Why is RNA more reactive than DNA

16. Biomolecules: nucleic acids

The entire genetic information is in the Deoxyribonucleic acids (DNA) contain. The translation of this information into the synthesis of proteins is carried out by the Ribonucleic acids (RNA) carried out. In addition to carbohydrates and polypeptides, nucleic acids are the third main type of biological polymer. Their monomer units are called Nucleotides:


The sugar content in DNA consists of 2-deoxyribose:

And you only find four different heterocyclic amine bases. Two are substituted pyrimidines, and two are substituted purines:

The sugar characteristic of RNA is ribose, and RNA also contains four bases. Thymine has been replaced by uracil:

16.1 Structure of DNA and RNA

Formally, the nucleotide can be assembled by first replacing the hydroxyl group on C1 of the sugar with one of the nitrogen bases. The resulting molecular unit is a Nucleoside.

Second, a phosphate residue is then introduced at C5 of the sugar. Since there are four different bases in DNA and RNA, there are also four each Nucleotides :

A polymer chain can then easily be built up by repeatedly creating phosphate ester bridges from C5 of the sugar unit of one nucleotide to C3 of the sugar of another. One end of the chain has a free hydroxyl group at C3 '(3 'end) and the other a phosphate unit at C5 (5 'end).

What is a Phosphate ester and which products are created by the hydrolysis of a phosphate ester?

Nucleic acids, especially DNA, can form extremely long chains with a molar mass in the billions (RNA molecules are much smaller than DNA molecules). In 1953, Watson and Crick proposed their famous hypothesis that DNA adopts a double helix structure consisting of two strands with a complementary base sequence. The crucial information on the basis of which this hypothesis was developed was that in the DNA of various organisms the ratio of adenine to thymine and guanine to cytosine was always one to one. This led to the assumption that two DNA chains are held together by hydrogen bonds in such a way that adenine and guanine in one chain always face thymine and cytosine in the other.

e.g .:

Adenine and thymine only form complementary hydrogen bonds with one another, but not with G or C. Similarly, G and C only form complementary hydrogen bonds with each other and not with A or T:

Due to other structural factors, the arrangement in which the hydrogen bonds are maximally formed and the steric repulsion is minimized (i.e. the most stable conformation) is the Double helix.

The two strands in a DNA double helix are not identical to each other but complementary. From the X-ray structures of DNA molecules it can be seen that the diameter of the double helix is ​​20 Å and that the helix rotates at 34 Å intervals. The two chains are linked by hydrogen bonds between base pairs. Adenine always pairs with thymine and guanine always pairs with cytosine. The genetic information lies in the base sequence.

On the other hand, RNA molecules do not normally appear as stable double-stranded forms, but take on single-stranded and more intricately folded forms.

16.2 DNA and RNA: Carriers of the genetic make-up

Watson and Crick postulated that the specific base sequence of a given DNA contains all the information necessary for a cell to divide. In addition, based on the exact complementarity of the double helix structure, they developed a model of how DNA looks replicated and passes on the genetic code:

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16.3 Chemical properties of DNA and RNA

A strong acid is needed to hydrolyze DNA, for example:

e.g .:

DNA is relatively stable under basic conditions, but RNA is rapidly degraded and yields 2'- and 3'-nucleoside phosphates:

In addition, the heterocyclic bases in DNA and RNA contain atoms and groups that are naturally good nucleophiles. They therefore react quickly with all molecules that contain an electrophilic center. Methyl iodide and dimethyl sulfate, for example, are reactive methylation agents (see chapter 8.4), and their reaction with DNA can also lead to mutations: