Source: Practice Questions, Molecular Biology I (UCF)
Tags: DNA vs RNA, transcription, template strand, non-template strand, coding strand, mRNA, nucleotide incorporation, Kozak sequence, start codon, point mutation, missense mutation, codon, gene expression, RNA polymerase
Difficulty: Intermediate Prerequisites: Basic nucleic acid structure (purines, pyrimidines, base pairing), the central dogma (DNA → RNA → protein), familiarity with 5′ and 3′ directionality.
This material sits at the bridge between molecular structure (what DNA and RNA are) and molecular function (how a gene gets expressed into protein). You need a firm grip on how DNA and RNA differ, how transcription copies one strand of DNA into mRNA, and how the ribosome then reads that mRNA to build a polypeptide. If you can follow a sequence from double-stranded DNA all the way through to an amino acid chain, and predict what happens when a single base changes, you are in strong shape for this part of the course.
DNA and RNA differ in sugar, strandedness, bases, and cellular location, but the claim that "DNA is only nuclear" is false because mitochondria and chloroplasts carry their own DNA. Transcription reads the template strand 3′→5′ to synthesise mRNA 5′→3′, and the resulting mRNA sequence matches the non-template (coding) strand with U replacing T. A single-base mutation in the coding strand feeds directly through to mRNA and can swap one amino acid for another (a missense mutation).
Template strand
The DNA strand that RNA polymerase reads during transcription. It is read in the 3′→5′ direction. In simple terms, this is the strand the enzyme physically sits on and uses as a recipe.
Non-template strand (coding strand / sense strand)
The DNA strand that is not read by RNA polymerase. Its sequence matches the mRNA (with T instead of U), which is why it is also called the coding strand. Think of it as the "preview" of what the mRNA will look like.
Kozak sequence
A short consensus sequence surrounding the AUG start codon in eukaryotic mRNA (commonly written as (gcc)gccRccAUGG, where R is a purine). It helps the ribosome recognise where to begin translation. In simple terms, it is the landing signal that tells the ribosome "start reading here."
Start codon (AUG)
The first codon translated by the ribosome, coding for methionine (Met). Every eukaryotic protein begins with Met, although it may be removed after translation.
Missense mutation
A point mutation that changes one codon so that a different amino acid is incorporated into the polypeptide. Think of it as a single spelling mistake that changes the meaning of a word.
S phase
The phase of the cell cycle during which nuclear DNA is replicated. RNA transcription, by contrast, is not restricted to S phase and occurs throughout interphase.
DNA is typically double-stranded; RNA is typically single-stranded.
DNA contains deoxyribose sugar; RNA contains ribose sugar.
DNA uses thymine (T); RNA uses uracil (U) in its place. The other three bases (A, G, C) are shared.
DNA is packaged into chromosomes with histone proteins; RNA is generally not.
DNA replication occurs during S phase of the cell cycle; RNA transcription occurs throughout interphase and is not confined to S phase.
The location trap. It is tempting to say "DNA is only in the nucleus," but this is incorrect. Mitochondria contain their own circular DNA (mtDNA), and in plants, chloroplasts do as well (cpDNA). Both of these organelles sit in the cytoplasm, so DNA is found outside the nucleus too. RNA, meanwhile, is found both in the nucleus (where it is made) and in the cytoplasm (where mRNA is translated).
This distinction is a tested exam point. If a question asks you to identify an incorrect statement about DNA and RNA, "DNA is only nuclear" is a classic wrong claim to watch for.
RNA polymerase binds to the template strand of DNA and reads it in the 3′→5′ direction.
The new RNA strand is synthesised in the 5′→3′ direction (antiparallel to the template).
Each RNA nucleotide added is complementary to the template base:
Template A → RNA U
Template T → RNA A
Template G → RNA C
Template C → RNA G
The resulting mRNA has the same sequence as the non-template (coding) strand of DNA, except every T is replaced by U.
Worked example: predicting the next nucleotide.
Given a template strand 5′-GGAACCCTGCTTCGGAGTCCCAATTAGCATGA-3′ and an RNA that begins 5′-UUGGGAC..., the polymerase is reading the template 3′→5′. The portion of template that produced UUGGGAC (reading 3′→5′) corresponds to the bases ...AACCCTG... on the template. The next template base in the 3′→5′ reading direction is A, so the next RNA nucleotide incorporated is U (since A on the template pairs with U in RNA).
Key point: always determine which direction the polymerase is travelling along the template before trying to predict the next base.
This section traces a single gene from its double-stranded DNA through to its polypeptide product.
Identifying the strands
Non-template (coding) strand: 5′-GTAGACTAGAGCCATGGTCAAGTTTACGATTCAGTCATAGAGATACAGAA-3′
Template strand: 3′-CATCTGATCTCGGTACCAGTTCAAATGCTAAGTCAGTATCTCTATGTCTT-5′
Writing the mRNA
The mRNA matches the non-template strand but with U in place of T:
5′-GUAGACUAGAGCCAUGGUCAAGUUUACGAUUCAGUCAUAGAGAUACAGAA-3′
Finding the start codon and Kozak context
The first AUG in the mRNA is at CCAUGG, which is a good match to the Kozak consensus.
Both the AUG start codon and a recognisable Kozak sequence are required for efficient translation initiation in eukaryotes.
Reading the open reading frame
Starting from AUG, the codons are:
Codon # | Codon | Amino Acid |
|---|---|---|
1 | AUG | Met (start) |
2 | GUC | Val |
3 | AAG | Lys |
4 | UUU | Phe |
5 | ACG | Thr |
6 | AUU | Ile |
Polypeptide: Met-Val-Lys-Phe-Thr-Ile...
Setting up the problem. A C→T mutation occurs in the non-template strand at the position corresponding to codon 5.
Codon 5 on the non-template strand is ACG (positions encoding Thr).
A C→T change turns ACG into ATG on the non-template strand.
The template strand correspondingly changes from TGC to TAC (since the complement of T is A, not G).
The mRNA now reads AUG instead of ACG at codon 5.
AUG codes for Met; ACG coded for Thr.
Result: Thr → Met, a missense mutation at position 5 of the polypeptide.
This is a single amino acid substitution. The protein still gets made, but one residue is different. Whether this matters functionally depends on how critical that position is to the protein's folding or active site.
Students often assume DNA is found only in the nucleus. It is not. Mitochondrial DNA (and chloroplast DNA in plants) resides in the cytoplasm.
Students sometimes confuse which strand RNA polymerase reads. The enzyme reads the template strand (3′→5′), not the non-template strand. The mRNA ends up matching the non-template strand, which is why it is also called the coding strand, but the polymerase never touches it directly.
When a mutation is described as occurring on the non-template strand, students may forget to also update the template strand (and therefore the mRNA). A C→T on the non-template strand means G→A on the template strand, which means C→U on the mRNA. Always trace the change through all three molecules.
Students sometimes number codons starting from the wrong AUG or forget that codon numbering begins at the start codon, not at the 5′ end of the mRNA. The 5′ UTR is not translated.
⚠️ "Identify the incorrect statement" questions about DNA vs RNA are a staple. Memorise the mitochondrial/chloroplast DNA exception.
⚠️ Nucleotide incorporation questions test whether you can correctly orient the template strand 3′→5′ and pick the complementary RNA base (remembering U, not T).
⚠️ Kozak sequence recognition and the role of AUG as a start codon come up in questions about translation initiation.
⚠️ Mutation-tracing problems (DNA → mRNA → amino acid) are high-yield. Be comfortable converting between non-template strand, template strand, and mRNA for any given base change.
True or false: DNA is found exclusively in the nucleus of eukaryotic cells.
Fill in the blank: RNA polymerase reads the template strand in the → direction and synthesises mRNA in the → direction.
True or false: The mRNA sequence is complementary to the non-template (coding) strand of DNA.
Fill in the blank: The Kozak sequence helps the ribosome identify the _______ codon for translation initiation.
True or false: A C→T mutation on the non-template strand will produce a C→U change in the mRNA.
Answers:
False (mitochondria and chloroplasts also contain DNA). 2. 3′→5′; 5′→3′. 3. False (it is identical to the non-template strand, with U replacing T; it is complementary to the template strand). 4. Start (AUG). 5. True.
Q: A question lists five statements comparing DNA and RNA. Which of the following is incorrect: "DNA is only nuclear, RNA is nuclear and cytoplasmic"?
A: This is incorrect because DNA is also found in the cytoplasm, within mitochondria (all eukaryotes) and chloroplasts (plants and algae). The claim that DNA is "only nuclear" is false.
Q: Given a template strand 5′-GGAACCCTGCTTCGGAGTCCCAATTAGCATGA-3′ and an RNA that begins 5′-UUGGGAC, what is the next nucleotide to be incorporated?
A: U. The polymerase reads the template 3′→5′. After the region encoding UUGGGAC, the next template base (in the 3′→5′ reading direction) is A, and the complement of A in RNA is U.
Q: In the gene sequence 5′-GTAGACTAGAGCCATGGTCAAGTTTACGATTCAGT...-3′ (non-template strand), what amino acid does codon 5 encode?
A: Thr (threonine). The reading frame from the AUG start codon gives codons AUG-GUC-AAG-UUU-ACG, and ACG codes for threonine.
Q: If a C→T point mutation occurs in the non-template strand at codon 5 (ACG → ATG), what is the effect on the protein?
A: The mRNA codon changes from ACG (Thr) to AUG (Met). This is a missense mutation: threonine at position 5 is replaced by methionine.
Q: Why is the non-template strand also called the coding strand?
A: Because its sequence is identical to the mRNA (with T in place of U). You can read codons directly from the non-template strand (substituting U for T) without needing to do a complementation step.
This material connects directly to DNA replication (Lectures 7–9), which covers the enzymology of copying both strands during S phase. Understanding template vs non-template here reinforces the concept of leading and lagging strand synthesis.
It also feeds forward into translation and the genetic code, where you will need to convert mRNA codons into amino acids using the codon table. The mutation analysis practised here is the foundation for understanding genetic diseases caused by point mutations (e.g. sickle cell anaemia, where a single missense mutation in the beta-globin gene changes Glu to Val).
The Kozak sequence discussion connects to gene regulation, specifically translational control: not all AUGs are equally efficient start codons, and upstream open reading frames (uORFs) can modulate how much protein is made from a transcript.
DNA vs RNA differences, template strand, non-template strand, coding strand, sense strand, antisense strand, mRNA transcription, RNA polymerase directionality, 5-prime to 3-prime synthesis, nucleotide incorporation, Kozak sequence, Kozak consensus, translation initiation, AUG start codon, methionine, open reading frame, ORF, codon numbering, point mutation, missense mutation, base substitution, Thr to Met, mitochondrial DNA, mtDNA, chloroplast DNA, cpDNA, S phase, cell cycle, molecular biology, UCF, eukaryotic gene expression