Viruses Flashcards
Viruses
• Viruses hijack or invade host cells by injecting its genetic information into a cell, recruiting cellular machinery
to manufacture many new viruses and promote further infection
• Compared to eukaryotic cells and even prokaryotic cells, viruses are much smaller and simpler in structure
• Lacking the structures and metabolic machinery found in a cell, a virus is an infectious particle consisting of
little more than genes packaged in a protein coat
• Viruses were first considered living because of their disease causing capabilities that paralleled them with
bacteria à simplest living life forms
• However, viruses cannot reproduce or carry out metabolism outside of a host cell
• Today biologist consider them between life forms and chemicals
• Uses of viruses:
o Helpful in first understanding genes and genetic material + replication + protein synthesis
o Helpful in understanding diseases
o Gene therapy
Structure of Viruses
• Viruses can range from 20nm to several hundred nm in dimeter -> too small for light microscope
• The structure of a virus consists of a nucleic acid enclosed by a protein coat and sometimes surrounded by
a membranous envelope
Viral genomes
• The genome of viruses may be: double-stranded DNA, single-stranded DNA, double-stranded RNA or
single-stranded RNA -> depends on type of virus
• Viruses are classified as either DNA or RNA viruses based on their genome
• In either case the genome is usually organized in a single linear or circular molecule of nucleic acid (although
some viruses can have multiple molecules)
Capsids and envelopes
• A protein shell enclosing the viral genome is known as a capsid
o Depending on the type of virus the capsid may be: rod-shaped, polyhedral, or more complex
(bacteriophage)
o Capsids are built from a large number of protein subunits called capsomeres
o Rod-shaped viruses are usually called helical viruses -> eg. Tobacco mosaic
o Adenoviruses are arranged in a polyhedral capsid with 20 faces -> infect respiratory tract
o Most complex capsids are found on bacteriophages which contain an elongated polyhedral head
attached to a protein tail with fibres
• Some viruses have accessory structures that help them infect host cells -> membranous envelope
o Viral envelopes, which are derived from membrane of host cell, contain host cell phospholipids and
proteins. They also contain glycoproteins (proteins with carbohydrates covalently attached) ->
influenza virus
Replicative cycles of viruses
• Viruses lack metabolic enzymes and equipment for making proteins, such as ribosomes
• They are therefore obligate intracellular parasites -> they can only replicate within a host cell
• Each particular virus can infe3ct cells of only a limited number of host species -> host range
• Viruses usually identify host cells by a ‘lock-and-key’ fit between viral surface proteins and receptor molecules
on the outside of host cells
• Viruses can also usually only affect certain parts (respiratory tract or certain immune system cells) of a host
cell
General features of Viral Replicative Cycles
• A viral infection begins when a virus binds to a host cell
and the viral genome makes it way inside
• The method of genome entry depends on the type of virus
and type of host cell
• Example bacteriophages use their tails to inject their DNA
into a bacterium. Other viruses are taken up by
endocytosis or by fusion (in the case of enveloped viruses)
of the viral envelope and host membrane
• Once inside the genome can reprogram the cell to
replicate the viral genome and manufacture viral proteins
• The host provides the nucleotides, enzymes, ribosomes,
tRNAs, amino acids, ATP and other components needed
for viral protein synthesis
• DNA viruses uses host cells DNA polymerases to
synthesize new genomes, using the viral DNA as a
template
• Whilst RNA viruses using virally encoded RNA polymerases
that can use RNA as a template
• Once the nucleic acids and capsomeres are produce they
assemble spontaneously forming new viruses
• The process ends with new viruses exiting the cell, which
usually results in death of host cell, the viruses can now
spread and infect other host cells
• This death and damage accounts for viral symptoms
Replicative cycles of phages
• Bacteriophages are double-stranded DNA viruses that infect bacteria
• Research led to discovery of two alternative replicative mechanisms:
o Lytic cycle
o Lysogenic cycle
The Lytic Cycle
• A phage replicative cycle that results in the death of a host cell
• The term lytic refers to the last stage of infection, where the bacterium lyses (breaks open) and releases the
phages produced within the cell
• Each of the phages can then infect a healthy cell, and few lytic cycles can destroy an entire bacteria
population in a few hours
• A phage that only replicates by a lytic cycle is a virulent phage
• Reasons why all bacteria have not been exterminated can be attributed to:
o Natural selection which favours mutant bacteria with surface proteins no longer recognizable by
phages
o Restriction enzymes cutting up viral DNA upon entering cell
o However, phages can also benefit from mutations, thus the parasite-host relationship is in a constant
evolutionary flux
The Lysogenic Cycle
• The lysogenic cycle allows replication of the phage genome without destroying the host
• Phages capable of using both modes of replication are called temperate phages (also called lambda)
• Infection of bacteria (E. coli) by this phage begins when the phage binds to the surface of the cell and
injects its linear DNA genome
• Within the host the DNA forms a circle
• What happens next depends on the replicative mode (lytic or lysogenic)
• During a lytic cycle the viral genes immediately turn the host cell into a lambda-producing factory, and the
cell soon lyses
• During a lysogenic cycle, the lambda DNA is incorporated into a specific site in E. coli chromosome, in
this way the viral DNA is known as a prophage
• However, this prophage remains silent in the bacterium until required
• Every time the E. coli divides, it replicates the phage DNA as well as its own DNA, such that each
daughter cell inherits a copy of the prophage
• This gives rise to large population of bacteria carrying the virus in prophage form
• The mechanism allows viruses to propagate without destroying host cell on which they depend
• The term lysogenic signifies that prophages are capable of generating active phages that lyse their host
cells
• This usually occurs when the lambda genome is induced to exit the bacterial chromosome and initiate a
lytic cycle (see figure below)
• An environmental signal usually triggers the switchover from lysogenic to lytic mode
Replicative cycles of Animal Viruses
• The replicative cycles of animal viruses depend on the type of viral genome (DNA or RNA) and on the
presence of a viral envelope
Viral envelopes
• A virus equipped with an envelope uses it to enter a host cell
• Protruding from the outer surface of this envelope are viral glycoproteins that bind to specific receptor
molecules on the surface of a host cell (see figure below for cycle)
• Ribosomes bound to the ER of the host cell make the protein parts of the envelope glycoproteins whilst
enzymes in the ER and Golgi body add the sugars
• These viral glycoproteins, derived from the host cell, are transported to the cell surface
• In a process similar to exocytosis, new viral capsids are wrapped in membrane as they bud from the cell
• In other words, the viral envelope is usually derived from the host cell’s plasma membrane
• The enveloped viruses are now free to infect other cells, this cycle does not necessarily kill the host cell, in
contrast to the lytic cycles of phages
RNA as Viral Genetic Material
• The RNA genome is transcribed into complementary RNA strands, which function both as mRNA and as
templates for the synthesis of additional copies of genomic RNA
• All viruses that use an RNA genome as a template for mRNA transcription require RNAàRNA synthesis
• These viruses use a viral enzyme capable of carrying out this process
• The RNA animal viruses with the most complicated replicative cycles are retroviruses
• These viruses contain an enzyme called reverse transcriptase, which transcribes an RNA template into DNA,
providing an RNA -> DNA flow, the opposite to the usual direction
• Refer to figure on next page for the replicative cycle of the retrovirus HIV
• Retroviruses are enveloped and contain two identical molecules of single-stranded RNA and two molecules
of reverse transcriptase
• After HIV enters a host cell, its reverse transcriptase molecules are released into the cytoplasm, where they
catalyse synthesis of viral DNA. The viral DNA then enters the nucleus and integrates with the DNA
chromosome -> this integration is called a provirus, which never leaves the host’s genome
• The Proviral DNA is transcribed and used as mRNA to synthesize proteins as well as genomes for new
viruses
Viral disease in plants
• Viral diseases in plants spread by two major routes:
• Horizontal transmission:
o A plant is infected from an external source
o The virus must get past the plants epidermis, which is possible if the plant has been damaged by
wind, injury or herbivores
o Herbivores pose a double threat as they may carriers of viruses too
• Vertical transmission:
o A plants inherits a viral infection from a parent
o Vertical transmission can occur in asexual propagation or in sexual reproduction via infected seeds
• Once a virus enters a plant cell and begins replicating, viral genomes and associated proteins can spread
throughout the plant by means of plasmodesmata (cytoplasmic connections that penetrate the walls between
adjacent plant cells)
