Lytic cycle:
Lytic or virulent phages are phages, which multiply in bacteria and kill the cell by lysis at the end of the
life cycle. Soon after the nucleic acid is injected, the phage cycle is said to be in eclipse period. During the
eclipse phase, no infectious phage particles can be found either inside or outside the bacterial cell.
Eclipse phase represents the interval between the entry of phage nucleic acid into bacterial cell and
release of mature phage from the infected cell. This phase is devoted to synthesis of phage components
and their assembly into mature phage particles. The phage nucleic acid takes over the host biosynthetic
machinery and phage specified m-RNA's and proteins are made. In some cases the early phage proteins
actually degrade the host chromosome. Structural proteins (head, tail) that comprise the phage as well
as the proteins needed for lysis of the bacterial cell are separately synthesized. Nucleic acid is then
packaged inside the head and then tail is added to the head. The assembly of phage components into
mature infective phage particle is known as maturation. In Lysis and Release Phase the bacteria begin to
lyse due to the accumulation of the phage lysis protein and intracellular phage are released into the
medium. It is believed that phage enzymes weaken the cell wall of bacteria. The number of particles
released per infected bacteria may be as high as 1000. The average yield of phages per infected bacterial
cell is known as burst size.
Lysogenic cycle:
Lysogenic or temperate phages are those that can either multiply via the lytic cycle or enter a dormant
state in the cell. In most cases the phage DNA actually integrates into the host chromosome and is
replicated along with the host chromosome and passed on to the daughter cells. This integrated state of
phage DNA is termed prophage. This process is known as lysogeny and the bacteria harboring prophage
are called lysogenic bacteria. Since the prophage contains genes, it can confer new properties to the
bacteria. When a cell becomes lysogenized, occasionally extra genes carried by the phage get expressed
in the cell. These genes can change the properties of the bacterial cell. This process is known as lysogenic
conversion or phage conversion.
Recombination and Genome Mapping in Viruses
Bacteriophage genomes also undergo recombination, although the process is different from that in bacteria. Because phages
themselves reproduce within cells and cannot recombine directly, crossing-over must occur inside a host cell. In principle, a virus
recombination experiment is easy to carry out. If bacteria are mixed with enough phages, at least two virions will infect each cell
on the average and genetic recombination should be observed. Phage progeny in the resulting lysate can be checked for alternate
combinations of the initial parental genotypes. Alfred Hershey initially demonstrated recombination in the phage T2, using two
strains with differing phenotypes. Two of the parental strains in Hershey’s crosses were h_r_ and hr. The gene h influences host
range; when gene h changes, T2 infects different strains of E. coli. Phages with the r_ gene have wild type plaque morphology,
whereas T2 with the r genotype has a rapid lysis phenotype and produces larger than normal plaques with sharp edges (figures
13.24b and 13.24c). In one experiment Hershey infected E. coli with large quantities of the h_r_ and hr T2 strains. He then plated
out the lysates with a mixture of two different host strains and was able to detect significant numbers of h_r and hr_ recombinants,
as well as parental type plaques. As long as there are detectable phenotypes and methods for carrying out the crosses, it is possible
to map phage genes in this way. Phage genomes are so small that often it is convenient to map them without determining
recombination frequencies. Some techniques actually generate physical maps, which often are most useful in genetic engineering.
Several of these methods require manipulation of the DNA with subsequent examination in the electron microscope. For example,
one can directly compare wild-type and mutant viral chromosomes. In heteroduplex mapping the two types of chromosomes are
denatured, mixed, and allowed
to rejoin or anneal. When joined, the homologous regions of the different DNA molecules form a regular double helix. In
locations where the bases do not pair due to the presence of a mutation such as a deletion or insertion, bubbles will be visible in
the electron microscope. Several other direct techniques are used to map viral genomes or parts of them. Restriction
endonucleases are together with electrophoresis to analyze DNA fragments and locate deletions and other mutations that affect
electrophoretic mobility. Phage genomes also can be directly sequenced to locate particular mutations and analyze the changes
that have taken place.