The LYTIC CYCLE (/ˈlɪtɪk/ LIT-ək ), is one of the two cycles of
viral reproduction , the other being the lysogenic cycle . The lytic
cycle results in the destruction of the infected cell and its
membrane. A key difference between the lytic and lysogenic phage
cycles is that in the lytic phage, the viral
* 1 Description
* 1.1 Penetrating * 1.2 Gene regulation biochemistry
* 1.3 Maturation and lysis
* 2 References
Viruses that only use lvytic cycle are called virulent viruses (in contrast to temperate viruses). The lytic cycle is a six-stage cycle. In the first stage, called "penetration", the virus injects its own nucleic acid into a host cell. In some viruses this genetic material is circular and mimics a bacterial plasmid . The virus hijacks the cell's replication and translation mechanisms, using them to make more viruses. Once enough virions have accumulated, specialized viral proteins are allowed to dissolve the bacterial cell wall . The cell bursts due to high internal osmotic pressure (water pressure) that can no longer be constrained by the cell wall. This releases progeny virions into the surrounding environment, where they can go on to infect other cells.
To infect a cell, a virus must first enter the cell through the
plasma membrane and (if present) the cell wall. Viruses do so by
either attaching to a receptor on the cell's surface or by simple
mechanical force.The binding is due to electrostatic interactions and
is influenced by pH and presence of ions such as Mg2+ and Ca2+. The
virus then releases its genetic material (either single- or
The virus's nucleic acid uses the host cell’s metabolic machinery
to make large amounts of viral components. In the case of
GENE REGULATION BIOCHEMISTRY
There are three classes of genes in the phage genome that regulate whether the lytic or lysogenic cycles will emerge. The first are the immediate early genes, the second is the delayed early genes and the third is the late genes.
* Immediate early genes: These genes code for two transcription
factors: N and cro. N is an anti-termination factor that is needed for
the transcription of the delayed early genes. cro has two functions.
The first function is to repress the activity of the repressor that is
needed to go into lysogeny. Note that a repressor coded by the CI gene
is needed to repress the lytic cycle from taking place. The second
function of cro is to initiate the transcription of the late genes
needed for the lytic cycle to go to completion.
* Delayed early genes: The immediate early gene N is required to
express the delayed early genes. In lytic cells, the delayed early
gene which is most important is Q. These genes are also used to
express late genes.
* The repressor: The repressor is needed to repress the lytic cycle
for lysogeny to proceed. It has 2 N domains that bind the
MATURATION AND LYSIS
After many copies of viral components are made, they are assembled into complete viruses. The phage then directs production of lysin (also called lysozyme), an enzyme that breaks down the bacterial cell wall , which allows extracellular fluid to enter the cell. The cell eventually becomes filled with viruses (typically 200-500) and liquid, and bursts, or lyses ; thus giving the lytic cycle its name. The new viruses are then free to infect other cells.
Lytic Cycle Without Lysis
Some viruses escape the host cell without bursting the cell membrane,
but rather bud/extrude off from it by taking a portion of the membrane
with them. Because it otherwise is characteristic of the lytic cycle
in other steps, it still belongs to this category, although it is
sometimes named the Productive Cycle.
* ^ bio scholar series * ^ Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms (11th ed.). Prentice Hall. ISBN 0-13-144329-1 . CS1 maint: Extra text: authors list (link ) * ^ Malys N (2012). " Shine-Dalgarno sequence of bacteriophage T4: GAGG prevails in early genes". Molecular Biology Reports. 39 (1): 33–9. PMID 21533668 . doi :10.1007/s11033-011-0707-4 .