Main principles and stages of the process
Replication is based on a template mechanism. This means that one DNA molecule acts as a template, based on which another molecule with an absolutely complementary structure is synthesized. The process is semiconservative: each daughter helix consists of one intact (conservative) strand inherited from the parent molecule and one newly synthesized strand.
Globally, there are three stages of replication:
- Initiation. Assembly of the enzyme complex at specific starting sites of the DNA. It is at this stage that the regulation of replication primarily occurs.
- Elongation. The actual lengthening of the polynucleotide chain.
- Termination. Completion of the doubling process.
Structure of the replication fork and asymmetric synthesis
To initiate synthesis, initiator proteins bind to the DNA, the double helix unwinds, and hydrogen bonds between nitrogenous bases break. This forms the replication fork, a Y-shaped structure that moves sequentially along the DNA. Because enzymes can only build a new strand in one direction (from the 5' end to the 3' end), synthesis proceeds asymmetrically and is semidiscontinuous:
- Leading strand (Leading strand). Synthesized continuously because its direction coincides with the movement of the replication fork. Requires only a single initiation event.
- Lagging strand (Lagging strand). Synthesized discontinuously in short fragments (called Okazaki fragments) because the direction of synthesis here is opposite to the movement of the fork. The length of such fragments is 1000–2000 nucleotide pairs in prokaryotes and 100–200 nucleotide pairs in eukaryotes.
Enzymatic machinery
The semiconservative model is ensured by the complex biochemical work of a whole array of enzymes (utilizing ATP energy):
- DNA helicase. Unwinds the double helix, breaking hydrogen bonds between the strands.
- SSB proteins (single-strand DNA-binding proteins). Bind to the resulting single-stranded DNA. They do not cover the nitrogenous bases, but prevent the strands from re-annealing.
- DNA topoisomerase. Relieves supercoiling (mechanical stress ahead of the fork). The enzyme makes a single-strand break, allows the strand to swivel, and reseals it.
- RNA primase. Synthesizes a short RNA primer. This is necessary because the main synthetic enzyme cannot start work "from scratch"—it needs a free 3'-OH end.
- DNA polymerase. The key enzyme that extends the polynucleotide chain (strictly 5' → 3'). It also possesses nuclease activity: 5'-3' exonuclease activity is needed to remove RNA primers, while 3'-5' exonuclease activity performs a proofreading function (removes erroneous nucleotides).
- DNA ligase. Seals nicks between Okazaki fragments on the lagging strand, restoring a robust phosphodiester bond.
Features of replication in prokaryotes and eukaryotes
Prokaryotes (e.g., E. coli) have a monoreplicon mechanism. They have only a single replication origin (ori, from origin), consisting of tandem highly conserved sequences. From this point, synthesis proceeds in both directions at equal speed until the chromosome is completely doubled. There are 5 known types of DNA polymerases (I, II, III, IV, V) responsible for replication and repair.
Eukaryotes feature a polyreplicon mechanism. Replication starts bidirectionally at multiple points simultaneously—ARS (autonomously replicating sequences). The region between adjacent points is called a replicon (hundreds of replicons are involved in the process). As synthesis proceeds, they form "replication bubbles," which then fuse. Eukaryotes have 7 types of polymerases: $\alpha, \beta, \gamma, \delta, \epsilon, \zeta, \eta$, localized in the nucleus, mitochondria, and plastids.
Briefly: answers to topical questions
What specific functions are performed by different types of DNA polymerases in eukaryotes (alpha, delta, epsilon, etc.)?
In eukaryotes, DNA polymerases ensure the synthesis of new strands and participate in the replication and repair of genetic material. There are 7 known types of these enzymes ($\alpha, \beta, \gamma, \delta, \epsilon, \zeta, \eta$), localized in the nucleus, mitochondria, and plastids.
Functions of the polymerases described in the sources:
- DNA polymerase $\alpha$ — possesses primase activity, synthesizes the RNA primer to initiate DNA synthesis.
- DNA polymerase $\delta$ — acts as the main replicative enzyme catalyzing the synthesis of the leading strand as well as the elongation of the lagging strand.
- DNA polymerase $\beta$ — participates in repair processes.
The specific functions of polymerases $\epsilon, \gamma, \zeta$, and $\eta$ are not detailed in the provided materials.
To remember the fork enzymes in order: Helicase "Hustles" (unwinds), Primase "Presents" (places the primer), Polymerase "Produces" (builds the chain), Ligase "Locks" (joins the fragments).
Class and exam questions
Why does synthesis on the lagging strand occur discontinuously?
DNA polymerase is capable of attaching nucleotides in only one direction—from the 5'- to the 3'-end. Since DNA strands are antiparallel and the fork unwinds in one direction, synthesis on the lagging strand is forced to proceed 'backwards' in short segments (Okazaki fragments).
Why are RNA primers needed if DNA is being synthesized?
DNA polymerase cannot initiate chain synthesis on its own; it strictly requires a free 3'-OH end to attach the first nucleotide. RNA primase lacks this limitation and can create a starting primer on a blank template.
What happens to the RNA primers at the end of replication?
They do not remain part of the mature DNA. Enzymes (via 5'-3' nuclease activity) remove the primers, the resulting gaps are filled with deoxyribonucleotides, and DNA ligase seals the remaining nicks.
What else to study on this topic
- The mechanism of relieving supercoiling using DNA topoisomerase
- Proofreading function of DNA polymerases (3'-5' exonuclease activity)
- Replication origins: structure of ori and ARS sites
- Specific functions of various types of DNA polymerases (I–V in E. coli and $\alpha$–$\eta$ in eukaryotes)
- Fusion of replicons and the spatial configuration of the replication bubble
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "DNA Replication" and 3,793 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in "Molecular Genetics"
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.