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a large nuclear structure complex termed thespliceosomewhere splicing takes

place. Introns are usually removed in a sequential manner from the 5^0 to the 3^0 end

and their number varies between different genes. Some eukaryotic genes such as

histone genes contain no introns whereas the gene for dystrophin, the gene respon-

sible for muscular dystrophy, contains over 250 introns. In some cases, however,

the same hnRNA transcript may be processed in different ways to produce different

mRNAs coding for different proteins in a process known asalternative splicing.

Thus a sequence that constitutes an exon for one RNA species may be part of an

excised intron in another. The particular type or amount of mRNA synthesised from

a cell or cell type may be analysed by a variety of molecular biology techniques

(Section 6.8.1).

Start site

Promoter

regions Exon1 Intron1 Exon2 Intron2 Exon3

Poly(A) site

Stop site

Transcription RNA polymerase II

hnRNA

Exon1 Intron1 Exon2 Intron2 Exon3

5 

3 

3 

5 

5  3 

Fig. 5.17Transcription of a typical eukaryotic gene to form heterogeneous nuclear RNA.

Exon1 Intron1 Exon2 Intron2 Exon3

hnRNA 5  (^3) 
Methylated G residue added to 5 end
AAAAAAA 3
Poly(A) tail added to 3 end
Me
5  G
Removal of introns in splicing reaction
Exon1 Exon2 Exon3
mRNA 5 
Me
G AAAAAAA 3
Fig. 5.18Post-transcriptional modifications of heterogeneous nuclear RNA.
159 5.5 Functions of nucleic acids