Chapter 18 Notes

The Genetics of Viruses and Bacteria

Concept 18.1

Researchers discovered viruses through the understanding of tobacco mosaic disease; which stunts growth and causes discoloration of leaves

Concept 18.1

Through 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.

Comparing the sizes of a virus, bacteria and animal cell.

Animation

How Flu Viruses Attack How A Virus Invades Your Body

Concept 18.1A 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

Concept 18.1

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

the capsid may be rod shaped, polyhedral, or more complex

Some viruses have viral envelopes, membranes that covers their capsids- Which are 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

Bacteriophage Animation

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 packaged sets of genes in transit from one host cell to another.

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 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

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

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 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

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

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 18.2

Many 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

Viral Diseases in Animals

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

Three processes contribute to new viruses

1. mutation of an existing virus- RNA does not proofread = high rate of mutation

Viral Diseases in Animals

2. spread of a virus from one host to another- ¾ of human diseases originated in another animal

3. dissemination of a disease from a small isolated population can lead to widespread epidemics- AIDS went unnoticed for decades

Viral Diseases in Plants More than 2000 types which results

in about $15 billion loss worldwide Have the same structure and mode of

replication as animal viruses Results are bleached or brown spots,

stunted growth, damaged flowers and roots and the streaking of tulips

Viral Diseases in Plants

Two modes of transmission:1. Horizontal: Plant is infected from

an external source because the virus can pass through the epidermis.

2. Vertical: The plant inherits a viral infection from a parent (asexual propagation or infected seeds).

The Simplest Infectious Agents

Viroids – circular RNA molecules, only several hundred nucleotides long, that infect plants

One viroid disease has killed over 10 million coconut palms in the Phillipines

They do not encode proteins but can replicated in the host cell

The Simplest Infectious Agents

Prions – infectious proteins Animation Appear to cause a number of

degenerative brain diseases in various animal species, ex. Mad Cow Disease

They are slow-acting with long incubation periods and they are virtually indestructible

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 molecules separate from the bacterial chromosome and can be transferred between cells

Concept 18.3

Bacteria reproduce asexually by binary fission

Mutations can cause some of the offspring to differ slightly in genetic makeup

Replication of a bacterial chromosome

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 and phenotype by the uptake of naked, foreign DNA from the surrounding environment

Ex. Bacteria of a harmless strain of S. pneumoniae can be transformed into pneumonia causing cells

Concept 18.3

Transduction: phages carry bacterial genes form one host cell to another as a result of aberrations in the phage reproductive cycle

The transferred DNA may recombine with the genome of the recipient cell bringing about a recombinant cell

Concept 18.3

Conjugation: sometimes referred to as bacterial “sex”, is the direct transfer of genetic material between two bacterial cells that are temporarily joined

One cell donates DNA and its “mate” receives the DNA

Concept 18.4

The control of gene expression enables bacteria to adjust their metabolism to environmental change

Operon: The operator, the promoter and the genes they control which is the entire stretch of DNA required for enzyme production

Concept 18.4

The lac operon: regulates the synthesis of the enzymes needed to metabolize lactose.

Animation

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.