Viruses Combined Set

Define virus

non cellular particle composed of genetic material enclosed in a capsid

Why can’t viruses reproduce indepdently?

They lack structures and processes for independent life and can only replicate inside a host cell

What does ‘obligate intracellular parasite‘ mean?

A virus must infect and use a host cell’s machinery to replicate, it cannot survive or reproduce alone

How big are viruses compared to cells?

20-300 nm,
smaller than bacteria and eukaryotic cells

Why is small size important for viruses?

It allows them to enter host cells and evade immune detection

Do viruses grow after assembly?

No — their size is fixed by the arrangement of capsid proteins

What type of genetic material do viruses contain?

Either DNA or RNA never both

What forms can viral genetic material take?

Single stranded or double stranded

linear or circular

What does genetic material do inside a host cell?

Direct’s the cell’s machinery to make viral proteins and replicate the viral genome

Why do RNA viruses mutate faster than DNA viruses?

RNA replication lacks proofreading mechanisms, leading to more errors.

What is a capsid

A protein shell made of repeating subunits — protecting the viral genome

Main functions of the caspid?

Protects the genome
Helps the virus attach to host cells
Enables efficient assembly with minimal genetic information

What are the three capsid shapes?

Icosahedral,
Helical
Complex

What is a viral envelope and what does it do

Lipid membrane that helps some viruses evade the immune system

Do all viruses have envelopes

No

Why are viruses considered acellulat

As they lack cytoplasm, organelles and membranes

do not have the internal structure of living cells

Why can’t viruses carry out metabolic processes?

They lack organelles and enzymes needed for energy production and protein synthesis

Do viruses contain many enzymes?

No

they carry only a few essential enzymes for replication

Why are viruses considered acellular?

lack cellular structure

do not share a single evolutionary origin

What contribiutes to the adaptabiity of viruses?

diverse solutions to parasitism

Three main shapes of viruses

Icosahedral
Helical
Complex

Icosahedral virus

Symmetrical

20 sided capsid

Coronavirus

Describe a helical virus

A rod shaped virus with a spiral capsid

What characterizes complex viruses

Icosahedral head
Helical tail

How small are viruses

20-300 nm

How large is bacteriophage lambda

up to 750nm

Do viruses grow?

No, their size is fixed which allows them to enter host cells

What forms of genetic material can viruses have?

DNA or RNA
SS or DS
Linear or Circular

Positive sense RNA?

RNA that acts directlty as mRNA

What is negative sense RNA?

RNA that must be transcribed into mRNA before translation

What type of genome do coronaviruses have?

Single stranded positive sense RNA

What is unique about HIV’s genome?

It contains two ss-RNA molecules that are reverse transcribed into DNA

What defines a naked virus?

A protein capsid

highly resistant to environmental stress

What defines an enveloped virus

A host derived lipid bilayer with viral glycoproteins

Why are enveloped viruses common in animals

Their lipid envelope helps them evade the immune system by mimicking host membranes

What organism does bacteriophage lambda infect?

E coli

Describe the structure of bacteriophage lambda

Icosahedral head with ds-DNA, protein tails and fibers

How does bacteriophage deliver its DNA

It injects DNA through the bacterial cell wall

What structural feature gives coronaviruses their name

Spike glycoproteins

What type of capsid do coronaviruses have?

Helical capsid

What type of virus is HIV?

A retrovirus.

What enzymes does HIV carry?

Reverse transcriptase, integrase, and protease

What is the function of reverse transcriptase in HIV?

Converts viral RNA into DNA for integration into the host genome

Why can viruses not reproduce independently

lack their own metabolic machinery,

rely entirely on host cell’s enymes, and energy systems

What do viruses depend on in the host cell to reproduce

Host cell’s metabolic machinery

Lytic Cycle

A virus infects a host cell
takes over it’s machinery to produce new viral particles and causes the host cells to burst releasing the viruses

What is the biological cost of the lytic cycle for the host cell?

Host cell is destroyed when it bursts

Lytic Cycle - Step 1

Attachment

Viral attachment proteins bind to specific receptor molecules on host cell surface

Lytic Cycle - Step 2

The phage infects its DNA into the host cell through a syringe-like tail;
only the viral DNA enters, while the capsid stays outside

Why does only viral nucleic acid enter the host cell while the capsid remains outside?

The tail structure injects the DNA directly into the cell, leaving the protein capsid behind.

What happens during Step 3 (Biosynthesis)?

Viral DNA hijacks host enzymes and ribosomes;

early genes replicate viral DNA

, late genes produce structural proteins,

host functions are suppressed.

What happens during Step 4 — Aseembly

Viral DNA and proteins self - assemble into complete bacteriophages, each with a capsid filled with viral DNA

What happens during Step 5 lysis

Viral enzymes break down the bacterial cell wall, causing the cell to burst and release hundreds of new viruses

Which enzyme degrades the bacterial cell wall during lysis and why is this necessary

Phage - produced lysins,

they break down the cell wall

Host cell bursts and releases new viruses

What is viral self assembly

Natural, energy free process by which capsid protein subunits spontaneously forma a symmetrical viral structure

One major advantage of the lytic cycle for viruses:

Rapid amplification — it produces large numbers of viral particles quickly.

What is a

What are the two life cycles bacteriophage λ can switch between

The lytic cycle and the lysogenic cycle.

What happens during Step 1 (Attachment) of the lysogenic cycle?

The phage attaches to E. coli by binding its tail proteins to maltoporin on the bacterial outer membrane.

What is maltoporin?

A protein in the E. coli outer membrane that bacteriophage λ specifically binds to

What happens during Step 2 (DNA Entry)?

The phage contracts its tail sheath and injects viral DNA into the cytoplasm; only DNA enters, the capsid stays outside

What happens to the viral DNA once inside the host?

It enters as a linear molecule with sticky ends and rapidly circularizes.

What enzyme inserts viral DNA into the host genome?

Integrase.

What is the viral DNA called once integrated into the host chromosome?

A prophage.

What happens during Step 4 (Prophage Replication)?

The host cell divides normally, passively copying the prophage along with its own DNA.

What ensures that each daughter cell inherits the prophage?

The prophage is replicated alongside host DNA during cell division.

What maintains lysogeny during Step 5 (Latency)?

Viral repressor proteins keep most viral genes switched off

What is lysogeny?

A dormant state where the viral genome is integrated and inactive while the host cell remains healthy

What type of virus is bacteriophage λ described as during lysogeny?

A temperate phage.

What triggers Induction

Environmental stressors
UV light
DNA damage
Chemicals

What happens during induction

The repressor is inactivated, the prophage is excised, and the virus enters the lytic cycle

What is the key meaning of induction in phage biology

Transition from lysogenic to lytic

What is the lysogenic cycle beneficial for the virus

It allows long term persistence without killing all the host cells, especially during poor conditions

Which protein dominates when the host is healthy and dividing

Cro

What viral strategy is favoured when Cro dominates

The lytic cycle

Which protein dominates when the host is stressed or starving

Cl (the repressor)

What viral strategy is favoured when Cl dominates

Lysogeny

What happens when host DNA is damaged

Cl is destroyed, Cro becomes active, and the virus switches to the lytic cycle

Why do viruses evolve so quickly?

Because they have short generation times, high mutation rates, and strong selective pressures.

Why does fast replication speed up viral evolution?

More replication events create more opportunities for mutations.

How fast do viruses replicate compared to multicellular organisms?

Within minutes to hours, versus decades for multicellular organisms.

Why do RNA viruses mutate faster than DNA viruses?

Their replication enzymes lack proofreading.

Which enzyme do DNA viruses use?

DNA polymerase.

Does DNA polymerase have proofreading?

Yes.

Which enzymes do RNA viruses use?

RNA polymerase or reverse transcriptase.

Do RNA polymerase and reverse transcriptase have proofreading?

No.

What is the consequence of no proofreading in RNA viruses?

They accumulate frequent errors and form genetically diverse populations.

Why can high mutation rates be beneficial for viruses?

They increase adaptability.

What selective pressures do viruses face?

Immune defenses, antiviral drugs, and transmission barriers.

How can mutations help viruses evade immunity?

By altering surface proteins so antibodies no longer recognize them.

What two factors must always be mentioned together when explaining rapid viral evolution?

Variation and selection.

What type of virus is influenza?

An RNA virus with eight genome segments.

Why does influenza mutate frequently?

Its RNA polymerase lacks proofreading.

What is antigenic drift?

Gradual accumulation of mutations in H and N genes.

What is the effect of antigenic drift?

Slight changes in surface proteins reduce immune recognition.

Why must the flu vaccine be updated annually?

Because antigenic drift produces new viral variants each year.

What is antigenic shift?

Sudden reassortment of genome segments when two influenza strains co-infect a host.

Why is antigenic shift dangerous?

It creates new strains with no pre-existing human immunity, potentially causing pandemics.

What is an example of antigenic shift?

The 2009 H1N1 swine flu pandemic.

What type of virus is HIV?

An RNA retrovirus.