Lesson 4 of 6 / DNA replication and gene expression
Transcribe the template strand
Which DNA strand determines the RNA sequence?
I can explain transcription with sequence direction and complementary pairing.
I can explain transcription, eukaryotic RNA processing and translation.Syllabus
Syllabus 9477, Core 2(c). Explain transcription, eukaryotic RNA processing and translation.
About 6 min
Make a guess. You are not marked.
The mRNA made from a gene has the same base sequence as which DNA strand (with U for T)?
Show the answer
The coding strand, not the template.
RNA is complementary to the template strand, so it matches the coding strand apart from U for T.
The key idea
RNA 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.
Watch out for this
A student says: "mRNA is complementary to the coding strand." What is wrong with this?
Show the answer
It is complementary to the template strand and matches the coding strand apart from U/T.
Check your understanding
Coding DNA is 5-prime ATG CCT 3-prime. Which is the RNA?
Show the answer
5-prime AUG CCU 3-prime
Replace T with U while retaining coding-strand order and direction.
Exam-style questions
Check your understanding
A DNA coding strand reads 5' ATG GCA TTC 3'. Which is the mRNA made from this gene?
Show the answer
5' AUG GCA UUC 3'
mRNA has the same sequence as the coding strand, with U in place of T, because it pairs with the template strand.
Quick recall
Which DNA strand determines the RNA sequence?