Structure of phages

THE ANATOMY OF PHAGES.

COMPOSITION.
All phages contain nucleic acid and protein, although some contain other
substances. The nucleic acid can be either DNA or RNA,
depending on the phage type and there can be sufficient to produce
code for 3-5 gene products or in excess of 100 gene products.

The Proteins can also vary in type and number, from only one or
two to quite a number of different types. Protein plays a part in the
infection process and also protects the nucleic acid from anything in the
environment that might attack it.

STRUCTURE.
Phage size varies generally between 24-200nm in length, with T-4
phages being among the largest at 200nm long & about 90nm wide.

The HEAD or CAPSID also varies in size and shape,
some being many-sided and others filamentous. Contained in the head is
the nucleic acid for which the head, composed of proteins, is the
protective covering.

The TAIL, possesed by many, but not all, phages is a hollow tube
through which the nucleic acid passes during injection into the host
bacterium. Some phages are quite simple and have no tail but the T-4
phage tail is surrounded by a contractile sheath which contracts during
infection of the host. More complex phages, like T-4, have a base
plate at the end of the tail, with one or more fibres
attached which, together with the base plate, are involved in the binding
of the phage to the bacteriums cell wall. For phages without base plates
and tails, other structures are involved in binding the phage to the
bacterial cell wall.

HOST INFECTION.
Absorbtion is the first phase in the infection process, which is mediated
by the tail fibres (or other structure) and is a reversible process. The tail
fibres fasten onto specific receptors on the bacterial cell wall; the type
of bacterium that the phage can attach to is usually determined by the
type of tail fibre the phage has. Different bacteria have different receptors,
making the phage very specific to its host. The bacterial receptors are
there for a different purpose than simply for the phage's benefit, but the
phage has evolved to use these receptors for its infection process.

IRREVERSIBLE ATTACHMENT. The attachment to the receptors is
a weak one and can be reversible but is made irreversible by the action
of the phage's base plate. Phages without base plates use different
methods of tightly binding to the bacterial cell.

THE MULTIPLICATION CYCLE OF PHAGES.
There are two types of phages:- Lytic and Lysogenic phages.

1. LYTIC PHAGES or VIRULENT PHAGES, are used in the
therapeutic treatment of bacterial infections, both in humans and animals.
They inject their nucleic acid into their host, multiply and emerge, killing
their host as they burst out, seeking more bacteria to
infect, until none are left. The phages are then expelled from the body.

The first stage is the ECLIPSE PERIOD. This is the time when the
nucleic acid is taking over the host's biosynthetic mechanism, telling it to
change production to phage early m-RNA and protein.
The early m-RNA is needed for phage DNA production, also shutting off
the host's DNA, RNA and biosynthesis and in some cases, will also
degrade the host's chromosomes. After phage DNA has been made,
late m-RNA and late proteins, which make up the structural
proteins of new phages, are produced. These late proteins are
needed for bacterial cell lysis.

In the INTRACELLULAR ACCUMULATION PHASE, the nucleic acid
and structural proteins that have been made are assembled and
accumulate within the host cell.

LYSIS AND RELEASE PHASE is where the newly made phages
burst out of the bacterium and are released into the medium. New
phages released can be as high as 1000, implying that phages
reproduce much faster than bacteria.

MRSA meet their match.
Thomas Hausler's excellent book 'Viruses-v-Superbugs'
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