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.