Difficulty: Intermediate | Prerequisites: Basic DNA structure, nucleotide base pairing
This material covers how cells copy their DNA before dividing, and how the information stored in DNA gets read out to build RNA molecules. These are the two upstream steps of the "central dogma" (DNA → RNA → protein). You need a solid grip on base pairing and the 5ʹ-to-3ʹ directionality of nucleic acids before diving in. Everything in the later units on mutations, gene regulation and cell division depends on understanding replication and transcription first.
DNA replication is semi-conservative: each parental strand serves as a template so that every new double helix contains one old strand and one new one. Transcription copies a gene's template strand into mRNA using RNA polymerase, and in eukaryotes the raw transcript is then processed (capped, tailed, spliced) before it leaves the nucleus. A team of enzymes, each with a distinct job, coordinates both processes.
Helicase
An enzyme that unwinds the two strands of the DNA double helix at the replication fork. Think of it as the zipper-puller that separates the two halves so each can be copied.
Topoisomerase
An enzyme that cuts DNA strands ahead of the replication fork, relieves the torsional strain caused by unwinding, and then rejoins the strands. In simple terms, it prevents the DNA from getting tangled as helicase works.
Single-strand binding proteins (SSBPs)
Proteins that stabilise the exposed single strands of DNA after helicase separates them, preventing the strands from snapping back together or being degraded.
Primase
An enzyme that synthesises short RNA primers on the template strand, giving DNA polymerase a free 3ʹ hydroxyl end to attach nucleotides to. Think of it as laying the first few bricks so the bricklayer (DNA polymerase) knows where to start.
DNA polymerase
The enzyme that adds nucleotide bases (dNTPs, where N = A, C, G or T) to the growing strand in the 5ʹ → 3ʹ direction. One form builds the new strand on the primer; another replaces the RNA primers with DNA.
DNA ligase
An enzyme that joins the Okazaki fragments on the lagging strand into one continuous strand. In simple terms, it is the molecular glue that seals the gaps.
Origin of replication
The specific site on the DNA molecule where helicase and polymerase bind and replication begins.
Telomere
A repetitive nucleotide sequence at the end of each chromosome that protects the chromosome from degradation and shortening during replication.
Telomerase
An enzyme found in germ-line cells, stem cells and cancer cells that adds nucleotides to the template strand at the telomere, extending it so that an RNA primer can bind and the chromosome does not shorten with each division.
DNA replication
The process of duplicating a DNA molecule, during which the parental strands separate and new complementary strands are synthesised.
Semi-conservative model (Watson and Crick)
The confirmed model of replication in which the two parental strands separate and each serves as a template for a new complementary strand. Every daughter molecule contains one old strand and one new strand.
Transcription
The process by which DNA is used as a template to make messenger RNA (mRNA). In simple terms, this is the cell reading one gene and writing it out as an RNA copy.
RNA polymerase
The enzyme that reads the DNA template strand and polymerises ribonucleotide triphosphates into an RNA transcript.
Promoter
A DNA sequence where RNA polymerase and its cofactors (transcription factors) bind to begin transcription.
Terminator
A DNA sequence that signals RNA polymerase to stop transcription and release the transcript.
Transcription unit
The stretch of DNA from promoter to terminator that is transcribed into a single RNA molecule.
Transcription factors
Proteins that recognise and bind to specific nucleotide sequences on DNA to promote or inhibit transcription.
Enhancer
A DNA sequence, sometimes thousands of base pairs away from the gene, that boosts transcription when bound by activator proteins.
Silencer
A DNA sequence that binds regulatory transcription factors and represses transcription of a nearby gene.
Operator
The binding site for a repressor protein, typically found in prokaryotic operons.
Lac operon
A set of prokaryotic genes whose mRNA encodes proteins that import and break down lactose; the operon is expressed only when lactose is present.
mRNA processing
The chemical modifications that convert the primary transcript into mature mRNA, including 5ʹ capping, 3ʹ polyadenylation and intron removal.
Intron
A non-coding sequence that is excised from the primary transcript and degraded during RNA splicing.
Exon
A coding sequence that remains in the mRNA after splicing and is translated into protein.
Spliceosome
A complex of proteins and small nuclear RNAs (snRNAs) that removes introns from the primary transcript.
Ribozyme
An RNA molecule (specifically snRNA within the spliceosome) that acts as an enzyme catalysing the splicing reaction.
Alternative splicing
A process by which the same primary transcript can be spliced in different ways, yielding different mature mRNAs and therefore different protein products from one gene.
Semi-conservative (correct model): Each parental strand serves as a template. After replication, every daughter molecule has one old strand and one new strand.
First round: two "half-old" molecules
Second round: two fully new molecules and two half-old molecules
Conservative (disproved): The original double helix stays intact; an entirely new copy is made alongside it.
Dispersive (disproved): Old and new DNA would be interspersed randomly within each strand.
Meselson and Stahl (1958) used nitrogen isotopes (heavy ¹⁵N vs. light ¹⁴N) and density-gradient centrifugation to prove the semi-conservative model correct. After one round of replication in light medium, all DNA was intermediate density (ruling out the conservative model). After two rounds, half was intermediate and half was light (ruling out the dispersive model).
Helicase unwinds the double helix at the origin of replication, creating a replication fork.
Single-strand binding proteins coat exposed single strands to keep them from re-annealing.
Topoisomerase works ahead of the fork, cutting and rejoining the DNA to relieve the torsional stress of unwinding.
Primase lays down short RNA primers, giving DNA polymerase a free 3ʹ-OH to extend from.
DNA polymerase adds complementary dNTPs in the 5ʹ → 3ʹ direction.
Leading strand: synthesised continuously in the same direction as fork movement.
Lagging strand: synthesised in short Okazaki fragments, each starting from its own RNA primer.
A second DNA polymerase removes the RNA primers and replaces them with DNA.
DNA ligase seals the nicks between Okazaki fragments to produce a continuous lagging strand.
Because DNA polymerase cannot replicate the very end of a linear chromosome (no room for a primer beyond the last base), chromosomes shorten slightly with each division. Telomeres, repetitive sequences at chromosome ends, act as a disposable buffer. In germ-line cells, stem cells and cancer cells, telomerase extends the template strand so that a primer can bind and the full length is maintained.
Transcription is the synthesis of an RNA copy from a DNA template. RNA polymerase reads the template strand 3ʹ → 5ʹ and builds the mRNA 5ʹ → 3ʹ.
Initiation: Transcription factors bind to the promoter, recruiting RNA polymerase. The complex opens the DNA and begins synthesis.
Elongation: RNA polymerase moves along the template strand, adding ribonucleotides complementary to the DNA (A pairs with U in RNA, not T).
Termination: The polymerase reaches the terminator sequence and releases the completed RNA transcript.
Enhancers increase transcription rate when bound by activator proteins. They can be thousands of base pairs away from the gene they regulate.
Silencers decrease transcription when bound by repressor proteins.
Operators (prokaryotes) are binding sites for repressor proteins, typically part of an operon structure.
The lac operon is a classic example: the genes for lactose import and breakdown are transcribed only when lactose is present and glucose is absent.
Before a eukaryotic mRNA can leave the nucleus, it undergoes three modifications:
5ʹ capping: A modified guanine nucleotide is added to the 5ʹ end, protecting the transcript from degradation and aiding ribosome recognition.
3ʹ polyadenylation: A poly-A tail (a string of adenine nucleotides) is added to the 3ʹ end, stabilising the mRNA and facilitating export from the nucleus.
Splicing: The spliceosome (proteins + snRNAs) removes introns and joins exons. The snRNAs within the spliceosome that catalyse this reaction are called ribozymes.
The same primary transcript can be spliced in different combinations, so one gene can produce multiple distinct mRNAs and therefore multiple different proteins. This is a major source of protein diversity in eukaryotes.
Students often think DNA polymerase can start a new strand from scratch. It cannot. It always needs an RNA primer laid down by primase first.
Students frequently confuse the leading and lagging strands. The leading strand is synthesised continuously; the lagging strand is made in short Okazaki fragments because polymerase can only work 5ʹ → 3ʹ.
The Meselson-Stahl experiment is sometimes dated to 1954 in lecture notes, but the experiment was published in 1958. The year 1953 is when Watson and Crick published their structure of DNA. Be precise if your exam asks for dates.
Introns are not "junk." They are removed from the transcript, but they play roles in gene regulation and enable alternative splicing. Calling them useless is an oversimplification.
⚠️ Know every enzyme at the replication fork and its specific function. Exam questions love asking "what would happen if enzyme X were missing?"
⚠️ Be able to distinguish the three replication models and explain how the Meselson-Stahl experiment ruled out each incorrect one.
⚠️ Understand the direction of synthesis (5ʹ → 3ʹ) and why the lagging strand requires Okazaki fragments.
⚠️ Know the three steps of eukaryotic mRNA processing (5ʹ cap, poly-A tail, splicing) and what each one does.
⚠️ Alternative splicing is a common exam topic because it explains how a relatively small genome can produce a much larger proteome.
True or False: DNA polymerase synthesises new strands in the 3ʹ → 5ʹ direction. (False, it works 5ʹ → 3ʹ.)
Fill in the blank: The enzyme that seals Okazaki fragments together is __________. (DNA ligase.)
True or False: In the semi-conservative model, after one round of replication both daughter molecules contain one old strand and one new strand. (True.)
Fill in the blank: Non-coding sequences removed from the primary transcript during splicing are called __________. (Introns.)
True or False: RNA polymerase requires a primer to begin transcription. (False, only DNA polymerase needs a primer.)
Q: Name the enzymes at the replication fork in the order they act, and state what each one does.
A: Helicase unwinds the double helix. Topoisomerase relieves torsional strain ahead of the fork. Single-strand binding proteins stabilise the separated strands. Primase synthesises RNA primers. DNA polymerase adds nucleotides to the primer. A second DNA polymerase replaces RNA primers with DNA. DNA ligase seals the gaps between Okazaki fragments.
Q: How did the Meselson-Stahl experiment distinguish the semi-conservative model from the conservative and dispersive models?
A: Bacteria were grown in heavy ¹⁵N medium, then transferred to light ¹⁴N medium. After one generation, all DNA was intermediate density (ruling out the conservative model, which predicted one heavy and one light band). After two generations, half was intermediate and half was light (ruling out the dispersive model, which predicted all molecules at an intermediate density that gradually shifted lighter).
Q: A eukaryotic primary transcript has five exons and four introns. How many different proteins could alternative splicing theoretically produce?
A: More than five. Different combinations of exons can be joined, so the number of possible distinct mRNAs exceeds the number of exons. The exact number depends on which exons are included or skipped in each variant.
Q: Why do chromosomes shorten with each round of replication, and which cells avoid this problem?
A: DNA polymerase cannot replicate the very end of a linear chromosome because there is no space for a primer beyond the final base. Telomeres absorb this shortening. Germ-line cells, stem cells and cancer cells express telomerase, which extends the template strand so the full length is maintained.
Q: What is the functional difference between an enhancer and a promoter?
A: The promoter is where RNA polymerase and general transcription factors assemble to initiate transcription. An enhancer is a distant regulatory sequence that, when bound by activator proteins, increases the rate of transcription at the promoter. The promoter is required for transcription to occur at all; the enhancer modulates how much occurs.
Replication errors feed directly into the mutations unit: if DNA polymerase misincorporates a base and proofreading misses it, that becomes a point mutation. Transcription connects forward to translation (the next step in the central dogma) and to gene regulation, because controlling when and how much mRNA is made is the cell's primary way of controlling which proteins are present.
DNA replication, semi-conservative replication, Meselson-Stahl experiment, replication fork, helicase, topoisomerase, primase, DNA polymerase, DNA ligase, Okazaki fragments, leading strand, lagging strand, telomere, telomerase, origin of replication, transcription, RNA polymerase, promoter, terminator, transcription unit, transcription factors, enhancer, silencer, operator, lac operon, mRNA processing, 5ʹ cap, poly-A tail, intron, exon, spliceosome, ribozyme, alternative splicing, central dogma, BIO 151 Exam 2