What are the main stages of protein synthesis in Biology?

Protein synthesis involves two main stages: transcription and translation. During transcription, which occurs in the nucleus, DNA is used as a template to synthesize messenger RNA (mRNA). In translation, which takes place in the cytoplasm, the mRNA is read by ribosomes to assemble amino acids into a polypeptide chain, forming a protein. This process is crucial for understanding the Nucleic Acids and Protein Synthesis topic in Cambridge International A Levels Biology.

How does mRNA differ from DNA in the context of protein synthesis?

mRNA, or messenger RNA, differs from DNA in several key ways. mRNA is single-stranded, while DNA is double-stranded. mRNA contains the sugar ribose, whereas DNA contains deoxyribose. Additionally, mRNA uses uracil in place of thymine, which is found in DNA. These differences are essential for the transcription stage of protein synthesis, where mRNA is synthesized from a DNA template, a fundamental concept in Cambridge International A Levels Biology.

What role do ribosomes play in protein synthesis?

Ribosomes are the cellular structures where protein synthesis occurs. They facilitate the translation stage by reading the sequence of codons on the mRNA and assembling the corresponding amino acids into a polypeptide chain. Ribosomes ensure that the genetic code is accurately translated into proteins, which is a critical aspect of the Nucleic Acids and Protein Synthesis topic in Cambridge International A Levels Biology.

Why is tRNA important in the process of protein synthesis?

Transfer RNA (tRNA) is crucial in protein synthesis because it transports the correct amino acids to the ribosome during translation. Each tRNA molecule has an anticodon that pairs with a specific mRNA codon, ensuring that the amino acids are added in the correct sequence to form a functional protein. Understanding the role of tRNA is vital for mastering the Nucleic Acids and Protein Synthesis topic in Cambridge International A Levels Biology.

What is the significance of the genetic code in protein synthesis?

The genetic code is a set of rules that defines how the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein. It is universal, redundant, and unambiguous, meaning each codon specifies only one amino acid, but multiple codons can code for the same amino acid. This code is fundamental to the translation process in protein synthesis, a key concept in the Nucleic Acids and Protein Synthesis topic for Cambridge International A Levels Biology.

Why is "Confusing codon and anticodon" a common mistake in Protein synthesis?

Why it happens: The two words sound similar and refer to complementary 3-base sequences. How to avoid it: Codon = mRNA. Anticodon = tRNA. The codon and anticodon are COMPLEMENTARY and ANTIPARALLEL to each other. Always specify which molecule when stating a base sequence. Source: 9700 Examiner Reports 2023-2024

Why is "Forgetting to replace T with U when transcribing mRNA" a common mistake in Protein synthesis?

Why it happens: Students copy the DNA letters directly. How to avoid it: RNA contains URACIL not thymine. When a DNA template has A, the mRNA has U (not T). Drill this with practice exercises. Source: 9700 Examiner Reports 2024

Why is "Not mentioning a STOP codon when describing translation" a common mistake in Protein synthesis?

Why it happens: Students focus on the start and forget how translation ends. How to avoid it: End your translation description with: 'Translation continues until a STOP codon (UAA, UAG or UGA) is reached, at which point the polypeptide is released'. Source: 9700 Examiner Reports 2024

Why is "Saying both DNA strands are transcribed" a common mistake in Protein synthesis?

Why it happens: Confusion with replication, where both strands are templates. How to avoid it: Only ONE strand of DNA (the TEMPLATE / antisense strand) is transcribed. The other strand (sense / coding strand) is not read. Source: 9700 Examiner Reports 2023

Why is "Saying tRNA 'makes' or 'synthesises' protein" a common mistake in Protein synthesis?

Why it happens: Oversimplification. How to avoid it: tRNA DELIVERS amino acids to the ribosome. The RIBOSOME (rRNA) catalyses peptide bond formation. Use precise language: 'tRNA brings amino acids; rRNA forms the peptide bond'. Source: 9700 Examiner Reports 2024

Why is "Stating that prokaryotes splice out introns" a common mistake in Protein synthesis?

Why it happens: Assuming eukaryote-specific features are universal. How to avoid it: Intron splicing, 5' cap and poly-A tail are EUKARYOTE features. Prokaryotic mRNA is translated immediately without modification — even before transcription is complete.

Nucleic Acids and Protein SynthesisCambridge International A Levels Biology (9700)

Protein synthesis

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Protein Synthesis — Cambridge International A Level Biology 9700 Study Notes (2025-2027 syllabus)

Transcription (DNA → mRNA in the nucleus, by RNA polymerase) and translation (mRNA → polypeptide at the ribosome, by tRNAs and rRNA), the codon-anticodon relationship, the properties of the genetic code (degenerate, non-overlapping, universal) and how mutations alter the protein produced.

At a glance

Central dogma: DNA → mRNA (transcription) → polypeptide (translation).
Transcription (nucleus): RNA polymerase unwinds DNA, reads template strand, joins free RNA nucleotides (A-U, T-A, G-C); pre-mRNA processed (introns out, exons joined, 5' cap + poly-A tail in eukaryotes).
Translation (cytoplasm, on ribosome): mRNA codons read 3 bases at a time; tRNA anticodons pair with codons; rRNA catalyses peptide bond between amino acids at P-site and A-site.
START codon = AUG (methionine). STOP codons = UAA, UAG, UGA.
Genetic code is: degenerate (multiple codons per amino acid, mostly differ in 3rd base = wobble), non-overlapping (each base read once), universal (same in nearly all organisms).
Polysomes = multiple ribosomes translating the same mRNA at once → faster protein production.
Mutations: silent (3rd base / wobble), missense (different amino acid), nonsense (premature stop), frameshift (insertion / deletion).

What you’ll learn

Mapped to the Cambridge IGCSE 9700 syllabus (2025-2027).

6.2.1 — Describe the process of transcription, including the role of RNA polymerase and the template strand.
6.2.2 — Describe the process of translation, including the roles of mRNA, tRNA, rRNA and ribosomes.
6.2.3 — Explain the meaning of codon, anticodon and the relationship between them.
6.2.4 — Explain that the genetic code is degenerate, non-overlapping and universal.
6.2.5 — Explain how a gene mutation (substitution, insertion, deletion) may affect the polypeptide produced and the function of the resulting protein.

Overview: from gene to protein

DNA holds the code; mRNA carries it; ribosomes read it; polypeptides are built one amino acid at a time.

Every protein in your body is encoded by a gene in your DNA. The information flow is summarised by the central dogma of molecular biology:

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{polypeptide}$

The two stages take place in different parts of the eukaryotic cell:

Transcription occurs in the nucleus. A copy of one gene is made as a single-stranded mRNA molecule using one DNA strand as a template.
Translation occurs in the cytoplasm at a ribosome (which may be free in the cytoplasm or attached to rough endoplasmic reticulum for secreted proteins). The mRNA is read three bases at a time (codon by codon), and the corresponding amino acids are joined by peptide bonds to form a polypeptide.

In prokaryotes there is no nucleus, so transcription and translation are coupled — ribosomes can begin translating an mRNA before transcription is complete. Prokaryotes also lack introns, so they do not splice mRNA.

DNA in nucleus → mRNA (transcription) → ribosome (translation) → polypeptide.
Eukaryotes: nucleus separates the two stages; mRNA must exit via nuclear pore.
Prokaryotes: no nucleus → transcription and translation are coupled.

Transcription — making mRNA from DNA

RNA polymerase unwinds DNA and copies the template strand into mRNA; introns are spliced out in eukaryotes.

Transcription is the synthesis of an mRNA molecule using one strand of DNA as a template. It occurs in five conceptual steps:

Binding of RNA polymerase. The enzyme RNA polymerase binds to a promoter region of DNA just upstream of the gene to be transcribed.
Unwinding. RNA polymerase unwinds the DNA double helix locally, breaking the hydrogen bonds between complementary bases and exposing the two strands.
Template recognition. Only one strand is used as a template — the template strand (also called the antisense strand). The other strand (sense / coding strand) has the same sequence as the mRNA (with T instead of U) and is not read.
RNA nucleotide alignment. Free RNA nucleotides (containing A, U, C, G) pair by complementary base pairing with the exposed template bases. Note that A in DNA pairs with U in RNA (uracil replaces thymine in RNA).
Polymerisation. RNA polymerase joins adjacent RNA nucleotides by forming phosphodiester bonds in the 5'→3' direction, building up a single-stranded pre-mRNA molecule.

In eukaryotes, the pre-mRNA undergoes post-transcriptional modification:

Splicing: non-coding introns are removed by a spliceosome; the coding exons are joined together to form the mature mRNA.
5' cap (a modified guanine) is added — protects mRNA from degradation and helps ribosome binding.
3' poly-A tail (a long string of A nucleotides) is added — also protects from degradation and helps export from the nucleus.

The mature mRNA then leaves the nucleus through a nuclear pore and enters the cytoplasm, where it can be translated.

RNA polymerase binds promoter and unwinds DNA.
Only the template (antisense) strand is read.
Free RNA nucleotides pair complementarily (A-U, T-A, G-C, C-G).
RNA polymerase joins them by phosphodiester bonds (5'→3').
Eukaryotes: introns spliced out + 5' cap + poly-A tail added before mRNA exits the nucleus.

See the full worked example for protein synthesis →

Translation — making polypeptides from mRNA

Ribosome reads mRNA codon-by-codon; tRNAs deliver amino acids; rRNA catalyses peptide bonds.

Translation is the synthesis of a polypeptide from an mRNA template. It happens on a ribosome in the cytoplasm and involves three phases: initiation, elongation and termination.

1. Initiation.

The small ribosomal subunit binds to the 5' end of the mRNA.
It scans until it finds the START codon AUG (which codes for the amino acid methionine).
A tRNA carrying methionine and bearing the anticodon UAC binds AUG in the ribosome's P-site.
The large ribosomal subunit then joins, creating a complete ribosome with a P-site (where the growing chain is held) and an A-site (where the next aminoacyl-tRNA arrives).

2. Elongation.

The codon at the A-site is read.
A tRNA whose anticodon is complementary to that codon binds, bringing its specific amino acid.
A peptide bond is formed between the amino acid on the P-site tRNA and the amino acid on the A-site tRNA — this is a condensation reaction catalysed by the ribosome's rRNA (so the ribosome is a ribozyme).
The ribosome then translocates one codon along the mRNA: the empty tRNA leaves from the E-site, the dipeptide-carrying tRNA moves from A-site to P-site, and a new codon enters the A-site.
The cycle repeats. The polypeptide grows one amino acid at a time.

3. Termination.

When the A-site reaches a STOP codon (UAA, UAG or UGA), no tRNA has a complementary anticodon.
Instead, a release factor protein binds, and the completed polypeptide is released from the final tRNA.
The ribosome dissociates into its subunits, ready to translate another mRNA.

Many mRNAs are translated by several ribosomes simultaneously (a polysome), allowing rapid production of many copies of the same polypeptide.

Aminoacyl-tRNA synthetases. Each tRNA is 'charged' with its correct amino acid by a specific enzyme that uses ATP. This step is what makes the genetic code work — the matching of codon to amino acid happens via the matching of tRNA to amino acid before translation, and codon to anticodon during translation.

Ribosome = rRNA + protein; has P-site and A-site for tRNAs.
START codon AUG (methionine); STOP codons UAA, UAG, UGA.
tRNA delivers amino acid; anticodon pairs with codon.
Peptide bond formed at ribosome (catalysed by rRNA).
Ribosome translocates codon-by-codon along mRNA.
Polysomes = several ribosomes translating one mRNA at once.

See the full worked example for protein synthesis →

Properties of the genetic code

Triplet, non-overlapping, degenerate, universal. Wobble in the 3rd base reduces mutation impact.

The genetic code is the set of rules that relates the base sequence of mRNA to the amino acid sequence of a polypeptide. It has four key properties:

Triplet code. Each amino acid is coded for by a sequence of three mRNA bases (a codon). With 4 bases and 3 positions there are $4^3 = 64$ possible codons.
Non-overlapping. Each base is read only once, in fixed triplets. The reading frame is set by the START codon AUG. (Codons do not overlap: e.g. the mRNA AUGCAU is read as AUG-CAU, not AUG-UGC-GCA-...)
Degenerate. 64 codons code for only 20 amino acids (plus 3 stop codons), so most amino acids are coded for by more than one codon. Codons for the same amino acid usually differ only in the third base (wobble position). This means many mutations in the third base are silent — they produce a synonymous codon with no change in amino acid.
Universal. The same codons code for the same amino acids in nearly all organisms — from bacteria to humans. This is strong evidence for the common ancestry of all life, and it is the basis of genetic engineering: a human gene can be expressed in bacteria because the bacterial ribosome reads it the same way.

There are some minor exceptions to universality (e.g. in mitochondrial DNA and a few protists), but these are rare and beyond the scope of 9700.

START and STOP codons.

AUG is the start codon (codes for methionine; sets the reading frame).
UAA, UAG, UGA are stop codons (no tRNA recognises them; release factor binds and ends translation).

Triplet (3 bases per codon).
Non-overlapping (each base read once).
Degenerate (many codons per amino acid; wobble in 3rd base).
Universal (same in nearly all organisms — basis of genetic engineering).
AUG = start; UAA / UAG / UGA = stop.

See the full worked example for protein synthesis →

Codon vs anticodon — how the message is decoded

Codon = mRNA, anticodon = tRNA. They pair by complementary base pairing during translation.

Decoding the genetic message depends on the relationship between two three-base sequences:

Codon — a 3-base sequence on mRNA that specifies one amino acid (or STOP).
Anticodon — a 3-base sequence on a tRNA that is complementary and antiparallel to the codon it binds.

When a tRNA enters the ribosome's A-site, its anticodon pairs with the mRNA codon by hydrogen bonds (complementary base pairing — A-U, U-A, G-C, C-G). The tRNA carries the specific amino acid that corresponds to that codon, ensuring the correct amino acid is added next in the growing polypeptide.

Worked example. The mRNA codon 5'-GCA-3' codes for the amino acid alanine. The tRNA that recognises this codon has the anticodon 3'-CGU-5' (or written 5'→3': 5'-UGC-3') and carries alanine.

The wobble position is the third base of the codon (and the first base of the anticodon, written 5'→3'). Wobble pairing is slightly less strict than standard Watson-Crick pairing — a single tRNA can sometimes recognise two or three related codons, reducing the total number of tRNAs the cell needs to make.

Reading a codon-amino-acid table. In the exam, you are often given a genetic code table and asked to translate a short mRNA sequence. The steps are:

Start at the START codon (AUG).
Read the next codon (3 bases).
Look it up in the table to find the amino acid.
Repeat until a STOP codon is reached.
Write out the amino acid sequence in order.

Codon = mRNA; anticodon = tRNA.
Codon-anticodon pair by complementary base pairing (antiparallel).
Each tRNA carries a specific amino acid matched to its anticodon.
Wobble position (3rd base of codon) tolerates some pairing flexibility.

See the full worked example for protein synthesis →

Effect of mutations on the polypeptide

Silent (no change), missense (different amino acid), nonsense (early stop), frameshift (everything downstream changed).

A gene mutation is a change in the base sequence of DNA. Because the base sequence determines the codon sequence and therefore the amino acid sequence, mutations can change the polypeptide produced:

Substitution — one base is replaced by another. Three possible outcomes:

Silent: new codon codes for the same amino acid (because the genetic code is degenerate). No change in protein. Most likely when the third base of a codon changes.
Missense: new codon codes for a different amino acid. Primary structure changes by one amino acid, which may or may not alter the tertiary structure / function.
Nonsense: new codon is a STOP codon (UAA, UAG, UGA). Translation terminates early and the polypeptide is shortened (truncated), almost always non-functional.

Insertion — an extra base is inserted. Frameshift: every codon downstream of the insertion is shifted by one base, so the amino acid sequence from that point onwards is completely different. The protein is almost always non-functional.

Deletion — a base is removed. Same effect as insertion: frameshift.

Why missense mutations vary in effect. Whether a missense mutation matters depends on:

Where the new amino acid is. If it is in the active site of an enzyme or a critical binding pocket, function is usually lost. If it is on the surface in a non-functional region, it may have little or no effect.
How similar the new amino acid is to the old one. Replacing one hydrophobic amino acid with another (e.g. valine → leucine) is often tolerated. Replacing a small polar amino acid with a large charged one is usually disruptive.

Sickle-cell anaemia is the textbook example: a single base substitution in the β-globin gene (CTC → CAC on the template, giving mRNA codon GAG → GUG) changes one amino acid (glutamic acid → valine) at position 6 of β-globin. The new hydrophobic valine causes haemoglobin molecules to stick together in low-oxygen conditions, distorting red blood cells into a sickle shape.

Substitution → silent / missense / nonsense.
Insertion / deletion → frameshift → downstream codons changed.
Effect depends on WHERE the mutation is and HOW different the new amino acid is.
Sickle-cell: missense (Glu → Val) at position 6 of β-globin.

See the full worked example for protein synthesis →

How it’s examined

Cambridge 9700 examines this on Paper 2 (AS) and Paper 4 (A2). Most-tested questions: (a) describe transcription (5-6 marks); (b) describe translation (6-8 marks) — emphasising tRNA, codon-anticodon, rRNA peptide bond and STOP codon; (c) explain the degenerate / universal / non-overlapping code (3-5 marks); (d) given an mRNA sequence, deduce DNA template and tRNA anticodons (3-4 marks); (e) explain effect of substitution / insertion / deletion mutations on protein (4-6 marks). Examiner reports flag confusion between codon and anticodon as the #1 error.

Sources: Cambridge International AS & A Level Biology 9700 syllabus (2025-2027); 9700 Examiner Reports 2022-2024; 9700/22 May/Jun 2024 question paper and mark scheme; 9700/42 May/Jun 2024 question paper and mark scheme; 9700/12 Oct/Nov 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Protein synthesis

Step-by-step solutions to past-paper-style questions on protein synthesis, written exactly the way a tutor would explain them at the board.

1Stepwise description of transcription (5 marks)
Extended• Adapted from 9700/22 May/Jun 2024• transcription, mRNA, Paper 2
Question
Describe the process of transcription. (5 marks)
Step-by-step solution
1. Step 1
  Location and start. Transcription occurs in the nucleus (of eukaryotes). The enzyme RNA polymerase binds to a promoter region just before the gene and causes the DNA double helix to unwind.
2. Step 2
  Template strand. Only one strand of DNA is transcribed — the template strand (also called the antisense strand). The other strand (sense / coding strand) is not used.
3. Step 3
  Free RNA nucleotides pair. Free RNA nucleotides line up against the exposed template by complementary base pairing: A in DNA pairs with U in RNA, T in DNA pairs with A in RNA, G with C, C with G.
4. Step 4
  Polymerisation. RNA polymerase joins adjacent RNA nucleotides by forming phosphodiester bonds, producing a single-stranded pre-mRNA molecule in the 5'→3' direction.
5. Step 5
  Post-transcriptional modification and export. In eukaryotes, introns are spliced out and exons joined; a 5' cap and 3' poly-A tail are added. The mature mRNA leaves the nucleus through a nuclear pore to be translated.
Answer
RNA polymerase unwinds DNA; one (template) strand transcribed; free RNA nucleotides pair by complementary base pairing (A-U, T-A, G-C); RNA polymerase joins them by phosphodiester bonds 5'→3'; mRNA processed (introns removed) and exported via nuclear pore.
Examiner tip
Mark scheme: (1) RNA polymerase / unwinds DNA; (2) one strand only / template; (3) complementary base pairing with U instead of T; (4) phosphodiester bonds / RNA polymerase; (5) mRNA leaves nucleus via nuclear pore. Stating that A pairs with U (not T) on the new mRNA is the most-missed mark.
2Stepwise description of translation (6 marks)
Extended• translation, ribosome, tRNA, Paper 2
Question
Describe the process of translation. (6 marks)
Step-by-step solution
1. Step 1
  Location and start. Translation takes place in the cytoplasm on a ribosome (a complex of rRNA and protein). The mRNA binds to the small ribosomal subunit; the large subunit joins to form the complete ribosome. The first codon read is the START codon AUG (which codes for methionine).
2. Step 2
  tRNA brings amino acids. Each tRNA has a specific anticodon (3 bases) and carries a specific amino acid attached to its 3' end. A tRNA whose anticodon is complementary to the codon at the ribosome's A site binds by H-bonds.
3. Step 3
  Peptide bond formation. Two tRNAs occupy the ribosome at once (P-site and A-site). The amino acid on the P-site tRNA is transferred to the amino acid on the A-site tRNA, forming a peptide bond by condensation. The reaction is catalysed by rRNA (the ribosome is a ribozyme).
4. Step 4
  Translocation. The ribosome moves one codon along the mRNA (towards the 3' end). The empty tRNA in the P-site moves to the E-site and leaves; the tRNA carrying the growing polypeptide moves from A-site to P-site; a new codon is now exposed at the A-site.
5. Step 5
  Repeat. The cycle (codon recognition → peptide bond → translocation) repeats codon by codon. The polypeptide grows one amino acid at a time.
6. Step 6
  Termination. Translation stops when a STOP codon (UAA, UAG or UGA) is reached. A release factor binds and the completed polypeptide is released. The ribosome separates into its two subunits and is reused.
Answer
mRNA binds ribosome; START codon AUG; tRNA anticodon pairs with codon at A-site; peptide bond forms (catalysed by rRNA); ribosome translocates; cycle repeats until STOP codon; polypeptide released.
Examiner tip
Mark scheme: (1) mRNA + ribosome / AUG start; (2) tRNA with anticodon brings amino acid; (3) codon-anticodon complementary pairing; (4) peptide bond by condensation; (5) ribosome moves codon-by-codon; (6) STOP codon (UAA/UAG/UGA) ends translation. Examiner reports flag candidates who never mention the STOP codon (loses 1 mark) or who confuse codon (mRNA) with anticodon (tRNA).
3Degenerate genetic code (3 marks)
Extended• genetic code, degenerate, Paper 2
Question
Explain what is meant by the term 'degenerate' in the context of the genetic code. (3 marks)
Step-by-step solution
1. Step 1
  More codons than amino acids. There are 64 possible codons (4³ combinations of A, U, C, G) but only 20 amino acids are coded for (plus 3 stop codons).
2. Step 2
  Multiple codons per amino acid. Most amino acids are coded for by more than one codon — for example, leucine is coded for by 6 codons. Only methionine (AUG) and tryptophan (UGG) have a single codon.
3. Step 3
  Wobble. The codons that code for the same amino acid usually differ in their third base (wobble position). A mutation in the third base often produces a synonymous codon, so the amino acid sequence is unchanged. This makes the code partially error-tolerant.
Answer
More than one codon codes for the same amino acid; 64 codons but only 20 amino acids; wobble position (3rd base) often varies without changing the amino acid.
Examiner tip
Mark scheme: (1) more than one codon per amino acid; (2) 64 codons / 20 amino acids; (3) wobble / 3rd base / silent mutations. 'Degenerate' is the specific keyword examiners look for — 'redundant' is accepted but 'degenerate' is preferred.
4Codon vs anticodon (3 marks)
Extended• codon, anticodon, Paper 2
Question
A region of mRNA has the base sequence 5'-AUG GCC AAG-3'. State the base sequence of (i) the DNA template strand and (ii) the tRNA anticodon that pairs with the second codon. (3 marks)
Step-by-step solution
1. Step 1
  DNA template. mRNA is complementary and antiparallel to the template strand, with U replaced by T. mRNA 5'-AUG GCC AAG-3' was read from a template that is 3'-TAC CGG TTC-5' (i.e. 5'-CTT GGC CAT-3' when written 5'→3').
2. Step 2
  The second codon on the mRNA is GCC (5'→3'). The anticodon on the tRNA is complementary and antiparallel to this codon.
3. Step 3
  Anticodon. Pairing G with C, C with G, C with G gives the anticodon 3'-CGG-5' (or written 5'→3': 5'-GGC-3'). The tRNA carrying this anticodon will deliver the amino acid alanine (for which GCC is the codon).
Answer
(i) DNA template: 3'-TAC CGG TTC-5' (or 5'-CTT GGC CAT-3'). (ii) tRNA anticodon for GCC: 3'-CGG-5' (or 5'-GGC-3').
Examiner tip
Mark scheme: (1) DNA template sequence correct (with T instead of U, antiparallel); (2) anticodon for GCC = CGG; (3) correct orientation / antiparallel pairing. Always state the direction (5'→3') to avoid ambiguity.
5Third-base mutations and degeneracy (4 marks)
Extended• Adapted from 9700/42 May/Jun 2024• mutation, degenerate, Paper 4
Question
A mutation changes the third base of a codon. Explain why this mutation is less likely to affect the protein produced than a mutation in the first base. (4 marks)
Step-by-step solution
1. Step 1
  Genetic code is degenerate. Most amino acids are coded for by more than one codon (64 codons → 20 amino acids), and the codons for a given amino acid typically differ only in the third base (the wobble position).
2. Step 2
  Third-base = often silent. A change in the third base therefore often produces a synonymous codon that still codes for the same amino acid — a so-called silent mutation. The amino acid sequence is unchanged.
3. Step 3
  First-base = usually changes amino acid. A change in the first base almost always produces a codon for a different amino acid (or a STOP codon). This is a missense or nonsense mutation.
4. Step 4
  Consequence on protein. A different amino acid alters the primary structure → may alter folding (tertiary structure) → may alter the active site / function. A change in the third base usually has no effect on the polypeptide and therefore no effect on the protein's function.
Answer
Genetic code is degenerate; codons for an amino acid often differ only in 3rd base; 3rd-base mutation often silent → same amino acid; 1st-base mutation usually changes the amino acid → alters protein structure / function.
Examiner tip
Mark scheme: (1) genetic code is degenerate / multiple codons per amino acid; (2) 3rd base = wobble / silent likely; (3) 1st base = different amino acid likely / missense; (4) link to protein structure / function. Examiner reports flag candidates who say 'all 3rd-base mutations are silent' — they are not, but they are MORE LIKELY to be silent than 1st-base mutations.

Model Answers — Protein synthesis

High-scoring sample answers for protein synthesis on the Cambridge International A Level 9700 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
9700/22 May/Jun 2024 Q3(b) (adapted)6 marks
Q (6 marks). Describe the process of transcription.
Model answer
Transcription is the production of an mRNA copy of a gene from a DNA template, and it takes place in the nucleus of eukaryotic cells.

The enzyme RNA polymerase binds to a promoter region of DNA just before the start of the gene. It causes the DNA double helix to unwind locally, breaking the hydrogen bonds between complementary bases and exposing the two strands.

Only one strand of the DNA — the template (antisense) strand — is transcribed. Free RNA nucleotides (containing A, U, C and G) line up against the template by complementary base pairing: where the template has A, an RNA nucleotide with U pairs; where the template has T, an A pairs; G with C, and C with G. (Note that U replaces T in RNA.)

RNA polymerase then catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides, building up the pre-mRNA strand in the 5'→3' direction.

In eukaryotes, the pre-mRNA undergoes post-transcriptional modification: non-coding introns are spliced out and the coding exons are joined together; a 5' cap and a 3' poly-A tail are added to protect the molecule.

The mature mRNA then leaves the nucleus through a nuclear pore and enters the cytoplasm, where it can be translated on a ribosome.
Why this scores
Why this scores 6/6. Mark scheme: (1) RNA polymerase / unwinds DNA / breaks H-bonds; (2) one strand only / template strand; (3) free RNA nucleotides / complementary base pairing; (4) U replaces T (A-U pairing on mRNA); (5) phosphodiester bonds / 5'→3'; (6) mRNA leaves nucleus via nuclear pore. Eukaryote-specific extras (introns / cap / tail) can substitute for any one of marks 5-6.
Question
9700/42 May/Jun 2024 Q6(a) (adapted)8 marks
Q (8 marks). Describe the process of translation.
Model answer
Translation is the synthesis of a polypeptide from an mRNA template; it takes place on a ribosome in the cytoplasm (or on rough ER for secreted proteins).

Initiation. The small ribosomal subunit binds to the 5' end of the mRNA and scans for the START codon AUG. A tRNA carrying the amino acid methionine and bearing the anticodon UAC binds to AUG in the ribosome's P-site. The large ribosomal subunit then joins, creating a complete ribosome with a P-site and an A-site (some ribosomes also have an E-site for tRNA exit).

Elongation. A second tRNA, with an anticodon complementary to the codon at the A-site, binds by hydrogen bonds, bringing its specific amino acid. Two amino acids are now held next to each other at the P- and A-sites.

A peptide bond is then formed between the amino acid on the P-site tRNA and the amino acid on the A-site tRNA. This is a condensation reaction, catalysed by ribosomal RNA (rRNA) — meaning the ribosome itself is a ribozyme.

The ribosome then translocates one codon along the mRNA (in the 5'→3' direction). The empty P-site tRNA exits via the E-site; the A-site tRNA (now carrying the dipeptide) moves to the P-site; a new codon is exposed at the A-site. The next aminoacyl-tRNA binds and the cycle repeats.

Termination. Translation continues codon by codon until a STOP codon (UAA, UAG or UGA) enters the A-site. No tRNA has an anticodon for these codons; instead, a release factor binds and the completed polypeptide is released from the final tRNA. The ribosome separates into its subunits, ready to be recycled.

Often several ribosomes translate one mRNA at the same time, forming a polysome, which speeds up protein production. The completed polypeptide is then folded (possibly with the help of chaperone proteins) into its functional 3-D shape.
Why this scores
Why this scores 8/8. Mark scheme: (1) ribosome / cytoplasm; (2) AUG = start codon; (3) tRNA carries specific amino acid / anticodon-codon pairing; (4) two tRNAs at P-site and A-site; (5) peptide bond / condensation; (6) catalysed by rRNA / ribosome; (7) ribosome translocates codon-by-codon; (8) STOP codon (UAA / UAG / UGA) terminates. Polysomes / chaperones can substitute for any one. Examiner reports note 'codon' / 'anticodon' must be used correctly — confusion between the two is the single biggest error.
Question
9700/12 Oct/Nov 2024 Q4 (adapted)3 marks
Q (3 marks). Explain what is meant by the term 'degenerate' in the context of the genetic code.
Model answer
The genetic code is described as degenerate because most amino acids are coded for by more than one codon. There are 64 possible codons (4³ combinations of the four bases A, U, C and G), but only 20 amino acids are coded for (plus 3 stop codons), so on average more than three codons correspond to each amino acid.

Where multiple codons code for the same amino acid, they usually differ only in the third base — the so-called wobble position. For example, the four codons GGU, GGC, GGA and GGG all code for glycine; they share the first two bases but vary in the third.

Only two amino acids are coded for by a single codon: methionine (AUG) and tryptophan (UGG). The degeneracy of the code reduces the impact of point mutations: a mutation in the third base of a codon is often silent (produces a synonymous codon and therefore the same amino acid), so the protein is unaffected. The code is also non-overlapping (each base is read only once) and universal (the same in nearly all organisms).
Why this scores
Why this scores 3/3. Mark scheme: (1) more than one codon codes for the same amino acid; (2) 64 codons vs 20 amino acids; (3) third base / wobble / silent mutations possible. Bonus information (universal, non-overlapping) is good but not required for the 3 marks.
Question
9700/42 May/Jun 2024 Q6(b) (adapted)4 marks
Q (4 marks). A mutation changes the third base of a codon. Explain why this mutation is less likely to affect the protein produced than a mutation in the first base.
Model answer
The genetic code is degenerate: most amino acids are coded for by more than one codon, and codons that code for the same amino acid usually differ only in their third base (the wobble position). For example, GGU, GGC, GGA and GGG all code for glycine.

A mutation in the third base therefore often produces a synonymous codon — one that codes for the same amino acid. This is a silent mutation: the amino acid sequence of the polypeptide is unchanged, so the primary structure, tertiary structure and function of the protein are also unchanged.

A mutation in the first base of a codon, by contrast, almost always produces a codon for a different amino acid (a missense mutation) or a STOP codon (nonsense). A different amino acid means a different R-group, which can alter the hydrogen bonds, ionic bonds and hydrophobic interactions stabilising the tertiary structure of the protein.

If the affected amino acid lies in or near a functionally critical region — for example, the active site of an enzyme or the haem-binding pocket of haemoglobin — the protein's function may be lost. A change in the third base is therefore much less likely to affect the protein because it is much more likely to be a silent mutation.
Why this scores
Why this scores 4/4. Mark scheme: (1) degenerate code / wobble position; (2) third-base change often gives synonymous codon / silent mutation; (3) first-base change usually changes amino acid; (4) link to tertiary structure / active site / function. Examiner reports flag candidates who claim 'all third-base mutations are silent' — they are not, but most are.
Question
9700/22 Oct/Nov 2024 Q5(b) (adapted)5 marks
Q (5 marks). Describe the role of tRNA and the role of the ribosome during translation.
Model answer
Transfer RNA (tRNA) is a single-stranded RNA molecule folded into a clover-leaf shape, stabilised by internal base pairing. Each tRNA has two functionally crucial regions: a three-base anticodon at one end and an amino-acid binding site at the 3' end (the CCA tail).

Each tRNA is specific: a particular anticodon is matched to a particular amino acid, which is attached to the tRNA by a specific aminoacyl-tRNA synthetase enzyme using ATP. During translation, the tRNA's anticodon base-pairs with the corresponding codon on the mRNA at the ribosome's A-site, delivering the correct amino acid to the growing polypeptide.

The ribosome is a complex of rRNA and proteins, made of a large and a small subunit. It performs three key roles:

It holds the mRNA in place so codons can be read in sequence.

It has binding sites (P-site and A-site) that can hold two tRNAs simultaneously, ensuring that the next amino acid is brought into position next to the growing chain.

Its rRNA catalyses the formation of the peptide bond (so the ribosome is a ribozyme).

The ribosome then translocates one codon along the mRNA, shifting the tRNAs and freeing the A-site for the next aminoacyl-tRNA. This continues codon-by-codon until a STOP codon is reached.
Why this scores
Why this scores 5/5. Mark scheme: (1) tRNA has anticodon + carries specific amino acid; (2) anticodon pairs with codon; (3) ribosome holds mRNA / has P-site and A-site; (4) ribosome (rRNA) catalyses peptide bond; (5) ribosome translocates codon-by-codon. Naming 'aminoacyl-tRNA synthetase' is bonus credit but not required.

Key Definitions and Keywords — Protein synthesis

Definitions to memorise and the exact keywords mark schemes credit for protein synthesis answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9700 sitting.

Transcription
Examiner keyword
The synthesis of an mRNA molecule from a DNA template, catalysed by RNA polymerase. Takes place in the nucleus in eukaryotes. Only one strand of DNA (the template / antisense strand) is read.
Translation
Examiner keyword
The synthesis of a polypeptide from an mRNA template at a ribosome. tRNAs deliver specific amino acids; peptide bonds are formed between adjacent amino acids until a STOP codon is reached.
Codon
Examiner keyword
A sequence of three adjacent bases on mRNA that codes for one amino acid (or a STOP signal). 64 codons exist; 61 code for amino acids and 3 are STOP codons.
Anticodon
Examiner keyword
A sequence of three bases on a tRNA molecule that is complementary and antiparallel to the codon on mRNA. Each tRNA's anticodon is specific to the amino acid it carries.
mRNA (messenger RNA)
Examiner keyword
A single-stranded RNA molecule that carries the genetic code from the DNA in the nucleus to the ribosome, where it is translated into a polypeptide.
tRNA (transfer RNA)
Examiner keyword
A folded single-stranded RNA molecule with a specific anticodon at one end and an amino acid binding site at the other. Delivers amino acids to the ribosome during translation.
rRNA (ribosomal RNA)
Examiner keyword
RNA that combines with proteins to form ribosomes. rRNA also catalyses peptide bond formation, making the ribosome a ribozyme.
RNA polymerase
Examiner keyword
The enzyme that catalyses transcription. It unwinds DNA at the promoter, reads the template strand and joins free RNA nucleotides by phosphodiester bonds in the 5'→3' direction.
Start codon
Examiner keyword
The codon AUG, which signals the start of translation and codes for the amino acid methionine.
Stop codon
Examiner keyword
One of three codons (UAA, UAG, UGA) that signal the end of translation. No tRNA has an anticodon for these codons; instead, a release factor binds and the polypeptide is released.
Degenerate genetic code
Examiner keyword
A genetic code in which most amino acids are coded for by more than one codon. With 64 codons but only 20 amino acids, codons that code for the same amino acid usually differ in the third (wobble) base.
Universal genetic code
Examiner keyword
The same codons code for the same amino acids in (almost) all organisms — evidence that all life shares a common ancestor.
Intron and exon
Examiner keyword
Introns are non-coding regions of a gene that are transcribed but then spliced out of the pre-mRNA before translation. Exons are the coding regions that are joined together to form the mature mRNA.

Common Mistakes and Misconceptions — Protein synthesis

The traps other students keep falling into on protein synthesis questions — taken from recent Cambridge International A Level 9700 examiner reports and mark schemes — and how to avoid them.

✕Confusing codon and anticodon
9700 Examiner Reports 2023-2024
Why it happens
The two words sound similar and refer to complementary 3-base sequences.
How to avoid it
Codon = mRNA. Anticodon = tRNA. The codon and anticodon are COMPLEMENTARY and ANTIPARALLEL to each other. Always specify which molecule when stating a base sequence.
✕Forgetting to replace T with U when transcribing mRNA
9700 Examiner Reports 2024
Why it happens
Students copy the DNA letters directly.
How to avoid it
RNA contains URACIL not thymine. When a DNA template has A, the mRNA has U (not T). Drill this with practice exercises.
✕Not mentioning a STOP codon when describing translation
9700 Examiner Reports 2024
Why it happens
Students focus on the start and forget how translation ends.
How to avoid it
End your translation description with: 'Translation continues until a STOP codon (UAA, UAG or UGA) is reached, at which point the polypeptide is released'.
✕Saying both DNA strands are transcribed
9700 Examiner Reports 2023
Why it happens
Confusion with replication, where both strands are templates.
How to avoid it
Only ONE strand of DNA (the TEMPLATE / antisense strand) is transcribed. The other strand (sense / coding strand) is not read.
✕Saying tRNA 'makes' or 'synthesises' protein
9700 Examiner Reports 2024
Why it happens
Oversimplification.
How to avoid it
tRNA DELIVERS amino acids to the ribosome. The RIBOSOME (rRNA) catalyses peptide bond formation. Use precise language: 'tRNA brings amino acids; rRNA forms the peptide bond'.
✕Stating that prokaryotes splice out introns
Why it happens
Assuming eukaryote-specific features are universal.
How to avoid it
Intron splicing, 5' cap and poly-A tail are EUKARYOTE features. Prokaryotic mRNA is translated immediately without modification — even before transcription is complete.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Protein synthesis — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Protein synthesis

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Stepwise description of transcription (5 marks)

2Stepwise description of translation (6 marks)

3Degenerate genetic code (3 marks)

4Codon vs anticodon (3 marks)

5Third-base mutations and degeneracy (4 marks)

Transcription

Translation

Codon

Anticodon

mRNA (messenger RNA)

tRNA (transfer RNA)

rRNA (ribosomal RNA)

RNA polymerase

Start codon

Stop codon

Degenerate genetic code

Universal genetic code

Intron and exon

✕Confusing codon and anticodon

✕Forgetting to replace T with U when transcribing mRNA

✕Not mentioning a STOP codon when describing translation

✕Saying both DNA strands are transcribed

✕Saying tRNA 'makes' or 'synthesises' protein

✕Stating that prokaryotes splice out introns

Practice questions

Past paper style quiz

Assess your understanding

Protein synthesis — frequently asked questions