Prokaryotes, Eukaryotes, and Bacterial Classification: Study Notes

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Bacteria are a major group of prokaryotic organisms, characterized by their simple cellular structure and lack of a membrane-bound nucleus.

Small size: Bacteria are typically much smaller than eukaryotic cells.
Unicellular: Most bacteria exist as single cells.
No membrane-bound nucleus: Genetic material is located in a region called the nucleoid.
Circular DNA: Bacterial DNA is usually a single, circular chromosome found in the cytoplasm.
Additional info: Bacteria may also contain plasmids, which are small, circular DNA molecules that carry extra genes.

Archaea are another group of prokaryotes, distinct from bacteria, and often found in extreme environments.

Prokaryotic: Like bacteria, archaea lack a membrane-bound nucleus.
No peptidoglycan in cell wall: Archaea have cell walls, but they do not contain peptidoglycan.
Membrane lipids: Archaea have unique membrane lipids that are ether-linked, unlike the ester-linked lipids in bacteria and eukaryotes.
Habitats: Archaea are often found in moderate to extreme environments, such as hot springs, salt lakes, and deep-sea vents.

Eukaryotes are organisms whose cells contain a true nucleus and other membrane-bound organelles.

Larger size: Eukaryotic cells are generally larger than prokaryotic cells.
Multicellular or unicellular: Eukaryotes can be single-celled (e.g., yeast) or multicellular (e.g., plants, animals).
Membrane-bound nucleus: The nucleus is surrounded by a double membrane and contains the cell's genetic material.
Linear chromosomes: Eukaryotic DNA is organized into multiple linear chromosomes.
Cell division: Eukaryotes divide by mitosis or meiosis.

Despite their differences, prokaryotes and eukaryotes share several fundamental cellular components and processes.

DNA and RNA: Both cell types use DNA as genetic material and RNA for protein synthesis.
Ribosomes: Both have ribosomes, though their structure differs.
Cytoplasm: The internal fluid where cellular processes occur.
Cilia and flagella: Structures for movement; present in some prokaryotes and eukaryotes.
Plasma membrane: A lipid bilayer that encloses the cell.
Cell wall: Present in most prokaryotes and some eukaryotes (e.g., plants, fungi), but not in animal cells.
Metabolic processes: Both perform essential metabolic reactions such as respiration and photosynthesis (in some cases).

Bacteria are commonly classified based on their cell shape, which is an important characteristic in microbiology.

Bacteria can also be classified based on how they group together:

Bacteria possess a single circular chromosome and may also contain plasmids, which are small, extra-chromosomal DNA molecules.

Plasmids: Carry genes that may provide advantages, such as antibiotic resistance.
Conjugation: The process by which bacteria transfer genetic material (usually plasmids) through a structure called a pilus.

Gram staining is a differential staining technique used to classify bacteria based on the structure of their cell walls.

Gram-positive bacteria: Have thick peptidoglycan cell walls and stain purple.
Gram-negative bacteria: Have thin peptidoglycan walls and an additional outer membrane; stain pink/red.

Crystal Violet (Primary Stain): Both Gram-positive and Gram-negative cells stain purple.
Iodine (Mordant): Forms a complex with crystal violet in both cell types.
Alcohol (Decolorizer): Dehydrates Gram-positive cell walls, trapping the stain; removes lipids and stain from Gram-negative cells, making them colorless.
Saffranin (Counterstain): Gram-positive cells remain purple; Gram-negative cells take up the counterstain and appear pink/red.

Step	Gram-Positive Effect	Gram-Negative Effect
Crystal Violet	Purple	Purple
Iodine	CV-I Complex forms	CV-I Complex forms
Alcohol	Cell wall shrinks, stain retained	Lipids removed, stain lost
Saffranin	Remains purple	Turns pink/red

Koch's postulates are a set of criteria used to establish a causal relationship between a microorganism and a disease.

The bacterium must be present in every case of the disease, but not in healthy individuals.
The bacterium must be isolated from the diseased host and grown in pure culture.
The specific disease must be reproduced when a pure culture of the bacterium is injected into a healthy, susceptible host.
The bacterium must be recovered from the experimentally infected host.

Example: Koch used these postulates to demonstrate that Bacillus anthracis causes anthrax.

Bacteria are commonly observed using light microscopy, often after staining to enhance visibility and differentiation.

Staining: Techniques such as Gram staining help distinguish bacterial types.
Slide preparation: Bacteria are fixed to slides to prevent washing away during staining.

Feature	Prokaryotes	Eukaryotes
Nucleus	No membrane-bound nucleus	Membrane-bound nucleus
DNA Structure	Circular DNA	Linear chromosomes
Cell Size	Small	Larger
Organelles	Few (no membrane-bound)	Many (membrane-bound)
Cell Wall	Present (peptidoglycan in bacteria)	Present in plants/fungi, absent in animals