Exam 2…

Mean = 72 (after adding 7 points to all tests)

Q. 19 - answers were messed up (real answer isc) so part of the 7 points was for that question.

Spend a little time going over the test…..

Bacterial Genetics - we will only discussmechanisms of genetic exchange (genetransfer) in bacteria….

Read pages 323-334 and 203-215.

Three modes of genetic exchange:

1. Transduction (figs. 8.16 and 13.11)2. Transformation (persp. 8.1, pg. 205)3. Conjugation (Figs. 8.18 - 8.22)

Transduction…..

Transfer of host DNA via bacteriophage

2 types: Specialized & Generalized Transduction

Generalized results from the lytic cycle ofcertain phage (see Fig. 8.16)

Specialized results from lysogeny followed bythe lytic cycle (Fig. 13.11)

In order to understand transductionwe need to know…..Viruses ofBacteria (bacteriophage or phage)

Always are naked viruses (no membrane)

why?

Huge variety - One species of bacteria can

have many different phage… Table 13.2

Important for:

1. Genetic transfer (transduction)

2. Control of Bacteria in Nature(read Persp. 13.1 pg. 330)

3. Some can cause bacteria to become pathogens (e.g. diphtheria - see Table 13.3)

Table 13.2. Note the variety of shapes,

nucleic acid content, and “life cycle”…

See Fig. 13.1c

Host ranges of phage:

Phage are usually very specific to the speciesthey infect - they attach to specific receptorson the outer layers of the bacterium -e.g. some phage of E. coli attach specificallyto the proteins of the flagellum…

see Figures 13.12 and 8.17

Phage for a particular bacterium also haveDNA methylation patterns like their host andthus avoid having their DNA cleaved byrestriction enzymes when the DNA entersthe cell…. See Figure 13.13

Bacteriophageattached to pilus ofE. coli

Phage tail fibersentwined aroundflagellum

Phage T4 attached tospecific receptors(cell wall proteins)of E. coli.

Fig. 13.12Virus interactions with host cells orReplication Cycles of phage…..

See Figs. 13.4 - 13.7

Fig. 13.04Fig. 13.4

Latent state =Lysogenic state

Fig. 13.5. Example of the lytic cycle: T4 of E.col

Fig. 13.6. Phage Lambda can undergo a lytic

or the lysogenic cycle

Fig. 13.7. Insertion of Lambda into a specificspot the bacterial chromosome….

Summary of lytic and lysogenic cycles

Consequences of the lysogenic cycle:

Cells are immune to further infection by thatphage

Can lead to specialized transduction (later)

Can cause “lysogenic conversion” = viral genesthat change the phenotype of the host cell - e.g.some phage have genes for toxin production andcan convert a non-pathogenic bacterium to apathogenic one….

See Table 13.3 for examples….

Table 13.3. Lysogenic conversion of bacteria

conferring pathogenic properties….

Prophage = the lysogenic phage incorporatedinto the bacterial chromosome…..

Back to Bacterial Genetics

Transduction…..

Transfer of host DNA via bacteriophage

2 types: Specialized & Generalized Transduction

Generalized results from the lytic cycle ofcertain phage (see Fig. 8.16)

Specialized results from lysogeny followed bythe lytic cycle (Fig. 13.11)

Fig. 8.16Generalized transduction results from the lytic cycle.

Chromosome is digested into small pieces - some of which end up

being packaged in virus particles…

Fig. 13.11. Specialized transduction by a temperate

phage. Results from the lysogenic cycle… Specific

pieces of DNA (near insertion sites) are transferred…

Fig. 13.11b. Specialized transduction by a temperate

phage

Bacterial Genetics

2. Transformation… uptake and

incorporation (into the chromosome) of“naked” DNA from the environment….Expression of this new DNA can alter thephenotype of the organism, e.g. converting anon-pathogen into a pathogen….

e.g. Streptococcus pneumoniae… Fig. 19.10. Streptococcus

pneumoniae is pathogenic only when it produces a capsule(which helps it avoid detection by antibodies and phagocytes)

Persp. 8.1, pg. 205. Experiment carried out by Griffin,

showing that something was transferred from dead,

pathogenic, S. pneumoniae to live nonpathogenic cells -

transforming them into pathogens…

It was later (1944) shown that the “transformingprinciple” was DNA (thus studies of transformation led to identification

of DNA as the genetic material in cells).

Fig. 8.14. Transformation of a cell from non-resistant (StrS) to resistant to streptomycin (StrR).

Fig. 8.15. There are many artificial ways to make

bacterial cells “competent” for transformation, e.g.

electroporation, and many chemical treatments that

make temporary holes in the cell wall and membrane

Transformation can take place between evenunrelated bacteria (but it is rarer thanbetween related because most foreign DNA isdegraded before it can be methylated - seefigure 8.26)

There are also genetic platforms (integrons)within bacterial chromosomes that canmobilize large pieces of DNA and integratelarge pieces of foreign DNA behind apromoter so that they are easilytranscribed…

Fig. 8.26

Bacterial Genetics

Conjugation (bacterial sex) Usually occursbetween a + strain (has a conjugative plasmid)and a - strain (no plasmid)….

Fig. 8.17. Conjugation between F+ and and F- E. coli…

Quick review of Plasmids - Extrachromosomal,

circular, double-stranded DNA molecules.Plasmids divide and copies go to both daughter cells

during asexual reproduction.Many plasmids can be transmitted between closely related

species but some are not limited to close relatives.

Not usually essential for a given organism, rather they allow the

organism to adapt to specific environmental conditions.

Therefore, plasmids are often unstable in a host bacterium due

to the increased metabolic load.

Also found in Archaea, Fungi and other euks………

All sorts of traits can be transferred viaconjugative plasmids - the most relevant traitsfor us are antibiotic resistance genes…. Butother important genes are also found onplasmids.. (Table 8.4)

Antibiotic resistance - usually by coding for an

enzyme that renders the antibiotic non-functional.

Beta-lactamase (inactivates Beta-lactams) is one

example. Penicillin is a Beta-lactam.

Special metabolic properties - some plasmids allow

bacteria to take advantage of situations that might be

otherwise harmful. Breakdown of complex organic

molecules is often plasmid encoded

Table 08.04

Virulence Plasmids - there are a number of ways that a

plasmid can confer virulence in a bacterium.

1) The production of one or more toxins that

can be directed toward the host or towards other

bacteria (bacteriocins).

2) The ability to form a capsule.

The recent anthrax scare is an example:

Virulent B. anthracis have 2 plasmids that encode for toxin

production and capsule formation. An avirulent strain used

for veterinary vaccination lacks the capsule forming

plasmid.

Other virulence factors on plasmids:

The production of siderophores that enable the bacterium to

scavenge iron in the body (e.g. S. aureus).

Adhesins - proteins or glycoproteins that are usually a

component of capsules or fimbriae that allow the bacterium

to adhere to specific cells.

The real scary thing about plasmids is thatmany of them have the above traits ANDare conjugative as well

Conjugation is brought about via informationstored on fertility plasmids (= conjugativeplasmids)… which contain genes for:

1. The F pilus2. Genes to mobilize the plasmid (Transfer factors)

3. An origin of replication

See figure 8.22 (Later in lecture…..)

Fig. 8.18. Conjugation - transfer of the F plasmid. Note that both cells are

F+ after the mating takes place…..

Fig. 8.19. Hfr formation via integration of the F-plasmid into the

host chromosome at specific insertion sequences (ISs)

Fig. 8.20. Conjugation involving an Hfr cell. Basically the

whole chromosome is now a giant F plasmid - but the whole thing is

rarely transferred..

Fig. 8.21. An F’ plasmid results if some chromosomal DNA is

excised when the plasmid pops back out….

Fig. 8.22.

The regions

of an R

Plasmid.

RTF =

resistance

transfer

factors

R= resistance

genes

Plasmids and a type of conjugation are also

involved in the transfer of genetic information frombacteria to some Eucaryotes.

Remember Agrobacterium tumefaciens (a Gm-,

alphaproteobacterium related to Rhizobium Fig.11.2)

has a Ti plasmid (tumor inducing plasmid) that

codes for:

1. A pilus

2. Transfer factors

3. Transferred DNA (T-DNA) that codes forplant hormone production and theproduction of opines

4. Metabolic genes that allows A. tumefaciensto eat opines

Persp. 8.2, pg. 211Table 8.3

Movement of DNA within cells

e.g.transposons

Transposition involves small segments of DNA

(transposons, "jumping genes") that can move

around chromosomes or plasmids.

e.g. F-plasmids have insertion sequences that

allow the plasmid to integrate into the

chromosome => Hfr cell.

Insertion sequences are very simple and

typically contain only the information needed for

insertion (see figure 8.24). Also used in experiments to disrupt

specific genes.

Transposons (Composite Transposon in Figure 8.24) are more

complex and may contain a number of genes that

confer e.g. antibiotic resistance and/or toxin

production.

Fig. 8.23. Movement of a transposon through a bacterialcommunity.

Fig. 8.24 a. Insertion sequence, b. Composite transposoncontaining an antibiotic resistance gene….

Fig. 08.24