key figures 4

Avery–MacLeod–McCarty experiment

setup:

• They took heat-killed virulent S strain bacteria (which by themselves can’t cause infection).

• Then they treated these extracts with different enzymes:

  • Protease → destroys proteins.

  • Ribonuclease (RNase) → destroys RNA.

  • Deoxyribonuclease (DNase) → destroys DNA.

  • Control → nothing destroyed.

• After treatment, they added live R cells (non-virulent) to see if they would transform into S cells.

results:

• Control (no enzyme): Transformation occurred → S cells appeared.

• Protease (no proteins): Transformation still occurred → proteins not required.

• RNase (no RNA): Transformation still occurred → RNA not required.

• DNase (no DNA): No transformation → without DNA, R cells stayed R.

DNA was the “transforming principle.

Hershey–Chase experiment

setup: They used a bacteriophage (a virus that infects bacteria).

to track what part of the virus entered the host cell, they labeled:

• DNA with radioactive phosphorus (³²P) → DNA contains phosphorus.

• Protein coat with radioactive sulfur (³⁵S) → proteins contain sulfur (DNA does not).

experiment

1. Phages with ³²P-DNA infected bacteria.

2. Phages with ³⁵S-protein coat infected bacteria.

3. After infection, they blended and centrifuged the mixture to separate the empty viral coats (outside) from the bacteria (inside)

results

• ³²P (DNA) → Found inside the bacteria → DNA entered the host cell.

• ³⁵S (protein) → Found outside, in the phage “ghosts” → protein did not enter.

• Only DNA was passed on to the next generation of phages.

  DNA, not protein, is the hereditary material that directs viral replication.

DNA and RNA

DNA: double stranded

C,A,T,G

A - T

C - G

RNA: single stranded, can unfold it self

sugar = ribose

A - U

G,C,A,U

genotype vs phenotype

genotype

• In this case, S. marcescens has the gene for producing a red pigment.

• The genotype does not change.

phenotype: Even though both plates contain bacteria with the same genotype, the phenotype changes depending on the environment.

• Left plate (28°C): Bacteria express the red pigment gene → colonies look red.

• Right plate (37°C): The gene is not expressed at this temperature → colonies look white/clear.

• So, the genotype is the same, but the phenotype differs due to environmental influence (temperature).

Transformation. transduction, and conjugation

1. Transformation: naked DNA is taken up from the environment

blocked by DNase (because it depends on free DNA floating around).

2. Transduction: genes are transferred between cells in a virus (see The Viral Life Cycle)

works with cell-free extracts and resists DNase (DNA is protected inside phage particles).

3. Conjugation: use of a hollow tube called a conjugation pilus to transfer genes between cells

   requires live, direct cell-to-cell contact (pilus), so cell-free extract won’t work.