Week 4 L1 - Life Cycle of Bacteriophage Flashcards Preview

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The five stages of viral replication?

1. Attachment
2. Penetration/Entry
3. Biosynthesis
4. Maturation/Assembly
5. Release


What is the difference in replication between T4 and λ, once inside the cell?

* T4 replicates only by the LYTIC CYCLE
* λ is a temperate phage which can replicate by either the LYTIC CYCLE or the LYSOGENIC CYCLE


What is transduction? What are the two types?

Transduction is the accidental transfer of bacterial DNA
between species by phages - may be generalised or specialised in nature. Bacteriophages act as vectors and transfer genes from one bacterium (donor) to another (recipient).
—GENERALISED: Random genes may be transferred
between bacterial cells. If the recipient cell is deficient in something and receives a wild type gene from the donor, this is advantageous for the recipient cell.
—SPECIALISED: During the change from the lysogenic to the lytic cycle, every so often by mistake, as the viral genome excises itself from the host genome via homologous recombination in preparation to form particles and get out of there, it takes a chunk of the bacterial DNA with it. Only specific genes are transferred between bacterial cells, on or near the attachment site, because the lambda genome integrates into the bacterial chromosome at a specific site between the gal region and the bio region. The site on the E. coli chromosome is called att-lambda – (Attachment site for lambda). It is homologous with a site called att in the lambda DNA, allowing direct insertion.



Transcription initiation sequence.



Group of genes transcribed from the same promoter.



Blocks transcription from the promoter.



The site at which the repressor binds to block transcription.



Induces transcription from the promoter.



Blocks the effect of a transcriptional terminator (by
interacting with RNA polymerases).


Lytic versus lysogenic cycle

A temperate bacteriophage has both lytic and lysogenic cycles. In the lytic cycle, the phage replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations.


cro protein

Reduces CII protein synthesis and regulator of cI, since cII activates transcription of cI. Encodes a regulator and represses the cI gene - promotes lytic growth.


N protein

Anti-terminator of TN and Tcro (i.e. transcription of N and cro) - early genes.



A gene that encodes lambda repressor, which maintains lysogeny.


What are the key factors to determine life cycles, i.e. the genetic switch to determine lytic or lysogenic cycles will occur?

cro and cI (lambda repressor) - oppose each other.


Transcription terminator

A section of nucleic acid sequence that marks the end of a gene or operon in genomic DNA during transcription.


Two types of bacteriophages used in lab experiments? What sort of genetic material do they contain?

T4 and lambda. Both contain DNA.


What is a plaque assay?

• A mixture of phages and bacteria is plated on a solid medium. The bacteria grow and produce a bacterial “lawn”.
• The lawn shows areas of clearing where viruses have infected the bacterium and burst their cells – “plaque”.
• The progeny viruses infect and kill neighbouring bacterial cells – producing large plaques or clearing the lawn.


What is a prophage?

The genetic material of a bacteriophage, incorporated into the genome of a bacterium and able to produce phages if specifically activated. A prophage is either integrated into the host bacteria's chromosome or more rarely exists as a stable plasmid within the host cell.


What is a lysogen?

A bacterial cell in which a phage exists as DNA in its dormant state (prophage).


What are temperate phages?

Phages that have a choice between lytic and
lysogenic pathways, eg. lambda


What does the genetic switch refer to?

The trigger for whether the lambda DNA will stay in the lytic cycle, go from the lytic to the lysogenic cycle, or go from the lysogenic cycle back to the lytic.
* If resources are abundant and there are enough nutrients to make many copies of the virus – LYTIC CYCLE
* If resources are limited or marginal – LYSOGENIC CYCLE
* Follows a distinct program of gene expression that is regulated
* Involves a complex genetic switch comprising of several DNA binding regulatory proteins and a set of operators.


O and P as gene products of lambda

Initiation of lambda DNA replication.


Q as a gene product of lambda

Transcription antiterminator protein for late genes (positive regulator), i.e. allows transcription or mRNA of late genes, such as S, R, RZ, head proteins, and tail proteins (phage structural proteins, or lysis proteins) - one promoter, many genes, i.e. operon. These proteins
• produce the viral structures
• pack lambda DNA, and
• lyse the host cell



Activator of transcription of cI (lambda repressor) and int. (integrase). Activate the PRE and PI promoters - cI and int proteins produced. Integrase promotes integration of phage DNA (lysogeny).



Stabiliser of cII.


Integrase (int)

Required for site specific recombination with chromosome, as seen in specialised transduction.


Relationship between cI and cro (two key lambda genes and their functions).

* When cI is on, cro is off - promotes lysogeny
* When cro is on, cI os off - promotes lytic growth
* Are in competition. Whichever repressor prevails, determines the state of the genetic switch, and of the expression of the lambda genome.
* Two proteins compete to bind OL and OR to decrease synthesis of proteins favouring the counter-pathway.


What are the two major promoters for the phage lambda genome?

PL (early promoter for the left operon - goes left), and PR (early promoter for the right operon - goes right).


What are the promoters PRE and PRM for?

PRE: transcription for repressor establishment.
PRM: transcription for repressor maintenance.


What are the operators OL and OR?

They are the operators that overlap with PL and PR.