Molecular Basis of Inheritance · TOPIC LESSON
Decode an RNA message while keeping sequence information, adaptors and ribosome states distinct.
BY THE END, YOU CAN
BEFORE YOU START
Original educational lesson. Work through the examples before revealing the practice answers. Check the current official syllabus separately; historical question frequency does not establish eligibility or predict future questions.
CONNECT THE CONCEPT TO THE EXAM
These selected examples are separate from the original practice below. Each identifies its exam, year and wording. They do not represent every paper or every selected year.
No source-verified exam questions have been published for this lesson yet. The worked examples and practice below are original teaching material, not past-exam questions.
01 / UNDERSTAND & APPLY
The familiar DNA → RNA → protein pathway connects transcription with translation. Transcription makes an RNA sequence using a DNA template. Translation uses an mRNA sequence to specify the amino-acid order of a polypeptide. DNA does not physically turn into RNA and then into protein; templates and products are different molecules, and many products can be made using the same information source.
Replication is DNA-directed DNA synthesis. Reverse transcription is RNA-directed DNA synthesis, and RNA-directed RNA synthesis occurs in the replication cycles of many RNA viruses. Therefore the simple classroom arrow chain is not an exhaustive list of all nucleic-acid information transfers. In particular, reverse transcription should not be treated as an impossible process merely because a simplified diagram leaves it out.
In the sequence-information meaning associated with Crick’s central dogma, information that has reached a protein’s amino-acid sequence is not read back as a template specifying a nucleic-acid sequence. Proteins can regulate transcription and catalyse nucleic-acid reactions, but catalytic influence is different from copying an amino-acid sequence back into a corresponding nucleotide sequence.
The distinction also matters for functional RNAs. A gene can produce an rRNA, tRNA or another functional RNA without making a polypeptide from that RNA. “All genes encode proteins” is too broad. An expression pathway can end with a functional RNA, even though protein-coding genes use mRNA as the intermediate in the familiar pathway.
Prion propagation involves changes in protein conformation, not reverse translation of amino-acid sequence into nucleic-acid sequence. It should not be used as a careless example of a protein being decoded into DNA. The lesson’s arrows track sequence-information transfer, not every kind of biological influence or inheritance.
Full-size scientific diagram. Swipe or scroll horizontally; keyboard users can focus the diagram and use arrow keys.
02 / UNDERSTAND & APPLY
A codon is a three-nucleotide unit read in an mRNA coding sequence. The standard genetic code assigns 61 codons to amino acids and three, UAA, UAG and UGA, to termination. Four RNA bases give 4³=64 triplets; a two-base code would provide only 4²=16 combinations, insufficient to distinguish the usual twenty amino acids with a simple one-word-per-amino-acid scheme.
The code is degenerate: several codons can specify the same amino acid. For example, UUU and UUC both specify phenylalanine, while GGU, GGC, GGA and GGG specify glycine. Degeneracy does not mean that one codon randomly specifies several different amino acids in the same standard reading context. The ordinary code is unambiguous at that level.
AUG specifies methionine in the standard code and commonly serves as an initiation codon. An internal AUG in an already established reading frame ordinarily adds methionine; it does not automatically restart translation. Start-site choice depends on initiation signals and context, not on treating every AUG in a long RNA as a new beginning.
The code is nearly universal rather than absolutely universal. Some organelles and organisms use codon assignments that differ from the standard table, and specialised recoding mechanisms exist. Unless a question explicitly supplies such a system, use the stated standard code. “Universal” is a useful introductory approximation, not permission to dismiss known exceptions.
Within an ordinary open reading frame, codons are read successively without overlapping and without a punctuation nucleotide inserted between each triplet. Biological genomes can nevertheless contain overlapping genes or programmed frame changes. Distinguish the default decoding rule for one message from a claim that no genome can organise information in more complicated ways.
03 / UNDERSTAND & APPLY
An RNA sequence written 5′-AUGGCUUUCUAA-3′ can be partitioned, from its stated start, as AUG | GCU | UUC | UAA. In the standard code this specifies methionine, alanine and phenylalanine, followed by termination. There are four codons in the displayed coding interval but only three amino-acid residues in the initial product.
If the same letters are divided from a different starting position, the triplets change. A codon table cannot choose the correct reading frame by itself. The problem must identify the start, provide sufficient initiation context, or explicitly ask for possible frames. Merely translating from the first nucleotide printed on the page can give a plausible-looking but wrong answer.
Codon tables normally use mRNA written 5′→3′. If a question gives a DNA coding strand in that direction, replace T with U to obtain the corresponding RNA sequence for the transcribed region. If it gives the template strand, first make the antiparallel complementary RNA. Do not translate a template-DNA triplet directly as if it were an mRNA codon.
Untranslated regions can lie before and after the coding sequence in mature mRNA. Eukaryotic introns are normally removed from the relevant pre-mRNA before its translated coding sequence is read. RNA length is therefore not automatically three times protein length: untranslated regions, termination codons and processing must be considered.
If a fully specified uninterrupted coding interval encodes n amino-acid residues and contains one following stop codon, its length is 3n+3 nucleotides. The amino-acid-specifying part alone is 3n nucleotides. These expressions exclude untranslated regions and assume no unusual recoding; saying which interval is counted prevents an off-by-three error.
04 / UNDERSTAND & APPLY
A transfer RNA has an anticodon region that recognises a codon and an amino-acid attachment end. The amino acid attaches to the 3′ terminal region, commonly described by the terminal CCA sequence, rather than to the anticodon itself. The two functions occupy different parts of the adaptor.
An aminoacyl-tRNA synthetase uses energy from ATP to attach an appropriate amino acid to its corresponding tRNA. This charging step is a major contributor to translation accuracy: the ribosome checks codon–anticodon matching, but that alone does not establish that the tRNA carries the chemically correct amino acid.
Codon and anticodon pair antiparallel. For codon 5′-AUG-3′, the complementary anticodon is 3′-UAC-5′, which becomes 5′-CAU-3′ if the answer requires anticodon notation in the 5′→3′ direction. An unlabelled “UAC” answer can hide a direction mistake. Always include ends when the question is testing pairing.
Wobble permits certain non-standard pairing relationships at the third codon position, corresponding to the first position of the anticodon when written 5′→3′. This helps explain why a cell does not need a separate tRNA species for every sense codon. It does not allow arbitrary pairing at every position or random amino-acid choice.
The tRNA’s cloverleaf drawing is a useful representation of secondary structure; the folded molecule has a three-dimensional shape. Likewise, an anticodon loop is not a free three-base molecule floating independently of the rest of the tRNA. Relating the recognition region to the attachment end makes the adaptor mechanism easier to understand than memorising the two labels separately.
05 / UNDERSTAND & APPLY
Ribosomes contain rRNA and proteins arranged in large and small subunits. The small subunit helps organise decoding of the mRNA, while the large subunit contains the peptidyl-transferase centre. The catalytic role of rRNA means the ribosome is not simply a protein enzyme decorated with passive RNA.
In the familiar bacterial model, 30S and 50S subunits combine into a 70S ribosome. Eukaryotic cytosolic ribosomes commonly have 40S and 60S subunits forming 80S particles. S refers to sedimentation behaviour, influenced by size and shape, so these numbers are not arithmetically additive masses. Organelle ribosomes also require their own qualified descriptions rather than assuming every eukaryotic ribosome is cytosolic 80S.
The A, P and E sites describe positions occupied by tRNAs during the elongation cycle. A is associated with an incoming aminoacyl-tRNA, P with the tRNA bearing the growing peptide before peptide transfer, and E with a departing deacylated tRNA. The positions are stages in a dynamic process, not permanent categories of tRNA molecules.
After peptide-bond formation, the growing chain is temporarily attached to the tRNA in the A site. Translocation then moves that peptidyl-tRNA into the P site. A diagram that labels the P site “where the peptide always is” misses this important intermediate. Tracking the chain before and after transfer resolves the apparent contradiction.
A polysome consists of several ribosomes translating the same mRNA. Each ribosome makes its own polypeptide while moving along the message in the same 5′→3′ direction. A polysome is not several ribosomes cooperatively stitching different pieces of one single polypeptide together.
Full-size scientific diagram. Swipe or scroll horizontally; keyboard users can focus the diagram and use arrow keys.
06 / UNDERSTAND & APPLY
Initiation assembles an mRNA, a small ribosomal subunit, an initiator tRNA and required factors into a correctly positioned complex, followed by association of the large subunit. The initiator tRNA occupies the P site in the standard model. It is an important exception to the simplified statement that all arriving charged tRNAs first enter the A site.
Bacterial initiation often uses an upstream ribosome-binding sequence that pairs with small-subunit rRNA to help position a start codon. Eukaryotic cytosolic initiation commonly involves recognition of the 5′ cap and scanning to an appropriate start site. Both descriptions have exceptions, so they should be introduced as common mechanisms rather than universal rules for every message.
The bacterial initiator commonly carries formylmethionine; the eukaryotic cytosolic initiator carries methionine. These initiation-specific roles do not mean that every methionine within a bacterial protein is formylated. Internal methionine is delivered during elongation by the corresponding elongator tRNA system.
The initial amino-acid residue may later be removed or modified, and proteins may undergo additional processing. Therefore the first residue of a mature isolated protein need not reveal the exact initiation event without further evidence. Translation produces a primary polypeptide whose final functional form may require additional steps.
07 / UNDERSTAND & APPLY
During ordinary elongation, an appropriate charged tRNA enters the A site and pairs with the next codon. Peptide transfer moves the growing chain from the P-site tRNA to the amino acid on the A-site tRNA. The ribosome then translocates by one codon, so the chain-bearing tRNA shifts into the P site and the uncharged tRNA can leave through the E site.
The message is read 5′→3′, while the polypeptide grows from its N terminus toward its C terminus. Adding a residue extends the carboxyl-terminal end of the chain. RNA polarity and protein polarity are different naming systems; writing “the protein grows 5′→3′” confuses nucleic-acid ends with amino-acid chemistry.
Translation consumes energy at several stages. ATP is used in amino-acid activation/charging, and GTP-dependent factors support steps including aminoacyl-tRNA delivery and translocation. The activated bond linking an amino acid or peptide to tRNA contributes to the chemistry of peptide transfer. Do not infer a universal total energy cost from one elongation step without stating what initiation, termination and charging costs are included.
When an in-frame stop codon enters the A site in the standard system, a release factor promotes release of the completed polypeptide from its tRNA. An ordinary stop codon does not encode a special “stop amino acid”, and it is not normally recognised by a standard aminoacyl-tRNA carrying such a residue. Ribosomal components are subsequently recycled.
A chain containing n residues has n−1 peptide bonds if it is one unbranched chain before cleavage. A stop codon contributes zero residues and zero extra peptide bonds. If a problem asks about several separate chains, apply the relationship to each chain rather than subtracting one only once from the pooled amino-acid count.
08 / UNDERSTAND & APPLY
A substitution can leave the encoded amino acid unchanged because of code degeneracy; this is a synonymous change at the coding level. It can instead change an amino acid, producing a missense change, or create a premature stop codon, producing a nonsense change. These labels describe immediate sequence consequences and do not by themselves determine the full biological effect.
A synonymous change is not guaranteed to have no biological effect, because RNA processing, stability or translation can sometimes be altered. Conversely, a missense change is not guaranteed to destroy protein function. The position, chemical substitution, structural context and other evidence matter.
An insertion or deletion not divisible by three within an uninterrupted translated coding region generally shifts the downstream reading frame. A three-nucleotide insertion can preserve the downstream frame while adding or altering residues near the insertion. Preserving the frame does not guarantee a functional protein, and a one-base change does not guarantee every downstream position remains translated before a stop is encountered.
To analyse a sequence change, write the original and altered RNA in the same 5′→3′ direction, mark the stated start and partition both into triplets. Compare the codons and then the amino-acid assignments. This explicit procedure is more reliable than declaring “mutation means frameshift” whenever a sequence differs.
Gene expression is regulated beyond the coding sequence itself. A change in initiation context, RNA processing or translation efficiency can alter protein amount without changing every amino-acid codon. Central-dogma arrows explain information transfer but do not supply a complete quantitative model of how much protein a cell makes.
09 / UNDERSTAND & APPLY
Use the standard code unless the question explicitly specifies an alternative.
ORIGINAL WORKED EXAMPLE
A model coding RNA is 5′-AUG GCU UUC UAA-3′, with initiation at the displayed AUG. Find the initial peptide and its peptide-bond count.
Answer Met–Ala–Phe, with two peptide bonds; the stop contributes no residue.
ORIGINAL WORKED EXAMPLE
The transcribed template segment is 3′-TAC CGA AAA ATT-5′ and its first triplet corresponds to the start. Determine the standard-code peptide.
Answer Met–Ala–Phe.
ORIGINAL WORKED EXAMPLE
Write an exactly complementary anticodon for 5′-GAA-3′ in the 5′→3′ direction, ignoring wobble.
Answer 5′-UUC-3′.
ORIGINAL WORKED EXAMPLE
A fully specified open reading frame produces an initial chain of 120 residues and ends with one stop codon. Count the coding interval including stop and the peptide bonds.
Answer 363 nucleotides including stop; 119 peptide bonds.
ORIGINAL WORKED EXAMPLE
Immediately after peptide transfer but before translocation, is the new growing chain on the P-site or A-site tRNA?
Answer The A-site tRNA at the specified instant.
ORIGINAL WORKED EXAMPLE
In a stated reading frame, UAU changes either to UAC or to UAA. Compare the standard-code consequences.
Answer UAU→UAC is synonymous; UAU→UAA is nonsense. Biological effect still depends on context.
ORIGINAL WORKED EXAMPLE
A protein binds DNA and increases transcription. Does this show protein-to-DNA sequence-information transfer?
Answer No. It is regulation of transcription, not reverse translation.
10 / UNDERSTAND & APPLY
The short invented sequences isolate decoding logic and are not official examination extracts.
ORIGINAL PRACTICE · 1
64 triplets, of which 61 specify amino acids.
Four possible bases at each of three positions give 4³; three triplets are stops.
ORIGINAL PRACTICE · 2
No.
It means multiple codons can specify the same amino acid in the standard code.
ORIGINAL PRACTICE · 3
UAA, UAG and UGA.
They normally recruit termination machinery rather than a stop-carrying tRNA.
ORIGINAL PRACTICE · 4
mRNA 5′→3′; peptide N terminus→C terminus.
Nucleic-acid polarity and protein polarity use different chemical labels.
ORIGINAL PRACTICE · 5
The P site.
Incoming elongator aminoacyl-tRNAs ordinarily enter the A site.
ORIGINAL PRACTICE · 6
Yes.
Start-site establishment is an initiation event; AUG within an established frame is normally a methionine codon.
ORIGINAL PRACTICE · 7
Svedberg coefficients are not additive masses.
Sedimentation depends on particle shape and other physical properties as well as size.
ORIGINAL PRACTICE · 8
40.
One unbranched chain has one fewer peptide bond than residues.
ORIGINAL PRACTICE · 9
126.
3×41+3=126; untranslated regions are excluded.
ORIGINAL PRACTICE · 10
No.
It can preserve the downstream frame while changing a crucial part of the protein.
ORIGINAL PRACTICE · 11
No.
Its RNA product contributes directly to ribosomal structure and function.
ORIGINAL PRACTICE · 12
Several separate polypeptide chains.
Each ribosome independently translates the same message.
KEEP THE BIG IDEAS
READ WITH CONTEXT
The teaching notes, worked explanations and original practice are written for this lesson. Separately labeled selected exam questions identify their exam, year, source and whether wording is paraphrased or an exact excerpt. Educational references do not certify current exam coverage.
Inspected standard-code reference; original short decoding examples, with near-universality and frame limitations explicit.
Inspected translation machinery and stages; intermediate A-site peptide state is explicitly distinguished.
Curricular reference for information transfer, genetic code and translation. No passages, figures or official questions copied.
Analytics use a separately reviewed, incomplete historical corpus. Opening a topic does not change its counts or denominator.