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6.2 Protein Synthesis

Syllabus
9700–2028–2029
Topic
6.2
Level
AS

Learning objectives

6.2.1Gene• Gene: sequence of nucleotides coding for a polypeptide6.2.2Universal genetic code and codons• Universal genetic code: triplets code for amino acids or start/stop codons6.2.3Transcription and translation roles• Transcription and translation roles:- RNA polymerase- mRNA and codons- tRNA and anticodons- Ribosomes6.2.4DNA strands in transcription• Transcribed (template) strand vs non-transcribed strand6.2.5Post-transcriptional modification• Post-transcriptional modification in eukaryotes: removal of introns, joining of exons6.2.6Gene mutations• Gene mutations: changes in base pair sequence6.2.7Types of mutations• Types of mutations: substitution, deletion, insertion6.2.8Effects on polypeptide produced• Effects on polypeptide produced

A gene is a DNA sequence that contributes to a functional product

A gene is a defined sequence of nucleotides in a DNA molecule that contains the information for producing a specific polypeptide. In the wider idea of gene expression, a DNA sequence is used to make an RNA message and, for a protein-coding gene, that message specifies an amino-acid sequence.

  • Information source: The ordered DNA bases in the gene carry the sequence information; the gene is a section of a DNA molecule, not the whole chromosome.
  • Expression chain: DNA base sequence → transcribed RNA information → codons read in order → amino-acid sequence → polypeptide.
  • Product boundary: The polypeptide sequence is the immediate product specified by this syllabus objective. Its folding and interactions can give the final protein its structure and function.
  • Expression boundary: Having a gene in the DNA does not mean it is being used at the same level in every cell or situation; gene expression refers to when and how the information is transcribed and used.

Because the DNA base order is copied into an RNA message and decoded into amino-acid order, changing the gene sequence can change the instructions available for the polypeptide. The gene supplies information; it is not itself the RNA message, amino-acid chain or finished protein.

A gene is not the entire chromosome, all of the DNA in a cell, or a protein molecule. It is a nucleotide sequence within DNA that can be expressed to specify a polypeptide; the later transcription and translation steps explain how that information reaches the product.

Codons are three-base instructions read on mRNA

A codon is a triplet of bases on mRNA read during translation. Each codon specifies one amino acid or acts as a start or stop signal; it does not directly name a complete protein.

  • Triplet and continuity: The mRNA sequence is read three bases at a time from a defined start point. The codons are read consecutively and do not overlap, so the reading frame determines which bases belong to each codon.
  • Mapping: A codon maps to an amino acid, and the ordered amino acids form the polypeptide. A start codon establishes where the coding sequence is read; a stop codon signals termination and does not add an amino acid.
  • Degeneracy: More than one codon can specify the same amino acid. This is degeneracy, not ambiguity: a given codon has one assigned meaning in the code.
  • Near-universality: Almost all organisms use the same codon assignments, which is why the code is described as universal at A-Level. No special exception is needed to apply the core mapping here.

The triplet rule provides enough combinations to assign amino acids, while the fixed reading frame prevents the message from being regrouped at every step. Translation therefore follows: mRNA codons → amino-acid sequence → polypeptide, with start and stop signals defining the usable coding run.

A codon is read on mRNA, not as an untranslated DNA triplet, and it specifies an amino acid or signal rather than an entire protein. A stop codon ends translation but is not incorporated as an amino acid. Do not shift the reading frame or treat overlapping groups as the standard code.

Transcription makes mRNA; translation builds a polypeptide

Protein synthesis converts genetic information into amino-acid sequence in two linked stages: transcription makes mRNA from a DNA template, and translation reads mRNA to assemble a polypeptide.

  1. RNA polymerase uses the DNA template strand and joins complementary RNA nucleotides to make mRNA.
  2. The mRNA leaves the nucleus and attaches to a ribosome, which reads its codons in order.
  3. Each tRNA carries a specific amino acid and has an anticodon complementary to an mRNA codon.
  4. The ribosome positions successive tRNAs and peptide bonds form between adjacent amino acids, lengthening the chain.
  5. A stop codon ends translation and the polypeptide is released to fold.

DNA base sequence is copied into mRNA codons; tRNA anticodons match those codons; the ribosome converts that sequence information into amino-acid order and a peptide-bonded polypeptide.

Transcription produces RNA, not a polypeptide; translation reads mRNA, not DNA directly. mRNA carries codons, while tRNA carries amino acids and uses anticodons.

The template is complementary; the coding strand matches mRNA

Only one DNA strand is used for a gene's transcription. The transcribed/template strand is complementary to mRNA; the non-transcribed/coding strand has the same 5-prime-to-3-prime base sequence as mRNA except DNA has T where RNA has U.

  1. Identify the template strand whose exposed bases RNA polymerase uses.
  2. RNA polymerase reads template DNA 3-prime to 5-prime and builds mRNA 5-prime to 3-prime.
  3. Template DNA 3-prime-TAC-5-prime gives mRNA 5-prime-AUG-3-prime.
  4. Coding DNA 5-prime-ATG-3-prime matches mRNA 5-prime-AUG-3-prime apart from T/U.
  5. Use both complementarity and direction to check strand identity before interpreting codons.

The template determines mRNA by complementarity. The coding strand is a useful check because its 5-prime-to-3-prime sequence matches mRNA apart from thymine being replaced by uracil.

Both DNA strands remain present, but only one is transcribed for a given gene. Do not call the coding strand the template, complement it twice or use T in mRNA.

Post-transcriptional processing turns a primary transcript into usable mRNA

In a eukaryotic cell, the first RNA made from a gene is a primary transcript. It contains both the gene’s coding exons and non-coding introns, so it must be processed before it becomes mature mRNA ready to leave the nucleus.

  1. Transcribe the whole gene region: Transcription produces a primary RNA transcript containing the exon and intron sequences that were present in the transcribed gene region.
  2. Identify the sections: Exons are the coding sequences retained for the message; introns are non-coding sequences that are not to be translated into the polypeptide.
  3. Remove introns: The intron sections are cut out of the primary transcript. This processing changes the RNA molecule, not the DNA gene itself.
  4. Join exons: The remaining exon sections are joined to form one continuous mature mRNA molecule. This joining step is splicing.
  5. Export the message: The mature mRNA leaves the nucleus through a nuclear pore and becomes available for the later translation stage. Translation is the next use of the processed message, not part of this card’s processing sequence.

Processing converts a mixed primary transcript into a continuous message: primary transcript (exons + introns) → introns removed → exons joined → mature mRNA → export from the nucleus. Without intron removal and exon joining, the RNA would not present the intended continuous coding sequence for later use.

Introns are removed from the RNA transcript, not deleted from the DNA template, and mature mRNA is not a protein. The primary transcript and mature mRNA are different RNA forms; the detailed codon/anticodon and peptide-bond process belongs to card 4578, not this post-transcriptional processing card.

A gene mutation changes DNA sequence, but its effect depends on context

A gene mutation is a change in the DNA base or base-pair sequence of a gene. It changes the stored sequence information; its biological consequence must be traced rather than assumed from the word “mutation”.

  • Substitution: one base pair is replaced by another. This may leave the encoded amino acid unchanged or may alter a codon, depending on the position and the new base.
  • Insertion: one or more base pairs are added to the sequence. The addition can change how downstream bases are grouped into codons, especially when the number added is not a multiple of three.
  • Deletion: one or more base pairs are removed. Like an insertion, a non-triplet change can shift the downstream reading frame; a triplet-sized change can have a different reach.
  • Consequence boundary: DNA change → possible mRNA/codon change → possible amino-acid or polypeptide change. The result depends on where the change occurs, how many bases are affected, and whether the relevant sequence is read or expressed.

A mutation matters through the expression chain, not by definition: a DNA sequence change may be silent, may alter one codon, or may regroup many downstream codons if the reading frame changes. A change outside the relevant coding information can have a different outcome, so mutation type alone is not enough to predict a polypeptide or phenotype.

Substitution, insertion and deletion describe different edits to DNA; they do not automatically describe the final protein effect. Not every mutation is harmful or changes an amino acid, and not every insertion/deletion causes a frameshift. Do not name a disease or phenotype without evidence for the specific sequence and expression context.

Substitution, insertion and deletion alter sequence in different ways

Substitution, insertion and deletion are three ways a gene’s DNA base sequence can change. Their different effects on the triplet grouping explain why some changes are local while others affect many downstream codons and the polypeptide produced.

  • Substitution — swap one base: One DNA base is replaced by another. It changes the triplet at that position but is not a frameshift, so it does not regroup all downstream triplets. Because the genetic code is degenerate, the amino-acid sequence may stay the same or may change at that triplet.
  • Insertion — add base(s): One or more bases are added to the DNA sequence. The added base changes the local triplet grouping and can shift the reading frame, changing downstream triplets and potentially many amino acids.
  • Deletion — remove base(s): One or more bases are removed. Like an insertion, a non-triplet change can shift the reading frame and alter downstream amino-acid instructions.
  • Comparison boundary: Substitution is local with respect to the reading frame; insertion/deletion can have a downstream frameshift effect. The actual polypeptide consequence still depends on the sequence position and how the changed codons are read.

The causal chain is DNA edit → altered triplet grouping or codon → possible amino-acid sequence change → possible polypeptide shape/function change. Insertions and deletions can propagate the change through later triplets, whereas a substitution does not automatically do so; the genetic code’s degeneracy means even a changed base need not change the polypeptide.

An insertion or deletion is not automatically the same as a substitution: check whether the reading frame is shifted. A mutation type predicts a mechanism of sequence change, not a guaranteed disease or protein outcome. This card compares the edits; the broader definition of mutation is card 4581 and the context-dependent polypeptide effect belongs to card 4583.

A mutation changes a polypeptide only through the expression chain

The effect of a gene mutation on a polypeptide must be traced through gene expression. A DNA change may alter the mRNA message, the amino-acid sequence or the amount of product, but it may also have little or no effect.

  1. DNA information: Start with the changed base sequence and ask whether it lies in the part of the gene that is read and expressed.
  2. RNA message: Transcription copies the relevant information into RNA; processing can produce the mature mRNA message used for expression.
  3. Codon reading: A ribosome reads the mRNA codons while tRNA anticodons bring amino acids. A changed codon or a shifted reading frame can change the amino-acid instructions, but the degenerate code means some base changes do not change the amino acid.
  4. Polypeptide outcome: The resulting chain may be unchanged, may contain a limited amino-acid change, or may be substantially altered/shortened when many downstream codons are affected.
  5. Function boundary: A changed amino-acid sequence can alter folding, shape or function, but a changed DNA base does not guarantee a changed polypeptide or phenotype. The final claim depends on the mutation’s position, reading context and the protein’s role.

The complete reasoning path is DNA change → RNA message/processing → codon and tRNA matching at the ribosome → amino-acid sequence → polypeptide shape and function. This chain explains both possibilities: a mutation can be buffered by the code or location, or it can propagate through the reading frame and produce a markedly different polypeptide.

Do not jump directly from “mutation” to “disease” or “non-functional protein”. Explain each link that is supported: DNA sequence, mRNA/codon, amino-acid chain, then shape/function. Cards 4581 and 4582 define and compare mutation edits; this card is the high-level completion and consequence check, not a repeat of their classifications.