Transcription and translation

Transcription occurs in the nucleus of eukaryotic cells. RNA polymerase II binds a promoter region upstream of the gene, unwinds the double helix, and synthesizes a complementary RNA strand in the 5′ to 3′ direction using one DNA strand as the template. The resulting pre-mRNA is then processed before it can leave the nucleus: a 7-methylguanosine cap is added at the 5′ end, a poly-A tail is added at the 3′ end, and the spliceosome removes introns and joins the exons. Alternative splicing lets a single gene produce more than one protein by combining exons in different arrangements.

Translation occurs in the cytoplasm at the ribosome. The mRNA is read three nucleotides at a time; each triplet is a codon specifying one amino acid. Transfer RNA molecules carry amino acids and pair their anticodons to the matching codons. Initiation begins at an AUG start codon, which encodes methionine. During elongation, the ribosome moves along the mRNA, forming peptide bonds between successive amino acids, and termination occurs when a stop codon — UAA, UAG, or UGA — enters the ribosome and a release factor frees the finished polypeptide.

The genetic code has properties that explain much of mutation biology. It is degenerate, meaning most amino acids are specified by more than one codon, so some base substitutions are silent and change nothing. It is nearly universal across organisms, which is why bacteria can be engineered to produce human proteins. And it is read in non-overlapping triplets, which is why an insertion or deletion that is not a multiple of three causes a frameshift and typically destroys the protein downstream of the mutation.

The MCAT tests transcription and translation under the transmission of genetic information from gene to protein. Expect questions on the direction of synthesis, the enzymes and processing steps involved, codon-anticodon pairing, and how specific mutation types — silent, missense, nonsense, and frameshift — alter the resulting protein.