Edexcel IGCSE Biology · Spec 3.16B-3.18B
Protein Synthesis (Triple)
How genes code for proteins through transcription and translation (protein synthesis).
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Protein Synthesis (Triple)
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Explained
From gene to protein, in two stages
A gene is a section of DNA that codes for a protein. The problem the cell has to solve is that DNA stays in the nucleus while proteins are assembled at ribosomes in the cytoplasm. The two stages of protein synthesis exist to bridge that gap.
DNA and RNA
DNA is a double helix of two strands, held together by base pairs. Adenine pairs with thymine and cytosine pairs with guanine, always.
RNA differs in four ways. It is single stranded rather than double. It uses uracil in place of thymine. It is not a helix. And its sugar is ribose rather than deoxyribose.
A sequence of three bases codes for one amino acid, and the order of amino acids is what makes one protein different from another.
Transcription
This happens in the nucleus. The two DNA strands separate over the length of the gene, and a complementary copy of one strand is built as messenger RNA.
Complementary means each base is matched by its pair, with one change: where the DNA has adenine, the mRNA has uracil rather than thymine. The finished mRNA then leaves the nucleus through a pore and travels to a ribosome.
The reason for making a copy is that DNA is too large to leave the nucleus and too valuable to risk. The mRNA is a disposable working copy of one gene.
Translation
This happens at the ribosome in the cytoplasm. The ribosome moves along the mRNA reading three bases at a time.
For each set of three, a transfer RNA molecule brings the matching amino acid, and the ribosome joins that amino acid to the growing chain. When the chain is complete it folds into a specific shape, and that shape is what allows the protein to do its job, which is why an enzyme's active site fits only one substrate.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for differences between DNA and RNA and lists four: DNA is double stranded while RNA is single stranded, DNA has thymine while RNA has uracil, DNA is a helix while RNA is not, and DNA has deoxyribose while RNA has ribose.
Its guidance column contains a detail worth having. Writing that DNA is a double helix is accepted as two of those marking points at once, because the phrase states both the double strand and the helix. One well chosen word can carry two marks.
Notice also that every point is written as a comparison, with both halves given. A difference question needs both sides. Saying that RNA is single stranded, without saying that DNA is double stranded, is only half a difference.
On the following part, about a drug that blocks protein production, the mark scheme credits a chain of reasoning: the complementary RNA binds to the messenger RNA, so translation cannot occur, so the enzyme is not made. It then allows separate marks for the background steps, that transcription makes the mRNA and that the RNA leaves the nucleus.
Keeping the two stages apart
Transcription happens in the nucleus and produces mRNA. Translation happens at a ribosome and produces a protein. The names help: transcription is copying something into the same language of bases, and translation is converting it into a different language of amino acids.
If a question asks where a stage takes place, the location is usually a mark on its own, and it is the part most often left out.
Spec 3.16B-3.18B
What you need to know
- Describe the structure of DNA and RNA
- Describe transcription, copying DNA into messenger RNA
- Describe translation, building a protein from that message
Active recall
Quick check
Answer each question before opening the answer.
Name the two stages of protein synthesis.
Transcription (DNA copied into mRNA in the nucleus) and translation (mRNA read at a ribosome to build the protein).
What decides which protein is made?
The sequence of bases (codons) in the gene, which sets the order of amino acids.
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