nucleotide
Meaning of nucleotide: Order is everything: the same few units in a different sequence spell a different protein, or a disease causing mutation.
Definition of nucleotide
A nucleotide is the repeating unit of DNA and RNA, built from three parts joined together: a five carbon sugar, a phosphate group and a nitrogen containing base. Linking them end to end through their sugars and phosphates creates a strand, and the order of the bases along that strand carries the genetic instructions a cell reads.
How do the parts fit together?
The sugar sits in the middle. A base attaches on one side and a phosphate group on the other, and it is the phosphate that links each unit to the next, producing a backbone of alternating sugar and phosphate with the bases pointing inward. In DNA two such strands pair up and twist around each other, held where the bases meet: adenine always opposite thymine, cytosine always opposite guanine. That fixed pairing is what makes copying possible, since either strand specifies the other exactly. A nucleoside is the same molecule without the phosphate, a distinction worth keeping because several drugs are deliberate nucleoside and nucleotide lookalikes.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | Adenine, cytosine, guanine, thymine | Adenine, cytosine, guanine, uracil |
| Usual form | Two strands wound together | Usually a single strand |
| Job | Long term store of the instructions | Working copy and assembly machinery |
How does the order of bases become a protein?
Bases are read in groups of three. Each triplet names one amino acid, and a chain of amino acids folds into a protein, so a stretch of sequence amounts to a set of assembly instructions. The cell copies the relevant stretch of DNA into RNA, then reads the RNA to build the protein. Changing a single base may be harmless, may swap one amino acid for another, or may halt the chain early and leave the protein unfinished, which is how a one letter difference produces a serious inherited condition. Genetic testing works by reading the order of nucleotides and comparing it against a reference. Not every base codes for protein, either. Some carry the switches deciding when a gene is used, and a methyl group attached to a cytosine can quiet a gene without altering the sequence at all.
What else do nucleotides do?
Storing information is only one of their jobs. ATP, the molecule cells spend to power almost everything they do, is a nucleotide with two extra phosphates attached, and energy is released as one is broken off. A close relative, cyclic AMP, relays signals from a hormone at the cell surface to the machinery inside. Several coenzymes that shuttle electrons through metabolism are built on the same chemical frame. That resemblance is exploited in medicine, since drugs designed to imitate nucleotides are picked up by copying enzymes and jam the process, which is the basis of a number of antiviral and cancer treatments.
Used in a sentence
Sequencing determined the exact order of nucleotides within the gene of interest.
Good to know
A nucleotide is sometimes confused with a nucleoside, which consists of only the sugar and base without the phosphate group that a complete nucleotide contains.
Word origin
Where “nucleotide” comes from
Formed from nucleus, Latin for kernel, reflecting the molecule's association with the genetic material housed in the cell nucleus, combined with the chemical suffix -ide.