DNA Replication (Practical)

Seven marks and a list of seven things to include. Treat the list as a checklist: every named item — the bases, the fork, both strands and all four enzymes — must appear with its role, not merely its name.

7 marks. Describe the process in order, and as each named item becomes relevant give it a sentence saying what it does. Do not list the enzymes separately at the end.

Replication begins when the enzyme helicase binds to the double helix and breaks the hydrogen bonds between the paired bases, unwinding and separating the two strands. This produces a Y-shaped region called the replication fork, at which both parent strands are exposed with their bases available for pairing. Each separated strand will serve as a template for the construction of a new complementary strand.

DNA polymerase cannot begin a new strand from nothing; it can only add nucleotides to an existing chain. The enzyme primase therefore synthesises a short RNA primer complementary to the template, which provides the starting point from which DNA polymerase can extend.

DNA polymerase then travels along each template strand, adding free nucleotides one at a time to the growing chain. Which nucleotide is added at each position is determined entirely by complementary base pairing: adenine is placed opposite thymine and cytosine opposite guanine, so the new strand is an exact complement of the template and each new double helix is identical to the original. DNA polymerase also proofreads as it works, checking each newly added base and removing it if it is incorrectly paired.

Because the two parent strands are antiparallel and DNA polymerase can build only in one direction, the two new strands are made differently. The leading strand is synthesised continuously, in one piece, following the replication fork as it opens. The lagging strand runs in the opposite direction, so it must be synthesised away from the fork in a series of short separate segments called Okazaki fragments, each requiring its own primer.

The RNA primers are subsequently removed and replaced with DNA, and the enzyme ligase then joins the adjacent Okazaki fragments by forming the covalent bonds that complete the sugar-phosphate backbone of the lagging strand, producing one continuous molecule.

The result is two DNA molecules identical to each other and to the original, each consisting of one original parent strand and one newly synthesised strand, which is why replication is described as semi-conservative.

Assess requires a judgement of value against criteria. Describing a paper or bead model of replication is not assessing it; the marks are for what such a model succeeds and fails at, and a verdict that follows.

5 marks. State what a model is for, give what models of replication do well with examples, give what they misrepresent, then judge.

A model is a simplified representation used to make a process observable and manipulable when the real process cannot be. DNA replication occurs at a molecular scale, far below the resolution of a light microscope, and at a speed of hundreds of base pairs per second, so it cannot be directly observed; a physical or diagrammatic model is therefore the only way most students can engage with the sequence of events. A model should be judged on whether it conveys the relationships accurately and on what it distorts.

Models demonstrate several features well. Building a complementary strand from cut-out or coloured components makes the base-pairing rule concrete and self-checking, since an incorrect pairing physically will not fit. A model shows the Y-shape of the replication fork and makes the antiparallel orientation of the strands visible, which in turn makes it clear why one strand must be assembled in fragments — a relationship that is genuinely difficult to grasp from text. Assigning students the roles of helicase, primase, polymerase and ligase shows the sequence in which the enzymes act and that each has one specific job.

Models also misrepresent the process in ways that can create misconceptions. Scale and speed are lost entirely: a model built in minutes cannot convey a process adding hundreds of nucleotides per second. Most models are two-dimensional and straight, so they omit the helical structure and the unwinding that helicase must perform. They are static and directed by the person building them, which suggests the enzymes act purposefully and in an orderly queue, whereas in a cell they are molecules arriving by diffusion and acting simultaneously at many replication origins. Proofreading and error correction are almost never modelled, so the accuracy of replication is presented as automatic rather than actively maintained.

Overall, models are highly valuable for demonstrating DNA replication, since the structural relationships they convey accurately — complementary pairing, the fork, antiparallel strands and the discontinuous lagging strand — are exactly the ones students find hardest and cannot observe directly. Their limitations concern scale, speed and the appearance of purpose rather than the relationships themselves, and each can be addressed by stating explicitly what the model does not represent. A model is therefore best judged not by how closely it resembles the process but by whether the relationships it does represent are the right ones, and by that criterion these models succeed.