A base pair is two bases that make up a rung of the DNA ladder.
A DNA nucleotide is made up of a base molecule, a sugar molecule, and a phosphoric acid molecule.
The bases are the letters that form the genetic code. C, G, T, and A are the code letters for cytosine, guanine, thymine, and adenine, respectively.
Guanine always forms a pair with cytosine, whereas adenine always forms a pair with thymine.
5' and 3' ends of DNA strand:
DNA is read in a specific direction, just as letters and words in English are read from left to right.
Furthermore, each end of the DNA chain is represented by a number.
One end of the DNA molecule is known as 5′ (five prime), and the other end is known as 3′. (three prime).
The numbers 3′ and 5′ refer to the number of carbon atoms in a deoxyribose sugar molecule that a phosphate group binds to.
The carbons in sugar are numbered clockwise, beginning with the oxygen atom.
This asymmetry gives the DNA strand a "direction," and the bases in the DNA are read from 5′ to 3′.
Because of this directionality and the fact that the synthesis proceeds in the 5′3′ direction, polynucleotide sequences are read and written in the 5′3′ direction.
Nucleic acid, like polypeptides, has an end-to-end chemical orientation.
On the 5′ carbon of its terminal sugar, the 5′ end has a phosphate group.
A DNA strand with the fifth carbon within the sugar ring at its terminus is said to have a five prime end.
On the 3′ carbon of its terminal sugar, the 3′ end has a free hydroxyl group.
The three prime ends of a molecule is the end that ends in a 3′ phosphate group.
Phosphate groups at the 5′ end of one nucleotide and hydroxyl groups at the 3′ end of another nucleotide can form phosphodiester bonds, connecting adjacent molecules.
The sugar-phosphate backbone is provided by this linkage, which provides the structural rigidity of DNA.
