Step by Step
1
Replication: DNA → DNA
DNA copies itself before cell division. Each new cell gets a complete copy of the genome. This happens in the nucleus.
2
Transcription: DNA → mRNA
A gene's sequence is copied into messenger RNA by RNA polymerase. This also occurs in the nucleus. The mRNA then exits to the cytoplasm.
3
Translation: mRNA → Protein
Ribosomes read the mRNA sequence and build a protein — one amino acid at a time. This happens in the cytoplasm (or on the rough ER).
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The exception: Reverse Transcriptase
Retroviruses like HIV carry reverse transcriptase — an enzyme that converts RNA back into DNA. This breaks the "one direction" rule and is why HIV can hide in the genome.
Exam tip: If a question mentions a retrovirus or HIV, reverse transcriptase is the key enzyme that defies the central dogma.
Applied Walkthrough
1
A gene for insulin production exists in pancreatic beta cells as a sequence of DNA base pairs.
2
RNA polymerase transcribes the gene into mRNA (still in the nucleus). The mRNA travels to the cytoplasm.
3
Ribosomes read the mRNA codons and assemble the insulin protein — one amino acid per codon.
4
The protein folds, is processed, and is secreted. DNA → RNA → Protein, exactly as the central dogma predicts.
Exam Application
Exams test whether you can correctly sequence the three steps (replication, transcription, translation), identify where each occurs (nucleus vs cytoplasm), and recognize that reverse transcriptase is the exception. Questions about retroviruses almost always hinge on this exception.
⚠ Common Trap
Students confuse transcription and translation. Remember: Transcription = copying into RNA (like transcribing notes). Translation = converting the RNA language into protein (a different language — hence "translation"). Transcription happens in the nucleus; translation happens in the cytoplasm.
✓ Quick Self-Check
1. What are the three steps of the central dogma in order?
Replication (DNA→DNA), Transcription (DNA→mRNA), Translation (mRNA→Protein).
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2. Where does transcription occur?
In the nucleus.
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3. Where does translation occur?
In the cytoplasm, on ribosomes (also on the rough ER).
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4. What enzyme breaks the central dogma rule, and in what virus?
Reverse transcriptase, found in retroviruses like HIV — converts RNA back into DNA.
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5. What does "central dogma" mean in plain English?
Genetic information flows in one direction only: from DNA to RNA to protein — never backward (with the exception of reverse transcriptase).
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