Test: Bacteriophage - MCAT MCQ

Bacteriophages are viruses that specifically infect bacteria but these are very simple structures consisting merely of a DNA/RNA molecule surrounded by a protective coat.

Enterobacteria phage (Lambda phage) infects the bacterial species E.Coli. It consists of an icosahedral head, measuring 50-60 nanometers in diameter and a flexible tail that is around 150 nanometers in length.

With some phage types the entire infection cycle is completed very quickly, possibly in less than 20 minutes. This type of rapid infection is called a lytic cycle, as a release of the new phage particles is associated with lysis of the bacterial cell.

With many lysogenic phages the phage DNA is inserted into the bacterial genome in a manner similar to episomal insertion. The integrated form of the phage DNA is called the prophage and bacterium is referred to as a lysogen.

Lambda phage is extensively used as a cloning vector. The lambda DNA molecule is 49 kb in size and has been intensively studied by the techniques of gene mapping and DNA sequencing.

The protective coat or capsid is made up of protein molecules which surround the nucleic acid molecule carrying a number of genes including several for the replication of phage.

The general pattern of infection, which is the same for all the phages is a three step process. The first step is the attachment of phage to the surface of bacterium and subsequent injection of its DNA into the cell, second step is replication of the injected DNA.

The characteristic feature of a lytic infection cycle is that phage DNA replication is immediately followed by synthesis of capsid proteins and the phage DNA molecule is never maintained in a stable condition in the host cell.

M13 is a lysogenic phage that follows a different infection cycle when M13 infects a bacterium, new phage particles are assembled and released continually from the cell. There is no integration of phage DNA into the bacterial chromosome and lysis of the cell also never occurs.

At either end of the lambda DNA molecule is a short 12 nucleotide stretch in which the DNA is single stranded. The two single strands are complementary and base pair with each other to form a circular completely double stranded molecule.