Lesson 4 of 6 / DNA replication and gene expression
Transcribe the template strand
Which DNA strand determines the RNA sequence?
In this lesson: Explain transcription with sequence direction and complementary pairing.
About 6 min
The key ideaRNA polymerase reads one DNA template strand and makes a complementary RNA molecule 5-prime to 3-prime.
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.
Check the coding strand too
The displayed template is 3'-TAC GGA ACT-5'. Its coding DNA partner is 5'-ATG CCT TGA-3'. The RNA is 5'-AUG CCU UGA-3': the coding-strand order with U in place of T.
RNA polymerase reads the template 3' to 5' and extends RNA 5' to 3'. A promoter selects the transcription unit; this short sequence shows pairing, not promoter recognition or a complete gene.
Explanation
Transcription begins when transcription machinery recognises a promoter and locally separates DNA strands. One strand serves as the template for that transcription unit. RNA polymerase links RNA nucleotides by phosphodiester bonds, using complementary base pairing to the template.
RNA is made 5-prime to 3-prime while the template is read 3-prime to 5-prime. RNA contains U opposite template A. The coding, or non-template, DNA strand has the same base order as the RNA when both are written 5-prime to 3-prime, except T in DNA replaces U.
As polymerase progresses, RNA separates and the DNA strands re-pair behind it. Transcription ends at an appropriate termination signal. Only the relevant region is copied; transcription is not replication of the entire genome and does not make a new DNA strand.
In eukaryotes, protein-coding genes first yield pre-mRNA in the nucleus, which must be processed before normal export and translation. In bacteria there is no nuclear envelope, allowing transcription and translation to be closely coupled. The same general information flow occurs in distinct cellular settings.
Step by step
- 1
Name the strand
Do not use coding and template interchangeably.
- 2
Use complementary bases
A in the DNA template pairs with U in RNA.
- 3
Write directions
Reverse-complement a template given 5-prime to 3-prime.
Worked example
Work through the evidence
Template DNA is 3-prime TAC GGA 5-prime. What RNA is transcribed?
One way to explain it
5-prime AUG CCU 3-prime.
Why this answer works
- T pairs with A, A with U, C with G and G with C.
- The RNA and template run antiparallel.
Is this true? "mRNA is complementary to the coding strand."
It is complementary to the template strand and matches the coding strand apart from U/T.