Viral Structure and Replication

Questions and Answers

How do viruses challenge our definition of living organisms?

Viruses complicate the definition of life because they exhibit characteristics of living organisms, such as replication, but cannot carry out metabolic processes independently.

What are the structural components of a virus?

A virus typically contains a nucleic acid genome (DNA or RNA) surrounded by a protein coat called a capsid, and some may have an outer lipid envelope.

What distinguishes the lytic cycle from the lysogenic cycle in bacteriophages?

The lytic cycle results in the immediate destruction of the host cell, while the lysogenic cycle integrates viral DNA into the host's genome, allowing for prolonged coexistence.

What unique features enable retroviruses to sustain chronic infections?

Retroviruses possess reverse transcriptase, which allows them to transcribe their RNA genome into DNA, integrating it into the host genome and evading the immune response. Signup and view all the answers

How does HIV compromise the immune system?

HIV targets and destroys CD4+ T cells, which are crucial for orchestrating immune responses, leading to a weakened immune system. Signup and view all the answers

What is a prion, and how does it replicate?

A prion is an infectious agent composed solely of misfolded proteins that induce other proteins to also misfold, leading to disease. Signup and view all the answers

What are the common features of prokaryotic cells?

Prokaryotic cells are unicellular organisms lacking a nucleus and membrane-bound organelles, characterized by their simple cell structure and rapid reproduction. Signup and view all the answers

What are the defining characteristics of eukaryotes?

Eukaryotes are distinguished by their complex cellular organization, including a true nucleus and membrane-bound organelles. Signup and view all the answers

Study Notes

Viruses and Living Organisms

Viruses challenge definitions of life due to the lack of cellular structure and dependence on host cells for replication.
They possess various structures, including a protein coat (capsid), genetic material (either DNA or RNA), and sometimes an outer lipid envelope.
Basic viral structures include surface proteins for host attachment, the nucleic acid core, and capsid shape variations (icosahedral, helical).

Viral Replication Mechanics

Viral replication occurs through two primary cycles: lytic and lysogenic.
Lytic Cycle: Virus injects DNA/RNA into the host cell, takes over cellular machinery, produces new virions, and eventually causes host cell lysis.
Lysogenic Cycle: Viral DNA integrates into the host genome, replicates along with the host DNA, and can later enter the lytic cycle.

Retroviruses and Chronic Infections

Retroviruses, like HIV, possess reverse transcriptase to convert RNA to DNA, allowing integration into the host genome.
This integration enables persistence in host cells, resulting in chronic infections due to undetectable viral activity.

HIV and Immune System Compromise

HIV targets CD4+ T cells, crucial for immune response, leading to CD4+ cell depletion.
The progressive loss of these cells results in weakened immune defenses, predisposing individuals to opportunistic infections.

Human Disease-Causing Virus

Virus Name: Influenza
Disease Name: Influenza
Viral Genome: Segmented RNA genome
Epidemiology: Rapid spread, seasonal outbreaks; primarily transmitted via respiratory droplets.

Prions and Viroids

Prions are misfolded proteins that induce abnormal folding in normal proteins, leading to neurodegenerative diseases.
Viroids are small, circular RNA molecules that infect plants and cause disease by disrupting normal cellular processes.

Prokaryotes Overview

Archaea, bacteria, and eukarya share common features like cellular organization and genetic coding but differ in cell structure and metabolic pathways.
Prokaryotic cells possess a simple structure, lacking a nucleus and membrane-bound organelles.

Bacteria and Archaea Features

Bacteria have peptidoglycan cell walls, while archaea have distinct membranes and may contain unique lipids.
Some archaea possess monolayer plasma membranes for stability in extreme environments.

Bacterial Cell Wall Structure

Gram-Positive: Thick peptidoglycan layer, retains crystal violet staining.
Gram-Negative: Thin peptidoglycan layer, outer membrane, does not retain crystal violet but takes up counterstain.

Prokaryotic DNA Exchange Mechanisms

Mechanisms include transformation (uptake of naked DNA), transduction (virus-mediated), and conjugation (direct transfer via pili).

Antibiotic Resistance Spread in Prokaryotes

Resistance genes can spread through horizontal gene transfer, plasmids, and selective pressure from antibiotic use.

Prokaryotic Energy and Carbon Acquisition

Prokaryotes can be autotrophic (using inorganic substances for energy) or heterotrophic (requiring organic compounds).

Role of Prokaryotes in Element Cycling

Prokaryotes are vital in biogeochemical cycles, contributing to nitrogen fixation, carbon cycling, and decomposition.

Symbiosis Examples

Symbiotic relationships include nitrogen-fixing bacteria in plant roots (e.g., Rhizobium) and gut flora aiding digestion in mammals.

Bioremediation with Bacteria

Certain bacteria can degrade environmental pollutants, such as oil spills, through metabolic processes in bioremediation techniques.

Human Disease-Causing Bacterium

Bacterial Species: Streptococcus pneumoniae
Disease Name: Pneumonia
Description of Genome: Circular DNA, plasmids may carry antibiotic resistance traits.
Epidemiology: Common in young children and elderly; spread via respiratory droplets.