Ch 2 · 6.2 Protein synthesis · AS & A Level Biology 9700

Topic 2 of 2 in Nucleic acids and protein synthesis — AS & A Level Biology 9700 — 58 lessons.

Introduction

The base sequence of DNA is converted into the amino acid sequence of a protein through two linked processes, transcription and translation, that together are known as gene expression. Cambridge questions test the genetic code vocabulary precisely, and expect the sequence of events at the ribosome to be described in order, so this lesson works through each stage in turn and ends with what happens when the code changes: gene mutations. (Syllabus section 6.2.)

Key ideas — Roles of mRNA, tRNA and ribosomes

Messenger RNA (mRNA) is synthesised as a complementary copy of one strand of a gene and carries the coded instructions for a polypeptide from the nucleus to a ribosome in the cytoplasm. Transfer RNA (tRNA) molecules each carry a specific amino acid at one end and have a complementary anticodon at the other end, which base-pairs with a codon on mRNA. Ribosomes, made of ribosomal RNA and protein, hold mRNA and tRNA in place and catalyse the formation of peptide bonds between adjacent amino acids as the polypeptide is built.

🧑‍🏫 Teacher's explanation

Think of protein synthesis like a construction site. The DNA is the master blueprint kept safely in the office (the nucleus). Because the blueprint is too precious to move, a photocopy is made; this photocopy is the mRNA. This mRNA travels to the construction site (the ribosome). The tRNA molecules are like specialized delivery trucks; each truck carries one specific building material (an amino acid) and has a unique key (the anticodon) that only fits a specific slot (the codon) on the blueprint. The ribosome is the foreman that coordinates the trucks and glues the materials together into a long chain.

📖 Definitions

🔍 Worked example — Easy

A DNA template strand has the following sequence of bases: 3' ext TAC - GGC - TTA - ACT 5'. Determine the sequence of the mRNA produced during transcription.

🔍 Worked example — Standard

A sequence of mRNA is 5' ext AUG - UUU - CGA - GCA - UAA 3'. If the tRNA anticodons are 3' ext UAA - AAA - GCU - CGU - AUU 5', identify the sequence of amino acids if the codons correspond to: AUG=Met, UUU=Phe, CGA=Arg, GCA=Ala, UAA=Stop.

🔍 Worked example — Exam-style

A DNA sequence is 5' ext TAC - GGC - TTA - ACT 3'. A mutation occurs where the third base of the second triplet (G) is deleted. Describe the effect on the resulting polypeptide.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Genes and the genetic code

A gene is a sequence of nucleotides that forms part of a DNA molecule and codes for a polypeptide.

🧑‍🏫 Teacher's explanation

Think of DNA as a massive, master architectural blueprint kept safely in a vault (the nucleus). You can't take the whole blueprint to the construction site (the ribosome), so you make a photocopy of just one specific section—this is transcription. This photocopy, the mRNA, carries the instructions to the workers (tRNA) who bring the actual bricks (amino acids) to build the structure.

📖 Definitions

🔍 Worked example — Easy

A DNA template strand has the sequence 3' ext TAC GGG ATT ACC 5'. Determine the sequence of the mRNA produced during transcription.

🔍 Worked example — Standard

A sequence of mRNA is 5' ext AUG UUU GGC AAA UAA 3'. If the DNA template strand was used, what is the sequence of the non-transcribed (coding) strand in the 5' o 3' direction?

🔍 Worked example — Exam-style

A DNA sequence undergoes a mutation where a single nucleotide is inserted into the middle of a gene. Explain the likely effect on the resulting polypeptide.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Transcription

Transcription occurs in the nucleus. RNA polymerase binds to DNA at the start of a gene, and the DNA double helix unwinds locally so one strand acts as a template. This is the transcribed (or template) strand; the other is the non-transcribed strand. RNA polymerase moves along the template strand, joining free RNA nucleotides that have base-paired with the template (using uracil in place of thymine), building a single strand of RNA, the primary transcript. In eukaryotes the primary transcript is then modified: the non-coding sequences (introns) are removed and the coding sequences (exons) are joined together to form mRNA, which leaves the nucleus through a nuclear pore. (Prokaryotes have no introns and no nucleus, so their mRNA is translated straight away.)

🧑‍🏫 Teacher's explanation

Think of DNA as a massive, permanent master blueprint kept safely in a high-security vault (the nucleus). You cannot take the master blueprint to the construction site (the ribosome) because it might get damaged. Instead, you make a portable, temporary photocopy of just one specific page (a gene). This photocopy is the mRNA.

📖 Definitions

🔍 Worked example — Easy

A segment of the template DNA strand has the following base sequence: 3' ext TAC GGG ATT ACC 5'. Determine the sequence of the resulting primary transcript (RNA).

🔍 Worked example — Standard

A DNA sequence is 5' ext TAC GGC TTA GCA 3'. If the non-transcribed strand is 3' ext ATG CCG AAT CGT 5', describe the sequence of the mRNA produced.

🔍 Worked example — Exam-style

A DNA sequence is 5' ext TAC GGC TTA GCA 3'. A mutation occurs where the third nucleotide (T) is deleted. Explain the effect on the resulting polypeptide.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Determine the sequence of the mRNA produced. [1] (b) Explain how a mutation involving the insertion of a single nucleotide into this DNA sequence would affect the resulting polypeptide. [3] [4 marks]

Key ideas — Translation, start codons and stop codons

Translation occurs at a ribosome in the cytoplasm. The ribosome attaches to mRNA at the start codon (AUG), which sets the reading frame and also codes for the first amino acid, methionine. A tRNA molecule with the complementary anticodon delivers its amino acid; the ribosome moves along the mRNA codon by codon, and successive tRNA molecules bring the correct amino acids, which are joined by peptide bonds catalysed at the ribosome. Translation ends when the ribosome reaches a stop codon, which does not code for an amino acid and has no corresponding tRNA, so the completed polypeptide is released.

🧑‍🏫 Teacher's explanation

Think of translation like a construction crew reading a blueprint. The mRNA is the blueprint, the ribosome is the construction site, and the tRNA molecules are the delivery trucks. Each truck carries a specific building material (an amino acid) and has a unique key (the anticodon) that only fits a specific instruction (the codon) on the blueprint.

📖 Definitions

🔍 Worked example — Easy

A segment of mRNA has the following sequence: 5' ext- AUG | UUU | GCA | UAG - 3'. Identify the number of amino acids that will be present in the resulting polypeptide chain.

🔍 Worked example — Standard

A tRNA molecule has the anticodon 3' ext- UAC - 5'. Determine the corresponding mRNA codon and the amino acid it would deliver if the codon codes for Methionine.

🔍 Worked example — Exam-style

A DNA template strand has the sequence 3' ext- TAC | GGG | CAT | ACT - 5'. Predict the resulting mRNA sequence and state how many amino acids are produced.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

(a) Determine the sequence of the mRNA produced during transcription. [1] (b) Explain how a substitution mutation, where the third base of the second triplet is changed from A to G, would affect the resulting polypeptide. [3] [4 marks]

Key ideas — Gene mutations

A gene mutation is a change in the sequence of base pairs in a DNA molecule. Because the base sequence determines the amino acid sequence, a mutation may result in an altered polypeptide. There are three types:

🧑‍🏫 Teacher's explanation

Think of a gene as a recipe in a cookbook. The recipe is written using a specific set of instructions (the genetic code). If you make a tiny typo by swapping one letter for another, the recipe might still work fine, or it might change one ingredient. However, if you accidentally skip a word or add an extra letter, the entire sentence structure breaks down, making the rest of the recipe impossible to follow. This is the essence of how mutations affect proteins.

📖 Definitions

🔍 Worked example — Easy

A DNA sequence for a segment of a gene is 5'-TAC-GGC-AAA-ACT-3'. If a substitution mutation occurs where the third base (A) is replaced by G, determine the new DNA sequence and state if the amino acid sequence is likely to change.

🔍 Worked example — Standard

A DNA template strand reads 3'-TAC-GGC-AAA-ACT-5'. A deletion mutation occurs where the second G is removed. Describe the effect on the resulting polypeptide.

🔍 Worked example — Exam-style

Compare the effects of a substitution mutation and an insertion mutation of a single nucleotide on the resulting polypeptide.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Explain why this mutation is likely to result in a non-functional polypeptide. [3] (b) Describe the difference between a substitution mutation and an insertion mutation. [2] [5 marks]

Worked example

Question. The template strand of a section of DNA has the base sequence TAC GGA ATT. (a) State the sequence of bases on the mRNA transcribed from this template strand. [2] (b) State the number of amino acids this sequence of mRNA would code for, and explain your reasoning. [2]

Ch 2 · 6.2 Protein synthesis · AS & A Level Biology 9700

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