Intron is noncoding segment of DNA removed by spliceosomes before mRNA leaves nucleus. Investigators have found that the simpler the eukaryote, the less likely that introns will be present. Ribozymes are RNAs with an enzymatic function restricted to cleaving RNA at specific locations. RNA could have served as both genetic material and as first enzymes in early life forms. Takes place in cytoplasm of eukaryotic cells.
Translation is second step by which gene expression leads to protein synthesis.
Amino acid-tRNA complex forms, then travels in cytoplasm to ribosome for protein synthesis. Subunits move separately through nuclear envelope pores into cytoplasm where they combine. Ribosomes can float free in cytosol or attach to endoplasmic reticulum. Polyribosomes are clusters of several ribosomes synthesizing the same protein. Nearly all polypeptides have signal sequences that target them for final location in cell.