Why is dna described as a double helix

The nitrogen bases can only pair in a certain way: A pairing with T and C pairing with G. Due to the base pairing, the DNA strands are complementary to each other, run in opposite directions, and are called antiparallel strands. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet. October 16, Licenses and Attributions. This technique is called X-ray diffraction. The DNA crystals resulted in a cross shape on the X-ray film which is typical of a molecule with a helix shape.

The cross pattern visible on the X-ray highlights the helical structure of DNA. Image credit: Wellcome Images. A double helix resembles a twisted ladder. Each 'upright' pole of the ladder is formed from a backbone of alternating sugar and phosphate groups.

Each DNA base adenine, cytosine, guanine, thymine is attached to the backbone and these bases form the rungs. There are ten 'rungs' for each complete twist in the DNA helix. Image credit: Wikimedia Commons. James and Francis suggested that each 'rung' of the DNA helix was composed of a pair of bases, joined by hydrogen bonds.

Illustration to show the structure of the DNA double helix. Image credit: Genome Research Limited. The concept that DNA was made of a sequence of paired bases along a sugar-phosphate backbone allowed James Watson and Francis Crick to draw two important conclusions:. The order of bases on each strand makes up the digital code that carries the instructions for life. However, a crucial contribution that enabled this discovery was made by Rosalind Franklin, who was not acknowledged at that time.

After her death, Crick said that her contribution had been critical. The double helix describes the appearance of double-stranded DNA, which is composed of two linear strands that run opposite to each other, or anti-parallel, and twist together.

Each DNA strand within the double helix is a long, linear molecule made of smaller units called nucleotides that form a chain. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences. Each base is also attached to a sugar molecule and a phosphate molecule.

Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. An important property of DNA is that it can replicate, or make copies of itself.