What Is the DNA Double Helix?

The DNA double helix is the most famous molecular structure in all of biology. When James Watson and Francis Crick, with the help of critical data from Rosalind Franklin, uncovered this structure in 1953 it revolutionized how scientists and eventually everyone thought about heredity.

It gave a simple explanation for how DNA is copied when a cell divides, how it is passed down from generation to generation, and how such a simple molecule could provide all the mind bending complexity that is life on Earth. It is considered one of the most important scientific discoveries for a reason.

 

What Is the Shape of DNA?

The double helix of DNA is, like its name implies, in the shape of a helix which is essentially a three dimensional spiral. The double comes from the fact that the helix is made of two long strands of DNA that are  intertwined—sort of like a twisted ladder.

Each strand of DNA (or side of the ladder) is a long, linear molecule made up of a backbone of sugars and phosphate groups. Connected to each sugar is a nitrogenous base. Each rung between the two strands of DNA is formed by pairs of these nitrogenous bases.

There are four types of bases in DNA, which are often called by their initials: adenine (A), thymine (T), cytosine (C), and guanine(G). When the base pairs interact, they match up in a very specific way: A always pairs with T, and C always pairs with G.

These bases pair up using a very weak type of chemical bond called a hydrogen bond. The A-T base pair is held together by two hydrogen bonds. The C-G pair is held together by three hydrogen bonds.  

 

The Double Helix and DNA Replication

DNA replication is the biological process by which a double-stranded DNA helix is copied, producing two identical replicas. The double helix structure is key to the process of DNA replication and is the basis of the most famous quote from the original paper by Watson and Crick describing the double helix for the first time:

"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

It is the specific base pairing in the central part of the DNA double helix that allows for DNA to be faithfully copied every time a cell divides.

The first step in DNA replication is the unwinding of the DNA helix. DNA polymerases, the proteins whose job it is to copy the DNA, can unwind the DNA because the hydrogen bonds between the pairs are so weak.  

Once the DNA strands are pulled apart, the DNA polymerase builds a new strand from each of the separated strands. It does so using the matching properties of the bases. If there is a T, for example, the DNA polymerase adds an A to the opposite strand—and so on. The end result is two identical copies of the original DNA.