What are nucleotides?
A nucleotide is a monomer (subunit) of a nucleic acid.
The genetic information of a cell is stored in molecules of deoxyribonucleic acid (DNA).
DNA is a polymer of nucleotides, that is, DNA is composed of a long strand of individual nucleotides.
Since a polymer of nucleotides is DNA, you can look at this the other way by noting than a nucleotide is a monomer of DNA.
A DNA nucleotide consists of three parts—a nitrogen base, a five-carbon sugar called deoxyribose, and a phosphate group.
There are four different DNA nucleotides, each with one of the four nitrogen bases (adenine, thymine, cytosine, and guanine).
The first letter of each of these four bases is often used to symbolize the respective nucleotide (A for adenine nucleotide, for example).
Two strands of nucleotides, paired by weak hydrogen bonds between the bases, form a double-stranded DNA molecule.
However, nucleotides are not present in just DNA. A nucleotide is really just a monomer of any nucleic acid, a category that also includes RNA (ribonucleic acid).
RNA differs from DNA in the following ways:
The sugar in the nucleotides that make an RNA molecule is ribose, not deoxyribose as it is in DNA.
The thymine nucleotide does not occur in RNA. It is replaced by uracil. When pairing of bases occurs in RNA, uracil (instead of thymine) pairs with adenine.
RNA is usually single-stranded and does not form a double helix as does DNA.