Biological Molecules - Structure of DNA and RNA

Describe structure of DNA nucleotide

• Polymer of nucleotides

• Each nucleotide formed from deoxyribose, phosphate group and nitrogenous base.

• Phosphodiester bonds between nucleotides

• Double helix held by H bonds

• H bonds between adenine, thymine, cytosine and guanine.

Describe how a phosphodiester bond is formed between two nucleotides within DNA molecule.

Condensation reaction. Between phosphate and carbon 3 of deoxyribose and is catalysed by DNA polymerase

How does structure of DNA relate to function?

• Stable structure due to sugar-phosphate backbone (covalent bonds) and double helix

• Double strand - so replication can occur using one strand as a template

• Weak H bonds for easy unzipping of two strands in a double helix

• Large molecule to carry lots of info.

• Complementary base pairing - allows identical copies to be made

DNA polymer occurs in pairs and pairs are joined together by H bonds between bases. HOW?

H bonds can only form between complementary base pairs. A-T C-G

Structure of RNA

• Nucleotide formed of ribose, nitrogenous base = uracil and phosphate group.

• Relatively short polynucleotide

• Single stranded

Function of RNA

Copy and transfer genetic code from DNA to ribosomes.

How are complementary base pairs formed?

• Purine adenine (A) pairs with pyrimidine thymine (T) – two hydrogen bonds are formed between these bases

• Purine guanine (G) pairs with pyrimidine cytosine (C) – three hydrogen bonds are formed between these bases

• This is known as complementary base pairing