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In-Depth Information
Exon 4
Exon 6
Exon 9
Exon 17
A
1 2
1
48
1
33
1 2
Genomic DNA
mRNA
Protein
B Differential use of alternative exons generates enormous receptor diversity.
Alternative Exons Observed
Alternative Exons Observed
cDNAs
Exon 4
Exon 6
Exon 9
cDNAs
Exon 4
Exon 6
Exon 9
1
1
2
14
26
6
27
16
2
1
5
5
27
6
30
17
3
1
8
11
28
6
34
7
4
1
33
20
29
6
36
16
5
1
43
20
30
6
37
23
6
2
4
13
31
7
6
5
7
2
18
30
32
7
7
12
8
2
25
8
33
7
18
5
9
2
28
13
34
7
19
5
10
2
37
30
35
7
20
21
11
2
39
13
36
7
24
9
12
3
3
13
37
7
42
8
13
3
3
32
38
8
4
26
14
3
15
13
39
8
6
16
15
3
27
7
40
8
19
15
16
3
24
13
41
8
30
25
17
3
37
19
42
8
48
24
18
4
30
4
43
9
35
3
19
4
42
32
44
10
16
2
20
4
46
8
45
10
37
14
21
5
6
8
46
10
42
25
22
5
21
2
47
10
48
26
23
5
26
19
48
12
7
16
24
5
26
19
49
12
15
16
25
5
19
18
50
12
18
9
FIGURE 6.28 Multiple forms of Dscam are generated by alternative splicing. A. The Dscam gene spans
61.2 kb of genomic DNA. Dscam mRNA extends 7.8 kb and comprises 24 exons. Mutually exclusive alterna-
tive splicing occurs for exons 4, 6, 9, and 17. 1 of 12 exon 4 alternatives, 1 of 48 exon 6 alternatives, 1 of 33
exon 9 alternatives, and 1 of 2 exon 17 alternatives are retained in each mRNA, as deduced from cDNA
sequence. Variable exons are shown in color: exon 4, red; exon 6, blue; exon 9, green; and exon 17, yellow.
Constant exons are represented by gray lines in genomic DNA and white boxes in mRNA. The splicing
pattern shown (4.1, 6.28, 9.9, 17.1) corresponds to that obtained in the initial cDNA clone. The alternatively
spliced exons 4, 6, 9, and 17 encode the N-terminal half of Ig2 (red), the N-terminal half of Ig3 (blue), the
entire Ig7 (green), and the transmembrane domain (yellow), respectively. B. Alternative exons are expressed.
RT-PCR was performed on total RNA isolated from 12-24 hour embryos. Fifty individual cDNA clones
(numbers 1-50) were isolated and sequenced across exons 4, 6, and 9. Alternative exons used in each cDNA
are indicated. Color coding of exons in schematic corresponds to scheme in A. 49 of the 50 cDNAs contain
unique combinations of alternative exons. (From Schmucker et al., 2000)
SUMMARY
Growing axons are often encouraged to enter the
target at one site and discouraged from exiting the
target at another site through repulsive barriers. There
are a variety of molecules within the target zone,
including gradients of Ephrins and CAMs, which con-
spire, often in combination, to encode different possi-
ble target cells along various axes and layers. The
incoming afferents are distinguished from each other
by the presence of different amounts of various recep-
Pathfinding to the vicinity of a correct target is only
the first step in the process of selecting appropriate
postsynaptic cells on which to synapse. Having come
to the doorstep of the target population, axons use a
variety of signals, such as relative changes in growth
factors, to enter the target and begin to arborize.
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