Microbio Exam #3 Questions With Complete Solutions
What happens when lambda infects and chooses lysogeny?
Its DNA circularizes and integrates into the bacterial genome to
wait for better conditions
What triggers lambda prophage to excise and go lytic?
Environmental stress or damage to the host cell.
What normally happens when lambda excises correctly?
It exits the chromosome cleanly and produces normal viral
particles.
What rare mistake leads to specialized transduction?
The prophage excises imprecisely and grabs adjacent bacterial
DNA when it comes out
Why is this defective phage genome still packagable?
It still contains the essential viral genes required to assemble
particles.
Why can the defective transducing phage not kill a new host
cell?
Because it lacks a complete viral genome.
What is the "gift" this defective phage gives to the new cell?
It delivers a specific piece of donor bacterial DNA that can
homologously recombine.
,Why does specialized transduction only move certain bacterial
genes?
Only genes directly next to the prophage insertion site can get
pulled out during faulty excision.
In lambda, what operon sits next to its integration site in E. coli?
The galactose utilization operon (gal).
What can specialized transduction do to phenotype?
It can instantly repair or complement defective pathways in the
recipient.
General vs Specialized transduction difference in one sentence
Generalized can transfer ANY random piece; specialized can
transfer ONLY DNA next to the prophage insertion site.
Why does specialized transduction require a temperate phage +
lysogeny first?
Because specialized transduction ONLY happens when a
prophage excises incorrectly from the chromosome — so the
phage MUST have integrated into the host genome first
(lysogeny)
Why does generalized transduction not require lysogeny?
Because generalized transduction happens during lytic
replication when the phage is packaging DNA — random
chromosomal fragments can accidentally get stuffed inside a
phage head WITHOUT the phage ever integrating first.
,Which transduction type results from packaging errors during
lytic assembly?
Generalized transduction.
What is "headful" packaging?
A viral packaging strategy where the phage fills the capsid until
it reaches the correct genome size — ANY DNA of ~correct
length can be packaged.
Why does sequence-specific packaging prevent transduction?
Only DNA with viral packaging signals gets packaged —
bacterial DNA is excluded.
Where are lambda att sites located?
One copy on the lambda phage genome and a matching copy on
the E. coli chromosome — these matching sequences allow site-
specific recombination for integration
Is lambda insertion homologous recombination like F plasmid
Hfr formation?
No. Lambda uses site-specific recombination at att sites (not
generalized homology along many sequences like IS elements).
What are lambda att sites?
Specific attachment sequences the lambda phage uses to align
with the bacterial chromosome for recombination.
What happens when lambda integrates at att sites?
It forms a chimeric junction that is partly viral DNA and partly
bacterial DNA.
, What is the host region next to lambda's att site in E. coli?
The galactose utilization (gal) operon.
What is a bacterial cell called once lambda is integrated?
A lysogen.
What is the integrated lambda DNA called inside the
chromosome?
A prophage.
What normally happens when lambda excises during induction?
It recombines correctly at the att sites and leaves with its exact
original genome.
What rare event leads to specialized transduction in lambda?
Faulty recombination at the att sites that pulls out adjacent
bacterial genes with the prophage.
What defective particle is produced when lambda accidentally
carries the gal operon?
A λ dgal particle ("lambda defective galactose").
Can a λ dgal particle cause a new viral infection?
No — it is missing viral genes required for productive infection.
Why is λ dgal still important biologically?
It can deliver the gal operon to a new cell and recombine,
restoring gal function in the recipient.
Why is λ dgal considered "defective"?
What happens when lambda infects and chooses lysogeny?
Its DNA circularizes and integrates into the bacterial genome to
wait for better conditions
What triggers lambda prophage to excise and go lytic?
Environmental stress or damage to the host cell.
What normally happens when lambda excises correctly?
It exits the chromosome cleanly and produces normal viral
particles.
What rare mistake leads to specialized transduction?
The prophage excises imprecisely and grabs adjacent bacterial
DNA when it comes out
Why is this defective phage genome still packagable?
It still contains the essential viral genes required to assemble
particles.
Why can the defective transducing phage not kill a new host
cell?
Because it lacks a complete viral genome.
What is the "gift" this defective phage gives to the new cell?
It delivers a specific piece of donor bacterial DNA that can
homologously recombine.
,Why does specialized transduction only move certain bacterial
genes?
Only genes directly next to the prophage insertion site can get
pulled out during faulty excision.
In lambda, what operon sits next to its integration site in E. coli?
The galactose utilization operon (gal).
What can specialized transduction do to phenotype?
It can instantly repair or complement defective pathways in the
recipient.
General vs Specialized transduction difference in one sentence
Generalized can transfer ANY random piece; specialized can
transfer ONLY DNA next to the prophage insertion site.
Why does specialized transduction require a temperate phage +
lysogeny first?
Because specialized transduction ONLY happens when a
prophage excises incorrectly from the chromosome — so the
phage MUST have integrated into the host genome first
(lysogeny)
Why does generalized transduction not require lysogeny?
Because generalized transduction happens during lytic
replication when the phage is packaging DNA — random
chromosomal fragments can accidentally get stuffed inside a
phage head WITHOUT the phage ever integrating first.
,Which transduction type results from packaging errors during
lytic assembly?
Generalized transduction.
What is "headful" packaging?
A viral packaging strategy where the phage fills the capsid until
it reaches the correct genome size — ANY DNA of ~correct
length can be packaged.
Why does sequence-specific packaging prevent transduction?
Only DNA with viral packaging signals gets packaged —
bacterial DNA is excluded.
Where are lambda att sites located?
One copy on the lambda phage genome and a matching copy on
the E. coli chromosome — these matching sequences allow site-
specific recombination for integration
Is lambda insertion homologous recombination like F plasmid
Hfr formation?
No. Lambda uses site-specific recombination at att sites (not
generalized homology along many sequences like IS elements).
What are lambda att sites?
Specific attachment sequences the lambda phage uses to align
with the bacterial chromosome for recombination.
What happens when lambda integrates at att sites?
It forms a chimeric junction that is partly viral DNA and partly
bacterial DNA.
, What is the host region next to lambda's att site in E. coli?
The galactose utilization (gal) operon.
What is a bacterial cell called once lambda is integrated?
A lysogen.
What is the integrated lambda DNA called inside the
chromosome?
A prophage.
What normally happens when lambda excises during induction?
It recombines correctly at the att sites and leaves with its exact
original genome.
What rare event leads to specialized transduction in lambda?
Faulty recombination at the att sites that pulls out adjacent
bacterial genes with the prophage.
What defective particle is produced when lambda accidentally
carries the gal operon?
A λ dgal particle ("lambda defective galactose").
Can a λ dgal particle cause a new viral infection?
No — it is missing viral genes required for productive infection.
Why is λ dgal still important biologically?
It can deliver the gal operon to a new cell and recombine,
restoring gal function in the recipient.
Why is λ dgal considered "defective"?
