Explain how the sequence of DNA bases directs protein synthesis through transcription and translation (North Carolina Standard Course of Study, Biology, LS.Bio.6).

What this topic is asking

North Carolina LS.Bio.6 asks how the sequence of DNA bases directs protein synthesis. For the Biology EOC you need the two steps, transcription (DNA to mRNA, in the nucleus) and translation (mRNA to protein, at the ribosome), the idea of a codon (three bases coding for one amino acid), the role of tRNA, and the gene-to-protein-to-trait chain. Items often test the codon-to-amino-acid arithmetic or the difference between the two steps.

Step one: transcription (DNA to mRNA)

In transcription, the DNA of one gene unwinds, and one strand is used as a template to build a complementary mRNA strand. RNA pairs with DNA the same way as DNA, with one change: RNA uses uracil (U) instead of thymine (T). So a DNA template reading T A C pairs with the mRNA A U G. The mRNA is a portable copy of the gene that can leave the nucleus and travel to a ribosome, which is why the cell makes mRNA rather than sending the precious DNA itself.

Step two: translation (mRNA to protein)

The codon arithmetic is a common EOC item. If an mRNA strand has $n$ bases, the number of amino acids it codes for is $\frac{n}{3}$, because each amino acid needs a three-base codon. For example, an mRNA 9 bases long codes for $\frac{9}{3} = 3$ amino acids.

The gene-to-protein-to-trait pathway

The reason protein synthesis matters is that proteins build and run the body, so the gene determines the trait through the protein it codes for:

• The order of DNA bases in a gene sets the order of codons in the mRNA.
• The order of codons sets the order of amino acids in the protein.
• The order of amino acids determines the protein's folded shape, and the shape determines its function.
• The protein's function shows up as a trait (for example, an enzyme that produces a pigment gives a color).

This chain explains why a change in the DNA (a mutation) can change a protein and so change a trait, which links this topic to mutations.

Try this

Q1. State where transcription and translation each occur. [2]

• Cue. Transcription in the nucleus; translation at the ribosome.

Q2. An mRNA molecule has 15 bases. State how many amino acids it codes for and why. [2]

• Cue. $15 \div 3 = 5$ amino acids, because each codon is three bases and codes for one amino acid.