Lecture 2: Mitosis and Meiosis – Cell Structure, Chromosomes, and the Cell Cycle

Cell Structure and Genetic Function

Types of Cells

Cells are the fundamental units of life, and their structure is closely tied to their genetic function. There are two main types of cells:

• Prokaryotic cells: Include bacteria and archaea. They lack a membrane-bound nucleus.

• Eukaryotic cells: Include protists, plants, fungi, and animals. They possess a membrane-bound nucleus and various organelles.

All cells share common features:

• Plasma membrane

• DNA

• Ribosomes

The Cell – Function Reflected by Genetics

Cells contain specialized structures (organelles) that perform distinct functions:

• Nucleus: Contains genetic material (DNA).

• Cytoplasm: Contains mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, centrioles, and cytoskeleton.

Nucleus and Nucleoid

• Nucleus (eukaryotes): Membrane-bound, houses DNA, site of ribosomal RNA (rRNA) synthesis, contains nucleolus and nucleolus organizer region (NOR).

• Nucleoid (prokaryotes): Not membrane-bound, region where DNA is located.

Cytoplasm and Cytoskeleton

• Cytoplasm: Includes extra-nuclear organelles and cytosol (colloidal material surrounding organelles).

• Cytoskeleton: Provides structural framework, shape, and movement. Composed of:

  • Microtubules (largest): a/b tubulin polymers

  • Intermediate filaments: vimentin/keratin fibers

  • Microfilaments (smallest): G actin polymers

Endoplasmic Reticulum and Golgi Apparatus

• Endoplasmic Reticulum (ER): Network of tubular channels. Two types:

  • Rough ER (RER): With ribosomes, site of protein synthesis

  • Smooth ER (SER): Lacks ribosomes, site of lipid synthesis

• Golgi Apparatus: Flattened sacs near nucleus, synthesizes large carbohydrates, modifies and packages proteins for secretion.

Mitochondria and Chloroplasts

• Mitochondria: Present in animals and plants, convert food into ATP (energy).

• Chloroplasts: Present in plants, algae, and protozoans, site of photosynthesis.

Lysosomes

• Cytoplasmic vacuoles with digestive enzymes, responsible for intracellular digestion.

Plasma Membrane and Cell Wall

• Plasma membrane: Surrounds all cells, selectively permeable, contains embedded proteins and glycoproteins for communication and transport.

• Cell wall: Present in plants (cellulose) and bacteria (peptidoglycan).

Movement of Materials In and Out of Cells

Transport Mechanisms

• Diffusion: Movement of solutes from high to low concentration.

• Osmosis: Movement of water from low to high solute concentration.

• Tonicity: Describes the effect of solute concentration on cell volume (isotonic, hypertonic, hypotonic).

• Active transport: Movement against concentration gradient, requires energy (e.g., sodium-potassium pump).

• Phagocytosis: Ingestion of large particles.

• Endocytosis: Engulfing small particles.

• Pinocytosis: Ingestion of fluids and small molecules.

• Exocytosis: Removal of waste/products from cells.

Chromosomes and Genetic Organization

Homologous Chromosomes

• Exist in diploid organisms as pairs (one from each parent).

• Carry genes for the same traits but may have different alleles.

Centromeres

• Constricted regions on chromosomes, binding site for kinetochore proteins.

• Maintain sister chromatid cohesion during division.

• Centromere position classifies chromosomes as metacentric, submetacentric, acrocentric, or telocentric.

Type	Description
Metacentric	Centromere in the middle
Submetacentric	Centromere slightly off center
Acrocentric	Centromere near one end
Telocentric	Centromere at the end

Somatic Cells and Chromosome Number

• Humans: 46 chromosomes (23 pairs), diploid number $2n=46$.

• Microorganisms: Often have a single set of chromosomes.

Chromosome Analysis

• Karyotype analysis: Arranges chromosomes to study number and structure.

• Uses fluorescent probes to "paint" chromosomes for identification.

Meiosis and Haploid Number

• Meiosis reduces chromosome number from diploid ($2n$) to haploid ($n$).

• Genome: The genetic information in a haploid set.

Organism	Haploid Number
Human	23
Fruit fly	4
Mouse	20
Yeast	16
Zebrafish	25
Additional info: Table includes more species in original.

Genetic Inheritance

• Homologous chromosomes have identical gene loci.

• Biparental inheritance: Each parent contributes one set of chromosomes.

• Alleles: Alternative forms of a gene.

• Sex chromosomes: Not homologous, but pair during meiosis.

Cell Cycle and Division

Cell Cycle Overview

The cell cycle is a series of events that cells go through as they grow and divide. It is vital for growth, repair, and maintenance in multicellular organisms.

• Efficient repair increases survival (e.g., skin, liver, immune cells).

• Some cells (neurons) are long-lived and do not divide; others (RBCs, skin) are short-lived and constantly renewed.

• Aging leads to decreased cell function and increased vulnerability.

• Cancer cells often escape normal cell cycle regulation.

Tissue Remodeling

• Occurs in response to stress or disease (e.g., chronic inflammation, fibrosis, hypertrophy, tissue remodeling after injury).

Phases of the Cell Cycle

• Interphase: Interval between divisions, includes:

  • G1 phase: Cell growth and preparation for DNA synthesis

  • S phase: DNA replication

  • G2 phase: Preparation for mitosis

  • G0 phase: Non-dividing, metabolically active state

• Mitosis: Division of the nucleus and cytoplasm

Cells in G0 are viable but not proliferative. Some can re-enter the cycle; others never do. Cancer cells often bypass G0.

Summary Table: Cell Cycle Phases

Phase	Key Event
G1	Cell growth, preparation for DNA synthesis
S	DNA replication
G2	Preparation for mitosis
M	Mitosis (nuclear and cytoplasmic division)
G0	Non-dividing, metabolically active

Example

Skin cells rapidly cycle through G1, S, G2, and M phases to replace lost or damaged cells, while neurons typically remain in G0 for life.

Additional info: Later slides and notes would cover mitosis and meiosis in detail, including their phases and genetic consequences.