Viruses and Bacteria

Ch. 18

Viruses

Parasite that requires a host cell in order to live They take the host cell hostage and use the cell to

create the proteins it needs to make new viruses Typically creates thousands of new viruses and

eventually kills the host cell

The range of organisms that a virus can attack is referred to as its host range

A new viral disease that harms humans (ex. Hanta virus) may result from a mutation in the virus that expanded its host range

Structure

Made of either DNA or RNA enclosed in a protein coat called a capsid

Many viruses also have a viral envelope which is a membrane that cloaks the capsids

A virus can only infect one specific type of cell The reason for this is that the virus binds to specific

receptors on the surface of the cell

Bacteriophages Most complex virus; Infects bacteria Also known as the shortened term “phages” Can reproduce by either the lytic cycle or lysogenic

cycle

Lytic Cycle: Phage enters host cell Takes control Replicates itself Causes cell to burst

This releases a new generation of phage viruses to go and do the same thing

Lysogenic Cycle Replicate without killing the host cell Incorporated into the host’s DNA Stays dormant and is called a prophage When host cell divides, prophage is replicated with it Leads to a population of infected cells Environmental trigger causes prophage to switch to the

lytic phase

Viruses capable of both types of reproducing are called temperate viruses

Retroviruses

Viruses that contain RNA instead of DNA and replicate unusually

Virus infects host cell, uses its own RNA to create complementary DNA (cDNA)

Reverse the usual flow of information from DNA to RNA This is why they are called retroviruses

Reverse transcription is made possible by an enzyme called reverse transcriptase

Newly made DNA integrates into a chromosome in the nucleus

The integrated viral DNA is called a provirus

Transduction

Phages carry bacterial genes from one host cell to another

There are two forms of transduction:

Generalized Transduction: moves random pieces of bacterial DNA as a cell is lysed by the phage and infects another (lytic cycle)

Restricted Transduction: moves specific pieces of DNA that were near the prophage site on the bacterial chromosome. Can only be done by a temperate phage.

Prions

Infectious proteins that can cause several brain diseases like “mad cow” disease in cattle

Prion is a misfolded version of a protein normally found in the brain

Causes normal proteins to misfold in same way Prions are not considered cells or viruses

Bacteria

Chromosomes are circular, double-stranded DNA tightly packed into a nucleoid

A nucleoid has no nuclear membrane Bacteria replicate DNA in both directions from a single

origin of replication This is called theta replication

Reproduction: Conjugation: direct transfer of genetic material

between two bacterial cells that are temporarily connected

Binary Fission: asexual reproduction

Plasmid: foreign, small, circular, self-replicating DNA molecule in a bacterium

A bacterium can have several plasmids

The first plasmid discovered is the F plasmid

F stands for fertility Bacteria that contain the plasmid are F+

Bacteria that do NOT contain the plasmid are F-

The F plasmid has the genes that produce pili Pili are cytoplasmic bridges that connect cells and

allow conjugation

Another plasmid worth knowing is the R plasmid

Resistance to antibiotics for the host bacteria R plasmid can also be transferred from one bacteria

to another by conjugation Bacteria that have the R plasmid have a big

advantage because they are resistant to antibiotics Resistant bacteria are more likely to survive and

reproduce quickly This is bad news for doctors!

The Operon Important to gene regulation Operon is a set of genes and “switches” that control the

expression of those genes Discovered in 1940’s by Jacob and Monod

Two types of operons:

Inducible: switched off until it is “induced” to turn on

Repressible: always on until it is not needed and becomes “repressed” (turned off)

Tryptophan Operon

A repressible operon Continuously turned on unless turned off by a corepressor Repressor is encoded by the regulatory gene and is initially

inactive This means RNA polymerase is free to bind to the promoter

and transcribe the structural genes, producing tryptophan When the inactive repressor combines with the corepressor

(tryptophan) it changes its shape and binds to the operator, stopping transcription

If tryptophan levels are high, no more is needed, so the production is turned off

Tryptophan is an allosteric effector

Lac Operon

Inducible Operon Turned off until it is “induced” to turn on

Repressor is initially active; binds to the operator located on the promoter, which blocks RNA polymerase from binding there, stopping production

Allolactose (derivative of lactose) acts as the inducer It binds to the repressor, changing its conformation (shape) Now that the shape has changed the repressor cannot bind

to the operator, which turns the operon on

Transposons Transposable genetic elements Also called “jumping genes” Discovered by Barbara McClintock

Two types:

Insertion sequences: made of only one gene, which codes for transposase, the enzyme responsible for moving the sequence from one place to another

Complex transposons: longer and include extra genes. One or more genes located between two insertion sequences