Everything Science Grade 12

2.6 CHAPTER 2. ORGANICMACROMOLECULES

nucleotide DNA polymer made upof

four nucleotides

phosphate sugar nitrogenous base

Figure 2.14: Nucleotidemonomers make up theRNA polymer

There are five differentnitrogenous bases: adenine (A), guanine (G),cytosine (C),
thymine (T) and uracil (U). It is the sequence of the nitrogenous bases in aDNA polymer
that will determine the genetic code for that organism. Three consecutivenitrogenous
bases provide the coding for one amino acid. So, for example, if the nitrogenous bases
on three nucleotides are uracil, cytosine and uracil (in that order), one serine amino
acid will become part of the polypeptide chain.The polypeptide chain is built up in
this way until it is long enough (and with the right amino acid sequence) to be a protein.
Since proteins control much of what happens inliving organisms, it is easy to see how
important nucleic acidsare as the starting pointof this process.

Table 2.2 shows some other examples of geneticcoding for different amino acids.

Table 2.2: Nitrogenousebase sequences and thecorresponding amino acid

Nitrogenous base sequence Amino acid

UUU Phenylalanine

CUU Leucine

UCU Serine

UAU Tyrosine

UGU Cysteine

GUU Valine

GCU Alanine

GGU Glycine