Non-functional "junk" DNA, called introns, [1] interrupt the functional
sections (exons), [2] which will eventually be expressed.
Both introns and exons are transcribed onto mRNA but enzymes cut out the
introns [3] and rejoin the exons [4] into the molecule of mRNA that then
leaves the nucleus. Only the sections of mRNA necessary for coding for the
required amino-acids [5] are retained.
The mRNA leaves the nucleus, via nucleopores, and travels in the cytoplasm
to ribosomes. Ribosomes are either found attached to the cell's internal
membrane system (rough endoplasmic reticulum) or free in the cytoplasm.
Ribosomes are made up of two subunits, a large one and a smaller one. Each
ribosome consists of ribosomal RNA (rRNA) and proteins (about 70 different
types).
