Why do genes have a spiral staircase shape?

Answer

Instead of genes, what is actually meant here is DNA.

DNA is a polynucleotide made up of mononucleotides, the building blocks. Such a DNA strand consists of two parts, a backbone on the one hand that is the same over the entire length of the chain and on the other hand four different bases, which form the variable side chains. The four different bases of DNA are the purines adenine and guanine and the pyrimidines cytosine and thymine. Symbolically, the bases are abbreviated with their initial letter, namely A, G, C and T. The purines (consisting of a five-membered ring linked to a six-membered ring) are larger than the pyrimidines (only a six-membered ring). The other building blocks of DNA are deoxyribose (a pentose, which is a sugar molecule made up of five carbon atoms) and phosphate (PO₄^3-). Deoxyribose and phosphate alternate constantly to form the constant backbone. The bases are each connected with one of their nitrogen atoms to the first carbon atom of deoxyribose. This is why the “steps” are equidistant from each other.

The most common model is the one proposed by Watson and Crick in 1953 (resulting in a Nobel Prize in 1962). Essential here is that a DNA molecule consists of a double helix. Two DNA chains are spirally wound around a common axis. One complete turn consists of 10 nucleotides (the building blocks) per chain and has a length of 3.4 nm. The diameter of the helix is ​​2 nm and is therefore constant. The bases of the two chains form base pairs (the “steps”) through hydrogen bonds, a bond. Adenine (A) can only form a base pair with thymine (T) and guanine (G) can only do this with cytosine (C).

Double-stranded DNA therefore consists of two chains with complementary sequences, A is complementary to T and C to G. If the chemical structure of the bases is examined, it appears that there is always a base pair between a purine and a pyrimidine. Simplistically speaking, a base pair always consists of two six-membered rings and one five-membered ring. It is important here that the distance between the two DNA backbones remains constant in this way. This is essential to get a stable structure. Other combinations either cause instability or cannot be formed because no hydrogen bonds are possible.

Two different clefts can be observed between the DNA backbones, namely a broad groove and a narrow groove. In these grooves, the bases have contact with their environment and can bind proteins. The latter is necessary to be able to perform various functions.