Characterizing and Classifying Prokaryotes, Eukaryotes, and Viruses

Chapter 11: Characterizing and Classifying Prokaryotes

Asexual Reproduction in Prokaryotes

Prokaryotes reproduce primarily through asexual methods, ensuring rapid population growth and genetic stability.

• Binary Fission: The most common method; a single cell divides into two identical daughter cells. Steps include DNA replication, cell elongation, septum formation, and cell separation.

• Budding: A new cell develops from a protrusion on the parent cell, eventually detaching. Less common than binary fission.

Archaea vs Bacteria: General Differences

Archaea and bacteria are both prokaryotes but differ in several fundamental ways.

• Cell Wall Composition: Bacteria typically have peptidoglycan in their cell walls; archaea lack peptidoglycan and may have pseudopeptidoglycan or other polymers.

• Membrane Lipids: Archaea have ether-linked lipids; bacteria have ester-linked lipids.

• Genetic Machinery: Archaea's transcription and translation processes are more similar to eukaryotes than bacteria.

• Habitat: Archaea often inhabit extreme environments; bacteria are found in diverse environments.

Extremophiles

Extremophiles are organisms that thrive in extreme environmental conditions.

• Thermophiles: Prefer high temperatures (often above 45°C).

• Halophiles: Thrive in high salt concentrations.

• Acidophiles: Grow in acidic environments (low pH).

• Barophiles: Live under high pressure, such as deep-sea environments.

Notable Bacterial Groups

Several bacterial genera and groups exhibit unique characteristics.

• Mycobacterium: Possess a waxy cell wall rich in mycolic acids, making them resistant to desiccation and many antibiotics. Includes pathogens like M. tuberculosis.

• Cyanobacteria: Photosynthetic bacteria capable of producing oxygen; important in aquatic ecosystems and nitrogen fixation.

• Clostridium/Bacillus: Both genera form endospores, allowing survival in harsh conditions. Clostridium includes species causing tetanus and botulism; Bacillus includes B. anthracis (anthrax).

• Alphaproteobacteria (Nitrifying Bacteria): Involved in nitrogen cycling; convert ammonia to nitrite and nitrite to nitrate, essential for soil fertility.

Example: Mycobacterium

Mycobacterium is notable for its slow growth, resistance to many antibiotics, and ability to survive within host cells, contributing to diseases like tuberculosis.

Chapter 12: Characterizing and Classifying Eukaryotes

Mitosis and Meiosis

Eukaryotic cells divide by mitosis (asexual) and meiosis (sexual), affecting chromosome number and genetic diversity.

• Haploid: Cells with one set of chromosomes (n).

• Diploid: Cells with two sets of chromosomes (2n).

• Ploidy Number: Refers to the number of chromosome sets in a cell.

• Mitosis: Produces two genetically identical diploid (or haploid) somatic cells; ploidy number remains unchanged.

• Meiosis: Produces four genetically diverse haploid gametes; ploidy number halves.

Equation for Ploidy Change

$\text{Mitosis:} \quad 2n \rightarrow 2n$ $\text{Meiosis:} \quad 2n \rightarrow n$

Alternation of Generations in Algae

Algae may exhibit alternation of generations, alternating between haploid (gametophyte) and diploid (sporophyte) stages.

• Gametophyte: Produces gametes by mitosis.

• Sporophyte: Produces spores by meiosis.

Binary Fission vs Multiple Fission (Schizogony)

• Binary Fission: One cell divides into two.

• Multiple Fission (Schizogony): One cell divides into many daughter cells simultaneously; common in some protozoans.

Protozoans and Their Morphology

Protozoans are unicellular eukaryotes with diverse morphologies.

• Alveolates: Characterized by membrane-bound sacs (alveoli) beneath the cell membrane. Includes ciliates, apicomplexans, and dinoflagellates.

• General Features: Lack cell walls, possess complex organelles, and exhibit varied locomotion (flagella, cilia, pseudopodia).

Prokaryotes vs Eukaryotes: General Differences

• Prokaryotes: Lack membrane-bound organelles, have circular DNA, reproduce asexually.

• Eukaryotes: Possess membrane-bound organelles, linear DNA, reproduce sexually and asexually.

Example: Alveolates (Apicomplexans)

Apicomplexans, such as Plasmodium (malaria parasite), have a unique apical complex for host cell invasion.

Chapter 13: Characterizing and Classifying Viruses, Viroids, and Prions

Key Characteristics of Viruses

Viruses are acellular entities that require host cells for replication and are not considered living organisms.

• Not Living: Lack cellular structure, metabolism, and independent reproduction.

• Obligate Intracellular Parasites: Must infect host cells to replicate.

Comparison Table: Living Cells vs Viruses

Feature	Living Cells	Viruses
Cellular Structure	Yes	No
Metabolism	Yes	No
Replication	Independent	Host-dependent
Response to Stimuli	Yes	No
Genetic Material	DNA (usually)	DNA or RNA

Virus Morphology

• Capsid: Protein shell surrounding the nucleic acid.

• Nucleic Acid: Can be DNA or RNA, single or double stranded.

• Envelope: Lipid membrane derived from host cell; not all viruses have envelopes.

• Surface Proteins: Determine host cell specificity.

Viral Genome Types

• +ssRNA: Positive-sense single-stranded RNA

• -ssRNA: Negative-sense single-stranded RNA

• dsDNA: Double-stranded DNA

• dsRNA: Double-stranded RNA

• +ssDNA: Positive-sense single-stranded DNA

+ssRNA and dsDNA viruses replicate similarly to host cell processes.

DNA/RNA Processing in Eukaryotic Cells

• DNA Processing: Occurs in the nucleus.

• RNA Processing: Transcription in the nucleus; translation in the cytoplasm.

Viroids and Prions

• Viroid: Infectious RNA molecule lacking a protein coat; affects plants.

• Prion: Infectious protein causing neurodegenerative diseases; lacks nucleic acid.

Lytic vs Lysogenic Cycle

• Lytic Cycle: Virus infects cell, replicates, and lyses cell to release progeny. Steps:

  1. Attachment

  2. Entry

  3. Synthesis

  4. Assembly

  5. Release

• Lysogenic Cycle: Viral genome integrates into host genome, remains latent, and can later enter lytic cycle.

Viruses and Cancer

Some viruses can cause cancer by disrupting host cell gene regulation, leading to uncontrolled cell division.

Virus Culturing in Laboratory

• Cell Cultures: Viruses are grown in living cells (e.g., tissue culture).

• Embryonated Eggs: Used for culturing certain viruses (e.g., influenza).

• Live Animals: Sometimes used for virus propagation and study.

Example: Influenza Virus

Influenza virus is often cultured in embryonated chicken eggs for vaccine production.