Add Met. Add Ala. Then stop.

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4px}.mobile-label{display:block;font-size:11px;font-weight:750;color:#52636a}.cell{padding:12px}.cell-grid{gap:8px}.readiness{grid-template-columns:1fr;gap:3px}.rhythm-grid{grid-template-columns:65px repeat(4,1fr);gap:2px}.rhythm-grid>*{font-size:11px;padding:12px 2px}.ribbon{grid-template-columns:57px 1fr 48px;gap:6px}}@media print{main{max-width:none;padding:0}body{background:#fff}button,.actions{display:none}.view[hidden]{display:block!important}.paper{break-inside:avoid}}</style></head><body class=""><main><header><span class="eyebrow">Toy mRNA · standard genetic code 1</span><h1>Add Met. Add Ala. Then stop.</h1><p>5′-AUG GCU UAA-3′ produces the illustrative two-residue trace Met–Ala. UAA terminates; it never becomes a third amino acid.</p><span class="fiction">Original illustrative example · local practice; answers are not stored</span></header><section class="paper"><div class="strip"><span>5′ → 3′</span><span>Declared frame: base 1</span><span>Positions 1–9</span></div><div id="codons" class="codons"></div><div class="pair" style="margin-top:22px"><div><span class="eyebrow">Current positions and codon</span><h2 id="current" class="large"></h2><p id="question"></p><label for="answer">Code-table result / action</label><select id="answer"><option value="">Choose an answer</option><option>Methionine (Met)</option><option>Alanine (Ala)</option><option>Termination</option><option>Add a “Stop” amino acid</option></select></div><div class="metric secondary"><small>Peptide trace after checked additions</small><p id="peptide" class="large" style="font-size:34px">Empty</p><p id="step-state">Not checked</p></div></div><div class="actions"><button id="check">Check and apply step</button><button id="next">Next codon</button><button id="reset">Reset sequence</button></div><p id="feedback" class="feedback" role="status"></p></section><details><summary>Alternate grouping from base 2</summary><p class="formula">UGG | CUU | AA</p><p>The final AA is incomplete. No base is added to complete it. This grouping comparison does not claim biological initiation at base 2.</p></details><aside class="note"><strong>Lookup convention</strong><p>NCBI’s code table displays T: AUG→ATG, GCU→GCT, UAA→TAA. Keep the supplied mRNA written with U. This is a toy sequence, not an identified natural protein or specimen analysis.</p></aside><footer>Changes and practice answers last only on this page. Keep actual observations, decisions and records in your established school, document or spreadsheet workflow.</footer></main><script>const $=id=>document.getElementById(id);const esc=s=>String(s).replace(/[&<>"']/g,c=>({'&':'&amp;','<':'&lt;','>':'&gt;','"':'&quot;',"'":'&#39;'}[c]));const status=s=>{$('feedback').textContent=s;};
const sequence=[{"codon":"AUG","positions":"1–3","result":"Methionine (Met)","residue":"Met"},{"codon":"GCU","positions":"4–6","result":"Alanine (Ala)","residue":"Ala"},{"codon":"UAA","positions":"7–9","result":"Termination","residue":null}];let step=0,peptide=[],checked=false,terminated=false;function validateMRNA(raw){if(!/^[ACGU]+$/.test(raw))return {error:'Use only A, C, G and U; invalid characters remain uncorrected.'};if(raw.length%3)return {error:'Incomplete triplet; no bases invented.'};return {valid:true};}function render(){const x=sequence[step];$('codons').innerHTML=sequence.map((c,i)=>'<div class="codon '+(i===step?'current':'')+'"><small>'+c.positions+'</small><br><b>'+c.codon+'</b></div>').join('');$('current').textContent=x.codon;$('question').textContent='Positions '+x.positions+': '+(step===2?'What happens now?':step===1?'What is added next?':'Which code-table result applies?');$('peptide').textContent=peptide.length?peptide.join('–'):'Empty';$('step-state').textContent=terminated?'Terminated · two residues':checked?'This step applied':'Not checked';$('next').disabled=!checked||terminated||step===2;$('check').disabled=checked;}$('answer').addEventListener('change',()=>{if(!checked)status('Answer selected. Check and apply this step.');});$('check').addEventListener('click',()=>{const a=$('answer').value,x=sequence[step];if(!a){status('Unanswered. Choose the table result or action.');return;}if(a!==x.result){status('Revise. At positions '+x.positions+', '+x.codon+' gives '+x.result+'. Stop is not an amino acid.');return;}checked=true;if(x.residue)peptide.push(x.residue);else terminated=true;render();status(step===2?'Termination. Retain Met–Ala; no amino acid is added.':x.codon+' → '+x.result+'. Trace: '+peptide.join('–')+'.');});$('next').addEventListener('click',()=>{if(!checked||terminated||step>=2)return;step++;checked=false;$('answer').value='';render();status('Unanswered. Inspect the next three bases.');});$('reset').addEventListener('click',()=>{step=0;peptide=[];checked=false;terminated=false;$('answer').value='';render();status('Reset to base-1 frame, AUG, empty peptide and unanswered state.');});globalThis.Model={sequence,validateMRNA,getState:()=>({step,peptide:[...peptide],checked,terminated})};render();status('Unanswered. Inspect AUG at positions 1–3.');</script></body></html>