DNA Strands

DNA Strands (a.k.a. Helixes) are an obstacle encountered in Checkpoint 1, Zen Mode, and the Local Multiplayer. They take the shape of a DNA strand, hence the name.

There are different types encountered in Checkpoint 1. Some are dangerous, and some are not. There are also some encountered on the left and right side. These are not dangerous.

Checkpoint 1
Checkpoint 1 Room 1 has many (harmless) helixes on the sides.
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Zen Mode
In Zen Mode, there are lots in one area.

Local Multiplayer
In the Local Multiplayer, there are two of them seen at any one time. They are (usually) dangerous to both players, however in rare cases they have been dodged.

Trivia

 * 13 Things You'll Only Understand If You Play Smash Hit had one point on destroying a huge chunk of one of the strands: (but wasting balls)

Significance in Marketing
DNA strands have their own screenshot in the Smash Hit Presskit, and their own trailer (link here, or here).