Genetic Manipulation 225into the genome, rather than at a defined site. However, both methods enjoy the
advantage that DNA enters the cell exactly as constructed and has not passed
through an intermediate vector giving the opportunity for gene rearrangement.
Transformation by the Ti plasmid ofAgrobacterium tumefaciens, shown dia-
grammatically in Figure 9.4, suffers from few disadvantages other than the limi-
tation that it does not readily infect some cereal crops. This potential problem has
been addressed by attempting to increase its host range (Godwin, Fordlloyd and
Newbury 1992) which has met with success, leading to improved transformation
procedures (Leet al. 2001). In essence, the wild-type plasmid contains genes
which causes the transfer of a piece of DNA, ‘T-DNA’, into a plant cell. This
piece is bordered by sequences of 24 base pairs in length which are repeats of
each other. This structure is fairly common in DNA and is described as direct
repeat. The T-DNA comprises genes which cause crown gall disease. These
Figure 9.4 Ti plasmid of Agrobacterium tumefaciens