6.5 A and B Forms of DNA

The B form of DNA is present under highly humid conditions. This was also the form of DNA that Watson and Crick chose to study.

The B form is also a right-handed double helix and is the structure that is the most relevant for double-stranded DNA.

The A form, on the contrary, is present in DNA fibers under conditions of low humidity.

The A form is also right handed, albeit it is “short and fat.” It it the form that it assumed by double-stranded RNA and DNA-RNA hybrid molecules.

6.5.1 Structural differences between A form and B form DNA

Structural Differences Between A Form and B Form DNA

Figure 6.21: Structural Differences Between A Form and B Form DNA

As seen in figure 6.21, the B form of DNA has its bases close to its helix axis (i.e., its center) which passes between hydrogen bonds.

The A form of DNA has its bases more on the outside; they are also strongly inclined with respect to the helix axis.

Differences in Grooves in A Form and B Form DNA

Figure 6.22: Differences in Grooves in A Form and B Form DNA

Furthermore, the B form of DNA has rather distinctive major and minor grooves (as seen in figure 6.22).

The A form of DNA has major and minor grooves of roughly equal width.

6.5.2 Miscellaneous information to note

The double helix models of DNA (i.e., B form DNA) are based on X-ray crystallography and reflect the structure of DNA in a cell in a very general way!

Variations in the structure of a double helix can involve variations in:

  1. Sugar and phosphate (backbone) conformation.
  2. base and complementary base orientation or position.
  3. base and neighboring base orientation or position.

Furthermore, the structure of nucleic acids is not homologous. It can vary in response to the local sequence and can be changed via interactions with other molecules.

Many DNA molecules are also bent - the helix axis does not follow a straight line and can bend depending on the DNA sequence (which are hugely influenced by the binding of various protein molecules).

DNA-Wrapped Histones

Figure 6.23: DNA-Wrapped Histones

In the cell’s nucleus, DNA is packaged around proteins of equal mass called histones (see figure 6.23). Because of this, most human DNA are strongly bent from being wrapped around histone proteins into nucleosomes.