DNA stays in the nucleus, but proteins are built at ribosomes in the cytoplasm. The cell sends a working copy of one gene out as messenger RNA (mRNA). Making that copy is transcription. It uses the same base pairing as DNA, with one change.
Key idea
RNA polymerase opens a gene and reads its template strand, building a complementary mRNA. RNA has U (uracil) instead of T, so the rules are: DNA A → RNA U, T → A, G → C, C → G.
Why it works
RNA is a nucleic acid too, but its sugar is ribose and it uses U in place of T. U pairs with A just as T does. Only one strand of the gene, the template, is read. The other strand, the coding strand, has the same sequence as the mRNA, except with T where the mRNA has U.
Worked example: From the template strand
A template strand reads 3′-TACGGATTC-5′. What mRNA is made?
- T→A, A→U, C→G, G→C, G→C, A→U, T→A, T→A, C→G.
Answer: 5′-AUGCCUAAG-3′
Worked example: From the coding strand
A gene's coding strand reads 5′-ATGTTCGGA-3′. What is the mRNA?
- The mRNA has the same sequence as the coding strand.
- Just swap each T for U.
Answer: 5′-AUGUUCGGA-3′
Worked example: Working backward
An mRNA reads 5′-AUGGCA-3′. What was the template strand?
- Each mRNA base paired with a template base: A with T, U with A, G with C, C with G.
Answer: 3′-TACCGT-5′
In eukaryotes, the new RNA is also processed before it leaves the nucleus: a cap and a tail are added, and stretches called introns are cut out. The finished mRNA then travels through a nuclear pore to the ribosomes, where translation begins.
Common mistake
Two slips are common. One is writing T in the RNA: RNA never has T, so every A on the template becomes U. The other is copying the template letter for letter. The mRNA is complementary to the template; it matches the coding strand. Check: no T anywhere, and each mRNA base pairs with the template base above it.