Introduction 
Normal hemoglobin beta chain 
Vol 
ine 
Histidi 
me 
Leucine 
Th 
reonme 
Prol 
ine 
Sickl e-cell anemia hemoglobin beta chain 
Vali 
ne 
Histidi 
ne 
Leucine 
Th 
reonme 
Proli 
ne 
Glutamic 
acid 
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Glutamic 
acid 
Glutamic 
acid 
Figure 12. A drawing of the hemoglo- 
bin molecule and the single-amino 
acid change in the P-chain of the mole- 
cule that causes sickle cell anemia. 
From Raven, P.H., and Johnson, 
G.B. 1988. Understanding Biology. St. 
Louis, MO: Times Mirror/Mosby Col- 
lege Publishing, p. 25 5. 
Figure 13. We can determine where particular genes 
are being expressed by using a radioactively labeled 
probe that selectively binds to the UNA transcript of 
the gene and examining the distribution of the radio- 
activity in a tissue autograph. This technique, known 
as in vitro hybridization, was used in this case to show 
the expression of one of the keratin genes in a hair 
follicle. The micrograph on the left is viewed under 
darkfield optics ( the exposed silver grains in the auto- 
radiograph appear as white dots ); that on the right is 
seen with conventional optics, and the radioactivity is 
marked by black dots. 
Courtesy of Elaine V. Fuchs. 
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