2. DNA Viruses
• Many viruses contain either single-stranded (ss) or
double-stranded (ds) DNA genomes.
• The genomes are replicated by direct DNA-to-DNA
copying using DNA polymerase, which requires most
DNA viruses to replicate in the host cell’s nucleus.
• One exception is the poxviruses that replicate in the
host cytoplasm, which means these viruses must
carry the gene for their own DNA polymerase.
3. RNA Viruses
• A large number of viruses contain either ssRNA or
dsRNA genomes, which are replicated by direct
RNA-to-RNA copying.
• Some of the single-stranded viruses, such as the
picornaviruses and coronaviruses, have their RNA
genome in the form of messenger RNA (mRNA).
These RNA viruses are referred to as positive-
strand viruses.
4. • Other ssRNA viruses, such as the
orthomyxoviruses and paramyxoviruses, have
RNA consists of complimentary RNA strands and
mRNA
• These genomes are referred to as negative-strand
viruses.
5. Retrovirus
• These are usually grouped with the RNA viruses,
the retroviruses are replicated indirectly through a
DNA intermediate (RNA-to- DNA-to-RNA).
• Each virion contains two copies of RNA.
• During the infection process, a DNA intermediate
will be formed using a reverse transcriptase
enzyme carried within the virion.
7. • The process of viral replication is one of the most
remarkable events in nature.
• A virus invades a living host cell a thousand or
more times its size, hijacks the metabolism of the
cell to produce copies of itself, and often destroys
the host cell when new virions are released.
• Replication has been studied in a wide range of
viruses and their host cells.
8. The Replication of Bacteriophages Is a Five-Step
Process
• One of the best studied processes of replication is
carried out by bacteriophages of the T-even group (T
for “type”). Bacteriophages T2, T4, and T6 are in this
group.
• They are large, complex, naked DNA virions with the
characteristic head and tail of bacteriophages.
9. • It is important to note that the nucleic acid in a phage
contains only a few of the many genes needed for
viral synthesis and replication.
• It contains, for example, genes for synthesizing viral
structural components, such as capsid proteins, and
for a few enzymes used in the synthesis; but it lacks
the genes for many other key enzymes, such as those
used during nucleic acid synthesis.
• Therefore, its dependence on the host cell is
substantial.
10. Phage replication in E. coli as a model for the
Lytic cycle.
1. Attachment
• The first step in the replication cycle of a virulent
phage occurs when phage and bacterial cells collide
randomly.
• If sites on the phage’s tail fibers match with a
complementary receptor site on the cell wall of the
bacterium, attachment will occur that consists of a
weak chemical union between phage and receptor
site.
• In some cases, the bacterial flagellum contains the
receptor site.
11. 2. Penetration
• Following attachment, the tail of the phage
releases lysozyme, an enzyme that dissolves a
portion of the bacterial cell wall.
• The tail sheath then contracts and the tail core
drives through the cell wall.
• As the tip of the core reaches the cell membrane
below, the DNA is ejected through the hollow tail
core and on through the cell membrane into the
bacterial cytoplasm.
• The ejection process takes less than two seconds
and the capsid remains outside.
12. 3. Biosynthesis
• Having entered the cytoplasm, production of new phage
genomes and capsid parts begins.
• As phage genes code for the disruption of the host
chromosome, the phage DNA uses bacterial nucleotides
and enzymes to synthesize multiple copies of its
genome.
• Messenger RNA molecules transcribed from phage DNA
appear in the cytoplasm, and the biosynthesis of phage
enzymes and capsid proteins begins.
• Bacterial ribosomes, amino acids, and enzymes are all
enlisted for biosynthesis.
• Because viral capsids are repeating units of capsomeres,
a relatively simple genetic code can be used over and
over.
13. 4. Maturation.
• Once the phage parts are made, they are
assembled into a complete virus.
• The enzymes encoded by viral genes guide the
assembly in a step-by-step fashion.
• In one area of the host cytoplasm, phage heads
and tails are assembled from protein subunits; in
another area, the heads are packaged with DNA;
and in a third area, the tails are attached to the
heads.
14. 5. Release.
• Mature phage particles now burst out from the
ruptured bacterial shell.
• For some phages, lysozyme, encoded by the
bacteriophage genes late in the replicative
cycle, degrades the bacterial cell wall.
• The mature bacteriophages are set free to infect
more bacterial cells.
15. • The time of attachment till release is known
as Burst time.
• For Bacteriophages, the burst time
aeverages between 20-40 min.
• At the end of cycle, 50-200 new phages
emerge from host cells. This is commonly
known as Burst size.
16. • Other phages interact with bacterial cells in a slightly
different way, called a lysogenic cycle. For example,
lambda phage also infects E. coli but may not
immediately cause cell lysis. Instead, the phage DNA
integrates into the bacterial chromosome as a prophage.
• Bacteriophages participating in this cycle are known as
temperate phages.
• The bacterial cell survives the infection and continues to
grow and divide normally.
• As the bacterial cell undergoes DNA replication and
binary fission, the prophage is copied and vertically
transferred to daughter cells.
• Thus, as cells divide, each daughter cell is “infected”;
that is, it contains the viral genome as a prophage.
17. • Such binary fissions can continue for an undefined
period of time. Usually at some point, the bacterial
cells become stressed (e.g., lack of nutrients,
presence of noxious chemicals).
• This triggers the prophage to excise itself from the
bacterial chromosomes and switch to a lytic cycle,
lysing the bacterial cells as new phages are
released.
