Funbio 4

Cell Ultrastructure: Nucleus & Plasma Membrane

• The lecture covers the cell ultrastructure, focusing on the nucleus and plasma membrane.
• The course is Fundamentals of Human Biology, code FUNBIO 4.
• The lecturer is Adrian Dervan PhD.
• The date of the lecture is October 2nd 2024.

Learning Outcomes

• Learners will describe the fluid mosaic model of the plasma membrane.
• Learners will discuss the differences between integral and peripheral membrane proteins.
• Learners will explain how membrane proteins function in semipermeable membranes.
• Learners will describe the structure of the nucleus, including the nuclear envelope, pores, and karyoplasm.
• Learners will differentiate between euchromatic and heterochromatic nuclei.
• Learners will explain how each organism has a characteristic number of chromosomes.

Cell Membranes - Plasma & Nuclear Membrane

• Animal and plant cells are eukaryotic.
• Eukaryotic cells contain a nucleus and other organelles, each enclosed in a membrane.
• Organelles, including the nucleus, plasma membrane, lysosomes, and mitochondria, are necessary for cell function.

Plasma membrane

• Under the electron microscope (EM), the plasma membrane appears as a three-layered structure between 7.5-10nm in diameter.
• All living cells have a plasma membrane.
• The lipid bilayer is the universal basis of cell membrane structure.
• It is composed of two lipid layers largely phospholipids.
• Phospholipids are amphipathic molecules, with hydrophobic tails facing inwards and hydrophilic heads facing outwards.
• These molecules have the ability to form a bilayer due to their chemical properties.
• The outer face has more saturated fatty acids (e.g., high cholesterol), making it more solid.
• The inner face has more unsaturated fatty acids, making it more fluid.
• The plasma membrane is a dynamic structure.

Cell Membrane: Fluid Mosaic Model

• Proposed by Singer & Nicolson (1972).
• Biological membranes can be considered as a two-dimensional liquid.
• The model consists of phospholipid molecules with embedded (and associated) proteins and lipids.
• Many cells have a cell coat or glycocalyx formed by carbohydrate side chains of proteins and lipids.
• Cell membranes are semipermeable.
• Membranes protect the cell and can keep other cells at a distance.
• Membranes enable cells to recognize each other and to communicate.

Membrane Proteins

• Two classes of membrane proteins: integral and peripheral.
• Integral membrane proteins are permanently attached to the membrane.
• Integral proteins can be transmembrane or integral monotopic membrane proteins.
• Peripheral membrane proteins are attached to integral membrane proteins or to the lipid regions of the lipid bilayer.

Integral Membrane Proteins

• Integral proteins are permanently embedded in the membrane.
• Transmembrane proteins span the entire membrane.
• Integral proteins can be folded as a-helices, or b-barrels.
• Some integral proteins act as anchors, transporters, enzymes, and receptors.

Peripheral Membrane Proteins

• Peripheral proteins are temporarily attached to the membrane.
• They are associated with peripheral regions of the lipid bilayer.
• Interactions are temporary.
• These proteins then dissociate from the membrane and enter the cytoplasm.

Plasma Membrane - Function

• All membranes act as semi-permeable membranes.
• Small, uncharged, lipid-soluble molecules readily pass through the membrane.
• Virtually any molecule will diffuse across a lipid bilayer.
• The rates of water and charged inorganic ions across membranes depend on carriers or channels.

Integral Transmembrane Proteins

• Span the entire plasma membrane.
• Usually folded as alpha-helices, or beta-barrels.
• Act as anchors, transporters, channels, enzymes, and receptors.

The Nucleus

• All cells in the human body contain a nucleus except mammalian red blood cells and platelets.
• These specialized cells have limited futures due to a lack of nuclei.

Prokaryote vs. Eukaryote

• Prokaryotes (bacteria and archaea) do not have a nucleus and the DNA is located in the cytoplasm.
• Eukaryotes (animals and plants) have a nucleus containing the genetic material.

The Nucleus - Functions

• Control of genetic information in the cell
• Control of heredity characteristics
• Control of protein and enzyme synthesis
• Control of cell division and growth
• Storage of DNA, RNA, and ribosomes
• Regulation of mRNA transcription to protein
• Production of ribosomes

The Nucleus - Structure

• One nucleus per typical cell, located in the cytoplasm as a rounded body.
• Nucleus composed of nuclear envelope, chromatin, nucleolus, nuclear pores, and nucleoplasm.

Nuclear Envelope

• Nucleus enclosed in a double-layered nuclear envelope.
• Isolates nuclear material from the cytoplasm.
• Has a perinuclear space (20-40nm wide) between the two membranes
• The outer membrane is continuous with the endoplasmic reticulum in the cytoplasm.

Nuclear Lamina

• Found adjacent to the inner face of the nuclear envelope.
• Provides internal structural support for the nucleus.
• Made of intermediate filaments (primarily lamins).
• Genetic mutations in lamins can cause progeria syndrome and Emery-Dreifuss muscular dystrophy.
• Function: support to nuclear envelope, aid in positioning of nuclear pores.

Nuclear Pore

• Thousands of pores occupying ~10-30% of the surface of the nuclear envelope.
• Ring-like arrangement, octagonal structure, over 50nm. diameter.
• Allows transport of water-soluble molecules. Includes RNA, ribosomes, proteins, carbs, and signal molecules.

Nucleoplasm

• Cytoplasm within the nucleus, a highly viscous liquid surrounding chromosomes and nucleoli.
• Contains various substances (nucleotides and enzymes).
• A network of fibers (nuclear matrix) is similar to cell cytoskeleton.
• Precise composition and arrangement not fully understood.

Chromatin Fibers

• Chromatin fibres are a collection of threads and fibres visible in the nucleoplasm during interphase, the stage of the cell cycle after cell division.
• Fine chromatin fibres (fibrils) have a 2nm diameter.
• These fibrils are about the size of a DNA molecule.

Chromatin

• Euchromatin is the lightly packed form of chromatin in which the DNA is actively transcribed.
• Heterochromatin is the tightly packed form of chromatin. DNA is not transcribed in this form. It's usually located at the periphery of the nucleus and plays a role in gene regulation and chromosome protection.

The Structure of Chromatin (details)

• DNA wraps around histone proteins forming nucleosomes.
• These nucleosomes then coil into higher-order structures (30-nm fibers, Looped domains etc).
• These structures are associated with chromosome structure in the context of cell division

Chromatin Packing

• Chromatin undergoes multiple coiling levels to compact the DNA.
• The basic unit of this packing in a nucleosome (DNA wrapped around histone octamers).
• These nucleosomes then repeat over the length of the chromatin fibre.

Number of Chromosomes in Species

• Different species have a characteristic number of chromosomes.
• Human cells have 46 chromosomes and 22 pairs of autosomes + 2 sex chromosomes.
• Trisomy 21 (3 copies of chromosome 21) leads to Down’s Syndrome.

Nucleolus

• One or more per typical cell nucleus.
• Dense basophilic (staining) material in the nucleoplasm which disappears during cell division.
• Re-appears organized by nucleolar organiser regions (NORs) on particular chromosomes.

Nucleolus Function

• Associated with specific chromosome regions for rRNA transcription.
• rRNA molecules are components of ribosomes.
• Ribosomes are closely linked to protein synthesis.
• Ribosomes contain approximately equal amounts of RNA and protein.
• Nucleolus size reflects cell activity.

This quiz explores key concepts related to the structure and function of the cell nucleus. It covers topics including the nuclear envelope, nuclear pores, nucleoplasm, and associated genetic mutations. Test your knowledge on the critical roles the nucleus plays in eukaryotic cells and the differences between nucleotides and nucleosides.