Protein synthesis — common mistakes

A common mistake in exams is confusing the DNA template (antisense) strand with the non-template (sense) strand. Remember, the mRNA sequence will be identical to the non-template DNA strand, but with Uracil (U) replacing Thymine (T). Always transcribe from the template strand or, if given the non-template, directly convert it to mRNA by replacing T with U. Also, be prepared to clearly distinguish between the features of protein synthesis in prokaryotes vs. eukaryotes, particularly the presence of introns/exons and simultaneous transcription-translation in prokaryotes.

The main stages are transcription and translation. Transcription is the synthesis of a messenger RNA (mRNA) copy from a DNA gene template in the nucleus. Translation is the synthesis of a polypeptide chain from the mRNA code at a ribosome in the cytoplasm.

Messenger RNA (mRNA) carries the genetic code from DNA to the ribosome. Transfer RNA (tRNA) acts as an adaptor, bringing specific amino acids to the ribosome that match the mRNA codons. Ribosomes, composed of ribosomal RNA (rRNA) and proteins, are the sites of translation, facilitating the binding of mRNA and tRNA and catalysing the formation of peptide bonds.

The genetic code is degenerate because most amino acids are specified by more than one codon, which can reduce the harmful effect of some point mutations. It is universal because the same codons specify the same amino acids in nearly all organisms. This universality provides strong evidence for a common evolutionary origin of life and allows for genetic engineering applications where genes can be transferred between species.

In eukaryotic genes, an intron is a non-coding sequence that is transcribed into pre-mRNA but is removed during a process called splicing. An exon is a coding sequence that is retained in the mature mRNA and is expressed to form part of the final polypeptide. Prokaryotic genes do not contain introns.