Source: Chapter 7 Review Sheet, Cell Biology, University of Florida
Difficulty: Intermediate | Prerequisites: DNA structure, base pairing, central dogma basics (Chapter 5–6)
Tags: transcription, RNA polymerase, DNA polymerase, template strand, coding strand, nontemplate strand, promoter, terminator, gene expression, PCB 3023, cell biology
This is the first half of the central dogma: DNA to RNA. You need a solid grip on DNA structure and complementary base pairing before any of this will click. Transcription is how the cell copies a gene's information into a portable RNA message, and it is the first major control point for gene expression. If you understand how replication works, transcription will feel familiar, but with a few critical differences that examiners love to test.
Transcription copies one strand of a gene's DNA into RNA using RNA polymerase. The enzyme reads the template strand 3' to 5' and builds the RNA 5' to 3', without needing a primer. Promoters tell the polymerase where to start (and which strand to read), while terminators signal where to stop.
Template strand (noncoding strand)
The DNA strand that RNA polymerase reads during transcription. It runs 3' to 5' relative to the direction of transcription. Think of it as the "original" the copier is reading from.
Nontemplate strand (coding strand / sense strand)
The DNA strand that is NOT read by RNA polymerase. Its sequence matches the RNA transcript (except with T instead of U). In simple terms, if you swap every T for a U, you have the mRNA sequence.
RNA polymerase
The enzyme that catalyses transcription. It reads the template strand 3' to 5' and synthesises the RNA chain 5' to 3'. Unlike DNA polymerase, it does not need a primer.
DNA polymerase
The enzyme that catalyses DNA replication. It also reads the template 3' to 5' and builds the new strand 5' to 3', but it requires a primer to begin synthesis.
Promoter
A DNA sequence upstream of (before) the transcription start site that RNA polymerase (or its associated factors) binds to in order to begin transcription. It is asymmetrical, which is what tells the polymerase which strand to read and which direction to go.
Asymmetrical sequence
A DNA sequence that reads differently on each strand. The promoter must be asymmetrical so the polymerase binds in only one orientation, ensuring it transcribes the correct strand in the correct direction.
Terminator
A DNA sequence downstream of the coding region that signals RNA polymerase to stop transcription and release the RNA transcript. The terminator is transcribed along with the coding region (unlike the promoter, which is only bound by protein, not transcribed).
Start site
The first nucleotide of the gene that is transcribed into RNA. The promoter sits upstream of this; the coding region begins at or near this point.
Coding region
The segment of the gene that is transcribed into RNA. It lies between the start site and the stop site.
Transcription start site vs. start codon
The transcription start site is where RNA synthesis begins. The start codon (AUG) is where translation begins. These are not the same position on the mRNA.
DNA replication copies the entire genome, so both strands serve as templates and the whole double helix is duplicated. The result is two complete double-stranded DNA molecules.
Transcription copies only one gene (or one stretch of DNA) at a time, and only one strand carries the meaningful sequence for that gene. Reading both strands simultaneously would produce two different, conflicting RNA messages from the same gene.
The key distinction: replication preserves all genetic information (both strands), while transcription extracts specific information (one strand per gene).
Reaction catalysed: DNA polymerase joins deoxyribonucleotides (dNTPs) into a DNA strand. RNA polymerase joins ribonucleotides (NTPs) into an RNA strand.
Energy source: Both use the incoming nucleoside triphosphate itself. Hydrolysis of the two terminal phosphates (pyrophosphate release) drives the reaction.
Template reading direction: Both read the template strand 3' to 5'.
New chain synthesis direction: Both build the new strand 5' to 3'.
Primer requirement: DNA polymerase requires a primer (short RNA or DNA) to begin. RNA polymerase does not need a primer; it can initiate a new chain from scratch.
The template strand (noncoding strand) is the one RNA polymerase reads. It runs 3' to 5' in the direction of transcription.
The nontemplate strand (coding strand / sense strand) has the same sequence as the RNA transcript (with T instead of U). It runs 5' to 3' in the direction of transcription.
These labels are gene-specific. A strand that serves as the template for one gene might be the nontemplate strand for a different gene elsewhere on the same chromosome. The terms describe a strand's role relative to a particular gene, not a fixed property of that strand.
The promoter is upstream of the start site (toward the 5' end of the nontemplate strand). It is bound by RNA polymerase (and transcription factors in eukaryotes) but is not itself transcribed.
The promoter is asymmetrical: its sequence differs on each strand, so RNA polymerase can only bind in one orientation. This determines which strand is read and which direction transcription proceeds.
The terminator is downstream of the coding region (past the stop site). It is transcribed along with the coding region, so terminator sequences appear at the 3' end of the RNA transcript.
Promoter: bound by protein (RNA polymerase and/or transcription factors) before transcription begins.
Terminator: also interacts with protein (and sometimes forms an RNA hairpin), but crucially, it is transcribed first, then triggers release.
This is a bread-and-butter exam skill. The steps:
You are given the template strand sequence (3' to 5').
Write the complementary RNA sequence (5' to 3'), substituting U for every A in the template. Remember: A pairs with U (not T) in RNA, and C pairs with G as usual.
Using a genetic code table, read the mRNA in triplets (codons) starting from the AUG start codon.
Each codon specifies an amino acid. Write the polypeptide sequence from N-terminus to C-terminus until you reach a stop codon (UAA, UAG, or UGA).
Example:
Template strand (3' → 5'): 3'-TAC GGA CTC ATT-5'
mRNA (5' → 3'): 5'-AUG CCU GAG UAA-3'
Polypeptide: Met-Pro-Glu-(stop)
Transcription is the target of several important drugs. Rifampicin, used to treat tuberculosis, works by blocking bacterial RNA polymerase. Understanding the differences between bacterial and eukaryotic RNA polymerases is how pharmacologists design antibiotics that kill bacteria without harming human cells.
Students often think the coding strand is the one the polymerase reads. It is not. The coding strand matches the RNA; the template strand is what the polymerase actually uses.
Students sometimes confuse the transcription start site with the AUG start codon. These are different positions. The start site is where RNA synthesis begins; the start codon is where protein synthesis begins. There is untranslated RNA (the 5'-UTR) between them.
Students frequently forget that the strand labels (template, coding) can switch for different genes on the same chromosome. These are not permanent labels for a given strand.
Some students think RNA polymerase needs a primer, like DNA polymerase does. It does not.
⚠️ Be able to identify template vs. coding strand from a diagram and write the RNA transcript. This is a near-certain exam question.
⚠️ Know the five points of comparison between DNA polymerase and RNA polymerase (reaction, energy source, direction, primer requirement, substrate type).
⚠️ Understand why the promoter must be asymmetrical, and know the spatial relationship between promoter, start site, coding region, stop site, and terminator.
⚠️ Remember: the promoter is bound but not transcribed. The terminator is both transcribed and bound.
⚠️ The review sheet warns that quiz and exam questions will be specific and in M/C and T/F format. Precise wording matters.
True or False: RNA polymerase reads the template strand in the 5' to 3' direction. → False. It reads 3' to 5'.
Fill in the blank: The ______ strand has the same sequence as the mRNA (with T instead of U). → Coding (nontemplate) strand.
True or False: The promoter is transcribed along with the coding region. → False. The promoter is bound by protein but not transcribed. The terminator is the one that is transcribed.
Fill in the blank: DNA polymerase requires a ______ to begin synthesis; RNA polymerase does not. → Primer.
True or False: The template strand for one gene is always the template strand for every other gene on the same chromosome. → False. Strand roles can switch between genes.
Q: Why does transcription use only one template strand while DNA replication uses two?
A: Replication must duplicate the entire genome, so both strands serve as templates to produce two complete double-stranded DNA molecules. Transcription only needs to copy one gene at a time, and only one strand carries the correct sequence for that gene's RNA product.
Q: Compare DNA polymerase and RNA polymerase with respect to primer requirement and substrate.
A: DNA polymerase requires a primer and uses deoxyribonucleoside triphosphates (dNTPs). RNA polymerase does not require a primer and uses ribonucleoside triphosphates (NTPs). Both read the template 3' to 5' and synthesise 5' to 3'.
Q: What makes the promoter asymmetrical, and why does that matter?
A: The promoter's DNA sequence reads differently on each strand (it is not a palindrome). This asymmetry ensures RNA polymerase binds in only one orientation, which determines which strand is read as the template and the direction of transcription.
Q: Given the template strand 3'-TAC AAA GCA ACT-5', write the mRNA sequence and the encoded polypeptide.
A: mRNA (5' → 3'): AUG UUU CGU UGA. Polypeptide: Met-Phe-Arg-(stop).
Q: Which sequence, the promoter or the terminator, is transcribed as part of the RNA? Which is bound by protein?
A: The terminator is transcribed (it appears at the 3' end of the RNA). Both are bound by protein: the promoter by RNA polymerase/transcription factors before transcription, and the terminator by proteins (or RNA structures) that trigger transcription to stop.
This material connects directly to mRNA processing (Part 2), since the raw transcript produced here must be modified before translation in eukaryotes. It also links to gene regulation, because controlling when and how strongly a gene is transcribed (via promoter activity and transcription factors) is the cell's primary method of regulating gene expression. Understanding transcription vs. replication is foundational for later topics on mutation, DNA repair, and biotechnology techniques like PCR.
transcription, RNA polymerase, DNA polymerase, template strand, coding strand, sense strand, antisense strand, nontemplate strand, noncoding strand, promoter, terminator, asymmetrical sequence, start site, stop site, coding region, 5' to 3' synthesis, 3' to 5' reading, primer, dNTPs, NTPs, gene expression, central dogma, PCB 3023, cell biology, University of Florida, Chapter 7