The T4 phage is tadpole shaped and consists of head, collar, tail, base plate and fibres (figure). The head is hexagonal which consists of about 2000 identical protein subunits. The long helical tail consists of an inner tubular core which is connected to the head by a collar. There is a base plate attached to the end of tail The base plate contains six spikes and tail fibres. These fibres are used to attach the phage on the cell wall of bacterial host during replication. A dsDNA molecule of about 50 μm is tightly packed inside the head. The DNA is about 1000 times longer than the phage itself.
