Explain how a gene's base sequence is transcribed into messenger RNA and translated into a sequence of amino acids, and how this gene-to-protein pathway produces an organism's traits (MA STE HS-LS1-1, HS-LS3-1).

What this topic is asking

The Massachusetts STE framework (HS-LS1-1) asks you to explain how the structure of DNA determines the structure of proteins, which carry out the functions of life. On the High School Biology MCAS this is the gene-to-protein pathway: how the base sequence of a gene is used to build a protein, and how that protein produces a trait. Questions are usually built on a model of the pathway, a base or codon sequence to read, or a base change to interpret. The crosscutting concept is structure and function, carried all the way from DNA bases to a whole-organism trait.

Genes, proteins, and traits

Recall from the chemistry of life that proteins do most of the cell's work and that a protein's amino-acid sequence sets its shape and so its function. The gene-to-protein pathway is how the cell turns stored information (DNA) into working molecules (proteins), and the proteins produce the organism's traits: an enzyme controls a chemical reaction, a pigment protein gives color, a structural protein builds a tissue.

Step 1: transcription

In transcription, the cell makes a working copy of a gene as messenger RNA (mRNA):

• The DNA double helix unwinds at the gene.
• One strand acts as a template, and free RNA nucleotides pair with it, following base pairing (in RNA, uracil (U) replaces thymine, so A on DNA pairs with U on RNA).
• The result is a single strand of mRNA carrying the gene's message.

Transcription happens in the nucleus. The mRNA then leaves the nucleus and travels to a ribosome. RNA differs from DNA in three ways worth remembering: it is single-stranded, it uses the sugar ribose, and it uses uracil instead of thymine.

Step 2: translation

In translation, the ribosome reads the mRNA and builds the protein:

• The ribosome reads the mRNA bases in groups of three called codons.
• Each codon specifies one amino acid. Transfer RNA molecules bring the matching amino acids.
• The amino acids are joined in order into a chain (a polypeptide), which folds into the finished protein.

Because every set of three bases codes for one amino acid, the order of bases in the gene sets the order of amino acids in the protein. Translation happens at the ribosome, the organelle introduced in cell structure and function.

Why a base change can matter (or not)

Because codons set amino acids, a change to the DNA base sequence (a mutation) can change a codon, which can change an amino acid, which can change the protein's shape and function. But not every base change has an effect. The genetic code is redundant: more than one codon can code for the same amino acid, so some base changes still specify the same amino acid and leave the protein unchanged (a silent change). This sets up the work in mutations and biotechnology.

Try this

Q1. Name the two stages of protein synthesis and where each happens. [2]

• Cue. Transcription (in the nucleus, making mRNA) and translation (at the ribosome, building the protein).

Q2. Explain how a gene determines a trait. [2]

• Cue. The gene's base sequence sets the order of amino acids in a protein; the protein's function (such as an enzyme controlling a reaction) produces the trait.