Binary Fission and Bacterial Growth: A Step-by-Step Guide

Hi, I’m Kathy with Level Up RN! In this lesson, we’ll cover binary fission—the primary method of bacterial reproduction—and explore the bacterial growth curve in detail.

At the end, I’ll quiz you on key concepts, so stay tuned! If you have our Level Up RN Microbiology Flashcards, grab the ones on binary fission and bacterial growth to follow along. Pay special attention to the bold red text on the back of the cards—these are high-yield facts you’re likely to be tested on!

What Is Binary Fission?

Binary fission is the process by which prokaryotic cells (such as bacteria) reproduce. Unlike mitosis in eukaryotic cells, binary fission is:

• Asexual – No genetic recombination occurs (except for mutations).
• Rapid – Some bacteria, like Escherichia coli (E. coli), can divide every 20 minutes under optimal conditions.
• Efficient – Each daughter cell receives an exact copy of the parent’s genetic material.

This ability to reproduce quickly gives bacteria an evolutionary advantage, allowing them to rapidly colonize environments and adapt to changes.

Steps of Binary Fission

Binary fission follows a highly regulated process to ensure the accurate distribution of genetic material:

1. DNA Replication

• The bacterial chromosome, which is a single circular DNA molecule, begins replication at a specific site called the origin of replication.
• As replication proceeds, two identical copies of DNA are formed.

2. Cell Elongation

• The cell elongates, and the two DNA molecules move toward opposite poles of the cell.
• This separation is guided by a protein network similar to the cytoskeleton in eukaryotic cells.

3. Septum Formation (Cytokinesis)

• A septum (a dividing wall) begins to form at the midpoint of the cell.
• This is controlled by the FtsZ protein, which assembles into a ring structure that helps constrict the cell.

4. Cell Separation

• The septum fully develops, and the two daughter cells detach, each receiving:
  • A complete copy of the bacterial chromosome.
  • A portion of the parent’s cytoplasm, ribosomes, and enzymes.

Generation Time: How Fast Do Bacteria Grow?

The time required for one bacterial cell to divide into two is called the generation time or doubling time.

• For E. coli, this is about 20 minutes.
• For Mycobacterium tuberculosis (the bacteria that causes tuberculosis), it takes 15–20 hours.
• The slower the generation time, the longer it takes for an infection to spread.

This is why some bacterial infections progress rapidly, while others develop over weeks or months.

Bacterial Growth Curve: Understanding Population Dynamics

When bacteria grow in a closed system (no additional nutrients, no waste removal), their growth follows a predictable four-phase curve:

1. Lag Phase (Adjustment Period)

• Bacteria do not divide immediately after entering a new environment.
• They synthesize enzymes, proteins, and RNA needed for replication.
• The duration depends on nutrient availability and bacterial adaptation.

➡ Key concept: Even though the cell count remains stable, bacteria are metabolically active.

2. Log (Exponential) Phase (Rapid Growth)

• Bacteria divide at their maximum rate, leading to exponential population growth.
• The growth rate depends on:
  • Nutrient availability
  • Oxygen levels (for aerobic bacteria)
  • Temperature and pH (bacteria thrive in specific conditions)

➡ Key concept: This phase is when bacteria are most susceptible to antibiotics, because many antibiotics target actively dividing cells.

3. Stationary Phase (Equilibrium)

• Nutrients start to deplete, and waste products accumulate.
• The rate of cell death equals the rate of new cell formation.
• Some bacteria form endospores, allowing them to survive in harsh conditions.

➡ Key concept: Pathogens in this phase often increase toxin production, which contributes to disease symptoms.

4. Death Phase (Decline)

• More cells die than are produced due to toxic waste buildup and nutrient exhaustion.
• Some bacteria resort to cannibalism, breaking down dead cells for nutrients.
• Others form dormant states, awaiting better conditions.

➡ Key concept: This phase explains why bacterial infections can resolve naturally—the bacteria eventually die if they run out of resources.

Memory Trick for the Growth Phases!

To remember that Lag comes before Log, think of the alphabet:

• L (Lag) comes before O (Log) in the word "Log".

This simple trick can help you recall the correct order of bacterial growth phases!

Quiz Time! Test Your Knowledge

Let’s see what you’ve learned!

1️⃣ Where does DNA replication begin during binary fission?

• Answer: The origin of replication

2️⃣ What is the term for the time required for a bacterial population to double?

• Answer: Generation time (or doubling time)

3️⃣ Which phase of the bacterial growth curve is characterized by exponential growth?

• Answer: Log phase

4️⃣ In which phase do dying cells outnumber new cells?

• Answer: Death phase