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DNA as the Genetic Material Cheat Sheet

A Education › Biology cheat sheet in English by UmeshJagtap

This cheat sheet summarizes key experiments proving DNA as genetic material, highlighting objectives, methods, results, and conclusions.

Biology And Dna Experiment Griffith Avery Macleod Mccarty Hershey Chase
3,868 views 583 downloads 27 Jul 2024 History
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Four Key Criteria for Genetic Material:

Inform­ation:
Contains instru­ctions to build an organism.
Replic­ation:
Capable of accurate copying (DNA replic­ation).
Transm­ission:
Passed from parent to offspring and between cells during division.
Variation:
Accounts for differ­ences within and between species.

Discovery of Genetic Material:

Early Hypotheses (Late 1800s):
August Weismann and Karl Nägeli proposed a bioche­mical basis for inheri­tance.
Chromosome Insight:
Chromo­somes, composed of proteins and DNA, identified as carriers of genetic inform­ation.

Griffi­th's Bacterial Transf­orm­ation Experi­ments:

Experiment Backgr­ound:
Studied Strept­ococcus pneumoniae:
Type S (smooth, virulent) strains produce a polysa­cch­aride capsule.
Type R (rough, non-vi­rulent) strains lack this capsule.

Experi­mental Steps:

Step 1:
Injected live type R bacteria into a mouse → Mouse survived, no live bacteria found.
Step 2:
Injected live type S bacteria into a mouse → Mouse died, live type S bacteria found in blood.
Step 3:
Injected heat-k­illed type S bacteria into a mouse → Mouse survived, no live bacteria found.
Step 4:
Mixed heat-k­illed type S with live type R bacteria → Injected into a mouse → Mouse died, live type S bacteria found in blood.

Conclu­sion:

Genetic material from heat-k­illed type S bacteria transf­ormed live type R bacteria.
This phenomenon was called "­tra­nsf­orm­ati­on" without knowing the bioche­mical nature of the transf­orming substance.
.

Transf­orm­ation Concept:

Living type R bacteria transf­ormed into type S, gaining the ability to produce a capsule.
This transf­orm­ation indicated transfer of genetic material.

Avery, MacLeod, and McCarty

Focus:Invest­igated bacterial transf­orm­ation, following up on Griffi­th's observ­ations to identify the bioche­mical nature of the genetic material.

Experi­mental Approach:

Question: What substance from dead type S bacteria transforms live type R bacteria?
Purifi­cation Process: Purified macrom­ole­cules (proteins, DNA, RNA) from type S Strept­ococcus pneumo­niae.
Found only purified DNA could convert type R to type S bacteria initially.

Detailed Experi­ment:

Step 1:
Mixed purified DNA from type S bacteria with type R bacteria.
Allowed DNA uptake by type R bacteria, converting some to type S.
Step 2:
Enzyme Treatments :
DNase: Digests DNA.
RNase: Digests RNA.
Protease: Digests proteins.
Step 3:
Aggregated type R cells (non-t­ran­sfo­rmed) removed by centri­fug­ation.
Step 4:
Type S cells (trans­formed) remain in the supern­atant.
Step 5:
Supern­atant plated on growth media to observe bacterial colony formation.
Step 6:
Control plates (without DNA extract) showed no type S colonies.

Conclu­sion:

DNA from type S bacteria alone could convert type R bacteria to type S, proving DNA as the genetic material.
Elimin­ation of transf­orm­ation with DNase confirmed DNA's essential role.

Hershey and Chase Experiment

 
Resear­chers: Alfred Hershey and Martha Chase (1952)
Objective: To determine whether DNA or protein is the genetic material in the T2 bacter­iop­hage, a virus that infects E. coli.
Virus Structure Compon­ents:
Capsid (phage coat): Made entirely of protein, consisting of a head, sheath, tail fibers, and base plate.
DNA: Found inside the head of the capsid.
Simpli­city: Composed of only DNA and proteins.

Experi­mental Design

Goal: To identify which component, DNA or protein, enters the bacterial cell and directs the synthesis of new viruses.
Key Insight: T2 phage injects its genetic material into the bacterial cell while the protein coat remains outside.

Method­ology

Labeling:
DNA labeled with 32P (radio­active phosph­orus).
Protein labeled with 35S (radio­active sulfur).
Infection Process:
E. coli cells are infected with either 32P-la­beled phage or 35S-la­beled phage.
Shearing Force:
Use a blender to detach phage coats from bacterial cells after allowing the phages to inject their genetic material.
Centri­fug­ation:
Separate heavier bacterial cells (pellet) from lighter phage coats (super­nat­ant).
Detection:
Measure the radioa­ctivity in the pellet and supern­atant using a Geiger counter.

Results

35S (Protein): Majority found in the supern­atant.
32P (DNA): Majority found in the bacterial pellet.

Conclu­sion:

DNA enters the bacterial cell, not protein. This indicates that DNA is the genetic material respon­sible for the production of new viruses.

Signif­icance

Impact: The experiment provided convincing evidence that DNA, not protein, is the genetic material.
Scientific Legacy: This study was crucial in establ­ishing DNA's role in heredity, greatly influe­ncing molecular biology.

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