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From DNA to RNA to Protein: The Central Dogma · Molecular Cell Biology · HS-LS1-1 · central dogma

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From DNA to RNA to Protein: The Central Dogma

with Medi

A glowing cell nucleus is cut open like a geode, revealing spiraling DNA strands inside; Medi, a sharp-eyed guide in a lab coat, unzips a double helix with one hand and holds up a freshly printed mRNA strand toward the light, explaining the molecular message being copied.

Explain the central dogma as the directional flow of genetic information from DNA to RNA to protein.
Describe the roles of RNA polymerase, mRNA, ribosomes, tRNA, and amino acids in transcription and translation.
Compare transcription and translation by identifying where each occurs, what goes in, and what comes out.
Predict how a mutation in a DNA template strand can alter the resulting protein.
Identify why proteins, not DNA or RNA, carry out most structural and functional work in a cell.

Key terms

Central dogma: The directional flow of genetic information from DNA to RNA to protein
Transcription: The nuclear synthesis of a complementary mRNA strand from a DNA template
Translation: The ribosomal assembly of a polypeptide by reading mRNA codons
Codon: A three-nucleotide unit of mRNA specifying one amino acid or a stop signal
Anticodon: The three-base region of tRNA that pairs complementarily with an mRNA codon

Transcription in the Nucleus

Transcription begins when RNA polymerase binds the promoter and unwinds the double helix. Reading the template strand in the 3-prime to 5-prime direction, it builds a complementary mRNA strand using A, U, G, and C, with uracil replacing thymine. After a termination signal, the primary transcript is processed in eukaryotes: introns are spliced out, a 5-prime cap and poly-A tail are added, and the mature mRNA exits through a nuclear pore.

Translation at the Ribosome

The ribosome reads mRNA three nucleotides at a time. Each codon is matched by a tRNA whose anticodon is complementary and whose other end carries the corresponding amino acid. As the ribosome advances, peptide bonds link incoming amino acids into a growing polypeptide. Translation begins at the start codon AUG and ends when a stop codon (UAA, UAG, or UGA) is reached, releasing the chain to fold into a functional protein.

Why Direction Matters

Information flows DNA to RNA to protein because each molecule has a distinct role. DNA is the stable, protected master copy that stays in the nucleus. mRNA is a disposable working copy that can be mass-produced and degraded on demand, giving the cell quantitative control. Proteins do the actual work of catalysis, structure, and signaling. The lone natural exception is reverse transcription in retroviruses, which copy RNA back into DNA.

Worked examples

Transcribe the DNA template strand 3'-TAC-GGA-5' into mRNA.

Read the template 3-prime to 5-prime and apply base pairing, remembering RNA uses uracil instead of thymine.
Template T pairs with mRNA A, template A pairs with mRNA U, template C pairs with mRNA G, giving the first codon AUG.
Continue with the second template triplet GGA: G pairs with C, G pairs with C, A pairs with U, giving CCU.

Answer: 5'-AUG-CCU-3' (start codon methionine, then proline).

Predict the protein effect when a codon for an amino acid mutates to a stop codon.

A stop codon signals the ribosome to release the polypeptide and end translation.
If it appears prematurely, the ribosome stops before the full sequence is built.
The result is a truncated polypeptide that usually cannot fold correctly and is nonfunctional.

Answer: Translation ends early, producing a truncated, likely nonfunctional protein.

Every trait you have — the enzymes in your stomach, the hemoglobin in your blood, the keratin in your fingernails — is built from proteins. But the instructions for making those proteins are locked inside DNA in the cell nucleus. How does a cell read those instructions and act on them? That is exactly what the central dogma describes. The central dogma states: DNA → RNA → Protein. Information flows in that direction. There is one well-known exception: retroviruses like HIV use an enzyme called reverse transcriptase to copy RNA back into DNA — but in normal human cells, the flow is always DNA to RNA to Protein. Step 1 — Transcription (in the nucleus): An enzyme called RNA polymerase binds to a region of DNA called the promoter and unzips the double helix. It reads the template strand of DNA (3′ to 5′) and builds a complementary strand of messenger RNA (mRNA) using RNA nucleotides — A, U, G, C (note: uracil U replaces thymine T). When the polymerase reaches a stop signal, it releases the mRNA. The mRNA is then processed (introns removed, a cap and tail added) and exits the nucleus through a nuclear pore. Step 2 — Translation (at the ribosome): The mRNA travels to a ribosome, which can be free in the cytoplasm or attached to the rough ER. The ribosome reads the mRNA in sets of three nucleotides called codons. Each codon is recognized by a transfer RNA (tRNA) molecule carrying a specific amino acid on one end and a complementary anticodon on the other. As the ribosome moves along the mRNA, tRNAs deliver amino acids one by one; peptide bonds link them into a growing polypeptide chain. A stop codon (UAA, UAG, or UGA) signals the ribosome to release the finished chain, which then folds into a functional protein. Why does direction matter? DNA stores information stably but cannot leave the nucleus. mRNA is a disposable working copy that can be made in many quantities and destroyed once unneeded. Proteins do the actual work — catalyzing reactions, forming structures, sending signals. The cell controls which genes are transcribed and how much mRNA is made, giving it precise control over its own chemistry. Quick hint for the activity: remember that in transcription, C pairs with G and A pairs with U (not T) in the mRNA copy.

Activity

Drag each molecule or event card into the correct stage of the central dogma pathway to build a working gene-expression timeline.

Practice

Compare transcription and translation by location, input molecule, and output molecule.

Given an mRNA strand, identify the start codon and the reading frame for the first amino acids.

Common mistakes to avoid

Proteins can write information back into DNAProteins lack nucleotide sequences, so under normal conditions information cannot flow from protein back into DNA.
Ribosomes read DNA directly in the nucleusRibosomes read mRNA in the cytoplasm or on the rough ER; mRNA must exit the nucleus first because ribosomes never read DNA.

Check your understanding

During transcription, which enzyme reads the DNA template strand and builds the mRNA?

A student claims that proteins carry genetic information back into DNA to update the cell's instructions. Which statement best explains why this claim contradicts the central dogma?

A point mutation changes a single DNA base in the template strand, converting a codon that specifies the amino acid valine into a stop codon in the mRNA. What is the most likely outcome?

Recap

The central dogma describes information flowing from DNA to RNA to protein. RNA polymerase transcribes a gene into mRNA in the nucleus, the mRNA travels to a ribosome, and tRNAs deliver amino acids codon by codon during translation, building the proteins that do nearly all cellular work.

Reflect

Why is it advantageous for the cell to keep DNA in the nucleus and send out disposable mRNA copies?