Protein synthesis for GCSE

Protein synthesis is how a cell uses the instructions in a gene to make a protein. For GCSE: in the nucleus, a matching molecule called mRNA is built along one strand of the gene (transcription). The mRNA goes to a ribosome in the cytoplasm, where carrier molecules bring amino acids in the order set by each group of three bases, and the ribosome joins them into a chain (translation). The chain then folds into the unique shape the protein needs to do its job.

Why cells make proteins

Proteins do most of the work in a cell. Enzymes, many hormones, antibodies and structural proteins such as collagen are all proteins. Each protein is a chain of amino acids in a particular order, and the order is set by a gene: a section of DNA. AQA GCSE Biology puts it this way: the order of bases controls the order in which amino acids are assembled to produce a particular protein.

The code works in threes. A sequence of three bases, called a triplet, codes for one amino acid. So a gene with 450 bases in its coding sequence codes for 450 ÷ 3 = 150 amino acids.

Protein synthesis steps

Transcription: making a copy

1. DNA is far too large to pass out through the nuclear membrane, so the cell works from a copy of the gene.
2. The two strands of DNA unzip at the gene.
3. One strand is used as a template. Free bases pair up with it (A with U, T with A, C with G, G with C) to build a single-stranded molecule called messenger RNA (mRNA). RNA uses U (uracil) in place of T.
4. The mRNA leaves the nucleus through a pore in the nuclear membrane and travels to a ribosome.

Translation: building the protein

1. The ribosome reads the mRNA three bases at a time.
2. Carrier molecules (called transfer RNA, or tRNA) each bring a specific amino acid that matches one triplet.
3. The ribosome joins the amino acids together in the order the triplets give, making a growing chain.
4. When the chain is complete, it is released and folds up to form a unique shape.

That shape is what lets the protein do its job: as an enzyme, a hormone, or part of a structure. AQA says proteins are synthesised on ribosomes according to a template, and that you do not need to know the structure of mRNA or tRNA, so the names and steps above are enough.

Worked example: from DNA to amino acids

A short section of the template strand of DNA reads TAC GGA TTT. Find the mRNA and the amino acids.

Each mRNA triplet is complementary to the DNA, and a codon chart gives the amino acid. You can try your own sequences in the DNA Translator, or translate one in this question from the Genetics and DNA course.

How mutations change proteins

Mutations are changes to the order of bases in DNA. AQA GCSE Biology says most mutations do not change the protein, or only change it slightly. A few code for an altered protein with a different shape:

• One changed base changes one triplet, which may code for a different amino acid.
• A different amino acid can change how the chain folds, so the protein has a different shape.
• An enzyme may then no longer fit its substrate, or a structural protein may lose its strength.

Not all DNA codes for proteins. Some non-coding parts switch genes on and off, so changes there can affect whether a gene is used at all. For the structure of DNA itself, see what is DNA.

Check your understanding.

Common mistakes

• Saying DNA leaves the nucleus. It stays inside; a copy (mRNA) leaves.
• Saying each base codes for an amino acid. It is three bases, a triplet, per amino acid.
• Saying the ribosome makes amino acids. It joins amino acids that carrier molecules bring.
• Forgetting folding. The finished chain must fold into its unique shape to work.
• Putting T in mRNA. RNA has U instead of T.

Practise with a short timed paper

DNA and protein synthesis come up in GCSE Biology. The daily GCSE Biology Higher paper is a short timed paper of 8 questions, with a new one every day.

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