The structure of ovalbumin gene
in chicken has been studied in considerable detail and it has been found that in the split gene of ovalbumin there are eight exons and seven introns (Figs. 29.7 and 29.8). The size of ovalbumin gene has been shown to be 7,700 base pairs although the mature mRNA for ovalbumin is only 1,872 nucleotides. It has also been shown, that entire ovalbumin gene having 7,700 base pairs is first transcribed to a precursor RNA to which a cap consisting of 7mG is added at the 5' end and poly A tail is added at 3' end. After the addition of cap and tail, the introns are excised, perhaps in two instalments, five introns being excised in the first step and the remaining two in second step. The exons produced due to splicing are then joined with the help of enzyme ligase to produce the mature mRNA of ovalbumin gene.
The different steps involved in the processing of precursor RNA into mRNA for ovalbumin gene
are shown in the Figure 29.9. The mechanism of RNA splicing is discussed in Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)
Fig. 29.7. The map of electron micrograph prepared after hybridizing mRNA (thin line) of the ovalbumin gene with its single stranded DNA (thick line). The loops A-G represent 7 different introns and segments L and 1-7 represent 8 exons.
Fig. 29.8. Schematic representation of ovalbumin gene showing 7 introns (A-G) and 8 exons (L, 1-7); number of base pairs in each exon are also given.
Fig. 29.9. Steps involved in the production of ovalbumin mRNA (1872 nucleotides long) from ovalbumin gene (7700 base pairs long); introns (A-G) and exons (L, 1-7).