Class Notes‎ > ‎Genetics‎ > ‎

Viruses and Other Infectious Particles

A Virus has a genome but can reproduce only within a host cell

The Discovery of Viruses: Scientific Inquiry

  • Researchers discovered viruses in the late 1800s by studying a plant disease, tobacco mosaic disease.

Structure of Viruses

  • A virus is a small nucleic acid genome enclosed in a protein capsid and sometimes a membranous envelope containing viral proteins that help viruses enter cells. The genome may be single- or double- stranded DNA or RNA

General Features of Viral Reproductive Cycles

  • Viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses. Each type of virus has a characteristic host range.

Reproductive Cycles of Phages

  • In the lytic cycle, entry of the viral genome in a bacterium programs destruction of host DNA, production of new phages, and digestion of the host's cell wall, releasing the progeny phages. In the lysogenic cycle, the genome of a temperate phage inserts into the bacterial chromosome as a prophage, which is passed on to host daughter cells until it is induced to leave the chromosome and initiate a lytic cycle. 

Reproductive Cycles of Animal Viruses

  • Many animal viruses have an envelop. Retroviruses (such as HIV) use the enzyme reverse transcriptase to copy their DNA genome into DNA, which can be integrated into the host genome as a provirus.

Evolution of Viruses

  • Since viruses can reproduce only within cells, they probably evolved after the first cells appeared, perhaps as packaged fragments of cellular nucleic acid.

Viruses, viroids, and prions are formidable pathogens in animals and plants

Viral Diseases in Animals

  • Symptoms may be caused by direct viral harm to cells or by the body's immune response. Vaccines stimulate the immune system to defend the host against specific viruses

Emerging Viruses

  • Outbreaks of "new" viral diseases in humans are usually caused by existing viruses that expand their host territory

Viral Diseases in Plants

  • Viruses enter plant cells through damaged cell walls (horizontal transmission) or are inherited from a parent (vertical transmission)

Viroids and Prions: The Simplest Infectious Agents

  • Viroids are naked RNA molecules that infect plants and disrupt their growth. Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals.

Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria

The Bacterial Genome and Its Replication

  • The bacterial chromosome is usually a circular DNA molecule with few associated proteins. Plasmids are smaller circular DNA molecules that can replicate independently of the chromosome. 

Mutation and Genetic Recombination as Sources of Genetic Variation

  • Because bacteria can proliferate rapidly, new mutations can quickly increase a population's genetic variation. Further diversity can arise by recombination of the DNA from two different bacterial cells.

Mechanisms of Gene Transfer and Genetic Recombination in Bacteria

  • New bacterial strains can arise by the transfer of DNA from one cell to another cell. In transformation, naked DNA from one cell to another cell. In transformation, naked DNA enters the cell from the surroundings. In transduction, bacterial DNA is carried from one cell to another by phages. In conjugation, and F+ donor cell. which contains the F plasmid, transfer plasmid DNA to an F-. The F factor of an Hfr cell, which is integrated into the bacterial chromosome. brings some chromosomal DNA along with it when it is transferred to an F- cell. R plasmids confer resistance to various antibiotics.

Transposition of Genetic Elements

  • DNA segments that can insert at multiple sites in a cell's DNA contribute to genetic shuffling in bacteria. Insertion sequences, the simplest bacterial transposable elements, consist of inverted repeats of DNA flanking a gene for transposase. Bacterial transposons have additional genes, such as those for antibiotic resistance.

Individual bacteria respond to environmental change by regulating their gene expression

Operons: The Basic Concept

  • Cells control metabolism by regulating enzyme activity or the expression of genes coding for enzymes. In bacteria, genes are often clustered into operons, with one promoter serving several adjacents genes. An operator site on the DNA swithces the operon on or off.

Repressible and Inducible Operons: Two Types of Negative Gene Regulation

  • In a repressible operon, binding of a specific repressor protein to the operator shuts off transcription. The repressor is active when bound to a corepressor, usually the end product of an anabolic pathway. In an inducible operon, binding of an inducer to an innately active repressor inactivates the repressor and turns on transcription. Inducible enzymes usually function in catabolic pathways

Positive Gene Regulation

  • Some operons are also subject to positive control via a stimulatory activator protein, such as a catabolite activate protein (CAP), which promotes transcription when bound to a site within the promoter.