Chp. 17 - Bio 190

General Properties of Viruses

• Viruses are nonliving particles with nucleic acid genomes needing living cells to replicate.
• Viruses are considered nonliving due to not being composed of cells, not using energy, and not carrying out metabolism, maintaining homeostasis, or reproducing on their own.
• Viruses infect all types of organisms.
• The tobacco mosaic virus was the first virus discovered.
• Many biologists study viruses due to their public interest in their disease-causing ability.

Genetic Properties of Viruses: Structure and Variation

• Viruses differ in their host range, structure, and genome composition.
• Host range is the number of species and cells a virus can infect, it may be broad or narrow.
• All viruses have a protein coat called a capsid, which varies in shape and complexity.
• Many viruses that infect animal cells have a viral envelope, a lipid bilayer derived from the host cell.
• Viral genomes can be composed of DNA or RNA, single- or double-stranded, and linear or circular.

Viral Reproductive Cycles

• Viral reproductive cycles consist of common steps, though details vary and alternative cycles may exist.
• The steps are attachment of the virus to the host cell surface, entry of the viral genome into the host cell, integration into the host cell's chromosome (in some viruses), synthesis of viral components using host cell machinery, viral assembly of new viruses, and release of new viruses into the environment.
• Lytic and lysogenic cycles are different types of viral reproduction.
• In the lytic cycle, new phages are produced, and the bacterial cell is lysed.
• In the lysogenic cycle, the integrated phage DNA (prophage) is replicated along with the host cell's DNA.

Genetic Properties of Bacteria

• Bacteria typically have circular chromosomes containing a few thousand genes composed of DNA and proteins.
• Chromosomes are usually circular and are tightly packed within a nucleoid region.
• Bacterial chromosomes are typically a few million base pairs long containing genes that encode proteins and a single origin of replication to initiate DNA replication.
• Bacteria utilize binary fission for reproduction.
• Binary fission is rapid cell division, whereby a single cell produces a visible colony of 10-100 million cells in less than a day.
• Cell division occurs by first replicating the DNA, then drawing the plasma membrane inward, and finally forming new cell walls to separate the daughter cells, creating genetically identical daughter cells.

Gene Transfer Between Bacteria

• Bacteria exhibit genetic diversity through mutations and horizontal gene transfer.
• There are three distinct methods of horizontal gene transfer in bacteria: conjugation, transformation, and transduction.
• Conjugation involves direct contact between two bacterial cells: a donor cell transfers genetic material (often a plasmid) to a recipient cell through a pilus.
• Transformation is the uptake of free DNA from the environment by a competent bacterial cell.
• Transduction is the transfer of genetic material from one bacterium to another by a bacteriophage. A bacteriophage picks up a segment of bacterial DNA, and transfers it to another bacterium.

Immune Defense Against Pathogens

• Antibodies help destroy pathogens in three major ways: neutralization, opsonization, and complement activation.
• Neutralization prevents viruses or toxins from binding to their targets.
• Opsonization tags pathogens for consumption by macrophages or neutrophils.
• Complement activation activates the complement system, leading to pathogen cell lysis.

Case Study: Vaccines

• Nevada has a diverse population making vaccine coverage challenging.
• Las Vegas has high vaccination rates due to proactive public health campaigns, access to healthcare, and high compliance with school requirements.
• Boulder City has lower rates due to vaccine hesitancy, misinformation, and limited public health outreach.