Bacterial Viruses

• Bacterial viruses are commonly referred to as bacterial phages, or simply phage.

Phage Lifecycle and Types

• Integrated viral DNA in bacteria is known as a prophage.
• Each time the bacterial host divides, it replicates the phage DNA, passing it to daughter cells.
• Under certain environmental signals, the viral genome can exit the bacterial chromosome and switch to the lytic cycle, leading to the destruction of the host.
• When the phage switches to lytic mode, the host cell bursts, releasing new phage particles.
• Some prophages expressed during lysogeny can produce toxins that are harmful to humans, similar to viruses like herpes, which can remain dormant in the body and reactivate under stress, causing symptoms like cold sores.

Bacterial Defense Mechanisms

• Natural Selection favors bacterial mutants with surface proteins that are unrecognized by specific phages, helping bacteria evade infection.
• Bacteria have restriction enzymes that can identify and cut foreign phage DNA into pieces, providing defense against phage attacks.
• The bacterial DNA is protected from these enzymes by a process called methylation, which compresses DNA so that it becomes unrecognizable to the enzymes.

CRISPR-Cas System

• CRISPR-Cas is a bacterial immune system that allows bacteria to protect against viral infections by storing phage DNA sequences and recognizing them in future attacks.
• CRISPR-associated proteins (Cas) interact with these stored sequences. If a phage infects the bacterium and it has survived the infection, it can block reinfection using this system.
• This system can be likened to human antibodies, where the bacterial cell retains a record of previous infections for improved defense in future encounters.

Viral and Bacterial Characteristics

• Viral DNA integrated into a host genome is called a provirus and is a permanent resident of the host cell. Unlike prophages, proviruses remain integrated and replicate alongside the host cell's own DNA.
• Retroviruses (like HIV) utilize reverse transcriptase to convert their RNA genome into DNA.

Vaccination and Antibodies

• Vaccines stimulate an immune response by introducing harmless derivatives of pathogens, which can lead to protective immunity.
• Misconceptions may arise about vaccines containing live virus; current vaccinations often use synthesized components rather than entire viruses.

Virus Characteristics

• Viruses can either have an RNA or DNA genome that is single-stranded or double-stranded, and they may have a membranous envelope.
• Many animal viruses possess both and utilize viral glycoproteins for host cell entry, much like a bouncer checking IDs at a club.
• The herpes virus and its ability to remain dormant until reactivated by stress is a key example of a virus that integrates into a host genome and causes disease.

Viral Infection Processes

• Infection mechanisms vary; viruses can transmit through horizontal (between individuals) or vertical (from parent to offspring) means, with prions (infectious proteins) also being a focus due to their ability to cause degenerative diseases.
• Plant viruses often have RNA genomes and can severely damage crops and agricultural products.

Gene Technology and Biotechnology

• DNA technology, like gel electrophoresis and polymerase chain reaction (PCR), allows for amplification and analysis of DNA sequences, serving as tools for genetic engineering and manipulating genes for practical purposes.
• DNA cloning refers to the process of creating multiple copies of a gene, which can be integrated into plasmids—a circular DNA that can replicate independently in bacterial cells.
• Techniques such as in situ hybridization and reverse transcription PCR facilitate understanding of gene expression, aiding in research related to health and disease.
• PCR enables rapid amplification of DNA segments, essential for studying ancient DNA (like woolly mammoths) or forensic applications.