John Hay
John Hay Biomedical
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Genetics of Disease (Medical Interventions)
Unit 1: Unit 1.1 Diagnostic Testing (ELISA prep)MI 1.1Biotechnology Research and Experiments

Perform Serial Dilutions

Use molecular-test evidence to perform serial dilutions accurately.

Builds on (2 levels back)inferred · high confidence
  • Control logic: Molecular results need positive and negative controls.
  • Signal interpretation: Bands, colors, curves, and E-values must be compared to a rule.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Use molecular-test evidence to perform serial dilutions accurately.

Step 1: Learn the key
Read the control [blank], compare the sample signal to the [blank], and report the result with one [blank].
Four tubes showing stock, 1:10, 1:100, and 1:1000 dilution steps
Step 2: Use the model
Read the figure, table, control, range, or protocol before choosing an answer.
Step 3: Name the limit
Say what the evidence can support and what it cannot prove yet.
Practice

Following the tubes in the figure, you start with a stock of 10^6 CFU/mL and perform four 1:10 dilutions in a row. What is the concentration in the final tube?

Reviewed
Four tubes showing stock, 1:10, 1:100, and 1:1000 dilution steps
  1. A.10^2 CFU/mL
  2. B.10^3 CFU/mL
  3. C.10^4 CFU/mL
  4. D.2.5 x 10^5 CFU/mL
Show the worked solution ▾

Answer: A. 10^2 CFU/mL

  1. Step 1: Count the 1:10 steps: Four 1:10 dilutions are done in a row.
  2. Step 2: Subtract from the exponent: Each 1:10 step divides by 10, lowering the exponent by 1. Four steps lower it by 4.
  3. Step 3: Compute: 10^6 -> 10^5 -> 10^4 -> 10^3 -> 10^2 CFU/mL.

Why it's right: Four tenfold dilutions lower the exponent by 4, so 10^6 CFU/mL becomes 10^2 CFU/mL.

Why the others miss:
  • B: That is only three 1:10 steps, not four.
  • C: That is only two 1:10 steps, not four.
  • D: Serial 1:10 steps divide by 10 each time; they do not subtract a fixed amount.

Aligned to Biotechnology Research and Experiments · reading level ~grade 9

Where you'd see this
  • In Unit 1.1 Diagnostic Testing (ELISA prep), this skill turns class evidence into a result another person can check.
Careers that use this
Molecular Technologist: Uses perform serial dilutions to make a decision from evidence without overclaiming.
Video library
Watch: Perform Serial Dilutions
Serial Dilution - Methods and Calculations
Bio-Resource
Guided notes

Fill these in as you work through the lesson.

Big idea: A serial dilution makes a sample weaker in equal tenfold steps, and each 1:10 step divides the concentration by 10.
Key terms: write the meaning
  • Serial dilution (repeated, equal dilution steps in a row):  
  • Dilution factor (how many times more dilute one step makes the sample):  
  • Stock (the starting, undiluted sample):  
  • Diluent (the liquid added to make the sample weaker):  
The rule

Each 1:10 step divides the concentration by  , so after n tenfold steps the exponent of 10 drops by  .

Check yourself
  1. If the stock is 10^6 CFU/mL, what is it after one 1:10 step? 
  2. How many 1:10 steps take 10^6 CFU/mL down to 10^2 CFU/mL? 
  3. To make 1 mL of a 1:10 dilution, how much sample and how much diluent do you add? 
Work one example

Stock = 10^6 CFU/mL. Four 1:10 steps: 10^6 -> 10^5 -> 10^4 -> 10^3 -> 10^2. Final tube = 10^2 CFU/mL.