DNA Replication: Microbiology Genetics Pre-Nursing, Pre-Med & Health Field Careers


Hi, I’m Kathy with Level Up RN! In this video, we’ll explore DNA replication in prokaryotic cells, an essential concept for your microbiology class. Understanding this process will help you grasp how cells duplicate their genetic material before division.


We’ll also cover key differences between prokaryotic and eukaryotic DNA replication. At the end of this video, I have a quiz to test your knowledge, so stick around!

If you have our Level Up RN Microbiology Flashcards, grab your DNA replication flashcards and follow along. Pay special attention to the bold red text—these are high-yield facts likely to appear on exams.

What is Semiconservative Replication?

DNA replication follows a semiconservative model, meaning that:

  1. The two strands of the parental DNA separate.
  2. Each original strand serves as a template to synthesize a new complementary strand.
  3. After replication, each daughter DNA molecule consists of:
    • One original (parental) strand
    • One newly synthesized strand

This mechanism ensures that genetic information remains consistent across cell generations while reducing the risk of mutations.

Step-by-Step Process of DNA Replication in Prokaryotic Cells

1. Initiation: Unwinding the DNA Helix

DNA replication begins at a specific site called the origin of replication (oriC). This site contains a short, AT-rich sequence that is easier to separate due to fewer hydrogen bonds between A-T pairs.

Key enzymes involved:

  • DNA Gyrase (Topoisomerase II):

    • Relieves supercoiling stress in the DNA ahead of the replication fork by introducing negative supercoils.
    • This is crucial because unwinding the helix generates torsional strain, which must be resolved to allow smooth replication.

  • Helicase (DnaB protein in bacteria):

    • Unwinds and separates the two DNA strands at the replication fork by breaking hydrogen bonds between complementary bases.
    • Requires ATP for energy.

  • Single-Stranded Binding Proteins (SSBs):

    • Stabilize the unwound DNA and prevent reannealing.

2. Formation of the RNA Primer

DNA polymerase cannot initiate DNA synthesis on its own—it needs a starting point.

  • RNA Primase (DnaG protein):
    • Synthesizes a short RNA primer (~10 nucleotides long) complementary to the parental DNA strand.
    • The primer provides a free 3′ hydroxyl (-OH) group, which DNA polymerase requires to start adding nucleotides.

Each new leading strand requires only one primer, while the lagging strand requires multiple primers (one for each Okazaki fragment).

3. Elongation: DNA Synthesis by DNA Polymerase III

DNA Polymerase III (Pol III) is the main enzyme responsible for synthesizing new DNA strands.

  • Adds nucleotides in the 5′ to 3′ direction (attaching new nucleotides to the 3′-OH of the growing strand).
  • Has proofreading activity (3′ → 5′ exonuclease function):
    • Detects and removes incorrectly paired nucleotides, reducing mutation rates.

Leading Strand vs. Lagging Strand

  • Leading Strand:

    • Synthesized continuously in the same direction as the replication fork.
    • Requires only one RNA primer.

  • Lagging Strand:

    • Synthesized discontinuously in short fragments called Okazaki fragments (~1000–2000 nucleotides in bacteria).
    • Each fragment requires its own RNA primer.
    • DNA polymerase must wait for helicase to open more of the replication fork before synthesizing each new fragment.

To visualize this, imagine a zipper:

  • The leading strand follows the zipper smoothly as it opens.
  • The lagging strand must stop, wait, and synthesize DNA in small sections as the zipper moves.

4. Primer Removal and DNA Repair

After elongation, RNA primers must be removed and replaced with DNA.

  • DNA Polymerase I:
    • Removes RNA primers using 5′ → 3′ exonuclease activity.
    • Replaces them with DNA nucleotides.

However, this process leaves small nicks (gaps) between DNA fragments.

  • DNA Ligase:
    • Seals the nicks by forming phosphodiester bonds, creating a continuous DNA strand.

5. Termination: Completing DNA Replication

Since prokaryotic cells have a circular chromosome, replication proceeds bidirectionally until the two replication forks meet at the termination site (ter site).

  • Tus (Terminus Utilization Substance) proteins bind to the ter sequences, blocking helicase and stopping replication.
  • The two circular DNA molecules (new daughter chromosomes) are still interlinked (catenated).
  • Topoisomerase IV resolves this by separating the two interlocked chromosomes, allowing them to be properly segregated into daughter cells.

Key Differences: DNA Replication in Prokaryotic vs. Eukaryotic Cells

Feature Prokaryotic Cells Eukaryotic Cells
Chromosome Structure Single, circular chromosome Multiple, linear chromosomes
Origin of Replication One per chromosome Multiple origins per chromosome
Replication Speed Fast (~1000 nucleotides/sec) Slower (~50–100 nucleotides/sec)
Okazaki Fragments Longer (1000–2000 nucleotides) Shorter (100–200 nucleotides)
Enzymes Uses DNA polymerase III & I Uses DNA polymerase α, δ, ε
End of Replication No telomeres, just terminator sequences Has telomeres to prevent chromosome shortening

Quiz: Test Your Understanding!

  1. Which enzyme unwinds and separates the DNA helix into single strands?

    • Answer: Helicase

  2. New nucleotides are added in which direction?

    • Answer: 5′ to 3′ direction only

  3. The lagging strand is synthesized discontinuously in what type of fragments?

    • Answer: Okazaki fragments

  4. Which enzyme removes RNA primers and replaces them with DNA?

    • Answer: DNA Polymerase I

  5. What is the role of DNA ligase?

    • Answer: Seals gaps (nicks) between Okazaki fragments

Final Thoughts

I hope this deep dive into prokaryotic DNA replication was helpful! If you found value in this video, hit the like button and subscribe to our channel. Share this video with your classmates and fellow nursing students!

Good luck with your studies, and I’ll see you in the next video.

What’s Improved?

✔ More scientific depth (enzymes, proofreading, termination mechanisms)
✔ Better readability (clear structure, bullet points, tables)
✔ Stronger analogies (zipper model)
✔ Engaging quiz for retention

Would you like any more refinements?

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