Lesson 4 of 6 / DNA and gene expression
Copy a gene into RNA
Which DNA strand is the template?
In this lesson: Describe transcription and relate template, coding and RNA sequences.
About 5 min
The key ideaRNA polymerase uses one DNA strand as a template, making a complementary RNA strand 5' to 3'. The DNA sequence remains in the chromosome.
Explore the idea
Pair RNA with the template
Work along the displayed template 3' to 5'. Pair T with A, A with U, and G with C to make the 5'-to-3' RNA.
Explanation
Transcription makes an RNA copy from a particular DNA region. RNA polymerase associates with the gene's starting region, locally separates the DNA strands and uses one strand as the template. Only a local stretch must be open at a time.
The template is read 3' to 5' while RNA is synthesised 5' to 3'. RNA nucleotides pair with template bases: A pairs with U in RNA, T with A, G with C and C with G. Covalent bonds join the RNA backbone as the chain extends.
The non-template, or coding, DNA strand has the same base order as the RNA when both are written 5' to 3', except that DNA has T where RNA has U. This is a comparison of sequences, not a claim that the coding strand is the one being copied by complementary pairing.
The RNA transcript separates and the DNA strands rejoin. For a eukaryotic protein-coding gene, the initial transcript is pre-mRNA that must be processed. Transcription does not join amino acids or convert the DNA molecule into RNA material.
Step by step
- 1
Find the template direction
RNA polymerase reads the template 3' to 5'.
- 2
Apply RNA complementarity
Use U opposite template A, with the other complementary pairs.
- 3
Keep the stages separate
The product is RNA, which can later be processed and translated.
Worked example
Transcribe a short template
A template segment is 3'-TAC GGA ACT-5'. Write the corresponding RNA.
One way to explain it
5'-AUG CCU UGA-3'. The RNA is complementary and antiparallel to the template, and contains U rather than T.
Why this answer works
- Start at the template's 3' end as displayed.
- Pair each base correctly.
- Label the RNA 5' to 3'.
Is this true? "mRNA is identical to the DNA template strand."
mRNA is complementary to the template. It resembles the coding strand when both are written 5' to 3', with U replacing T.