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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, which was not thought to be sufficiently complex to code for all the operations of a cell.

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are genes composed of dna or protein
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
experiments showing dna is the genetic material
Experiments showing DNA is the genetic material

SMOOTH COLONY (S)

ROUGH COLONY (R)

There are two strains

of Streptococcus pneumoniae.

R strain is benign

(Lacks a protective

capsule, recognized

and destroyed by

host’s immune system)

S strain is virulent

(Polysaccharide capsule prevents detection by

host’s immune system)

experiments by avery macleod and mccarty 1944
Experiments by Avery, MacLeod and McCarty (1944)

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

Heat-killed

S cells

Lipids

Carbohydrates

Add

proteinases

Add

DNAse

Add

RNAse

Add

deoxy-

ribonuclease

Add

proteinases

Add

ribonuclease

2. Treat solutions with

enzymes to destroy

proteins, RNA, or DNA.

Sample should

contain

NO DNA

Sample should

contain

NO PROTEIN

Sample should

contain

NO RNA

Sample should

Contain

NO PROTEIN

Sample should

Contain

NO RNA

Sample should

Contain

NO DNA

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

occurs.

Add R cells

Add R cells

Add R cells

S cells

appear

S cells

appear

No S cells

appear

Transformation occurs

No transformation occurs

Conclusion: Transformation cannot occur unless DNA is present.

Therefore, DNA must be the hereditary material.

are genes composed of dna or protein6
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 protein7
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.
how do viruses work
How do viruses work?

Virus particle

Virus particle

Host cell genome

Host cell genome

1. Viral genome

enters host cell.

1. Viral genome

enters host cell.

DNA

mRNA

2. Viral genome

is replicated and

transcribed.

Virus particle

Host cell genome

New viruses released

1. Viral genome

enters host cell.

DNA

mRNA

4. Particles assembled

inside host.

2. Viral genome

is replicated and

transcribed.

Protein

3. Viral particles produced

experiments by hershey and chase 1952
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
the structure and replication of genetic material

Sister chromatids

DNA replication

Centromere

One chromosome

(Unreplicated state)

One chromosome

(Replicated state)

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
free dna nucleotides are called dntps
Free DNA nucleotides are called dNTPs

P

Structure of dNTPs

P

P

CH2

5'

Base

O

4'

1'

2'

3'

OH

review of dna structure dna and rna are polymers made of nucleotide subunits

Nucleotide

Pyrimidines

NH2

O

O

H3C

NH

O

N

NH

N

5

–O

Nitrogenous

base

P

O

O

N

N

N

O

O

O

O–

1

4

Phosphate

group

H

H

H

2

3

Cytosine

(C)

Uracil

(U)

Thymine

(T)

5-carbon

sugar

Purines

NH2

O

Deoxyribose

Ribose

N

N

NH

N

OH

HO5CH2

OH

HO5CH2

O

O

H

H

H

H

4C

1C

4C

1C

N

N

N

N

NH2

C3

2C

C3

2C

H

H

H

H

H

H

OH

OH

OH

H

Adenine

(A)

Guanine

(G)

Review of DNA structure: DNA and RNA are polymers made of nucleotide subunits
slide17
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'

slide18

T

T

A

A

DNA is a

double

helix.

C

G

G

C

G

C

C

G

A

A

T

T

T

A

T

A

C

G

C

G

G

C

G

A

T

A

T

A

T

T

A

C

G

C

G

T

A

A

T

G

C

T

A

A

T

G

G

C

C

A

T

T

A

Cartoon of

base pairing

Cartoon of

double helix

Space-filling model of

double helix

DNA is double stranded with the 2 strands held together by hydrogen bonds between complementary bases
distinguishing between models of dna replication
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
distinguishing between models of dna replication23
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.

14N

15N

15N

15N

15N

14N

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 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 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 3)
figure 16 9 the meselson stahl experiment tested three models of dna replication layer 4
Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 4)