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<title>Normal cell — how division is controlled</title>
<desc>A wall of cards covering RB1 the brake, p16 keeping the brake on, how RB and E2F interact through a phosphate toggle, what E2F is, the genes E2F switches on, protein production, and p53 the damage guard.</desc>
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<text x="30" y="36" font-size="20" font-weight="700" fill="#23303A">NORMAL CELL — how division is controlled</text>

<!-- RB1 -->
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<text x="48" y="90" font-size="15" font-weight="700" fill="#3A6B7E">RB1 — THE BRAKE</text>
<text x="48" y="118" font-size="12.5" fill="#23303A">RB1 is a gene in every normal human cell.</text>
<text x="48" y="138" font-size="12.5" fill="#23303A">Its RB protein is present permanently.</text>
<text x="48" y="158" font-size="12.5" fill="#23303A">A phosphate tag toggles it ON / OFF — a brake</text>
<text x="48" y="176" font-size="12.5" fill="#23303A">that's always installed; the cell presses or</text>
<text x="48" y="194" font-size="12.5" fill="#23303A">releases it.</text>
<text x="48" y="220" font-size="12.5" font-weight="600" fill="#23303A">JOB: stop the cell dividing until it's</text>
<text x="48" y="234" font-size="12.5" font-weight="600" fill="#23303A">genuinely supposed to.</text>

<!-- p16 -->
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<text x="498" y="90" font-size="15" font-weight="700" fill="#3A6B7E">p16 — KEEPS THE BRAKE ON</text>
<text x="498" y="118" font-size="12.5" fill="#23303A">p16 blocks the enzyme CDK.</text>
<text x="498" y="138" font-size="12.5" fill="#23303A">With CDK blocked, no phosphate goes onto RB,</text>
<text x="498" y="158" font-size="12.5" fill="#23303A">so RB stays active — the brake stays on.</text>
<text x="498" y="184" font-size="12.5" fill="#23303A">It's a protein; its gene is CDKN2A.</text>
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<!-- RB & E2F interact -->
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<text x="48" y="294" font-size="15" font-weight="700" fill="#3A6B7E">HOW RB &amp; E2F INTERACT — the brake cycle</text>
<text x="48" y="322" font-size="12.5" fill="#23303A">• E2F sits parked on the DNA at the start switches of the division genes, ready to fire.</text>
<text x="48" y="344" font-size="12.5" fill="#23303A">• RB binds directly onto E2F, clamping it shut — the division genes stay OFF.</text>
<text x="48" y="366" font-size="12.5" fill="#23303A">• A legitimate "time to divide" signal makes the enzyme CDK stick a phosphate tag onto RB.</text>
<text x="48" y="388" font-size="12.5" fill="#23303A">• Phosphate <tspan font-weight="700" fill="#BC6B2E">ON</tspan> → RB changes shape → it can no longer hold E2F → RB lets E2F go (cell divides).</text>
<text x="48" y="410" font-size="12.5" fill="#23303A">• After division, phosphate <tspan font-weight="700" fill="#3A6B7E">OFF</tspan> → RB returns to shape → it re-grips E2F (brake back on).</text>

<!-- E2F -->
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<text x="48" y="480" font-size="15" font-weight="700" fill="#BC6B2E">E2F — THE "GO" SWITCH</text>
<text x="48" y="508" font-size="12.5" fill="#23303A">A protein called a transcription factor (its job:</text>
<text x="48" y="526" font-size="12.5" fill="#23303A">switch other genes on by binding to DNA).</text>
<text x="48" y="550" font-size="12.5" fill="#23303A">When free, it latches onto DNA and turns on the</text>
<text x="48" y="568" font-size="12.5" fill="#23303A">whole batch of genes needed to copy DNA and</text>
<text x="48" y="586" font-size="12.5" fill="#23303A">divide.</text>
<text x="48" y="610" font-size="12.5" fill="#23303A">It docks at a gene's start switch (its promoter)</text>
<text x="48" y="628" font-size="12.5" fill="#23303A">and signals "read this now", which lets the</text>
<text x="48" y="646" font-size="12.5" fill="#23303A">machinery make the protein.</text>
<text x="48" y="664" font-size="12.5" font-weight="600" fill="#23303A">It recruits RNA polymerase.</text>

<!-- Genes E2F turns on -->
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<text x="498" y="480" font-size="15" font-weight="700" fill="#BC6B2E">GENES E2F TURNS ON</text>
<text x="498" y="508" font-size="12.5" fill="#23303A">DNA-copying machinery, e.g. MCM2–7 &amp; PCNA:</text>
<text x="510" y="528" font-size="12.5" fill="#23303A">• MCM2–7 — the "unzipper" that opens the DNA.</text>
<text x="510" y="548" font-size="12.5" fill="#23303A">• PCNA — clamp that holds the copier onto</text>
<text x="522" y="566" font-size="12.5" fill="#23303A">the DNA strand.</text>
<text x="498" y="590" font-size="12.5" fill="#23303A">Raw materials: enzymes that make the DNA</text>
<text x="498" y="608" font-size="12.5" fill="#23303A">"letters" to copy with.</text>
<text x="498" y="638" font-size="12.5" font-weight="600" fill="#23303A">E2F → genes ON → their proteins are built →</text>
<text x="498" y="656" font-size="12.5" font-weight="600" fill="#23303A">those proteins copy the genome &amp; split the cell.</text>
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<!-- Protein production -->
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<text x="48" y="726" font-size="15" font-weight="700" fill="#BC6B2E">PROTEIN PRODUCTION</text>
<text x="48" y="754" font-size="12.5" fill="#23303A">RNA polymerase reads the gene's DNA.</text>
<text x="48" y="774" font-size="12.5" fill="#6E7F8C">↓</text>
<text x="48" y="794" font-size="12.5" fill="#23303A">Writes a working copy of the recipe (mRNA).</text>
<text x="48" y="814" font-size="12.5" fill="#6E7F8C">↓</text>
<text x="48" y="834" font-size="12.5" fill="#23303A">Ribosomes read the copy and assemble the</text>
<text x="48" y="852" font-size="12.5" fill="#23303A">protein.</text>

<!-- p53 -->
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<text x="498" y="726" font-size="15" font-weight="700" fill="#3A6B7E">p53 — THE DAMAGE GUARD</text>
<text x="498" y="754" font-size="12.5" fill="#23303A">A protein; its gene is TP53.</text>
<text x="498" y="774" font-size="12.5" fill="#23303A">When DNA is damaged or something's wrong,</text>
<text x="498" y="792" font-size="12.5" fill="#23303A">p53 steps in and either:</text>
<text x="510" y="814" font-size="12.5" fill="#23303A">(a) halts the cell and orders a repair, or</text>
<text x="510" y="834" font-size="12.5" fill="#23303A">(b) if damage is too severe, triggers the</text>
<text x="522" y="852" font-size="12.5" fill="#23303A">cell to self-destruct.</text>

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