Proteins are made by tiny machines in the cell called ribosomes. Ribosomes are in the main body of the cell, but DNA is only in the nucleus of the cell. The codon is part of the DNA, but DNA never leaves the nucleus. Because DNA cannot leave the nucleus, the cell nucleus makes a copy of the DNA sequence in RNA. This is smaller and can get through the holes – pores – in the membrane of the nucleus and out into the cell.