How Do Genes Work?

Are Genes Composed of DNA or Protein?

Chromosomes, the known carriers of genes, are made of DNA and proteins

Proteins are more variable in structure than DNA, whichwas not thought to be sufficiently complex to code for allthe operations of a cell.

DNA = a polymer made of 4 different types of nucleotides (adenine, cytosine, guanine, thymine)

Protein = a polymer made of 20 different types of amino acids

There are two strains of Streptococcus pneumoniae.

ROUGH COLONY (R) SMOOTH COLONY (S)

R strain is benign(Lacks a protectivecapsule, recognizedand destroyed byhost’s immune system)

S strain is virulent(Polysaccharide capsule prevents detection byhost’s immune system)

Experiments showing DNA is the genetic material

Experiments by Griffiths, 1928

Conclusion:

Sample shouldContainNO PROTEIN

Adddeoxy-ribonuclease

Sample shouldContainNO DNA

Addribonuclease

Sample shouldContainNO RNA

Addproteinases

Sample shouldcontainNO PROTEIN

AddDNAse

Sample shouldcontainNO DNA

AddRNAse

Sample shouldcontainNO RNA

Add R cellsAdd R cells

S cellsappear

S cellsappear

No S cellsappear

Transformation occurs No transformation occurs

Conclusion: Transformation cannot occur unless DNA is present.Therefore, DNA must be the hereditary material.

Add R cells

Addproteinases

Heat-killedS cells Lipids

Carbohydrates

1. Remove lipids and carbohydrates from a solution of heat-killed S cells. Proteins, RNA, and DNA remain.

2. Treat solutions withenzymes to destroyproteins, RNA, or DNA.

3. Add a portion of each sample to a culture containing R cells. Observe if transformationoccurs.

Experiments by Avery, MacLeod and McCarty (1944)

Are Genes Composed of DNA or Protein?

Transformation experiments provide the first evidence that genes are DNA.

• Griffith: material from dead virulent Streptococcus can transform benign Streptococcus into a virulent strain.

• Avery et al.: extracted material from dead virulent Streptococcus and treated it with enzymes to destroy either DNA or RNAor proteins. Only DNA-destroying enzymes prevented transformation, hence DNA is the genetic material.

Are Genes Composed of DNA or Protein?

Not everyone was convinced

Hershey and Chase (1952): T2 virus experiments convinces skeptics that DNA is the genetic material since the material injected by the virus into host cells is DNA, not protein.

Bacteriophages are viruses that infect bacterial cells

Host cell genome

Virus particle

1. Viral genomeenters host cell.

Host cell genome

Virus particle

1. Viral genomeenters host cell.

2. Viral genomeis replicated andtranscribed.

DNAmRNA

Host cell genome

Virus particle

1. Viral genomeenters host cell.

2. Viral genomeis replicated andtranscribed.

DNAmRNA

3. Viral particles produced

Protein

New viruses released

4. Particles assembledinside host.

How do viruses work?

Experiments by Hershey and Chase (1952)

Protein contains sulfur, but not phosphorous

DNA contains phosphorous, but not sulfur

Growing viruses with radioactive sulfur will label proteins but not DNA

Growing viruses with radioactive phosphorous will label DNA but not proteins

Experiments by Hershey and Chase (1952)

The structure and replication of genetic material

DNA structure was determined by James Watson and Francis Crick using data from Rosalind Franklin

In order to pass on genetic material from parent (cell) to offspring (cell), the genetic material must be duplicated: DNA replication

One chromosome(Unreplicated state)

One chromosome(Replicated state)

Centromere

Sister chromatids

DNA replication

Figure 16.0 Watson and Crick

Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA

P

P

P

CH25'

3'

O

OH

Base

Structure of dNTPs

Free DNA nucleotides are called dNTPs

1'

2'

4'

Review of DNA structure: DNA and RNA are polymers made of nucleotide subunits

Nucleotide

Ribose Deoxyribose

P O

O

O–

–O

Phosphategroup

ON

4

3 2

1

5 Nitrogenousbase

5-carbonsugar

O OHH

HOHHH

OHHO5CH2

OH

2C

HH

HOH OH

H

N

O

H

N

NH2

Pyrimidines

NH

O

H

N

O

NH2

NN

N

H

N

Cytosine(C)

Adenine(A)

Uracil(U)

Purines

Thymine(T)

Guanine(G)

H3CNH

N

H

H

N N NH2

NH

ON

C3

4C 1C2CC3

1C4C

HO5CH2

O

O

A single strand of nucleotides is made when a

phosphodiester bond is formed between the 3’ C of

one nucleotide and the 5’ C of another

5'

3'

DNA is double stranded with the 2 strands held together by hydrogen bonds between complementary bases

5´

3´ 5´

3´ 3´

3´5´

5´T

G

C

T

T

T

C

G

A

C

T

G

T

T

G

A

C

G

A

A

G

C

G

T

A

A

C

A

C

A

T A

G C

GC

A T

TA

C G

G

A T

T A

C G

T A

A T

C G

A T

Cartoon ofbase pairing

Cartoon of double helix

Space-filling model ofdouble helix

3´5´

3´5´DNA is adouble helix.

Figure 16.6 Base pairing in DNA

Unnumbered Figure (page 292) Purine and pyridimine

Distinguishing between Models of DNA Replication

Three different models of how DNA might replicate were proposed based on DNA structure.

• Semi-conservative replication

• Conservative replication

• Dispersive replication

Figure 16.8 Three alternative models of DNA replication

Distinguishing Between Models of DNA Replication

The Meselsohn and Stahl experiment determines which model is correct.

• 15N was fed to growing E. coli cells to mark DNA (“heavy” DNA), then cells were switched to 14N.

• Newly synthesized DNA will incorporate 14N (“light” DNA.

15N 15N 15N 14N 15N14N

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 1)

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 2)

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 3)

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 4)