(3)
1. Beskrive og tegne hvordan et nukleotid i en nukleinsyre er sammensat af komponenterne: base (purin/pyrimidin), sukker, fosfat
Devlin, s. 29-31;, fig. 2.5
Devlin, s.33 - fig. 2.8
Stryer, s.118-120, fig. 5.3

Nucleic acids are linear polymers consisting of repeating nucleotide units.

Each nucleotide consists of three components:
 

·                    a pentose sugar - D-ribose (in RNA) and 2-deoxy-D-ribose (in DNA).

The deoxy prefix in deoxyribose in DNA indicates that the 2´ carbon atom of the sugar lacks the oxygen atom that is linked to the 2` carbon atom in ribose in RNA
 

·                    a phosphate group - monofosfat, difosfat or trifosfat.

The sugars in nucleic acids are linked to one another by phosphodiester bridges of a monophosphat group.  Specifically, the 3`-OH group of the sugar moiety of one nucleotide is esterified to a phosphate group, which is, in turn, joined to the 5`-hydroxyl group of the adjacent sugar.

DNA and RNA consist of polymers joined by 3`- 5` phosphodiester bonds.

·                    heterocyclic base - purines and pyrimidines.

The major purine derivates are guanine (G) and adenine (A), which are found in  both DNA and RNA and are attached to the sugar at N-9.

The 2 major pyrimidine derivates are cytosine (C), uracil (U) and thymidine (T). Cytosine is present in both DNA and RNA. Uracil is found generally in RNA, while thymidine in DNA. They are all linked to the sugar through N-1 position.

The bond between the base and the sugar is a beta-glycosidic linkage.

A difference should be made between:

• nucleoside - a base glycosylated with either pentose sugar

• nucleotide -  phosphate esters of nucleosides, contains: base, sugar and a phosphate ester

• oligonucleotide - linear sequence of under 50 nucleotides

• polynucleotide - linear sequence of over 50 nucleotides

• backbone - the chain of sugars linked by phosphodiester bridges

tilbage til molekyær biologi