DNA to protein
Transcription and translation are a sequence of conversions, and every question is about doing one of them in the right direction. This page works through DNA → mRNA → anticodon → amino acid with a full example, shows how to read the codon table the exam gives you, and sorts out what each kind of mutation does.
- 1. The two stages
- 2. Base pairing, in every direction
- 3. One strand, all the way through
- 4. Reading the codon table
- 5. Mutations
- 6. The biotechnology questions
The two stages
The idea: transcription copies one gene into mRNA inside the nucleus. Translation reads that mRNA at a ribosome and builds the polypeptide.
| Transcription | Translation | |
|---|---|---|
| Where | Nucleus | Ribosome, in the cytoplasm or on rough ER |
| Input | The template strand of DNA | mRNA, read in codons of three |
| Output | mRNA | A chain of amino acids |
| Main enzyme or machinery | RNA polymerase | Ribosome plus tRNA |
| Starts and stops at | A promoter and a terminator | The start codon AUG; a stop codon UAA, UAG or UGA |
DNA never leaves the nucleus — that is the reason mRNA exists. Replication is a third process that copies the whole molecule before a cell divides; it is not part of making a protein.
Base pairing, in every direction
The idea: A pairs with T in DNA and with U in RNA; G always pairs with C. Every conversion in this unit is that one rule, applied carefully.
| Going from | To | Rule |
|---|---|---|
| DNA template strand | mRNA | Complementary, with U wherever the template has A |
| DNA coding strand | mRNA | Identical, with U replacing every T |
| mRNA codon | tRNA anticodon | Complementary, both RNA, so A pairs with U |
| mRNA codon | Amino acid | Look it up in the codon table |
Watch out: the template (antisense) strand is the one transcribed. The coding (sense) strand is its partner and reads the same as the mRNA. Transcribing the wrong strand gives an answer that is complementary to the right one — a costly slip, and an easy one to check for.
Chargaff's rule falls out of the same pairing: in double-stranded DNA, %A = %T and %G = %C. If a sample is 30% adenine, it is 30% thymine, leaving 40% for guanine and cytosine together — 20% guanine.
One strand, all the way through
The idea: do it in a table, one row per conversion, and never skip a row.
| Stage | Sequence |
|---|---|
| DNA template strand | TAC · GGA · TTT · ACT |
| DNA coding strand | ATG · CCT · AAA · TGA |
| mRNA | AUG · CCU · AAA · UGA |
| tRNA anticodons | UAC · GGA · UUU · ACU |
| Amino acids | methionine · proline · lysine · stop |
- The polypeptide is three amino acids long, not four — a stop codon ends translation and codes for nothing.
- TAC on the template is always the start, because it transcribes to AUG.
- The anticodon looks like the template with U for T, which is a useful check that you have not drifted.
Reading the codon table
The idea: the table is printed on your data pages, so these marks come from reading it in the right order — not from memory.
- Work with mRNA, never DNA. Convert first if you were given a DNA strand.
- Split into threes from the start codon. A codon is three bases, and the grouping is what the reading frame means.
- First base down the left column, second base across the top, third base down the right column. Every codon in the table is read in that order.
- Check for stop codons — UAA, UAG, UGA — before you keep going.
Why the code is called redundant
Several codons can specify the same amino acid: CCU, CCC, CCA and CCG are all proline. That redundancy is why some base substitutions change nothing at all, which is what a silent mutation is.
Mutations
The idea: substitutions affect one codon. Insertions and deletions shift the reading frame and wreck everything after them.
| Mutation | What changes | Effect on the protein |
|---|---|---|
| Substitution — silent | One base, but the codon still codes for the same amino acid | None |
| Substitution — missense | One base, one different amino acid | Usually small; sometimes severe, as in sickle cell disease |
| Substitution — nonsense | A codon becomes a stop codon | The protein is cut short and usually does not work |
| Insertion or deletion | The reading frame shifts from that point on | Every downstream amino acid is wrong — usually a non-functional protein |
A frameshift late in a gene does less damage than one at the start, because fewer codons are read incorrectly. And a mutation only matters to the next generation if it is in a gamete — a mutation in a body cell affects that person, not their children.
The biotechnology questions
The idea: four techniques cover nearly every applied question in this unit.
| Technique | How it works | Used for |
|---|---|---|
| Restriction enzymes | Cut DNA at specific sequences, leaving matching sticky ends | Cutting a gene out and opening a plasmid so they can be joined by ligase |
| Recombinant plasmids | A human gene is inserted into a bacterial plasmid; the bacteria transcribe and translate it | Producing human insulin — it works because the genetic code is essentially universal |
| PCR | Repeated heating and cooling with polymerase doubles the DNA each cycle | Amplifying a tiny sample before it can be analyzed |
| Gel electrophoresis | DNA is negatively charged, so it moves toward the positive electrode; small fragments travel farther | Comparing fragment patterns — DNA fingerprinting, paternity, forensics |