Chapter 28: Nucleosides, Nucleotides and Nucleic Acids

Deoxyribose Nucleic Acid : DNA

• There 2 important biological processes involving DNA:
• replication to produce a "copy" of the DNA
• protein biosynthesis via RNA
• DNA is based on the carbohydrate deoxyribose and the pyrimidine bases cytosine and thymine and the purine bases adenine and guanine.
• The structure is the famous double helix discovered by Watson and Crick (shared Nobel Prize1962).
• There are 3 forms of DNA : A-DNA (compact), B-DNA (common) and Z-DNA (left hand double helix)
• B-DNA involves two strands of nucleic acids twisted around each other with a right hand twist in an antiparallel fashion.
• Antiparallel means the sense of the individual strands is in opposite directions.
• The double helix is stabilised by H-bonding between the nitrogen bases in the interior of the helix.
• Note how the bases in the helix are π-stacked parallel to each other 3.4 Angstroms apart.
• Careful inspection of the DNA helix shows a wide gap = the major groove, and a narrow gap = the minor groove between the strands.

		Show backbone Show bases Reset
The double helix of a partial strand of DNA

• An important observation was mad e by Chargaff : the amounts of adenine (A) and thymine (T) were about equal, around 19.5% and so were the amounts of guanine (G) = cytosine (C) at 30.3%.
• This led to the realisation that the hydrogen bonding interactions between specific base pairs was important : A=T and G≡C
• These interactions are shown below:

G-C base pair	A-T base pair

 

• This interaction is so specific that it means that these are the only pairings that are observed :  A=T and G≡C only

Show A bases Show C bases Show G bases Show T bases Show A-T pairs Show C-G pairs Reset

---

© Dr. Ian Hunt, Department of Chemistry