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53 Cards in this Set

  • Front
  • Back

Are bacterial viruses host dependent?


What does host dependent mean?

Cannot survive without infecting bacteria

Require host machinery to produce progeny phages

What is host specificity

Each phage has its specific range of host bacteria

Are they beneficial to the host?

No, they are harmful to their hosts

What is the most abundant and diverse form of DNA replicating agent on the planet?

Marine phages

What is the ratio of marine phage to ocean in 1ml?

5x10^7 per ml

What do marine phages do for bacterial population explosions?

Essential in preventing bacterial population explosions (algal blooms)

Who discovered marine phages?

Twort and d' Herelle independently

What is the most common genome of phages?

dsDNA genomes

Name the parts of a virus

How many tailed phages are there?


How many phages in total?


What are the categories in categorising a phage?

Host bacteria, genome, morphology, ability to lysogenise

What are the 3 basic types of phage?

Tailed phages (dsDNA)

Filamentous phages (ssDNA)

Icosahedral phages (ssDNA, ssRNA, dsDNA)

Example of lambda phage

Linear dsDNA, icosahedral lysogenic cycle

How much average size gene products do simple phage have enough nucleic acid to code for?

3-5 average size gene products

How many gene products do more complex phages have to code for?

over 100 gene products

What do tail fibres attach to?

Specific receptors

What are some examples of specific receptors?

Surface proteins, LPS, pili and lipoprotein

What is the role of the sheath?

Serves as a channel for DNA/RNA injection

3 characteristics of the lytic cycle

+lysis of host and release progeny particles

+viral genes fully expressed

+virulent phage

What are 3 characteristics of the lysogenic cycle?

+phage DNA inserted into the host chromosome sometimes at a specific site-prophage

+most of phage functions are switched off

+temperate phage

What are the 5 stages of the infection of host cells by phages --lytic cycle ?

1. Adsorption

2. Injection fo genomic nucleic acid

3. Transcription & Translation

4. Replication of DNA

5. Maturation & release

3 characteristics of adsorption?

Highly specific

Interaction between tail & cell surface receptors

Host range mutation may occur in phages

What did the Hershey and Chase Experiement (1952) show?

The material inside the virus is not proteins but nucleic acids (genetic information)

4 steps in viral injection of DNA

Landing, pinning, tail contraction and penetration & DNA injection

What are the 3 types of genes in sequential transcription of phage genes (Transcription & translation- 1)

Immediate early and delayed early genes

Middle genes- transcribed throughout infection

Late genes

What occurs in immediate early and delayed early genes?

Shut off cellular macromolecule synthesis

Replication of phage DNA

What occurs in middle genes- transcribed through infection?

DNA replicates and recombines

What occurs in late genes?

Capsid proteins & enzymes for lysing cells

What else occurs in transcription and translation?

utilisation of ribosomes

What is the main component of the head capsid?

Most abundant T4 protein

Describe the viral DNA replication- M13

Host enzymes carring in the infecting ss viral genome and a duplex DNA replicative form

Replication occurs (bi-directoinal ori)

Gene II nicks + strand rolling in a circle replication

Gene II nicks completed + strand and circularisatoin of completed + strand

What occurs during maturation and release?

Assembly of the capsid
Association of DNA and capsid
Lysis of cells and release of phages (~100 progeny phage released, each cycle 45-60mins)

What forms when there is bacteria + phage after phage plaque formation?

lysis can be seen- where all the bacteria have died as the phage killed it

What is lysogeny?

The indefinite persistence of the phage DNA in the host cells, without phage production

What can temperate phages produce?

phenomenon of lysogeny

What are prophages?

Integrated, noninfectious form of phages

What is repression?

Phage inserts itself via integration to host's chromosome to be replicated everytime bacteria replicates

What is a repressor?

Phage coded protein that binds to a site on the phage DNA (operator)

What is the role of the repressor?

Shuts off transcription of most phage genes except repressor gene

Is repression specific?

Yes, immunity to superinfection with same phage

Describe integration of DNA into bacterial chromosome

Cut & paste of DNA , recombination

What are some phage resistant systems?

Blocking of phage DNA injection

Abortive infection systems

Evolutionary arms race between bacteria and their phages

What is the abortive infection system?

interfering with phage DNA replication, RNA transcription, phage development and morphogenesis

What is naturally occurring phage resistance

Adsorption interference

DNA injection inhibition

Restriction/modification (R/M)

Abortive infection (Abi)

What is genetically engineered phage resistance?

Per (phage encoded resistance)

Antisense RNA

Briefly describe phage resistance in lytic cycle

Restruction modification (R/M) -> Abortive infection -> Adsorption interference

Importance of bacteriophges

Early mode for molecular genetics and biology

Therapy-antibiotic resistance

Important component of modern biotechnology

Advantages of bacteriophages

Very specific- affects targeted bacterium only

Chance of developing secondary infects reduced

No side affects

Phage able to reproduce as long as host bacteria is available

Development of resistance may lead to virulence

Selection of new phages can overcome resistance

Production simple and relatively inexpensive

Advantages of antibiotics

Can be used without knowing the ID of the bacteria

Disadvantage of bacteriophages

Disease causing bacteria must be identified before therapy

Disadvantage of antibiotics

Non-specific action that targets pathogen and normal flora leading to secondary infections
Multiple side effects: yeast infection, allergies , intestinal disorders
Repeated administration is needed
Antibiotic resistant bacteria remains pathogenic
Development of new antibiotic takes years
Production is expensive