Viruses Essential question: Describe how the structure of viruses and

cells are similar? Different?

What is a virus?

Viruses are particles that are

composed of two macromolecules

Nucleic acid (DNA or RNA)

Protein

All are pathogens- an agent that causes illness or

disease in an organism

The capsid is made of protein and functions to protect the

nucleic acid for the external environment

The capsid can vary in shape

Capsid (protein coat)

– inside contains either RNA or DNA

Surface Marker

DNA or RNA

Capsid (protein coat)

1. Bacteriophage—viruses that infect bacteria

2. Flu (influenza), HIV

Are viruses organisms?

No, viruses are not organisms for the following reasons:

viruses cannot reproduce on their own

viruses cannot grow on their own

Viruses can only replicate (produce more of themselves)

in a host cell

What organisms do viruses infect?

Viruses can infect all organisms: animals, plants,

bacteria

Viruses that specifically infect bacteria are called

bacteriophages

How do viruses enter an

organism?

Through an open wound: cut or scrape

Mouth

Nose

Eyes

Ears

Genital area

Do viruses infect all cells?

No, virures attack specific cells based on structure

The virus can only recognized the host cell by it

receptor molecules

The virus then attaches its surface proteins into the

host cells receptor molecules (shape specific)

Viruses cannot infect cells that do not have receptor

molecules it can recognize

Certain viruses can only attack

certain cell types. They are said to

be specific.

It’s like the pieces of a puzzle.

The ends have to match up so

only certain pieces fit.

Surface

Markers

Receptor Sites

Example: The rabies virus only attacks brain or

nervous cells.

Viru

s

Cell

Surface Markers

Receptor Sites

Virus

A virus recognizes cells it can infect by matching its surface marker with a receptor site on a cell.

Cell

Once a viruses attaches to a cell,

what happens?

After a virus attaches to a host cell, it can infect a cell

in one of two ways

Lytic cycle

Lysogenic cycle

Both cycle cause infections, but have two different pathways

with different steps

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

Lysogenic

Cycle

There are 4 steps in the Lysogenic Cycle

NNNNNNNNNNN

NNNNNNNNNNN

NNNNNNNNNNN

NNNNNNNNNNN

N

nnnn

nnnn

nnnn

nnnn

nddd

d

ATTACHMENT

PENETRATION

INTEGRATION

REPLICATION

Lysogenic

Cycle

Steps 1 and 2 are identical to the first two

steps in the lytic cycle.

NNNNNNNNNNN

NNNNNNNNNNN

NNNNNNNNNNN

NNNNNNNNNNN

N

nnnn

nnnn

nnnn

nnnn

nddd

d

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

Cycle of Lytic and Lysogenic

Essential question What is the difference between the lytic and lysogenic cycles?

How do humans defend

themselves against viruses

Skin

Immune system- white blood cells

Vaccines- a substance the stimulates the body’s own

immune response against invasion by foreign microbes

or viruses

Chickenpox, flu, measles, mumps, herpes

Viruses can be prevented with vaccines, but NOT treated

with antibiotics.

A virus is an infectious agent made up of

nucleic acid (DNA or RNA) wrapped in a

protein coat called a capsid.

Viruses have no nucleus, no organelles, no

cytoplasm or cell membrane—Non-cellular

vs

This is why it does NOT belong to any

kingdom.

RNA or DNA core

(center), protein coat

(capsid)

Copies itself only inside

host cell--REPLICATION

DNA or RNA

NO

NO

NO

YES

Cell membrane, cytoplasm,

genetic material,

organelles

Asexual or Sexual

DNA and RNA

YES—Multicellular Organisms

YES

YES

YES

Structure

Reproduction

Genetic Material

Growth and

Development

Response to

Environment

Change over time

Obtain and

Use Energy

Structure

Reproduction

Genetic Material

Growth and

Development

Response to

Environment

Change over time

Obtain and

Use Energy