3-D model of flu virus

Syllabus Requirements

Structure of viruses using a bacteriophage and a retrovirus as examples.

Main distinguishing features between viruses and living organisms.

Details of lytic and lysogenic life cycles are expected.

CHARACTERISTICS OF VIRUSESViruses: do not have a cellular structure

(they are acellular)

do not carry out many of the processes characteristic of life

Viruses are on the borderline between living and non-living

KingdomProtoctista

Viruses do not: regulate transport of materials into and out of

themselves

perform any metabolic functions

viruses can only reproduce by invading living cells

REPRODUCTION

The only vital function viruses have in common with organisms is:

VIRUSES:

are all parasitic most cause disease are obligate intracellular parasites, i.e. can

only live parasitically inside other cells

T2 phages attack E. coli

VIRUSES have a simple structure

viruses consist of a: small piece of nucleic acid:

either DNA or RNA, surrounded by a protein or lipoprotein coat

CHARACTERISTICS OF VIRUSES each type of virus will:

recognise infect

Receptor

only certain types of cell, i.e. viruses are highly specific to their hosts

What is a ‘virus’?

A virus (from the Latin virus meaning toxin or poison) is a microscopic infectious agent that can only reproduce inside a host cell

What is a ‘virion’?

Viruses outside host cells

Discovery of viruses

the first virus was discovered in the 1890s—it was an agent that causes tobacco mosaic disease

the agent was crystallised in 1930s

Characters of viruses that assign them as non-living:

a) They can be crystallised [transforming viral components into organised particles (solid)]

b) Outside the cell, they behave like inert chemicals.

Virus crystal.

c) They do not show all the 7 vital functions.

Characters of viruses similar to living things:

a) They multiply within host cells.b) They possess genetic material, either DNA or

RNA.c) There are definite races or strains.d) They exhibit mutations.

Question: [SEP, 2007]

Use your knowledge of biology to explain why Viruses are not included in the Five-Kingdom classification scheme. (5 marks)

A prerequisite to be placed in one of the 5 kingdoms, an organism must be alive. It is arguable whether viruses are living or not. They may be considered as non-living as they lack the usual cell structure and do not carry out all of the 7 vital functions. They depend on their host for energy, enzymes and raw materials. They are obligate intracellular parasites. They are much smaller than any known organism. They possess non-living features as they can be crystallised. On the other hand, they resemble organisms in having genetic material. They have been described as being borderline between living and non-living.

Size of viruses ranges: 20-300 nm

 1 nm = 1/1,000,000,000 of a meter

Can viruses be seen by a light microscope?

No. By an electron microscope.

are about 50 times smaller than bacteria can pass through filters which retain bacteria

Viruses

Structure of a Virus

1. Core

2. Capsid

All contain:

Structure of a Virus

1. Core

2. Capsid

3. Envelope

Some contain:

Structure of a Virus

1. Core at the interior contains: DNA or RNA

single-stranded or double-stranded

one or more enzymes needed to start the

process of reproduction within the host cell

2. Capsid made of capsomeresFunctions of capsid: a protective coat of protein around the core establishes what kind of cell the virion can

attach to infects the cell

CAPSID

3. Envelope – in a few animal viruses

e.g. HIV & influenza virusesan additional lipoprotein layer around the capsid

How is the envelope formed?

Derived from the cell surface membrane of the

host cell

Give a reason for this statement:

Viruses are not affected by antibiotics that target bacterial cell walls or ribosomes.

Viruses do not possess cell walls and ribosomes like

bacteria.

What is a ‘bacteriophage’ or ‘phage’?A virus that infects bacteria.

E. coli-infecting viruses are the best studied-Include the “T” series (T1, T2, etc.)

T2 bacteriophages infecting a bacterium

One of two things can happen once a

bacteriophage inserts its genetic material inside a bacterium.

Phage reproduces immediately and kills the host cell - lytic cycle - cell bursts and

releases progeny viruses.

Phage postpones reproduction by

integrating into the host cell’s genome - lysogenic cycle.

1

2

MULTIPLICATION OF BACTERIOPHAGES

(LYTIC CYCLE)

Lytic Cycle involves 5 steps:

1. Attachment (adsorption)

Phage attaches by tail fibers to host cell

2. Penetration Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell

3. Biosynthesis Production of phage DNA and proteins

4. Maturation Assembly of phage particles

5. Release Phage lysozyme breaks cell wall

Attachment:Phage attaches to host cell.

Penetration:Phage penetrates host cell and injects its DNA.

Biosynthesis: production of phage DNA and proteins

1

2

3

Bacterial cell wall

Bacterial chromosome

Capsid DNA

Capsid

Sheath

Tail fiber

Base platePin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

Lytic Cycle

4 Maturation:Viral components are assembled into virions.

Tail

5 Release:Host cell lyses and new virions are released.

DNA

Capsid

Tail fibers

LYSOGENIC CYCLE

Prophage:the dormant stage of the phage

Lytic phage: called virulent

Lysogenic phage: called temperate

Which type of reproduction? some viruses reproduce only by the lytic cycle others undergo both types of reproductive

cycles

has RNA e.g. HIV

A retrovirus

Life cycle of a

retrovirus

cDNA – complementary DNA

Question: [MAY, 2007]This question is concerned with viruses. (a) What is a virus? (2)

A microscopic infectious agent that can only reproduce inside a host cell.

(b) Viruses are often classified using a taxonomic system that is

separate from that used to classify all other forms of life. Why is this so? (2)Structurally and functionally they differ from other forms of life. They are acellular, consisting of a protein coat and genetic material contained in it.

(c) Write short notes on each of the following:(i) retroviruses; (2)

Have RNA instead of DNA. Make a strand of DNA using reverse transcriptase once inside the host. This single DNA strand is made a double strand using the same enzyme as in HIV.

(ii) bacteriophages. (2)

Viruses that infect bacteria and inject their DNA into them for replication. They have a capsule, a neck and tail fibres which anchor them to a bacterium e.g. T4 phage.

Question: [MAY, 2009]

How may a new strain of virus arise? (1) By mutations in the genetic material. A new virus is formed when two viruses

combine within a host.