If \(80 \%\) of the base pairs in a duplex DNA molecule \((12.5 \mathrm{kbp})\) are in the B-conformation and \(20 \%\) are in the Z-conformation, what is the length of the molecule?

DNA is a complex molecule that twists and turns to form various structures, with the B-conformation being the most common form found in natural conditions such as within human bodies. It's characterized by a right-handed helix, with about 10.5 base pairs per turn, and a helical twist of 36 degrees. Base pairs in B-DNA are relatively upright and perpendicular to the helical axis, resulting in a uniform appearance.

B-DNA is most conducive to biological function as its major and minor grooves allow easy access to proteins such as transcription factors, which is crucial for processes like gene expression. The width of the B-conformation is approximately 2 nanometers, with adjacent base pairs spaced at 0.34 nanometers apart, a measurement that's vital for understanding the physical length of DNA.

Z-DNA is less common than B-DNA and has a left-handed helical shape with a zigzag sugar-phosphate backbone, which is where its name originates. It's believed to occur in cells under certain conditions, such as high salt concentration or when the sequence has alternating purines and pyrimidines. This conformation has approximately 12 base pairs per turn and a helical twist that is more extended than in B-DNA.

Z-DNA plays a role in the regulation of gene expression, and its discovery was crucial for understanding DNA's flexibility and its ability to adopt different conformations depending on chemical and environmental factors. The inter-base pair distance in Z-DNA is typically 0.38 nanometers, which is a key factor when calculating its contribution to the overall length of a DNA molecule.

Base pairs are the fundamental building blocks of the DNA double helix. Composed of nitrogenous bases, they form pairs through hydrogen bonds: adenine with thymine and guanine with cytosine in DNA. The pairing is specific and follows the rules of base complementarity.

Base pairs are not just the rungs of the DNA ladder but also dictate the genetic information encoded within the DNA molecule. Additionally, the sequence and number of base pairs determine the shape and length of DNA, influencing its conformation and thus its function. The understanding of base pairs is essential for any molecular calculation regarding DNA, such as forecasting the length of a DNA strand with given information about its conformation and the number of bases present.

Calculating the molecular length of DNA involves considering both the type of conformation (B or Z) and the number of base pairs. Since the distance between base pairs differs in B-conformation (0.34 nm) and Z-conformation (0.38 nm), you first need to calculate the length that each conformation contributes separately.

For example, if a DNA strand consists of 10 kilo base pairs (kbp) in B-conformation and 2.5 kbp in Z-conformation, you would multiply the number of base pairs in each conformation by their respective distances (10 kbp x 0.34 nm for B-DNA and 2.5 kbp x 0.38 nm for Z-DNA), then sum up these lengths to obtain the total molecular length of the DNA. This precise method allows for an accurate determination of an otherwise too diminutive length to visualize or measure directly.