Biology Reference
In-Depth Information
producing nonproductive transcripts from many 'non-expressed' genes and the
cost of turning off the transcription of many thousands of 'leaky' genes com-
pletely.
3.3 Exon structure and evolution
The average cellular gene in vertebrates contains about eight exons (Sharp, 1994).
A comprehensive analysis of 5061 exons from some 2705 intron-containing human
genes has yielded a classification scheme for exons according to their transcrip-
tional or translational boundaries (Zhang, 1998). This scheme (depicted in
Figure
3.2
) employs 12 different categories: (1) 5
terminal untranslated exons, (2) 3
ter-
minal untranslated exons, (3) 5
terminal exons having a 5
untranslated sequence
(UTR) followed by a coding sequence, (4) 3
untrans-
lated sequence (UTR) followed by a coding sequence, (5) internal exons having a
5
terminal exons having a 3
UTR followed by coding sequence, (6) internal exons having a cod-
ing sequence followed by a 3
portion of 5
UTR, (7) internal untranslated exons,
(8) internal translated exons, (9) exons containing the complete coding sequence
but which do not contain the transcriptional end, (10) exons containing the com-
plete coding sequence but which do not contain the transcriptional start, (11)
exons containing the complete coding sequence and both the transcriptional start
and the transcriptional end, and (12) exons containing the complete coding
sequence but neither the transcriptional start nor the transcriptional end.
Although no doubt subject to ascertainment bias, the sample of exons studied by
Zhang (1998) exhibited the following frequencies by type for the first 8 categories
of exon: (1) 271, (2) 38, (3) 482, (4) 553, (5) 174, (6) 69, (7) 34, (8) 3440. Exons in cat-
egories (3) and (7) were in general found to be <100 bp in length whereas exons in
categories (2) and (4) were mostly 300-500 bp in length. There appears to be virtu-
ally no minimum size for an internal translated exon (category 8) in human genes;
the smallest include the 4 bp exon 3 of the skeletal muscle troponin (
TNNI1
;
19q31) gene and two 3 bp exons (10 and 14) in the cardiac myosin binding protein
C (
MYBPC3
; 11p11.2) gene. As far as the maximum size of an internal exon is con-
cerned, the human
F8C
(3106 bp; Xq28),
APOB
(7572 bp; 2p23-p24) and
MUC5B
(10,690 bp; 11p15.5) genes are currently league leaders.
Zhang (1998) found that the size of 5
portion of 3
UTR varied between human genes from
0 bp to 2077 bp with a mean of 136 bp. The size of the 3
UTR varies from -2 bp
to 3427 bp with a mean of 589 bp (Zhang, 1998). The -2 value, for the human
-
D
-galactosidase A (
GLA
; Xq) gene, is due to the fact that the coding sequence
including the stop codon ends at nucleotide 11268 whilst the poly(A) addition site
is located at 11266.
3.4 Introns early or introns late?
Were introns used in the assembly of the first genes or were they added only later to
previously contiguous coding sequences? The '
introns early
' theory or simply the
'
exon theory of genes
' proposes that the genes encoding complex extant proteins
