But that's an inconvenient way to talk about a protein-coding DNA sequence: everything's not only complementary but also backwards.įor the sake of ease and clarity, scientists tend to ignore the bottom strand (they call it the "non-coding" or "antisense" strand). It would be more accurate to say that the DNA sequence of the "start codon" on the bottom strand is CAT. That means we'd have to write the sequence of the bottom strand like this: The scientific standard is to write a nucleotide sequence from 5' to 3'. That is, the 5' (5-prime) and 3' (3-prime) ends of the two DNA strands face in opposite directions: The chemical structure of DNA gives it a polarity, and the two complementary DNA strands are anti-parallel. And if we're being literal about the actual nucleotides in the DNA strand that are read to build the mRNA's AUG start codon, we might consider the start codon on a DNA molecule to be TAC.īut that's not quite right. While our shorthand version shows just the top strand, it's actually the bottom strand that RNA polymerase reads to build an mRNA molecule.
If we wanted to, we could include the sequences of both strands: It's a shortcut, and it's tidier to look at, and it's how DNA sequences are typically written. We've shown the sequence of just one of the DNA strands. Here's a DNA sequence, with the start codon in red: The key thing to remember is that DNA is double stranded. If AUG on an mRNA molecule means "start,"Īnd the DNA template is complementary to the mRNA copy, Scientists generally consider AUG to be a start codon in mRNA sequence and ATG to be a start codon in a DNA sequence. It's not a mistake when we say that ATG is a start codon.
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