2.6 Structure of DNA and RNA

What are nucleic acids?

The genetic material of the cells and are composed of recurring monomeric units called nucleotides.

What is each nucleotide made of?

* 5-carbon pentose sugar
* Phosphate group
* Nitrogenous base

Structure of the nucleotide

* Nitrogenous base is attached to the 1’ - carbon atom
* Phosphate base is attached to the 5’ - carbon atom

What are the two types of nucleic acids present in cells?

* DNA, a more stable double stranded form that stores the genetic blueprint for cells
* RNA, a more versatile single stranded form that transfers the genetic information for decoding

Nitrogenous bases in RNA & number of strands

Adenine, Cytosine, Guanine, Uracil + single stranded

Nitrogenous bases in DNA & number of strands

Adenine, Cytosine, Guanine, Thymine + Double stranded

Drawings of DNA and RNA (you need to know how to draw these)

How the structure of DNA is formed

* Phosphate group of one nucleotide attaches to sugar of another nucleotide (at 3’)
* This is called a phosphodiester bond and water is produced as a by-product
* The two chains are held together through hydrogen bonding between complementary nitrogenous bases
* In order for the bases to be facing each other and thus able to pair, the strands must be running in opposite directions 
* To be able to do this the strands must be antiparallel

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Who proposed the correct structural organization of the DNA molecule?

James Watson and Francis Crick.

What were Watson and Crick able to find out?

* DNA strands are antiparallel and form a double helix
* DNA strands pair via complementary base pairing (A = T ; C Ξ G)
* Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)

Errors in earlier versions of Watson and Crick DNA model

* The first model generated was a triple helix
* Early models had bases on the outside and sugar-phosphate residues in the centre
* Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity

Controversy with first DNA model

The final construction of a correct DNA molecule owed heavily to the X-ray crystallography data generated by Franklin

* This data confirmed the arrangement of the DNA strands into a helical structure
* The data was shared without Franklin’s knowledge or permission and contributed profoundly to the final design
* Hence, Franklin is now recognised as a key contributor to the elucidation of DNA structure