The upper diagram shows a di-nucleotide
(with green bases),
with a tri-phosphate group attached to the 5'
carbon of the first (upper) nucleotide, a 3' 5' phosphodiester bond linking
the 3' carbon of the upper nucleotide to
the 5' carbon of the lower nucleotide, and
an exposed 3' carbon with an attached -OH
group on the second (lower) nucleotide. Addition of the
third nucleotide (with a blue base)
onto the bottom of the second nucleotide requires
clipping of the outer two PO4
groups and formation of a new 3' 5' phosphodiester bond. The original 5' end on the first
nucleotide remains unmodified, and the new 3' end is
from the newly-added third nucleotide. Polynucleotides thus "grow" from an
unchanging 5' end by succesive modification of the 3'
end: we define this as growing in the 5'3'
direction. Do NOTattempt to think
about the process any other way:
Nucleic acid synthesis
processes occur only in the 5'3' direction.