AP Bio Study Guide

Big Idea 3.A

How does DNA become a protein?

DNA is transcribed into mRNA (messenger RNA) inside the nucleus.

Then, this RNA is processed or modified in the nucleus, including alternative splicing. A 5' cap and poly-A tail are added to the 5' and 3' ends of the RNA strand, respectively. The 5' cap protects the transcript from being broken down. The poly-A tail is a sequence of 100-200 adenine (A) nucleotides, which stabilizes the transcript and assists in export from the nucleus into the cytosol.

The RNA is then transported into the cytoplasm. At the ribosome, each triplet of nucleotides is turned into an amino acid. The resulting amino acid sequence is a polypeptide, which forms a part of (or the whole of) a protein.