CELS191 Lecture 9: Nucleus - Comprehensive Study Notes

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Professor David Orlovich

Professor David Orlovich specializes in the evolution of plants and fungi, and molecular ecology.

Lecture 9: Nucleus - Objectives

By the end of this lecture, you should be able to:

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

Recap: Endomembrane System and Protein Synthesis

Proteins transported via the endomembrane system originate from genes in the nucleus. RNA carries information from these genes out of the nucleus, where ribosomes translate it into proteins.

Ribosomes: Structure and Function

Structure

Ribosomes are complexes made of ribosomal RNAs and proteins, found in two locations:

Bound ribosomes: attached to the rough ER
Free ribosomes: in the cytoplasm

Function

Ribosomes carry out translation. The more protein synthesis a cell requires, the more ribosomes it contains.

The Nucleus: Overview

The nucleus is the most prominent organelle in eukaryotic cells, typically 5-10 µm in diameter. Most cells have one nucleus. It contains most of the cell’s genes (with some additional genes found in the mitochondria and chloroplasts) and serves as the repository of inherited genetic information, acting as the cell’s control center.

The Nucleus: Structure

The nucleus is surrounded by the nuclear envelope and contains a prominent region called the nucleolus. It also contains tightly packaged DNA and has channels called nuclear pores.

The Nuclear Envelope

The nuclear envelope is composed of two membranes: an outer and an inner membrane, with a perinuclear space in between. The outer membrane is continuous with the endoplasmic reticulum. Each membrane is a phospholipid bilayer.

Nuclear Lamina

The inner surface of the nuclear envelope is lined by the nuclear lamina, which is composed of intermediate filaments. The nuclear lamina maintains the shape of the nucleus and helps organize the packing of DNA within the nucleus.

Defective Nuclear Lamina

In certain genetic disorders, an altered form of lamin protein results in cells that cannot produce a typical nuclear lamina. Consequently, the nuclei are unable to maintain a spherical shape, limiting the ability of affected cells to divide, resulting in atypical features and premature aging.

Nuclear Pores

Nuclear pores are channels made of proteins (nucleoporins) that form the Nuclear Pore Complex. There are approximately 1000 nuclear pores per cell. They span the nuclear envelope and control the movement of molecules in and out of the nucleus (nucleo-cytoplasmic exchange).

Nucleus to Cytoplasm

mRNA, tRNA, and ribosomal subunits move from the nucleus to the cytoplasm. mRNA (messenger RNA) carries information from a gene. tRNA (transfer RNA) and ribosomal subunits are needed to build proteins.

Cytoplasm to Nucleus

Control signals, building materials, and energy move from the cytoplasm to the nucleus (e.g., signals for when to turn a gene on or off, the building “blocks” for RNA, and energy for chemical synthesis).

The Nucleolus

The nucleolus is a prominent nuclear structure within non-dividing cells. It is a non-membrane-bound specialized region within the nucleus and is responsible for making ribosomal RNA and ribosomal subunits, which are then transported out of the nucleus and assembled as ribosomes in the cytoplasm. There may be two or more nucleoli per cell, depending on the species and stage of the cell cycle.

DNA Organization

DNA is a nucleotide polymer. Each human cell contains about 2.5 meters of DNA, which is 250,000 times the diameter of the nucleus. Therefore, DNA must be very well packed but also accessible.

Levels of DNA Organization

The DNA double helix is about $2 nm$ in diameter.
The helix interacts with specific proteins called histones (H2-H4). This interaction forms a $10 nm$ diameter fiber. Each 'bead' is called a nucleosome.
Further interactions between the DNA and another histone (H1) cause the $10 nm$ fiber to coil to form the $30 nm$ fiber.
This $30 nm$ fiber then loops to form $300 nm$ fiber.
During cell division, the $300 nm$ fibers coil to form metaphase chromosomes.

Chromosomes and Karyotypes

Chromosomes can be displayed as a karyotype, which can be used to screen for chromosomal variants. During interphase, the location of chromosomes within the nucleus is not random. The pairs are generally not located together.

Cancer cells can have very disorganized chromosomes.

Euchromatin and Heterochromatin

Euchromatin

Euchromatin is less electron-opaque and contains genes being used by that cell. Transcriptional machinery needs to access specific DNA regions to begin transcription. Euchromatin is less tightly packed, so allows access

Heterochromatin

Heterochromatin is more electron-opaque and contains genes not being used by that cell. There is a dynamic relationship between euchromatin and heterochromatin.

Review: Cellular Components

Cells contain a plasma membrane, endomembrane system, a cytoskeleton, and a nucleus, each with a distinct structure, composition, and function.

Summary

The nucleus is the control center of the cell, surrounded by a double membrane called the nuclear envelope and supported structurally by the nuclear lamina. The nucleus contains most of a cell’s genes.
Nuclear pores are channels made by nucleoporins inserted in the nuclear envelope. Nuclear pores control the movement of molecules in (control signals, building materials, and energy) and out (mRNA, tRNA, and ribosomal subunits) of the nucleus.
The nucleus contains a specialized nucleolus, where ribosomal RNA and ribosomal subunits are made à transported to the cytoplasm for assembly. DNA is packaged tightly around histone proteins forming 10-nm fibers and nucleosomes, which are further packaged into larger (30 nm) and larger (300 nm) fibers and, during cell division, into chromosomes.
Euchromatin (less tightly packed) contains genes being used by the cell. Heterochromatin (tightly packed) contains genes not being used by the cell.

Objective-Based Questions

What is the nuclear envelope made of?
What lines the inner surface of the nuclear envelope? What is the function of this structure?
What can move in via nuclear pores?
What can move out via nuclear pores?
What is the function of the nucleolus?
How is DNA organized within the nucleus? What is the role of histones in this organization?
What is the difference between euchromatin and heterochromatin?

Lecture Objectives

Describe the basic structure and function of the nucleus: The nucleus is the control center of the cell, surrounded by a double membrane called the nuclear envelope and supported structurally by the nuclear lamina. It contains most of the cell’s genes.
Describe the structure of the nuclear pore complex and its role in nucleo-cytoplasmic exchange: Nuclear pores are channels made by nucleoporins inserted in the nuclear envelope. They control the movement of molecules in (control signals, building materials, and energy) and out (mRNA, tRNA, and ribosomal subunits) of the nucleus.
Describe the organization of DNA within the nucleus: The nucleus contains a specialized nucleolus, where ribosomal RNA and ribosomal subunits are made and transported to the cytoplasm for assembly. DNA is packaged tightly around histone proteins, forming 10-nm fibers and nucleosomes, which are further packaged into larger (30 nm) and larger (300 nm) fibers and, during cell division, into chromosomes.
Outline the functional differences between euchromatin and heterochromatin: Euchromatin (less tightly packed) contains genes being used by the cell, while heterochromatin (tightly packed) contains genes not being used by the cell.