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Overview

🌳 Concept tree

Lesson 1→Identification of DNA as Genetic Material

├── The question

├── Evidence 1 — Bacterial transformation (Griffith, then Avery)
├── Evidence 2 — Bacteriophages (Hershey and Chase)
├── Is DNA the genetic material of every organism?

├── Evidence 3 — The quantity of DNA in cells

├── Worked example 1 — interpreting a Griffith-type result
└── Worked example 2 — predict the Hershey–Chase outcome

Lesson 1: Identification of DNA as Genetic Material · 1 / 9

The question

Observation of dividing cells shows that the chromosomes of the nucleus separate and every daughter cell ends up with the same number of chromosomes as the original.
That strongly suggested chromosomes carry the hereditary information.
But a chromosome is made of two substances DNA and protein.
Which one is the genetic material?

At first, proteins looked like the better candidate.
Genes have to hold a huge variety of instructions; proteins are a diverse, complex group built from 20 different amino acids that can be combined in endless sequences, whereas DNA seemed to have only four kinds of nucleotide.
Scientists therefore expected the proteins to carry the information.
By the 1940s, experiments showed this was wrong DNA carries the genetic information.
This discovery launched molecular biology, the study of the molecular basis of inheritance.

Lesson 1: Identification of DNA as Genetic Material · 2 / 9

Evidence 1 — Bacterial transformation (Griffith, then Avery)

Griffith’s experiment (1928).

The British scientist Frederick Griffith was studying pneumonia bacteria while looking for a vaccine.
He used mice and two strains of Pneumococcus:

Comparison aspect	Main idea	Keywords
S strain (smooth, virulent)	causes fatal pneumonia	causesfatalpneumonia
R strain (rough, non-virulent)	does not kill	doeskill
•	His four groups of mice	fourgroupsmice

Injected with live S bacteria → mice died.
Injected with live R bacteria → mice lived.
Injected with heat-killed S bacteria → mice lived.
Injected with heat-killed S mixed with live R bacteria → some mice died, and live S bacteria were recovered from their bodies.

Conclusion some hereditary material from the dead S bacteria passed into the living R bacteria and converted them into virulent S bacteria.
He named this bacterial transformation.
However, he could not say what the transferring material was.
Avery and colleagues.

They extracted the substance from S bacteria that caused transformation and analysed it: it was DNA.
A strain of bacteria absorbed DNA from another strain and acquired the characteristics of the donor, and – importantly – this new trait was passed on to later generations.
The objection. Critics argued that the extract was not pure and that traces of protein might have caused the change.
The crucial control experiment.

Avery’s group treated the transforming extract with deoxyribonuclease (DNase), an enzyme that completely hydrolyses DNA to nucleotides but does not affect protein or RNA.
After treatment, transformation did not occur.
Therefore DNA was the transforming substance, i.e.
the genetic material.

Lesson 1: Identification of DNA as Genetic Material · 3 / 9

Evidence 2 — Bacteriophages (Hershey and Chase)

A bacteriophage (phage) is a virus that infects bacteria.
Its particle is a DNA molecule enclosed in a protein coat, with a tail-like extension that attaches to the bacterial cell wall.
After attachment, the bacterium bursts open after a time (about 32 minutes in the book’s account), releasing many new phages.
So something passed from phage to bacterium that carried the instructions for making new phages.
Hershey and Chase exploited a chemical difference:

DNA contains phosphorus but no sulphur.
Protein contains sulphur but (almost) no phosphorus.

They grew one batch of phages whose protein was labelled with radioactive sulphur (³⁵S) and another batch whose DNA was labelled with radioactive phosphorus (³²P).
They let each batch infect bacteria, then separated the phage coats from the bacteria (by agitation and centrifugation) and measured radioactivity inside and outside the bacterial cells.

Comparison aspect	Main idea	Keywords
Result	nearly all the labelled DNA (³²P) entered the bacteria the labelled protein (³⁵S) stayed outside	nearlylabelledenteredbacteria
Conclusion	only the viral DNA enters and it directs the bacterium to build new viruses	viralentersdirectsbacterium
•	Hence the genes of the phage are DNA	Hencegenesphage

Lesson 1: Identification of DNA as Genetic Material · 4 / 9

Is DNA the genetic material of every organism?

The two experiments show that the genes of pneumonia bacteria and phages are DNA.
Are all genes made of DNA?
No – some viruses contain RNA instead, and in them RNA is the genetic material (for example, the AIDS virus, influenza virus, polio virus).
These viruses are exceptions, and form a small part of life’s forms; for nearly all living things, DNA is the genetic material.

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Evidence 3 — The quantity of DNA in cells

In a given species (for example chicken), all somatic (body) cells contain the same amount of DNA, but their content of protein varies from one cell type to another.
Gametes contain half the DNA amount of somatic cells. This fits what we expect of genetic material: a zygote forms from a male and a female gamete; if each gamete had the full amount, the quantity would double every generation. Protein quantity does not follow this pattern.
Proteins are continually made and broken down in cells, but DNA, once made, is remarkably stable – as a hereditary store should be.

Lesson 1: Identification of DNA as Genetic Material · 6 / 9

Worked example 1 — interpreting a Griffith-type result

Situation a student mixes heat-killed S bacteria with live R bacteria, but first treats the mixture with an enzyme that destroys DNA, and injects it into mice.
What happens, and why?
Answer The mice survive and no live S bacteria are found.
The enzyme destroyed the transforming principle (DNA), so the R bacteria stayed non-virulent.
This is exactly the logic of Avery’s DNase experiment.

Lesson 1: Identification of DNA as Genetic Material · 7 / 9

Worked example 2 — predict the Hershey–Chase outcome

If the genetic material were protein, where would the radioactivity be after infection by ³⁵S-labelled phages and agitation?
Inside the bacteria.
The real result, with radioactive sulphur found outside, showed the opposite, so the protein is not the genetic material.
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Part 9

📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

DABOUR HUB ACADEMY • EGYPTIAN HIGH SCHOOL (THANAWEYA AMMA)
EXPLANATION Lesson 1: Identification of DNA as Genetic Material
Course Biology (Senior 3) | Chapter 5 – Molecular Biology – DNA Structure and Function | Official Ministerial Syllabus 2026/2027
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1. Academic Lesson Overview & Pedagogical Objectives

The historical journey proving DNA—not protein—is the genetic material: Griffith's bacterial transformation, Avery's enzymatic verification, Hershey-Chase bacteriophage experiments, and DNA quantity measurements.

2. Hierarchy Classification Tree (Cognitive Concept Map)

Below is the hierarchical structural tree organizing the conceptual core of this lesson:

Proof of DNA as Genetic Material

├── 1. Griffith's Transformation Experiment (Pneumococcus pneumoniae)
│ ├── Smooth Strain (S) Virulent, lethal capsule
│ ├── Rough Strain (R) Non-virulent, live non-capsulated
│ └── Heat-killed S + Live R → Mice die! Living S recovered (Transformation occurred!)
├── 2. Avery, MacLeod & McCarty (Active Principle): Isolated transforming substance = DNA!
├── 3.
Decisive Experiment (Deoxyribonuclease Enzyme): Hydrolyzes DNA completely without affecting protein → Transformation halted completely!
├── 4.
Hershey & Chase Bacteriophage Experiment: Radioactive Phosphorus-32 (DNA) vs.
Radioactive Sulfur-35 (Protein) → 32P entered bacteria; 35S stayed outside!
└── 5.
Quantitative Cell Evidence: Somatic cells have constant 2n DNA; Gametes have exactly 1n DNA; Protein quantities fluctuate widely.

3. The Master Teacher's Blackboard & Exam Rules

Comparison aspect	Main idea	Keywords
•	Extracted high-yield rules golden equations and trap warnings from top Egyptian teachers	Extractedhighyieldrules
• RULE 1	Deoxyribonuclease (DNase) breaks phosphodiester bonds of DNA without touching proteins or RNA	DeoxyribonucleaseDNasebreaksphosphodiester
• RULE 2	Phosphorus is in DNA (phosphate group) but absent in proteins Sulfur is in proteins (methionine cysteine) but absent in DNA	Phosphorusphosphategroupabsent
• RULE 3	Gametes contain exactly HALF the DNA of somatic cells (n vs 2n) proving DNA is the hereditary unit	GametescontainexactlyHALF
• TRAP	Griffith did NOT explain the biochemical mechanism of transformation Avery and the DNase enzyme provided the conclusive molecular proof	Griffithexplainbiochemicalmechanism

Lesson 1: Identification of DNA as Genetic Material · 9 / 9

Curriculum

Biology & Geology

Chapter 1 - Support and Movement in Living Organisms

Chapter 2 - Hormonal Coordination in Living Organisms

Chapter 3 - Reproduction in Living Organisms

Chapter 4 - Immunity in Living Organisms

Chapter 5 - Molecular Biology - DNA Structure and Function

Chapter 6 - Molecular Biology - RNA, Protein Synthesis and Genetic Engineering

Chapter 7 - Geology, Earth Sciences and Ecology

Lesson 1: Identification of DNA as Genetic Material