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3. Beskrive polariteten af nukleinsyrer (5'-3' - ender) og angive at strengene i en DNA dobbelthelix er antiparallele
Stryer, s. 120
Devlin, s.39

A DNA chain has a polarity. One end of the chain has a free 5`-OH group (or a 5`-OH group attached to a phosphate), whereas the other end has a 3`-OH group, neither of which is attached to another nucleotide.

By convention, the base sequence is written in the 5`- to - 3` direction. This also underlines the fact that the DNA-molecule consists of polymers that are joined by  3`- 5` phosphodiester bonds.

The double helix DNA molecule contains 2 strands that are antiparallel to each other. This basically means that the two strands are aligned in opposite direction. This supports the complementarity principle.

fx.
if two adjacent bases in the same strand, thymine (T) and cytosine (C), are connected in 5`-3` direction, their complementary bases adenine (A) and guanine (G) will be linked in 3`-5` direction.

This antiparallel alignment produces a stable association between strands to the exclusion of the alternate parallel arrangement. The 2 strands have a very high affinity for each other.

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