DNA Structure & Function (SQA National 5 Biology): Revision Note

DNA structure

• DNA (deoxyribonucleic acid) is a molecule that contains genetic information; it is the encoded instructions for making proteins

• DNA is found in the nucleus of cells in structures called chromosomes

The double helix and base pairs

• DNA consists of a double-stranded helix held together by base pairs

• The four different bases present in DNA are:

  • adenine (A)

  • cytosine (C)

  • thymine (T)

  • guanine (G)

• The bases within DNA always pair up as follows:

  • adenine always pairs with thymine (A-T)

  • cytosine always pairs with guanine (C-G)

• This A-T and C-G pairing pattern is known as complementary base pairing

DNA function

• A gene is a section of DNA that codes for a protein

• The sequence of bases (A, T, G, C) within a gene determines the sequence of amino acids within a protein

  • Each sequence of three bases (triplet) codes for one amino acid, e.g.:

    • CAG codes for the amino acid valine

    • TTC codes for the amino acid lysine

    • GAC codes for the amino acid leucine