The Search for the Genetical Material: Experiments

Amino Acids

What is the monomer of proteins?

1. Carbon (C)

2. Hydrogen (H)

3. Oxygen (O)

4. Nitrogen (N)

What are the elements that comprise proteins?

Nucleotides

What is the monomer of DNA?

1. Carbon (C)

2. Hydrogen (H)

3. Oxygen (O)

4. Nitrogen (N)

5. Phosphorus (P)

What are the elements that comprise DNA?

Proteins & DNA

What are the key components of chromosomes?

Genes

What were found in chromosomes?

20

How many variety of protein monomers are there?

4

How many variety of DNA monomers are there?

Proteins have a greater structural variety and it makes sense considering that they have a multitude of functions such as being enzymes, hormones, and structural components among others.

Between proteins and DNA, which has greater structural variety?

Its Phosphate Groups

What about DNA makes it acidic?

It has a negative charge due to the bonding of its components: Phosphorus and Oxygen atoms. These bonds leave extra electrons, making the DNA negatively charged.

Describe DNA in terms of charge. What gives DNA this charge?

Griffith’s Experiment (1928)

This experiment refers to the study where two strains of pneumonia bacterium were tested on mice.

Frederick Griffith

What is the full name of the scientist who conducted the Griffith’s Experiment (1928)?

Streptococcus pneumoniae

What is the full scientific name of the bacterium used in Griffith’s Experiment (1928)?

1. S (Smooth) Strain

   • Virulent/Pathogenic

2. R (Rough) Strain

   • Nonvirulent/Nonpathogenic

What are the two strains of bacterium and describe each.

The cells in this bacteria strain have an outer capsule (made out of polysaccharides), which protects them from the animal’s immune system.

What makes the S Strain bacteria pathogenic?

The cells in this bacteria strain do not have an outer capsule to protect itself from being detected, thus it is immediately eliminated by the animal's immune system.

Why is the R Strain nonpathogenic?

The heat-killed S strain alone does not cause disease, which suggests that live bacteria are required for virulence. This means that the polysaccharide capsule of the S strain is not enough by itself to cause infection; the bacteria must be alive and capable of reproducing.

In Griffith’s Experiment (1928), what does the setup with the heat-killed S cells tell you?

1. The presence of S cells at the end of the experiment signifies that the R cells could not have just acquired an outer capsule as a conversion from R cell to S cell occurred. Rather, that give evidence to the theory that R cells gained the pathogenic components (DNA) of the S cells that are what determined the composition of the S cells in the first place.

2. Additionally, the fact that the resulting S cells are able to reproduce – forming more S cells – means that the initial R cells indeed gained the genetic material and not just the capsule of the S cell.

How did the Griffith’s Experiment (1928) cross out the possibility that the R cells simply used the dead S cells’ capsules to become pathogenic?

The R cells went through what Griffith called the “transforming principle” and taken up the genetic material from the heat-killed S cells, which allowed them to “transform” into smooth-coated bacteria and become virulent.

What had caused the non-pathogenic R cells to become pathogenic in the presence of heat-killed S cells?

This is because heat denatures most proteins, so it’s more likely that the genetic material that transferred from the heat-killed S cell’s to the R cells is DNA.

In the end of the experiment, why did Griffith conclude that it was the DNA that transferred from the heat-killed S cells to R cells’ and not protein?

Transformation

In Griffith’s Experiment (1928), some chemical component of the dead pathogenic cells caused a heritable change in the nonpathogenic cells. Griffith called this _____________?

Griffith’s Experiment (1928)

What study led to the identification of DNA as the genetic material?

Proteins & DNA

What are the main components of chromosomes?

• This was because it has plenty known building blocks (200 that have their own specificities in function compared to DNA (4).

• Plus, not much was known about DNA back then.

Why was protein the most likely candidate to be behind the genetic code of life back in the day?

Friedrich Miescher (1871)

On an early study of DNA, what is the full name of the scientist who isolated an acidic substance from the nuclei of white blood cells.

1. Nuclein

2. Nucleic Acid

The acidic substance from the nuclei of white blood cells was first named 1._______ which sooner became 2._______ _____.

Avery, McCarty, and MacLeod's Experiment (1944)

What experiment attempted to identify Griffith’s “transforming agent” by purifying the chemicals from heat-killed S cells that was tested on live R cells for transformation?

1. Oswald Theodore Avery

2. Maclyn McCarty

3. Colin Munro MacLeod

What are the full names of the scientists involved in the Avery, McCarty and MacLeod's Experiment (1944)?

They isolated the biomolecules in the heat-killed S cells that were the possible transformative agents and allowed them to mix with the R strain cells.

How exactly did Avery, McCarty and MacLeod refine Griffith's experiment?

No transformation occurred (no S cells reproduced).

In Avery, McCarty, and MacLeod’s Experiment (1944), what was the result for the setup with RNA?

No transformation occurred (no S cells reproduced).

In Avery, McCarty, and MacLeod’s Experiment (1944), what was the result for the setup with Proteins?

A transformation occurred (S cells reproduced).

In Avery, McCarty, and MacLeod’s Experiment (1944), what was the result for the setup with DNA?

No transformation occurred (no S cells reproduced).

In Avery, McCarty, and MacLeod’s Experiment (1944), what was the result for the setup with Lipids?

No transformation occurred (no S cells reproduced).

In Avery, McCarty, and MacLeod’s Experiment (1944), what was the result for the setup with Carbohydrates?

The molecule that carries the heritable information is DNA.

In Avery, McCarty, and MacLeod’s Experiment (1944), based on the results of the fractionation of the S cell extract into classes of molecules, what can we infer?

1. Lytic Cycle

2. Lysogenic Cycle

What are the two cycles in the Viral Life Cycles?

Lytic Cycle

In what viral life cycle are new viruses released?

Lysogenic Cycle

In what viral life cycle does the host cell temporarily remain alive?

Lysogenic Cycle

In what viral life cycle will the host cell reproduce?

Lysogenic Cycle

In what viral life cycle is the viral DNA integrated with the host cell’s DNA?

Hershey and Chase's Experiment (1952)

What experiment used bacteriophages to confirm that DNA was the genetic material?

1. Alfred Day Hershey

2. Martha Cowles Chase

What are the full names of the scientists involved in Hershey and Chase's Experiment (1952)?

Escherichia coli

What bacteria was used in Hershey and Chase's Experiment (1952)?

T2 phage

What bacteriophage was used in Hershey and Chase's Experiment (1952)?

DNA

The T2 phage genome (located inside its head) consists of what biomolecule?

Proteins

All the other parts of a T2 phage consist of what biomolecule?

1. 35S (Sulfur)

2. 32P (Phosphorus)

What were the labels of the radioactive isotopes placed in the T2 phage?

Proteins/Amino Acids

What biomolecules in the T2 phage does the 35S correspond to?

DNA

What biomolecules in the T2 phage does the 32P correspond to?

1. Supernatant

2. Pellet

After infection, agitation, and centrifugation, the 1.__________ and 2._______ were analyzed by Hershey and Chase.

Pellet

In centrifugation, this portion is the solid and denser material that settles on the bottom of the test tube.

Supernatant

In centrifugation, this portion is the clear/translucent liquid remaining above the pellet. It contains smaller and lighter components.

Supernatant

In the setup with radioactive 35S, where was radioactivity detected, in the pellet or supernatant?

Pellet

In the setup with radioactive 32P, where was radioactivity detected, in the pellet or supernatant?

• When proteins were labeled, radioactivity remained outside the cells (supernatant), but when DNA was labeled, radioactivity was found inside the cells (pellet).

• This means that the phage's DNA entered the bacterial cells and the phage's proteins did not. Therefore, DNA, not protein, functions as the genetic material of the T2 phage.

What can be inferred from the results of Hershey and Chase's Experiment (1952)?

Chargaff's Rules (1950)

What set of principles focuses on the base composition analysis of DNA?

Chargaff's Rules (1950)

What study involved extracting DNA from different organisms, hydrolyzing them into individual nucleotides, and analyzing them chemically.

Erwin Chargaff

What is the full name of the scientist behind Chargaff's Rules (1950)?

• A = T & G = C

• A & T > G & C

What is the ratio of the DNA's base compositions according to Chargaff's Rules (1950)?

Watson & Crick's Model of DNA (1953)

What study refers to the discovery of the double helix structure of DNA?

"Photo 51"

What is the name of the picture that corresponds to the X-ray crystallography photo that helped Watson & Crick model the 3d structure of DNA?

Rosalind Elsie Franklin

What is the full name of the scientist who made the X-ray crystallography image — Photo 51?

Maurice Hugh Frederick Wilkins

What is the full name of he scientist who took Photo 51 and showed it to Watson without Franklin's knowledge?

1. James Dewey Watson

2. Francis Harry Compton Crick

What the are full names of the scientists involved in the Watson & Crick's Model of DNA (1953)?

It revealed the structure of monomers that repeated at regular intervals in DNA and how it had a corkscrew shape of a helix.

What did Franklin's X-ray crystallography revealed in DNAs structure?

Outside of the helix

Based on Watson & Crick's Model of DNA (1953), where is the DNA's Sugar-Phosphate Backbone located?

This is because it is hydrophilic in nature, making it compatible with the surrounding aqueous environment.

Why is the Sugar-Phosphate Backbone of DNA located outside its helix?

This is due to the negatively charged phosphate groups with in it.

Why is the Sugar-Phosphate Backbone of DNA hydrophilic in nature?

This is because it is non-polar and hydrophobic in nature.

Why are the Nitrogenous Bases of DNA located inside of the helix?

This position of the Nitrogenous Bases protect it from chemical damage and enable stable hydrogen bonding.

What is the main reason why it's important that the DNA's Nitrogenous Bases are located inside of the helix?

Adenine (A) and Guanine (G)

What Nitrogenous Bases are Purines?

They have a double-ring structure.

What is the structure of Purines?

Cytosine (C) and Thymine (T)

What Nitrogenous Bases are Pyrimidines?

They have a single-ring structure.

What is the structure of Pyrimidines?

The purine-pyrimidine pairing

What maintains the constant width of the double helix?

~2 nm

What is the diameter of a DNA molecule?

0.34 nm

The Nitrogen Bases inside the DNA helix are stacked ___ __ apart.

2

How many hydrogen bonds does a pair of Adenine and Thymine have?

3

How many hydrogen bonds does a pair of Cytosine and Guanine have?

Sugar and Phosphate Backbone

What does the outside of the DNAs double helix consist of?

Pairs of N bases are stacked on the inside.

What is inside of the DNA helix?

This bonding pattern suggests a possible copying mechanism for the genetic material.

What does the Nitrogen Bases bonding with its complement only and not with any of the other bases suggest?

Replication and Protein Synthesis

What are the two main functions of genes?

Watson and Crick, together with Wilkins

Who are the scientists (surnames only) that received the Nobel Prize for the elucidation of DNA structure?

Rosalind Elsie Franklin

Who is the scientist (full name) who could not be honored for the elucidation of DNA structure because the Nobel Prize can not be awarded posthumously?

Meselson & Stahl’s Experiment (1958)

What study demonstrated and supported the semiconservative model of DNA replication by Watson and Crick?

1. Matthew Stanley Meselson

2. Franklin William Stahl

What are the full names of the scientists involved in the Meselson & Stahl’s Experiment (1958)?

1. Conservative Model

2. Semiconservative Model

3. Dispersive Model

What are the three models of DNA replication that were hypothesized?

Conservative Model

This model of DNA replication is where the two parental strands reassociate after acting as templates for new strands, restoring the parental double helix.

Semiconservative Model

This model of DNA replication is where the two strands of the parental molecule separate, where each functions as a template for synthesis of a new complementary strand.

Dispersive Model

This model of DNA replication is where each strand of both daughter molecules contains a mixture of old and newly synthesized DNA.

Escherichia coli

What is the full scientific name of the bacteria used in Meselson & Stahl’s Experiment (1958)?

1. E. Coli

2. ¹⁵N

3. ¹⁴N

Meselson and Stahl cultured 1.__ ____ for several generations in a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 2.___. They then transferred the bacteria to a medium with only 3.___, a lighter isotope. A sample was taken after the first DNA replication; another sample was taken after the second replication. They extracted DNA from the bacteria in the samples and then centrifuged each DNA sample to separate DNA of different densities.

Hybrid (¹⁵N-¹⁴N) DNA

What kind of DNA did the first replication in the ¹⁴N medium produce?

Conservative Model

• This is because it is the only model out of the three that expected one Heavy (¹⁵N-¹⁵N) DNA and one Light (¹⁴N-¹⁴N) DNA, but the outcome of the first replication only resulted in one Hybrid (¹⁵N-¹⁴N) DNA.

What model did the first replication in the ¹⁴N medium rule out and why?

Semiconservative Model and Dispersive Model

• This is because both of these models expected one Hybrid (¹⁵N-¹⁴N) DNA, which aligns with the first replication's results.

What models did the first replication in the ¹⁴N medium prove and why?

Light (¹⁴N-¹⁴N) DNA and Hybrid (¹⁵N-¹⁴N) DNA

What kinds of DNA did the second replication in the ¹⁴N medium produce?

Dispersive Model

• This is because this model expected only one Heavy (¹⁵N-¹⁴N) DNA, but the outcome of the second replication resulted in one Light (¹⁴N-¹⁴N) DNA and one Hybrid (¹⁵N-¹⁴N) DNA.

What model did the second replication in the ¹⁴N medium rule out and why?

Semiconservative Model

• This is because the model expected one Light (¹⁴N-¹⁴N) DNA and one Hybrid (¹⁵N-¹⁴N) DNA, which aligns with the first replication's results.

What model did the second replication in the ¹⁴N medium prove and why?