DNA – Deoxyribonucleic acid Polymer Nucleic acids are biopolymers, or large biomolecules, essential to all known forms of life. They are composed of monomers, which are nucleotides made of three components: a 5-carbon sugar, a phosphate group, and a nitrogenous base. DNA is a type of nucleic acid Individual unit called a (mono)nucleotide DNA is the genetic code to synthesise proteins

Nucleotide Structure 3 components: Pentose sugar Organic base Phosphate sugar base Stay the same Changes Contains nitrogen & carbon Pentose sugar (5 Carbon atoms) 3 components: Pentose sugar (ribose in DNA) Phosphoric acid Organic base (always contains nitrogen)

Bondings The base and sugar join with a: glycosidic bond The phosphate and sugar join with an: ester bond Both require a condensation reaction to occur 

2 condensation reactions occur in the formation of a nucleotide: Phosphate 2 condensation reactions occur in the formation of a nucleotide: H Ester bond OH Glycosidic bond OH H Base Sugar

Nucleotide 3 components: Pentose sugar; deoxyribose Phosphate group Nitrogenous base; 4 types: Join by condensation phosphate base Stay the same sugar Changes Contains nitrogen & carbon Ribose sugar in RNA Deoxyribose sugar in DNA Pentose sugar (5 Carbon atoms)

You do not need to know this structure for CIE

In the backbone of DNA the 5' carbon of one deoxyribose is linked to the 3' carbon of another by a phosphate group. The 5' carbon of this deoxyribose is again linked to the 3' carbon of the next, and so forth.

Phosphodiester Bonds: the sugar-phosphate-backbone

4 Bases Purines – Pyrimidines – Double ringed structure Planar Single ringed structure

Complementary Base Pairing *Purines always pair with pyrimidines* A joins to T (2 hydrogen bonds) C joins to G (3 hydrogen bonds) see

Hydrogen bonding Explain how the interaction between base pairs fits the definition of hydrogen bonding.

Bonding relationships Type of Base Purine Pyrimidine Structure Bonding relationships Adenine Guanine Thymine Cytosine = hydrogen bond

5’ 3 hydrogen bonds 3’ C G 0.34nm A T 3’ 2 hydrogen bonds 5’ 2nm

DNA jigsaw… Sketch the pieces joined in the correct arrangement for DNA. Label: a sugar, a phosphate and a base pair

Answer… (are there other possibilities?)

The bigger picture… Can you suggest any other reasons that Adenine always pairs with Thymine and Cytosine always pairs with Guanine?

The bigger picture… Can you suggest any other reasons that Adenine always pairs with Thymine and Cytosine always pairs with Guanine? Consider the size of the base pairs relative to the distance between the sugar-phosphate backbone.

2 strands run in opposite directions. Strands are twisted to form a double helix. The bases link the 2 strands together via H bonds and van der Waals’ forces between one base pair and the next. Base pairs are always complementary T & A and G & C.

A DNA strand acts as a template for replication.