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Learning objectives

  • Describe the lytic and lysogenic life cycles
  • Describe the replication process of animal viruses
  • Describe unique characteristics of retroviruses and latent viruses
  • Discuss human viruses and their virus-host cell interactions
  • Explain the process of transduction
  • Describe the replication process of plant viruses

All viruses depend on cells for reproduction and metabolic processes. By themselves, viruses do not encode for all of the enzymes necessary for viral replication. But within a host cell, a virus can commandeer cellular machinery to produce more viral particles. Bacteriophages replicate only in the cytoplasm, since prokaryotic cells do not have a nucleus or organelles. In eukaryotic cells, most DNA viruses can replicate inside the nucleus, with an exception observed in the large DNA viruses, such as the poxviruses, that can replicate in the cytoplasm. RNA viruses that infect animal cells often replicate in the cytoplasm.

The life cycle of viruses with prokaryote hosts

The life cycle of bacteriophages has been a good model for understanding how viruses affect the cells they infect, since similar processes have been observed for eukaryotic viruses, which can cause immediate death of the cell or establish a latent or chronic infection. Virulent phages typically lead to the death of the cell through cell lysis. Temperate phages , on the other hand, can become part of a host chromosome and are replicated with the cell genome until such time as they are induced to make newly assembled viruses, or progeny virus es .

The lytic cycle

During the lytic cycle of virulent phage, the bacteriophage takes over the cell, reproduces new phages, and destroys the cell. T-even phage is a good example of a well-characterized class of virulent phages. There are five stages in the bacteriophage lytic cycle (see [link] ). Attachment is the first stage in the infection process in which the phage interacts with specific bacterial surface receptors (e.g., lipopolysaccharides and OmpC protein on host surfaces). Most phages have a narrow host range and may infect one species of bacteria or one strain within a species. This unique recognition can be exploited for targeted treatment of bacterial infection by phage therapy or for phage typing to identify unique bacterial subspecies or strains. The second stage of infection is entry or penetration . This occurs through contraction of the tail sheath, which acts like a hypodermic needle to inject the viral genome through the cell wall and membrane. The phage head and remaining components remain outside the bacteria.

This figure outlines the stages of the lytic cycle. Step 1 is attachment when the phage attaches to the surface of the host. The bacteriophage is shown sitting on the surface of the bacterial host cell. Step 2 is penetration when the viral DNA enters the host cell. The image shows DNA from within the virus being injected into the host DNA. Step 3 is biosynthesis when the phage DNA replicates and the phage proteins are made. The image shows various pieces of virus being built within the cell. Step 4 is maturation when the new phage particles are assembled. This shows the viral components being put together in the cell. The fifth step is lysis when the cell lyses and the newly made phages are released. This shows the cell bursting and built viruses being released.
A virulent phage shows only the lytic cycle pictured here. In the lytic cycle, the phage replicates and lyses the host cell.

The third stage of infection is biosynthesis of new viral components. After entering the host cell, the virus synthesizes virus-encoded endonucleases to degrade the bacterial chromosome. It then hijacks the host cell to replicate, transcribe, and translate the necessary viral components (capsomeres, sheath, base plates, tail fibers, and viral enzymes) for the assembly of new viruses. Polymerase genes are usually expressed early in the cycle, while capsid and tail proteins are expressed later. During the maturation phase, new virions are created. To liberate free phages, the bacterial cell wall is disrupted by phage proteins such as holin or lysozyme. The final stage is release. Mature viruses burst out of the host cell in a process called lysis and the progeny viruses are liberated into the environment to infect new cells.

Questions & Answers

short not on medical microbiology
Massah Reply
Hy
Iqra
hope you fine
Arif
love you
Arif
Assalamualaikum
Suhaib
arai yeh micro kai sth kis ko love hua
Umer
icrobiology is the scientific study of these microorganisms. Microorganisms are those organisms that are too small to see with the naked eye and include things like bacteria, fungi, and viruses
Suhaib
iqra iqbal
Arif
What is gram syaining?
Arif
to differentiate between gram positive and gram negative bacty
Umer
True
Arif
where you are from
Arif
i m frm sopore..kashmir
Umer
i m frm baramulla .. Kashmir
Umer
and u
Umer
umer
Arif
and i m from afghanistan lovely country
Arif
yes
Umer
thanks
Arif
medical microbiology means study of microorganisms which are beneficial or harmful for the body. mostly which are studied under microscope by structure and shape appearance. e.g bacteria, viruses, bacteriophage, fungus hyphae and some blood and other body fluids parasites e.g plasmodium etc.
Rana
arif bhai gram staining in which we are given different dyes to the bacterial cell wall and cytoplasm and nucleus.e.g giemsa stain
Rana
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Omar baba crystal violet iodine 70% alcohol fuchicine or safranine after every step washing must be done
Rana
hello
Cristy
*Fuchsin stain
Cristy
*Safranin
Cristy
thanks
Ganesh
difference between epidermophyton trichophyton and microsporum
Ankita Reply
plz answer the question
Iqra
epidermophyton type of fungi causes superficial and cutaneous mycoses trichophyton is also fungi type including parasitic varieties causes dermatophytosis microsporum is also type of fungi causes dermatophytosis.
Rana
ok
Iqra
Is this app perfect for preparation of exam?
Ankita
Ankita I totally disagree with your respected opinion. it's just minor helpful
Rana
disagree by me ?
Ankita
what is infectious
Fred Reply
Many poeple are die due to covid-19 virus
AMINU Reply
they become sick due to millions bacteria, fungi and some insects that make plants as their host.
charles Reply
what are microbes and what are their effects to humans
charles Reply
they are organism that cannot be seen with the naked eye
Angel
classify microorganisms.
Satarupa Reply
protozoa' bacteria' virus' algai' archea'
Cabdifitaax
protozoa
Ayesha
Thank you
Satarupa
bacteria,fungi, protozoa and virus
Angel
mutant having a requirement for a certain growth factor is called a(n)
Purushoth Reply
what is atome
Marie Reply
is the smallest part of any material that cannot be broken up by chemical means
Feisal
How does forest tree become sick
Emmanuel
Aseptate hyphae are seen in
lab Reply
yes
Ahmed
w
Ahmed
hello
Apdikrim
what is ADT
Feisal
hii
Salman
how does this work bro
Feisal
CHEMISTRY
Apdikrim
biology
Ahmed
I.m student
Ahmed
I.m question what is difference between arthropod and insect?
Ahmed
Arthropod is a large division of jointed-foot Invertebrates such as Insects, centipedes and others so Insect is a part of the division or the phylum
hilina
thanks
Ahmed
ur welcome
hilina
describe binary fission
Queen Reply
in which parent cell devides into two daughter cells
Iqra
single parent cell
Tean
who is the father of microbiology
Mary Reply
Antonie van Leeuwenhoek , he is father of microbiology and Louis Pasteur is father of modern microbioloy.
Arshad
Anton van leeuwenhoek ( He descovered bacteria in 1679)
Satarupa
what is the role of protein in virus genome
Rana Reply
what is microbiological
Apdikrim Reply
Science is a good Subject
AMINU Reply
yes it is
Madrine

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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