2.6 Structure of DNA and RNA

U1: The nucleic acids DNA and RNA are polymers of nucleotides.

There are two types of nucleic acids present in cells - DNA and RNA - DNA (deoxyribonucleic acid) is a more stable double stranded form that stores the genetic blueprint for cells - RNA (ribonucleic acid) is a more versatile single stranded form that transfers the genetic information for decoding Both DNA and RNA are polymers of nucleotides, however key differences exist in the composition of DNA and RNA nucleotides: http://ib.bioninja.com.au/_Media/nucleotide-rna-vs-dna_med.jpeg

U2: DNA differs from RNA in the number of strands present, the base composition and the type of pentose.

DNA and RNA are both polymers of nucleotides, however differ in a few key structural aspects: - Number of strands present - Composition of nitrogenous bases - Type of pentose sugar differences between DNA and RNA: http://ib.bioninja.com.au/_Media/dna-vs-rna_med.jpeg

U3: DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

Nucleic acids are composed of nucleotide monomers which are linked into a single strand via condensation reactions - The phosphate group of one nucleotide attaches to the sugar of another nucleotide (at the 3'- hydroxyl (-OH) group) - This results in a phosphodiester bond forming between the two nucleotides (and water is produced as a by-product) - Successive condensation reactions result in the formation of long polynucleotide strands Two polynucleotide chains of DNA are held together via hydrogen bonding between complementary nitrogenous bases - Adenine (A) pairs with Thymine (T) via two hydrogen bonds - Guanine (G) pairs with Cytosine (C) via three hydrogen bonds In order for the bases to be facing each other and thus able to pair, the strands must be running in opposite directions - The two strands of DNA are described as being antiparallel As the antiparallel chains lengthen, the atoms will organise themselves into the most stable energy configuration - This atomic arrangement results in the double-stranded DNA forming a double helix (~10 - 15 bases per twist)

S1: Drawing simple diagrams of the structure of single nucleotides of DNA and RNA, using circles, pentagons and rectangles to represent phosphates, pentoses and bases.

Nucleic acids are the genetic material of the cell and are composed of recurring monomeric units called nucleotides Each nucleotide is comprised of three principal components: - 5-carbon pentose sugar (pentagon) - Phosphate group (circle) - Nitrogenous base (rectangle) Both the phosphate group and nitrogenous base are attached to the central pentose sugar - The nitrogenous base is attached to the 1'- carbon atom (right point) - The phosphate base is attached to the 5'- carbon atom (left point) diagram: http://ib.bioninja.com.au/_Media/nucleotide-schematic_med.jpeg

A1: Crick and Watson's elucidation of the structure of DNA using model making.

The structural organisation of the DNA molecule was correctly proposed in 1953 by James Watson and Francis Crick - These British scientists constructed models to quickly visualise and assess the viability of potential structures Their efforts were guided by an understanding of molecular distances and bond angles developed by Linus Pauling, and were based upon some key experimental discoveries: - DNA is composed of nucleotides made up of a sugar, phosphate and base - Phoebus Levene, 1919 - DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) - Erwin Chargaff, 1950 - DNA is organised into a helical structure - Rosalind Franklin, 1953 (data shared without permission) Making DNA Models Using trial and error, Watson and Crick were able to assemble a DNA model that demonstrated the following: - 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) As Watson and Crick's model building was based on trial and error, a number of early models possessed faults: - 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 The Rosalind Franklin Controversy 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