Protein Synthesis — Print · Grade 10 Science

Protein Synthesis

Learning goals

• I can describe transcription (DNA to mRNA) and translation (mRNA to protein).
• I can explain how the genetic code is read in codons and how mutations can change a protein.

Readings for this unit

• From an allele symbol to a measured characteristic

DNA holds the master instructions for life — but instructions are useless until something acts on them. That something is proteins: the molecules that build your tissues, carry oxygen, digest food, fight infection, and speed up nearly every chemical reaction in your body. Genetics summarizes one important information pathway for protein-coding genes with a tidy pathway sometimes called the central dogma: DNA to RNA to protein. In this pathway, information in a protein-coding gene is first copied into a portable messenger, and that messenger is then read to assemble a protein.

The first step, transcription, happens inside the nucleus. A section of DNA — one gene — unzips, and one strand serves as a template while the cell builds a matching strand of mRNA (messenger RNA). The pairing rules are almost the same as in DNA, with one twist: RNA uses the base uracil (U) wherever DNA would use thymine, so a DNA template written 3′-A-T-G-5′ is copied into mRNA written 5′-U-A-C-3′. This mRNA is a disposable working copy of the gene, and unlike the precious original DNA, it can safely leave the nucleus and travel out to where proteins are made.

Out in the cytoplasm, the mRNA docks with a ribosome, the molecular machine that carries out translation. The ribosome reads the mRNA three bases at a time, and each three-base 'word' is called a codon. Most sense codons specify an amino acid — one link in the chain — while stop codons signal termination rather than adding an amino acid, and as the ribosome moves down the message, it links the amino acids together in exactly the order the codons dictate. One special codon, AUG, acts as the 'start' signal, and three others act as 'stop' signals that tell the ribosome the protein is finished.

The rulebook that matches each codon to its amino acid is the genetic code, and two features of it are worth remembering. First, it is nearly universal — a bacterium, a banana, and a person all read the same codons the same way, which is powerful evidence that all life shares a common ancestor, and also why a human gene can be placed into bacteria to mass-produce a medicine like insulin. Second, it is redundant: there are 64 possible codons but only 20 amino acids, so several different codons can call for the same amino acid.

This whole pathway is also where mutations make their mark. Because a protein's shape and job depend on its exact sequence of amino acids, a change in the DNA can ripple downstream into the protein. Some changes are silent — thanks to the code's redundancy, an altered codon may still specify the same amino acid, leaving the protein unchanged. Others swap one amino acid for another, and a single such swap is what causes sickle-cell disease. The most disruptive change of all can turn an ordinary codon into a 'stop,' cutting the protein short before it is complete. That range — from no effect at all to dramatic — is exactly why mutations can be neutral, harmful, or occasionally helpful.

Transcription uses DNA information to produce RNA, while translation uses messenger RNA information to build a polypeptide. Some genes produce functional RNAs that are not translated. The common DNA→RNA→protein model describes an important pathway without covering every biological case.

A codon is a sequence of three mRNA bases specifying an amino acid or a translation signal. Multiple codons can specify the same amino acid. Read the supplied code table for mRNA, and keep template DNA, coding DNA and mRNA conventions distinct.

A protein's final function depends on folding, processing and cellular context as well as amino-acid sequence. A sequence change can have no detectable effect, disrupt function or alter it under particular conditions. Do not assume every mutation produces a visible trait change.

Codon Length

number of mRNA bases = 3 x (number of amino acids)

each amino acid is coded by one 3-base codon (a complete gene also needs a start and a stop codon).

Worked examples

Avoid these mistakes

• A codon table typically reads mRNA codons, not a DNA template directly.

Words to know

transcription

The process of copying a gene's DNA into a strand of messenger RNA (mRNA).

mRNA (messenger RNA)

A single-stranded working copy of a gene that carries its instructions out of the nucleus to a ribosome.

codon

A group of three mRNA bases that codes for one specific amino acid (or a start/stop signal).

translation

The process of reading mRNA codons at a ribosome to build a chain of amino acids.

ribosome

The cell structure that reads mRNA and links amino acids together to build a protein.

amino acid

One of the 20 building blocks that link together in a specific order to form a protein.

stop codon

An mRNA codon such as UAA that signals termination instead of adding another amino acid.

How the code was cracked

In the early 1960s, Marshall Nirenberg and colleagues broke the genetic code with a clever trick: they built an artificial mRNA made entirely of uracil (UUU-UUU...) and found it produced a protein of pure phenylalanine. So UUU means phenylalanine. One synthetic RNA at a time, researchers mapped all 64 codons. The result is a compact dictionary: 64 codons, one start signal (AUG), three stop signals, and just 20 amino acids.

One gene, several proteins

In your cells, a freshly transcribed mRNA is edited before use: non-coding stretches called introns are snipped out and the coding pieces (exons) are joined together. Because the cell can join those pieces in different combinations — alternative splicing — a single gene can direct the building of several different proteins. It's one reason humans manage such complexity with only about 20,000 protein-coding genes, far fewer than scientists once expected.

Why some antibiotics target ribosomes

Bacterial ribosomes are built slightly differently from the ribosomes in your own cells. Several antibiotics, including tetracycline and erythromycin, exploit that difference by jamming the bacterial ribosome so it can't finish translation — halting the bacteria's protein production while leaving your cells largely untouched. Understanding translation isn't just theory; it's the basis of drugs that have saved countless lives.

The code is (almost) universal

A remarkable fact is that nearly every organism on Earth — bacteria, oak trees, jellyfish, humans — reads the same codons to mean the same amino acids. This near-universality is strong evidence that all life descends from a common ancestor. It also has a powerful practical payoff: scientists can insert the human gene for insulin into bacteria, and because the bacteria read the code the same way, they faithfully build human insulin, which is now manufactured this way for people with diabetes.

Breaking the one-way street

The central dogma usually flows one way: DNA to RNA to protein. But some viruses run it backward. Retroviruses such as HIV carry their genes as RNA and use an enzyme called reverse transcriptase to copy that RNA back into DNA, which then sneaks into the host's genome. Discovering this exception reshaped biology — and reverse transcriptase became an everyday laboratory tool for studying genes.

In the real world

mRNA vaccines, including several COVID-19 vaccines, work by delivering a piece of mRNA into your cells so that your own ribosomes translate it into a harmless viral protein — training your immune system without ever using the live virus.

Exit check

Trace information from a protein-coding gene to a protein.