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Genetics of Viruses and

Bacterial Gene Control

C18-1 & 18-4

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18-1 What is a virus? Is it alive?

DNA or RNA enclosed in a protein

coat

Viruses are not cells

Extremely tiny

Electron microscope

Smaller than ribosomes

~20-50 nm

1st discovered in plants (1800’s)

Tobacco mosaic virus

Couldn’t filter it out

Couldn’t reproduce on media like

bacteria

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Viruses called bacteriophages can infect

and set in motion a genetic takeover of

bacteria, such as Escherichia coli

Viruses lead “a kind of borrowed life”

between life-forms and chemicals

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Structure of Viruses

Viruses are not cells

Viruses are very small

infectious particles

consisting of nucleic

acid enclosed in a

protein coat and, in

some cases, a

membranous envelope

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Variation in Viruses

A package of genes in transit from 1 host cell to another

Plant virus pink eye influenza bacteriophage

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Viral Genomes

Viral nucleic acid varies

double stranded DNA dsDNA

Single stranded DNA ssDNA

double stranded RNA dsRNA

Single stranded RNA ssRNA

Linear or circular molecule of nucleic acid

Smallest viruses have only 4 genes, while largest

have several hundred

Depending on its type of nucleic acid, a virus is called

a DNA virus or an RNA virus

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Viral protein coat

Capsid

Crystal-like protein

shell

1-2 types of proteins

Many copies of the

same potein =

capsomere

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Fig. 19-3

RNA

Capsomere

Capsomereof capsid

DNA

Glycoprotein

18 250 nm 70–90 nm (diameter)

Glycoproteins

80–200 nm (diameter) 80 225 nm

Membranousenvelope RNA

Capsid

Head

DNA

Tailsheath

Tailfiber

50 nm50 nm50 nm20 nm

(a) Tobacco mosaicvirus

(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

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Viral Envelope

Lipid bilayer membranes cloaking the

viral capsid

• Envelopes are derived form host

cell membranes

• Glycoproteins on the surface

• To hide from the immune

system

Chicken pox

Herpes Simplex

Ebola

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Generalized Viral Life Cycle Parasties

Lack enzymes for metabolism

Lack ribosomes for protein synthesis

Need host cell “machinery” to

reproduce.

Entry

Viral DNA/RNA takes over the host cell

Reprograms host cell to copy viral

nucleic acids and build viral proteins

Self Assembly

Nucleic acid molecules & capsomeres

then self assemble into viral particles

Exit the cell

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Viral Host Range Host range

Each type of virus can infect & parasitize on a limited range of

host cells.

Identify host cells via “lock & key” fit

Between proteins on the viral coat & receptors on host cell

surface.

Broad host range

Rabies –can ifect all mammals

Narrow host range

Cold virus – on the cells of the upper repsiratory tract of

humans

AIDS virus = binds only the specific white blood cells

Zoonotic – ability to jump species range to another species.

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Lytic lifecycle of phages

Ew. I don’t

feel so good

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Lytic Cycle

Infection - The viral genome is injected by the viral

particle into the host cell.

The viral genome takes over host cell “machinery”

to:

Reproduce its genetic material

Synthesize viral proteins (capsomers)

Assembly – once the cell is filled with viral

components. The componenets self assemble and

cause the cell to burst – releasing 1000’s – 10,000s

new viral particles.

The Cell Dies – the infection rages on.

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Lysogenic lifecycle of phages.

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Lysogenic lifecycle

Infection – Viral genome enters the cell.

Integration – The viral genome is

integrated into the host cells genome.

Latency – a span of time, without

infection. Host cell is not affected, carries

out life processes, divides and behaves as

any other uninfected cells.

Activation – the viral genes become active

“Triggered” and go into a lytic phase.

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Reproductive cycle of animal viruses Enveloped viruses – modified

lysogenic special case

Infect the host cell

Replicate components

(Genome & protein)

Self -Assemble

Leave the cell by budding off

Don’t necessarily kill the host

cell, leaving behind an

infected cell still able to

produce more viral particles.

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RNA as viral genetic material

Viruses that have RNA as

their genetic material

(retroviruses) also have an

enzyme within their capsid –

reverse transcriptase.

to infect a host cell the viral

RNA must be reverse

transcribed from RNA into

DNA before it can be

integrated into the host

genome.

HIV is one of these viruses –

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Provirus The integrated viral

DNA never leaves

the host cell.

Copies can be

made to

proliferate new

viral particles.

Provirus is

replicated every

time the host cell

divides – more

and more infected

cells.

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Evolution of viruses

Viruses generally have more in common

genetically with their target host than with other

viruses.

However, there are a few highly conserved viral

genes that most viruses share.

It is not thought that they share a common

ancestry with either prokaryotes or eukaryotes.

One hypothesis: Earliest viruses were naked

DNA/RNA without a protein coat, which

eventually evolved to increase transmission.

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Defense against viruses Bacteria have defenses against phages

Natural selection favors bacterial mutants with

receptor sites that are no longer recognized by a

particular type of phage

Bacteria produce restriction enzymes that

recognize and cut up foreign DNA

Modifications to the bacteria’s own DNA

prevent its destruction by restriction enzymes.

Its an escalating WAR!

Natural selection favors phage mutants resistant

to the bacterial defenses.

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AP Biology 2007-2008

18-4 Control of

Prokaryotic (Bacterial) Genes

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Bacterial metabolism

Bacteria need to respond quickly to

changes in their environment

if they have enough of a product,

need to stop production

why? waste of energy to produce more

how? stop production of enzymes for synthesis

if they find new food/energy source,

need to utilize it quickly

why? metabolism, growth, reproduction

how? start production of enzymes for digestion

STOP

GO

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Remember Regulating Metabolism?

Feedback inhibition

product acts

as an allosteric

inhibitor of

1st enzyme in

tryptophan pathway

but this is wasteful

production of enzymes

= inhibition-

-Oh, I

remember thisfrom our

Metabolism Unit!

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Different way to Regulate Metabolism

Gene regulation

instead of blocking

enzyme function,

block transcription

of genes for all

enzymes in

tryptophan pathway

saves energy by

not wasting it on

unnecessary

protein synthesis

= inhibition-

Now, that’s a good idea from a lowly bacterium!

-

-

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Gene regulation in bacteria

Cells vary amount of specific enzymes

by regulating gene transcription

turn genes on or turn genes off

turn genes OFF example

if bacterium has enough tryptophan then it

doesn’t need to make enzymes used to build

tryptophan

turn genes ON example

if bacterium encounters new sugar (energy

source), like lactose, then it needs to start

making enzymes used to digest lactose

STOP

GO

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Bacteria group genes together Operon

genes grouped together with related functions example: all enzymes in a metabolic pathway

promoter = RNA polymerase binding site single promoter controls transcription of all genes in

operon

transcribed as one unit & a single mRNA is made

operator = DNA binding site of repressor protein

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So how can these genes be turned off?

Repressor protein

binds to DNA at operator site

blocking RNA polymerase

blocks transcription

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operatorpromoter

Operon model

DNATATA

RNApolymerase

repressor

repressor = repressor protein

Operon:

operator, promoter & genes they control

serve as a model for gene regulation

gene1 gene2 gene3 gene4RNA

polymerase

Repressor protein turns off gene by

blocking RNA polymerase binding site.

1 2 3 4mRNA

enzyme1 enzyme2 enzyme3 enzyme4

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mRNA

enzyme1 enzyme2 enzyme3 enzyme4

operatorpromoter

Repressible operon: tryptophan

DNATATA

RNApolymerase

tryptophan

repressor repressor protein

repressortryptophan – repressor protein

complex

Synthesis pathway model

When excess tryptophan is present,

it binds to tryp repressor protein &

triggers repressor to bind to DNA

blocks (represses) transcription

gene1 gene2 gene3 gene4

conformational change in

repressor protein!

1 2 3 4

repressortrpRNA

polymerase

trp

trp

trptrp

trp trp

trptrp

trp

trp

trp

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Tryptophan operonWhat happens when tryptophan is present?

Don’t need to make tryptophan-building

enzymes

Tryptophan is allosteric regulator of repressor protein

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mRNA

enzyme1 enzyme2 enzyme3 enzyme4

operatorpromoter

Inducible operon: lactose

DNATATARNA

polymerase

repressor repressor protein

repressorlactose – repressor protein

complex

lactose

lac repressor gene1 gene2 gene3 gene4

Digestive pathway model

When lactose is present, binds to

lac repressor protein & triggers

repressor to release DNA

induces transcription

RNApolymerase

1 2 3 4

lac lac

laclac

lac

lac

lac

conformational change in

repressor protein!

lac

lac

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Lactose operonWhat happens when lactose is present?

Need to make lactose-digesting enzymes

Lactose is allosteric regulator of repressor protein

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cAMP + CAP = Positive Control

cAMP: accumulates when glucose is scarce

cAMP binds to CAP (catabolite activator protein)

Active CAP binds to DNA upstream of

promoter, ↑ affinity of RNA polymerase to

promoter, ↑ transcription

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Jacob & Monod: lac Operon

Francois Jacob & Jacques Monod

first to describe operon system

coined the phrase “operon”

1961 | 1965

Francois JacobJacques Monod

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Operon summary

Repressible operon

usually functions in anabolic pathways

synthesizing end products

when end product is present in excess,

cell allocates resources to other uses

Inducible operon

usually functions in catabolic pathways,

digesting nutrients to simpler molecules

produce enzymes only when nutrient is

available

cell avoids making proteins that have nothing to do,

cell allocates resources to other uses

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Don’t be repressed!

How can I induce you

to ask Questions?