4 Concept 18.1Through many researchers, it was found that the disease was caused by a particle that was much simpler and smaller than bacteria.It was also found that this pathogen could only reproduce within the host it infected.
5 Comparing the sizes of a virus, bacteria and animal cell.
6 AnimationHow Flu Viruses AttackHow A Virus Invades Your Body
7 Concept 18.1 A virus is a genome enclosed in a protective coat - a virus can be a DNA virus or an RNA virus depending on the kind of nucleic acid
8 Concept 18.1The protein shell that encloses the viral genome is called the capsidthe capsid may be rod shaped, polyhedral, or more complexSome viruses have viral envelopes, membranes that covers their capsids- Which are derived from the membrane of a host cell
12 Concept 18.1Viruses are obligate intracellular parasites: they can only reproduce within a host cell- viruses lack the enzymes for metabolism and have no ribosomes for making proteinsViruses are merely packaged sets of genes in transit from one host cell to another.
13 Concept 18.1Host range: the limited range of host cells each type of virus can infect and parasitize- viruses identify their host cells by a “lock-and-key” fit between proteins on the outside of the virus and specific receptor molecules on the surface of the cell.
14 Concept 18.1 Viral reproductive cycle Animation - the virus injects its DNA into the host cell- the viral DNA uses host nucleotides and enzymes to replicate itself- the viral DNA also uses host resources to produce its capsid- once assembled, the virus leaves
17 Concept 18.1Phages replicate using either the lytic or lysogenic cycleLytic cycle: reproductive cycle that culminates in the death of the host.- during the last stage of the cycle the cell breaks open (or lyses) and releases the phages that were produced
21 Concept 18.1- phage l, the cause of e.coli, is an example of a temperate phage.- the phage DNA will incorporate with the bacterial DNA to become a prophage- generally an environmental change will cause a virus to go from the lysogenic cycle to the lytic cycle
23 Concept 18.1 Reproductive Cycle of an Enveloped Virus - Glycoproteins on the envelope bind to receptor molecules on the host cell- Viral envelope fuses with the cell’s membrane and the capsid and genome enter the cell
24 Concept 18.1- enzymes remove the capsid and the genome enters the cell- genome uses the host organelles to make and assemble new viruses- viruses exit the cell
26 Concept 18.1Retrovirus- most complicated reproductive cycle of viruses- refers to the reverse directional flow of the genetic information- contain reverse transcriptase: transcribes DNA from an RNA template
27 Concept 18.1 - RNA DNA directional flow - DNA integrates as a provirus in the host cell- ex. HIV (human immunodeficiency virus)
30 Concept 18.2Many of the temporary symptoms associated with viral infections, such as fever and aches, are a result of the body defending itself from infection.Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mount defenses against infection. Animation
31 Viral Diseases in Animals Emerging Viruses- recently seen in the last 25 yrs.- ex. Ebola, Hantavirus, and SARS virusThree processes contribute to new viruses1. mutation of an existing virus- RNA does not proofread = high rate of mutation
32 Viral Diseases in Animals 2. spread of a virus from one host to another- ¾ of human diseases originated in another animal3. dissemination of a disease from a small isolated population can lead to widespread epidemics- AIDS went unnoticed for decades
33 Viral Diseases in Plants More than 2000 types which results in about $15 billion loss worldwideHave the same structure and mode of replication as animal virusesResults are bleached or brown spots, stunted growth, damaged flowers and roots and the streaking of tulips
34 Viral Diseases in Plants Two modes of transmission:Horizontal: Plant is infected from an external source because the virus can pass through the epidermis.Vertical: The plant inherits a viral infection from a parent (asexual propagation or infected seeds).
35 The Simplest Infectious Agents Viroids – circular RNA molecules, only several hundred nucleotides long, that infect plantsOne viroid disease has killed over 10 million coconut palms in the PhillipinesThey do not encode proteins but can replicated in the host cell
36 The Simplest Infectious Agents Prions – infectious proteins AnimationAppear to cause a number of degenerative brain diseases in various animal species, ex. Mad Cow DiseaseThey are slow-acting with long incubation periods and they are virtually indestructible
37 Concept 18.3Bacteria have a short generation span which helps them adapt to changing environments- the main part of a bacterial genome is a double-stranded circular molecule- bacteria also have plasmids: other small circular DNA molecules separate from the bacterial chromosome and can be transferred between cells
38 Concept 18.3 Bacteria reproduce asexually by binary fission Mutations can cause some of the offspring to differ slightly in genetic makeup
40 Concept 18.3 Genetic recombination produces new bacterial strains - ex. two mutant strains of bacteria are unable to grow on a nutrient medium b/c they are unable to synthesize necessary amino acids; if we mix the two strains, and they grow, we will know that the strains shared genes
42 Concept 18.3Transformation: the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environmentEx. Bacteria of a harmless strain of S. pneumoniae can be transformed into pneumonia causing cells
44 Concept 18.3Transduction: phages carry bacterial genes form one host cell to another as a result of aberrations in the phage reproductive cycleThe transferred DNA may recombine with the genome of the recipient cell bringing about a recombinant cell
46 Concept 18.3Conjugation: sometimes referred to as bacterial “sex”, is the direct transfer of genetic material between two bacterial cells that are temporarily joinedOne cell donates DNA and its “mate” receives the DNA
48 Concept 18.4The control of gene expression enables bacteria to adjust their metabolism to environmental changeOperon: The operator, the promoter and the genes they control which is the entire stretch of DNA required for enzyme production
49 Concept 18.4The lac operon: regulates the synthesis of the enzymes needed to metabolize lactose.Animation
52 Concept 18.4 -w/out lactose, no enzymes are made for metabolism - the regulatory gene, lacI is producing a repressor (prevents the transcription process) which blocks the RNA polymerase from working