However, the translation to protein is still systematic and colinear, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on.įigure 15.3 Instructions on DNA are transcribed onto messenger RNA. The translation to protein is a bit more complex because three mRNA nucleotides correspond to one amino acid in the polypeptide sequence. The copying of DNA to RNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every nucleotide read in the DNA strand. Because the information stored in DNA is so central to cellular function, it makes intuitive sense that the cell would make mRNA copies of this information for protein synthesis, while keeping the DNA itself intact and protected. The decoding of one molecule to another is performed by specific proteins and RNAs. The flow of genetic information in cells from DNA to mRNA to protein is described by the Central Dogma ( Figure 15.3), which states that genes specify the sequence of mRNAs, which in turn specify the sequence of proteins. The Central Dogma: DNA Encodes RNA RNA Encodes Protein It is the variety of amino acid side chains that gives rise to the incredible variation of protein structure and function. The side chain may be nonpolar, polar, or charged, as well as large or small. Each amino acid is composed of an amino group ( N H 3 + N H 3 +), a carboxyl group (COO -), and a side chain (blue). They know there's another strand, and they know how to figure out what its sequence is if they need to.Figure 15.2 Structures of the 20 amino acids found in proteins are shown. Instead, they refer to the sequence of the "coding" or "sense" strand: the one that's almost identical to mRNA-the difference of course being that every T in DNA is replaced by a U in RNA. 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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