores?
  • What is the function of the nucleolus in ribosome production?
  • How is DNA organized within the nucleus, and what role do histones play?
  • Explain the primary differences between euchromatin and heterochromatin in terms of structure and function.