DNA
polynucleotide
chain
growth
During DNA synthesis,
new
nucleotides
are added to the existing 3' end by a 3' 
5'
phosphodiester bond (R
- O - P - O
- R). The original 5' end remains
unmodified. Polynucleotides thus "grow" by modification of
the 3' end,
in the 5'3' direction. Nucleic
acid biochemical processes occur only in
the 5'3'
direction.
Figure
after © 1997 Klug &
Cummings; All text © 2011 by Steven M. Carr