23 
the DNA double hehx (one 
side of the twisted ladder). A 
strand of mRNA is comple- 
mentary to the DNA from 
which it is transcribed, 
except that each adenine of 
the DNA is paired with a 
uracil (U), instead of with a 
thymine. For example, the 
stretch of DNA bases ATCG 
is transcribed into the mRNA 
sequence UAGC. 
After additional processing, 
the mRNA leaves the nucleus 
through pores in the nuclear 
membrane and carries its 
message into the cytoplasm, 
while the DNA remains safely 
in the nucleus. Once in the 
cytoplasm, the messenger 
RNA moves to tiny organelles 
called ribosomes, the factories 
in which the next step of 
protein manufacture, called 
translation, takes place. 
G 
(( (I H H ii 
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To replicate before cell 
division , the DNA double 
helix separates and 
unwinds and each strand 
acts as a templa te for 
the forma tion of a mirror 
image according to the 
rules of base pa iring: A 
with T, and G with C. 
This results in two 
daughter DNA molecules 
whose sequences are 
identical to those of the 
original DNA . 
! 
!r \i 
22 X Y 
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14 
15 
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