The Protein Synthesis Studio

The Protein Synthesis Studio

Your DNA is a set of instructions, but instructions for what? For building proteins: the enzymes, hormones and structures that run your body. This module (Higher tier) follows the code from gene to finished protein through two stages, transcription and translation, and then asks the question the whole topic exists for: what happens when one base changes.

The words for it

Five terms, each of which will be put to work rather than recited:

Why send a copy?

The gene stays in the nucleus and a copy of it, mRNA, is sent out instead. Why does the cell work this way rather than sending the DNA itself?

• The ribosomes are out in the cytoplasm, but DNA is far too large to leave the nucleus and is the cell's only master copy, so a short, disposable copy of the one gene needed is sent instead
• mRNA travels faster through the cytoplasm than DNA would
• Ribosomes can only read RNA because they are themselves made of DNA
• mRNA is more stable than DNA, so it survives the journey better

The two stages

Protein synthesis is one process in two halves, and almost every exam question turns on keeping them apart:

Transcribe it yourself

RNA pairs with DNA in the usual way, with one exception: RNA contains no thymine, so wherever the DNA template carries adenine the mRNA carries _____ instead. Working along a DNA template strand that reads TACGGA, the mRNA transcribed from it reads _____. The enzyme that builds it is RNA _____, and the finished strand leaves the nucleus through a nuclear _____.

Label translation

This diagram shows translation at a ribosome. Drag each label onto the correct part.

Sequence, shape, function

The polypeptide does not stay a straight chain. It folds into a specific three-dimensional shape, and which shape it folds into is decided by the order of its amino acids. That shape is the whole point. An enzyme works because its active site is exactly the shape of its substrate; a hormone works because it fits its receptor; a structural fibre is strong because of how its chains lie against each other. So the chain runs: base order → amino-acid order → folding → shape → function. Break the chain anywhere near the start and everything downstream of the break is at risk.

What if it were missing?

The whole pathway

Put the stages of protein synthesis in order, from the gene to the finished protein.

• The DNA unwinds at the gene
• RNA polymerase builds an mRNA strand complementary to the DNA
• The mRNA leaves the nucleus and moves to a ribosome
• The ribosome reads the mRNA in three-base codons
• tRNA brings the amino acid matching each codon
• The amino acids join into a polypeptide, which folds into the protein

One amino acid out

An enzyme is made with a single amino acid different from usual, at a position deep inside the folded molecule. It no longer breaks down its substrate at all. What is the best explanation?

• The different amino acid changed how the chain folds, and that altered the shape of the active site, so the substrate no longer fits it
• The enzyme is now one amino acid shorter, so it is too small to work
• The substrate has changed shape to match the new enzyme
• The enzyme was used up in the reaction, so it cannot work again

One base changes

A single base in a gene is altered. Follow the consequences down the chain, level by level.

• The base is changed in the DNA. What is the first thing downstream that changes?
• That codon now calls for a different amino acid. What happens to the polypeptide?
• The protein is an enzyme, and it still works perfectly. How can that be, given that its sequence has changed?
• In a second gene, one base is changed and the enzyme now does nothing whatever. What is the most likely difference between the two cases?

One base, one letter, one disease

Sickle cell anaemia is the textbook case, and its scale is worth pausing on. In the gene for one of haemoglobin's chains, a single base is different. That changes one codon, which calls for one different amino acid, at position 6 of a chain that is 146 amino acids long. That one substitution changes how the haemoglobin molecules sit against each other. At low oxygen they stick together into long fibres, which pull the red blood cell out of its disc shape into a stiff crescent. Those cells carry oxygen poorly and jam in narrow capillaries, which is what causes the pain and the damage.

Which are true?

Select ALL THREE statements that are TRUE.

• A changed amino acid far from an enzyme's active site can still stop it working, because folding depends on the whole chain
• mRNA is short-lived by design: a permanent messenger would keep a protein being made long after it was needed
• A gene 300 bases long codes for roughly 100 amino acids, because each amino acid needs a codon of three
• Translation happens inside the nucleus
• mRNA is an identical copy of the DNA template strand
• One base codes for one amino acid

Explain the route

Exam practice. In about 50 words, explain how the order of bases in a gene ends up determining what a protein does. Include:

• what the base order sets directly, and which stage does that
• what happens to the finished chain of amino acids
• why the shape it takes decides the protein's function

Studio summary

In transcription, the enzyme RNA _____ builds an mRNA strand complementary to the DNA. The mRNA moves to a _____, where it is read in _____ of three bases. tRNA brings the matching amino acids, which are joined into a polypeptide. That chain then _____ into a specific shape, and the shape is what decides the protein's function.