Viruses have a simple structure consisting usually of nucleic acid packed into a protein head. They lack the metabolic machinery for isolated mutiplication and must invade a host cell in order to reproduce. This parasitic lifestyle gives rise to some interesting reproductive cycles which we will explore in this chapter.
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[The Lytic Cycle]
[The Lysogenic Cycle]
- A virus is an obligate intracellular parasite.
- This means it must invade a host cell in order to reproduce.
- An isolated virus can not replicate itself or carry on metabolism because it lacks many of the
enzymes and structures necessary for reproduction, protein synthesis and ATP generation.
- Therefore, it invades and takes control of a
hostís metabolic machinery in order to multiply.
- Viruses are extremely small, ranging in size 20 to 14000 nm in length.
- They are structured to lead a parasitic life, composed of mostly nucleic acid surrounded by a protein coat.
- Viral Nucleic Acid:
- A virus contains only one type of nucleic acid which is either DNA or RNA. Yet, this genetic material can exist in many different forms.
- It can be double- stranded DNA, single- stranded DNA, double- stranded RNA or single- stranded RNA.
- The nucleic acid can be circular or linear.
- The Protein Coat:
- The protein coat or capsid as it is called, surrounds the nucleic acid and protects it from the environment.
- A capsid is composed of repeating units called capsomeres.
- The proteins making up the capsomere is determined by the viral nucleic acid.
- A virus may also be covered by an envelope composed of lipids,proteins and carbohydrates.
- The envelope may also have spikes projecting from the surface.
- Whether a virus has an envelope or spikes is again determined by the nucleic acid.
- A virus can only infect and reproduce within certain host cells.
- Viruses identify their hosts by specific receptor molecules on the
outside of the host cell.
- Some viruses have broad host ranges and can
infect many different types of cells.
- Other viruses, on the other hand,
have extremely narrow ranges and can infect only very specific cells.
- The general life cycle of a
virus can be described by:
- 1)Recognition of Host
- 2)Genetic Material Enters Host
- 3)Replication Using Host Nucleotides
- 4)Protein Synthesis Using Host Enzymes,
Ribosomes, tRNA, ATP
- 5)Self-assembly Of Capsids And Packaging Of Genome
- 6)Release From Host
- There are many variations of this cycle depending on the type of
virus and the host.
- We will now examine bacteriophage lifecycle.
- A bacteriophage (phage) is a virus that infects bacteria.
- The phage T4 and the phage lambda, for example, both infect E. coli.
- Phages, like all viruses, are obligatory intracellular parasites and must
invade a host cell in order to reproduce.
- Viruses can multiply by two
alternate mechanisms: the lytic or the lysogenic cycle.
- T4 multiplies
by the lytic cycle which kills the host and lamba multiplies by the
lysogenic cycle which does not cause death of the host cell.
- In lysogeny, the phage DNA remains latent in the host until it breaks
out in a lytic cycle.
- The lytic cycle can be described as follows:
- The T4 phage is a complex virus and has several tail fibers.
- T4 uses these fibers to attach to complementary receptor sites on E. coliís cell
- Weak chemical bonds form between the attachment and
receptor sites, adhering the virus to the host.
- T4 injects its DNA into E. coli by releasing the enzyme, phage
lysozyme, which breaks down a portion of the cell wall.
- T4 contracts is tail sheath driving the tail core into E. coli.
- The viral DNA passes through the core and into the cell.
- The capsid remains outside.
- Host protein synthesis is stopped by viral degradation of host DNA,
and interference of host transcription and translation.
- T4 uses host
nucleotides to replicate its DNA and host ribosomes, enzymes and
amino acids to synthesize its enzymes and proteins.
- During this time
no complete phages are found in the host and is called the eclipse
- Spontaneous assembly of capsids and packaging of DNA inside the
- Tails fibers join the complex.
- Release of the viruses from the host.
- The viral enzyme lysozyme, lyses
(breaks open) E. coliís plasma membrane and cell wall.
- The E. coli cell dies.
- Burst time is the time from attachment of a virus to lyses and release
of the new phage particles.
- Burst time ranges from 20 to 40 minutes.
- Burst size is the number of viruses released from the cell at the burst
- Burst size ranges from 50 to 200 viruses.
- Some viruses, like phage lambda, can enter a lytic cycle or
alternatively, may enter a lysogenic cycle.
- Phages that can multiply by
either cycle are lysogenic or temperate phages.
- When lambda enters a lysogenic cycle, its DNA recombines with
E. coliís chromosome.
- The inserted phage DNA is called a prophage.
- Repressor proteins, which are products of the prophage, prevent
transcription of the other prophage genes.
- As a result, the synthesis
and release of new phages is repressed.
- Every time the bacterial chromosome is replicated, prophage DNA
is replicated, as well.
- The prophage can remain latent in the bacterial
chromosome for many generations.
- A spontaneous event at any time
may cause the virus to break out of its latent state and enter the lytic