Cell Structure, Genetic Material, and Microbial Diversity: Microbiology Study Notes

Endosymbiotic Theory and Organelle Origins

Definition and Supporting Evidence

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells, forming a symbiotic relationship and becoming permanent organelles.

• Double Membranes: Both mitochondria and chloroplasts have double membranes, consistent with engulfment by another cell.

• Own DNA: These organelles contain their own circular DNA, similar to bacterial genomes.

• Ribosomes: Their ribosomes resemble those of bacteria (70S), not eukaryotic ribosomes (80S).

• Replication: They replicate independently within the cell by binary fission, like bacteria.

• Genetic Similarity: Mitochondrial DNA is similar to aerobic bacteria (e.g., Rickettsia), and chloroplast DNA is similar to cyanobacteria.

How to Remember: "Ancient bacteria became part of us!"

Evidence: Membranes, DNA, ribosomes, replication, and genetic similarity.

Genetic Material in Prokaryotes, Eukaryotes, and Viruses

Types and Organization

• Prokaryotes (e.g., bacteria):

  • Main genetic material: Circular double-stranded DNA (chromosome)

  • Extra genetic material: Plasmids (small, circular DNA molecules)

• Eukaryotes (e.g., plants, animals, fungi):

  • Main genetic material: Linear double-stranded DNA (in nucleus)

  • Organelles: Mitochondria and chloroplasts also have their own circular DNA

• Viruses:

  • Genetic material can be DNA or RNA

  • DNA can be single-stranded (ssDNA) or double-stranded (dsDNA)

  • RNA can be single-stranded (ssRNA) or double-stranded (dsRNA)

  • Shape: Can be linear or circular

Type	Main Genetic Material	Extra Genetic Material
Prokaryotes	Circular dsDNA	Plasmids
Eukaryotes	Linear dsDNA (nucleus)	Circular DNA (organelles)
Viruses	DNA or RNA (ss or ds)	None

Cell Structure: Prokaryotes vs. Eukaryotes

Cell Wall, Nucleus, and Organelles

• Cell Wall:

  • Prokaryotes: Most have a cell wall (e.g., peptidoglycan in bacteria)

  • Eukaryotes: Plants (cellulose), fungi (chitin); animals and protozoa do not have a cell wall

• Nucleus:

  • Prokaryotes: No true nucleus; DNA in nucleoid region

  • Eukaryotes: True, membrane-bound nucleus

• Organelles:

  • Prokaryotes: No membrane-bound organelles

  • Eukaryotes: Many membrane-bound organelles (mitochondria, ER, Golgi, etc.)

Unicellular vs. Multicellular

• Prokaryotes: Mostly unicellular (single-celled)

• Eukaryotes: Can be unicellular (e.g., yeast, protozoa) or multicellular (plants, animals, fungi)

Major Eukaryotic Organelles and Their Functions

• Nucleus: Contains genetic material (DNA); controls cell activities and gene expression

• Mitochondria: "Powerhouse" of the cell; produces ATP through cellular respiration

• Chloroplast: Site of photosynthesis; converts light energy to chemical energy (plants/algae)

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes and processes proteins

  • Smooth ER: No ribosomes; synthesizes lipids and detoxifies chemicals

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport

• Lysosomes: Digestive enzymes; break down waste, old organelles, and macromolecules

• Peroxisomes: Break down fatty acids and detoxify harmful substances

• Vacuoles: Store nutrients, water, and waste; large central vacuole in plant cells helps maintain shape

• Ribosomes: (Not membrane-bound) Synthesize proteins; found in cytoplasm or on rough ER

• Cytoskeleton: Provides structural support, helps with cell movement and transport within the cell

Nucleus, Nucleolus, and Nucleoid

Definitions and Functions

• Nucleus: Eukaryotes; membrane-bound, contains DNA

• Nucleolus: Inside nucleus; makes rRNA

• Nucleoid: Prokaryotes; DNA region, no membrane

Cell Division: Mitosis, Meiosis, and Binary Fission

Comparison and Key Features

• Mitosis:

  • Occurs in eukaryotic cells

  • Produces two genetically identical daughter cells

  • Used for growth, repair, and asexual reproduction

  • One cell division cycle

  • Maintains chromosome number (diploid → diploid)

• Meiosis:

  • Occurs in eukaryotic cells (gamete formation)

  • Produces four genetically unique daughter cells

  • Used for sexual reproduction

  • Two cell division cycles (meiosis I and II)

  • Reduces chromosome number by half (diploid → haploid)

• Binary Fission:

  • Occurs in prokaryotic cells (bacteria and archaea)

  • Produces two genetically identical daughter cells

  • Simple process: DNA replication, cell elongation, division

  • No nucleus or mitotic spindle involved

Endocytosis and Exocytosis

Definitions and Types

• Endocytosis: Cell takes in materials by engulfing them with its membrane

  • Phagocytosis: "Cell eating"; engulfs large particles (e.g., bacteria)

  • Pinocytosis: "Cell drinking"; takes in small droplets of fluid and dissolved substances

  • Receptor-mediated endocytosis: Uses receptor proteins to bind and take in specific molecules (e.g., cholesterol uptake)

• Exocytosis: Cell releases materials to the outside by fusing a vesicle with the plasma membrane; used to secrete substances like hormones, neurotransmitters, or waste products

Major Groups of Eukaryotes

Characteristics and Examples

• Protista: Mostly unicellular; very diverse; can be autotrophic (algae) or heterotrophic (protozoa); examples: Amoeba, Paramecium, algae

• Fungi: Mostly multicellular (yeast are unicellular); cell wall made of chitin; heterotrophic (decomposers); reproduce by spores; examples: mushrooms, molds, yeast

• Plantae: Multicellular; cell wall made of cellulose; autotrophic (photosynthesis); examples: mosses, ferns, flowering plants

• Animalia: Multicellular; no cell wall; heterotrophic (ingest food); examples: insects, fish, mammals, humans

Fungi: Hyphae and Mycosis

Structure and Pathogenicity

• Hyphae: Long, thread-like filaments that make up the body (mycelium) of fungi; help absorb nutrients from the environment

• Mycosis: Infection caused by a fungus; can affect skin, nails, lungs, or other organs

• Opportunistic Pathogens: Cause disease mainly in people with weakened immune systems (e.g., HIV/AIDS, cancer patients)

• True Pathogens: Can infect healthy individuals; example: Histoplasma capsulatum (causes histoplasmosis)

• Toxin-Producing Fungi: Produce mycotoxins that can cause illness when ingested, inhaled, or contacted; example: Aspergillus species produce aflatoxins

Protozoa: Major Groups and Motility

Classification and Examples

• Amoeboids (Sarcodina): Move and feed using pseudopodia ("false feet"); example: Amoeba

• Flagellates (Mastigophora): Move using one or more flagella; example: Trypanosoma

• Ciliates (Ciliophora): Move using many short cilia; example: Paramecium

• Sporozoans (Apicomplexa): Non-motile; move by gliding or are carried by hosts; example: Plasmodium (causes malaria)

Flagella: Prokaryotic vs. Eukaryotic

Structure and Function

• Prokaryotic Flagella:

  • Found in bacteria

  • Made of the protein flagellin

  • Structure: simple; basal body, hook, filament

  • Movement: rotates like a propeller (clockwise or counterclockwise)

  • Powered by proton motive force (H+ gradient)

• Eukaryotic Flagella:

  • Found in protists, some animal cells (e.g., sperm)

  • Made of microtubules (9+2 arrangement) and covered by the cell membrane

  • Structure: complex; many proteins and a plasma membrane covering

  • Movement: whips back and forth (undulating motion)

  • Powered by ATP

Feature	Prokaryotic Flagella	Eukaryotic Flagella
Structure	Flagellin, simple	Microtubules, complex
Movement	Rotation	Undulation
Energy Source	Proton gradient	ATP
Membrane Covering	No	Yes

Ribosomes: Free vs. Bound

Types and Functions

• Free Ribosomes: Float freely in the cytoplasm; make proteins that function within the cytosol (e.g., enzymes for glycolysis, cytoskeletal proteins)

• Bound Ribosomes: Attached to the rough endoplasmic reticulum (RER); make proteins that are:

  • Secreted from the cell (e.g., hormones, antibodies)

  • Sent to organelles like lysosomes

  • Inserted into cell membranes

Cytoskeleton Elements

Types and Processes

• Microfilaments (Actin Filaments): Made of actin protein; involved in cell movement (muscle contraction, cell crawling), cell shape maintenance

• Intermediate Filaments: Made of various proteins (e.g., keratin, vimentin); provide structural support, resist mechanical stress, anchor organelles

• Microtubules: Made of tubulin protein; involved in cell division (mitotic spindle), intracellular transport (moving organelles and vesicles), cell motility (cilia and flagella)

REMEMBER: Microfilaments = movement, Intermediate filaments = strength, Microtubules = transport & division!

Genome, Chromosomes, and Plasmids

Definitions and Differences

• Genome: Complete set of genetic material in an organism; includes all DNA (or RNA in some viruses), containing all genes and non-coding sequences

• Prokaryotic Chromosomes: Usually a single, circular DNA molecule; not membrane-bound; contains most genetic information

• Eukaryotic Chromosomes: Multiple, linear DNA molecules; located inside the nucleus (membrane-bound); associated with histone proteins for packaging

• Prokaryotic Plasmids: Small, circular, extra-chromosomal DNA; replicate independently; often carry genes for antibiotic resistance or other advantages

• Eukaryotic Plasmids: Rare

Feature	Prokaryotes	Eukaryotes
Chromosome Shape	Circular	Linear
Plasmids	Common	Rare

Nucleotides and Bases in DNA and RNA

Components and Pairing

• Three Components of a Nucleotide:

  1. Phosphate group (PO4-)

  2. Pentose sugar: Deoxyribose in DNA, ribose in RNA

  3. Nitrogenous base

• Four Bases in DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

• Four Bases in RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

• REMEMBER: DNA uses Thymine, RNA uses Uracil!

Complementary Base Pairing

• DNA is made of two strands forming a double helix. Each base on one strand pairs with a specific partner on the opposite strand:

• Adenine (A) pairs with Thymine (T) (DNA) or Uracil (U) (RNA)

• Cytosine (C) pairs with Guanine (G)

• These pairs are held together by hydrogen bonds:

  • A-T pairs have 2 hydrogen bonds

  • G-C pairs have 3 hydrogen bonds

Tip: To write a complementary DNA sequence, replace each base with its partner!

Directionality and Antiparallel Arrangement

• Nucleic acids have directionality, with one strand running 5' to 3' and the other 3' to 5'.

• This antiparallel arrangement is essential for DNA replication and function.

Formula for base pairing:

$\text{A} = \text{T} \, (\text{DNA}) \, ; \, \text{C} = \text{G}$

$\text{A} = \text{U} \, (\text{RNA}) \, ; \, \text{C} = \text{G}$

Summary Table: Key Differences Between Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	No (nucleoid)	Yes (membrane-bound)
Organelles	No membrane-bound	Many membrane-bound
Chromosomes	Circular	Linear
Cell Wall	Usually present	Plants/fungi only
Ribosomes	70S	80S
Cell Division	Binary fission	Mitosis/meiosis

Additional info:

• Some context and examples were inferred for clarity and completeness.

• Tables were recreated to summarize comparisons and classifications.