A 2.3 viruses

What is a virus?

A small infectious particle that invades cells and reproduces inside them.

Why are viruses small?

They must be small enough to enter host cells.

Why do viruses have fixed size?

They do not grow; they self‑assemble inside host cells.

What is the viral capsid?

A protein coat that encloses the viral genome.

Do viruses have cytoplasm or many enzymes?

No — only a few essential enzymes for infection/replication.

What types of genetic material can viruses have?

DNA or RNA; single‑ or double‑stranded; circular or linear.

What is an enveloped virus?

A virus that uses host plasma membrane to form an envelope.

What is a non‑enveloped virus?

A virus lacking an envelope; often more resistant.

Explain the steps of the lytic cycle.

1. The virus attaches to the maliporin on the outer membrane of E.Coli ( the bacterium) 

• The virus attaches to the cell 

2. The DNA of the virus can enter the host via the pore in maltoporin and in the inner membrane of the bacterium 

• The virus inserts its genetic material into the cell

3. The viral DNA ends will join to form a circle and is then replicated 100 times - this is how new viruses are created ( replication) 

4. Then this new DNA is transcribed to form mRNA 

• Messenger RNA copies the viral genes. These are then translated to make viral proteins (transcription) 

5. The MRNA is used to synthesize proteins using the host cells ribosome in a linear structure 

6. These proteins will then self assemble to form capsids with the viral DNA inside each capsid  

•  The viral proteins can make holes through the wall and membranes of the host cell 

• The entire cell bursts  and the virus can then spread to other cells 

What happens to the host cell in the lytic cycle?

It is destroyed (burst).

Explain the steps of the lysogenic cycle.

1. The virus attaches to the maliporin on the outer member of E.Coli ( the bacterium)

• The virus attaches to the host cell 

2. The DNA of the virus can enter via maltoporin 

• The genetic material (DNA) of the virus  is injected and inserted into the host cell 

3. The viral DNA ends will join to form a circular DNA

4. The circular DNA is integrated into the host DNA ( done through the enzyme Intergrase)

• End result : The viral DNA becomes become part of the host DNA ( of the cell)

5. The virus now exists inside the host DNA - when the bacterial host replicates its own DNA it will also replicate the proggae with it aka the dna of the virus 

• Both of the daughter cells contain the prophage (the dna of the virus)

• Under stressful conditions the prophage Dna is removed from the bacterial chromosome and enters the lytic cycle

What is a prophage?

Viral DNA integrated into host DNA.

Why must viruses have evolved after cells?

Viruses require cells to replicate and use the same genetic code.

Why do viruses likely have multiple origins?

Their structures and genomes are extremely diverse.

What is the progressive hypothesis?

Viruses evolved from cellular genetic elements that gained mobility.

What is the regressive hypothesis?

Viruses evolved from cells that lost components over time.

Does the lysogenic cycle kill the host cell?

No — minimal harm until switching to lytic.

Why do viruses evolve rapidly?

Quick generation times, high mutation rates, strong natural selection.

Why do viruses mutate frequently?

Their replication enzymes often lack proofreading.

Why does HIV mutate rapidly?

Reverse transcriptase has no proofreading → many errors.

Why does influenza mutate rapidly?

RNA replicase lacks proofreading + 8 RNA segments recombine.

What is viral replication dependent on?

Host cell machinery (ribosomes, enzymes).

Why do viruses avoid killing hosts too quickly?

They need living cells to replicate.

What is lysis?

Breaking the host cell open to release viruses.

What host protein does bacteriophage bind to in E. coli?

Maltoporin.

: What happens after viral DNA replication in the lytic cycle?

Viral DNA is transcribed to mRNA.

What synthesizes viral proteins?

Host ribosomes.

How do viral proteins exit the host cell?

They form holes in membranes → cell bursts.

What triggers prophage to leave host DNA?

: Stressful conditions.

What happens after prophage excision?

Virus enters the lytic cycle and produces new viruses.