Envelope

Protein ShellCapsid

Nucleic Acid

Receptors

DNA or RNA

Membrane around virus

Proteins that help virus get into proper host

Attachment

The virus attaches to

receptors on the host cell.

The virus is able to get into the cell in one of three ways.

Entry into the Cell

Fusion of the Membranes

The viral envelope and

cell membrane fuse, allowing

the virus to enter the cell.

The virus is engulfed by the cell membrane.

Endocytosis

Injection

E. coli Bacteriophage T4

What happens once the genetic material is inside the cell?

One of two different processes may occur:

1. The virus takes over the cell and begins to replicate right away, killing the host cell in the process.

2. The virus doesn’t take over or kill the host cell right away, but does begin to replicate.

Lytic Cycle

There are 5 steps in the Lytic Cycle

RELEASE (LYSIS)

ATTACHMENT

Lytic Cycle

1. Attachment – the virus attaches to receptors on the host

ATTACHMENT

Lytic Cycle

2. Penetration – the nucleic acid moves across the cell membrane into the host cell

Lytic Cycle

3. Replication and Synthesis – the virus degrades the host nucleic

acid and uses the host to make new viral components.

Lytic Cycle

4. Assembly – viral components are assembled into new viruses.

Lytic Cycle

RELEASE / LYSIS

5. Release / Lysis – fully assembled viruses are released when the host cell bursts

Lytic Cycle

Viruses that are in a lytic cycle are described as VIRULENT

RELEASE / LYSIS

ATTACHMENT

T4 Bacteriophage Lysing E. coli Bacterium

Bacteriophage Lambda

(λ)

Lysogenic Cycle

There are 4 steps in the Lysogenic Cycle

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ATTACHMENT

PENETRATION

INTEGRATION

REPLICATION

Lysogenic Cycle

Steps 1 and 2 are identical to the first two steps in the lytic cycle.

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ATTACHMENT

PENETRATION

Lysogenic Cycle

INTEGRATION

3. Integration – Phage DNA integrates into bacterial DNA

Lysogenic Cycle

INTEGRATION

When it becomes integrated into the host DNA, the viral genome is referred to as a PROPHAGE.

Lysogenic Cycle

REPLICATION

4. Replication – The integrated prophage replicates when bacterial DNA replicates

Lysogenic Cycle

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Viruses that enter into a lysogenic cycle are known as

TEMPERATE VIRUSES

What is special about Bacteriophage λ?

ATTACHMENT

PENETRATION

REPLICATION AND SYNTHESIS

ASSEMBLY

RELEASE / LYSIS

INTEGRATION

REPLICATION

Bacteriophage λ attached to an E. coli Bacterium

Replication of Viral Nucleic Acids

DNA Viruses – replication of viral DNA is similar to the

process that the host cell uses to replicates its own DNA

DNA DNA

Replication of Viral Nucleic Acids

Most RNA Viruses – replication of viral RNA is dependent upon

an enzyme that is encoded on the viral genome

RNA RNA

Replication of Viral Nucleic Acids

– RNA viruses that convert their RNA into DNA

using an enzyme that is included with the viral genome

RNA DNA

Retroviruses

Replication of Viral Nucleic Acids

Retroviruses – This DNA is inserted into the host genome, where it can be transcribed back into viral RNA during normal cellular activity.

Replication of Viral Nucleic Acids

Retroviruses – Once the DNA is integrated into the host genome, some retroviruses may become LATENT, where they remain

inactive for long periods of time.

Replication of Viral Nucleic Acids

Human Immunodeficiency Virus (HIV), the virus that causes AIDS, is an example of a retrovirus that

can become latent.

Retroviruses

Virus hijacks the host cell and keeps making more copies of itself until the host bursts open

Lytic Cycle

Lysogenic Cycle

HIV

Virus integrates into the host genome and replicates when the host divides. When triggered it can enter the lytic cycle.

RNA viruses that convert their RNA to DNA which then integrates into the host genome.

Retrovirus that infects humans which can remain latent for long periods of time.

HIV Infection

When not active, the virus is described as being latent

Lytic and Lysogenic Cycles

Lytic and Lysogenic Cycles

ATTACHMENT

PENETRATION

REPLICATION AND SYNTHESIS

ASSEMBLY

RELEASE / LYSIS

INTEGRATION

REPLICATION

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Lytic Cycle

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Lysogenic Cycle

Lytic and Lysogenic

Cycles