Structural highlights
- Most RNA and RNA-DNA duplex in this form
- Shorter, wider helix than B.
- Deep, narrow major groove not easily accessible to proteins
- Wide, shallow minor groove accessible to proteins, but lower information content than major groove.
- Favored conformation at low water concentrations
- Base pairs tilted to helix axis and displaced from axis
- Sugar pucker C3'-endo (in RNA 2'-OH inhibits C2'-endo conformation)
Take the Tour
The tour starts with the view. Now look at this .The backbone is yellow and the bases are magenta. Note that the major groove (at the top, when you have just clicked the button) is very deep.
Now change the display to make the show the connecting the phosphate atoms. Now the bases are easier to see. Notice how they are stacked upon each other but not perpendicular to the axis of the double helix. They are also displaced to the side of the axis. The result is a wide, short helix. Note also that the backbone forms a smooth, continuous curve.
You can .You are looking into the major groove and the colors of the base pairs alternate. You can also .
Each base pair stacks on the next similarly, as shown from . This is the .
B-DNA stacks similarly, but compare this with Z-DNA, which behaves much differently. Essentially all helical RNA is in A form, but DNA can also be found in A form under certain conditions (particularly in RNA-DNA hybrids). The 2'-OH of ribose favors the C3'-endo sugar pucker necessary for A-form geometry.
You can compare it with the DNA forms by looking at this 3D red-blue stereo picture of A, B, and Z DNA
