Chapter 18 Notes The Genetics of Viruses and Bacteria

Concept 18.1

Researchers discovered viruses through the understanding of tobacco mosaic disease - stunts growth and causes discoloration of leaves It was found that the pathogen could only reproduce within the host it infected

Concept 18.1

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 - the protein shell that encloses the viral genome is called the capsid

Concept 18.1

- the capsid may be rod shaped, polyhedral, or more complex Some viruses have viral envelopes, membranes that cloak their capsids - derived from the membrane of a host cell

Concept 18.1

The most complex capsids are found among bacteriophages (or phages), viruses that infect bacteria

Concept 18.1

Concept 18.1

Viruses are obligate intracellular parasites: they can only reproduce within a host cell - viruses lack the enzymes for metabolism and have no ribosomes for making proteins Viruses are merely packages sets of genes in transit between hosts

Concept 18.1

Host 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.

Concept 18.1

Viral reproductive cycle - 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

Concept 18.1

Concept 18.1

Phages replicate using either the lytic or lysogenic cycle Lytic 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

Concept 18.1

Concept 18.1

Lysogenic cycle: replication of the phage genome without destroying the host Temperate phages: viruses that are capable of using both modes of reproducing within a bacterium

Concept 18.1

- phage l 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

Concept 18.1

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

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

Concept 18.1

Concept 18.1

Retrovirus

- 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

Concept 18.1

- RNA  DNA directional flow - DNA integrates as a provirus in the host cell - ex. HIV (human immunodeficiency virus)

Concept 18.1

Concept 18.2

Emerging Viruses - ex. Ebola, Hantavirus, and Nipah virus - recently seen in the last 25 yrs.

3 process contribute to new viruses - mutation of an existing virus - RNA does not proofread = high rate of mutation

Concept 18.2

- spread of a virus from one host to another - ¾ of human diseases originated in another animal - dissemination of a disease from a small isolated population - AIDS went unnoticed for decades

Concept 18.2

Concept 18.3

Bacteria 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 moecules separate from the bacterial chromosome

Concept 18.3

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

Concept 18.3

Concept 18.3

Transformation: the alteration of a bacterial cell’s genotype by the uptake of naked (no cell w/ it), foreign DNA from the surrounding environment

Concept 18.4

The control of gene expression enables bacteria to adjust their metabolism to environmental change The lac operon: regulates the synthesis of the enzymes needed to metabolize lactose.

Concept 18.4

Concept 18.4

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

Concept 18.4

Concept 18.4

- when lactose is present, an inducer binds to the repressor making it inactive - RNA polymerase can then begin the transcription process and the gene can be expressed.