Transduction Box 119

In the third form of genetic transfer in bacteria, bacteriophages act as carriers of DNA from one cell to another. In order to appreciate the way in which this is done, it is necessary to recall the sequence of events in phage replication cycles discussed in the previous chapter (see Figure 10.11).

Generalised transduction occurs in virulent phages, that is, those with a lytic life cycle. Sometimes, the enzymes responsible for packaging phage DNA into its protein coat package instead similarly sized fragments of degraded chromosomal DNA (Figure 11.32). Despite containing the wrong DNA, this transducing phage particle is still infective, since this is dependant on its protein element. Thus following infection of another bacterial cell, the DNA can be incorporated by recombining with the homologous segment in the recipient cell. Since any chromosomal fragment can be mistakenly packaged in this way (as long as it finds an area of homology), all genes are transferred at a similar (low) frequency.

Specialised transduction results in a much higher efficiency of transfer for specific genes, however it is limited to genes having a particular chromosomal location. Recall from Chapter 10 that in lysogenic life cycles, the phage DNA is integrated into the host chromosome, and later, perhaps after many rounds of cell division, excised again before re-entering a lytic cycle. If this excision does not happen precisely, some of the adjoining chromosomal DNA, carrying a gene or two, may be incorporated into the phage particle (we saw a similar mechanism in the case of F' plasmid formation). Upon infecting another cell, the transduced genes would undergo recombination and become incorporated into the recipient's chromosome (Figure 11.33). Although limited to genes in the vicinity of the lysogenic phage's integration, this is a highly efficient form of transfer, since the genes become stably integrated into the host cell.

Transduction experiments, like those involving conjugation, can be used to determine the relative positions of genes on a bacterial chromosome.

Figure 11.32 Generalised transduction. During the lytic cycle of a phage, the host DNA is degraded (b), and a fragment may be mistakenly packaged into a newly synthesised phage particle (c). Upon infecting a new host cell, the transducing phage releases its DNA (e); although unable to replicate, this can undergo recombination with a homologous sequence on the host chromosome (f)

Figure 11.32 Generalised transduction. During the lytic cycle of a phage, the host DNA is degraded (b), and a fragment may be mistakenly packaged into a newly synthesised phage particle (c). Upon infecting a new host cell, the transducing phage releases its DNA (e); although unable to replicate, this can undergo recombination with a homologous sequence on the host chromosome (f)

Was this article helpful?

0 0
How To Win Your War Against Bronchitis

How To Win Your War Against Bronchitis

Sick And Tired Of Your Constant Cough? Is Your Bad Immune System Leading You To The Path Of Fever And Sore Chest? You Sure Have A Reason To Panic BronchitisThere Is Always A Way Out And, This Is It Finally Discover Some Of The Most Effective Tips That Can Curb Bronchitis, And Its Repeated Bouts Learn How To Keep The Chronic Cough, And Sore Chest Away Breathe Free, And Feel The Whiff Of Fresh Air, With No Hassles

Get My Free Ebook


Post a comment