Mechanism of DNA Replication — Revision Notes

⚡ 30-Second Revision

Semi-conservative: — Each new DNA has one old, one new strand.
Enzymes:

- Helicase: Unwinds DNA ($H_2$ bonds). - Topoisomerase: Relieves supercoiling. - SSBPs: Stabilize single strands. - Primase: Synthesizes RNA primers. - DNA Polymerase: Synthesizes DNA $5' \rightarrow 3'$. - DNA Ligase: Seals nicks.

Direction: — Always $5' \rightarrow 3'$ synthesis.
Leading Strand: — Continuous synthesis, towards fork.
Lagging Strand: — Discontinuous synthesis, away from fork, forms Okazaki fragments.
Prokaryotes: — Single origin, DNA Pol I (primer removal/gap fill), Pol III (main synthesis).
Eukaryotes: — Multiple origins, Pol $alpha, delta, epsilon$, Telomerase for telomeres.

2-Minute Revision

DNA replication is the semi-conservative process of creating two identical DNA copies from one original molecule. It starts at specific origins of replication where DNA helicase unwinds the double helix, forming a replication fork.

Single-strand binding proteins (SSBPs) stabilize the separated strands, and topoisomerases relieve torsional stress. DNA primase synthesizes short RNA primers, as DNA polymerase can only add nucleotides to a pre-existing 3'-OH group.

DNA polymerase then synthesizes new DNA in the 5' to 3' direction. The leading strand is synthesized continuously towards the replication fork. The lagging strand is synthesized discontinuously in short Okazaki fragments, moving away from the fork, each requiring a new primer.

RNA primers are removed (by DNA Pol I in prokaryotes, RNase H/FEN1 in eukaryotes) and replaced with DNA. Finally, DNA ligase seals the remaining nicks, creating continuous strands. Eukaryotes also use telomerase to replicate the ends of their linear chromosomes, preventing shortening.

5-Minute Revision

DNA replication is a highly accurate, semi-conservative process ensuring genetic fidelity during cell division. The process initiates at specific DNA sequences called origins of replication (Ori). In prokaryotes, there's typically one Ori, while eukaryotes have multiple.

At the Ori, initiator proteins bind, followed by DNA helicase, which unwinds the DNA double helix by breaking hydrogen bonds, creating a Y-shaped replication fork. Single-strand binding proteins (SSBPs) then bind to the separated single strands to prevent re-annealing and degradation.

Ahead of the fork, topoisomerases (like DNA gyrase in prokaryotes) relieve the torsional stress (supercoiling) caused by unwinding.

DNA polymerase, the main enzyme, can only synthesize DNA in the 5' to 3' direction and requires a free 3'-OH group to start. This is provided by short RNA primers synthesized by DNA primase. On the leading strand, which has a 3' to 5' template, synthesis is continuous towards the replication fork from a single primer.

On the lagging strand, with a 5' to 3' template, synthesis is discontinuous. Primase lays down multiple RNA primers, and DNA polymerase synthesizes short DNA segments called Okazaki fragments, moving away from the fork.

In prokaryotes, DNA Polymerase III is the primary replicase, while DNA Polymerase I removes RNA primers (using 5' to 3' exonuclease activity) and fills the gaps. In eukaryotes, DNA Pol $alpha$ synthesizes primers, and Pol $delta$ and $epsilon$ are the main replicases.

After primer removal and gap filling, DNA ligase seals the nicks between DNA fragments, forming continuous strands. Eukaryotes also employ telomerase, a reverse transcriptase, to replicate the ends of their linear chromosomes (telomeres), preventing progressive shortening with each replication cycle.

Prelims Revision Notes

1
Definition: — DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule.
2
Nature: — Semi-conservative (Meselson-Stahl experiment proved this). Each new DNA molecule has one parental and one newly synthesized strand.
3
Direction of Synthesis: — Always $5' \rightarrow 3'$ for the new strand.
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Key Enzymes & Their Functions:

* Origin of Replication (Ori): Specific DNA sequence where replication starts. * Initiator Proteins: Recognize and bind to Ori. * DNA Helicase: Unwinds DNA double helix by breaking H-bonds (requires ATP).

* Single-Strand Binding Proteins (SSBPs): Stabilize separated single strands, prevent re-annealing. * Topoisomerase (e.g., DNA Gyrase in prokaryotes): Relieves supercoiling ahead of the replication fork.

* DNA Primase: Synthesizes short RNA primers (provides 3'-OH for DNA polymerase). * DNA Polymerase: Synthesizes new DNA strands. * Prokaryotes: DNA Pol III (main replicase, high processivity, 3' to 5' proofreading).

DNA Pol I (removes RNA primers via 5' to 3' exonuclease, fills gaps via 5' to 3' polymerase, 3' to 5' proofreading). * Eukaryotes: DNA Pol $alpha$ (primer synthesis), DNA Pol $delta$ (lagging strand synthesis, repair), DNA Pol $epsilon$ (leading strand synthesis, repair), DNA Pol $gamma$ (mitochondrial DNA).

* DNA Ligase: Seals nicks (phosphodiester bonds) between DNA fragments (e.g., Okazaki fragments).

1
Replication Fork: — Y-shaped structure where DNA unwinding and synthesis occur.
2
Leading Strand: — Synthesized continuously, $5' \rightarrow 3'$, towards the replication fork. Requires one primer.
3
Lagging Strand: — Synthesized discontinuously, $5' \rightarrow 3'$, away from the replication fork. Forms Okazaki fragments. Requires multiple primers.
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Okazaki Fragments: — Short DNA segments on the lagging strand.
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Primer Removal: — RNA primers are removed and replaced with DNA.
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Termination: — Replication forks meet. In eukaryotes, telomeres are replicated by telomerase to prevent shortening of linear chromosomes.

Vyyuha Quick Recall

Helping Students Prepare Perfectly Leads To Outstanding Results:

Helicase: Unwinds DNA
SSBPs: Stabilize strands
Primase: Lays RNA Primers
Polymerase: Synthesizes DNA
Ligase: Seals nicks
Topoisomerase: Relieves Tension
Okazaki: Fragments on lagging strand
Replication: Semi-conservative