Lecture 6 - DNA as Genetic Material

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Questions and Answers

What was demonstrated by Griffith's experiments in 1928?

All bacteria are harmful

S cells can survive without a capsule

R cells are pathogenic

Transformation occurs in bacteria (correct)

Which of the following statements is true regarding the R and S strains of Streptococcus pneumoniae?

S strains are rough in appearance

S strains are non-pathogenic

R strains produce a gelatinous capsule

R strains do not cause disease (correct)

In the Avery-MacLeod-McCarty experiments, which treatment resulted in no transformation of R cells to S cells?

DNase treatment (correct)

Protease treatment

Heat treatment

RNase treatment

What conclusion can be drawn from the results of the Avery-MacLeod-McCarty experiments regarding genetic material?

DNA is the genetic material Signup and view all the answers

In the bacteriophage T2 experiments conducted by Hershey and Chase, which component was found to carry genetic information?

DNA Signup and view all the answers

What was the method used to isolate the genetic material in the Avery-MacLeod-McCarty experiments?

Using heat-killed virulent strains Signup and view all the answers

Which enzyme was used to destroy RNA in the Avery-MacLeod-McCarty experiment?

RNase Signup and view all the answers

What structural feature of phage T2 is responsible for protecting its genetic material?

Icosahedral head Signup and view all the answers

What is the concept of the inborn error of metabolism illustrated by albinism and alkaptonuria?

Defective inherited factors for specific metabolic steps Signup and view all the answers

What does the term 'auxotroph' refer to in the context of Beadle and Tatum's experiments?

A mutant requiring an additional nutrient to grow Signup and view all the answers

Which organism did Beadle and Tatum use as a model for their experiments?

Neurospora crassa Signup and view all the answers

What was concluded about the relationship between genes and enzymes from Beadle and Tatum's studies?

One gene controls one enzyme Signup and view all the answers

What does the growth pattern on minimal medium supplemented with arginine suggest about the mutant strains?

They require arginine because of a mutation Signup and view all the answers

How is an arginine auxotroph identified through Beadle and Tatum's experimentation?

Through its failure to grow on minimal medium but growth with arginine Signup and view all the answers

What process allows for the identification of nutritional requirements in the mutant Neurospora strains?

Observing growth on minimal medium Signup and view all the answers

What outcome indicates that a haploid organism contains a mutation affecting a metabolic step?

Failure to grow on minimal medium Signup and view all the answers

What role does complementation play in arginine biosynthesis among different mutants?

It allows one nucleus to compensate for the mutation in the other, enabling product formation. Signup and view all the answers

If the arg-1 mutant is complemented by the arg-2 mutant, what effect does this have on arginine synthesis?

The heterokaryon can grow in the absence of arginine. Signup and view all the answers

What would be the expected outcome if a gene controlled every step of arginine biosynthesis?

Every nucleus would exhibit defects in the same gene. Signup and view all the answers

What was discovered about nucleic acid in the year 1869?

It was termed 'nuclein' and found to be rich in phosphorus. Signup and view all the answers

What would likely happen if two auxotrophs derived from different asci were mated?

They might complement each other's mutations. Signup and view all the answers

How are the defects in nuclei described in the processing of precursor 1 and precursor 2?

One nucleus converts precursor 1 while the other can convert precursor 2. Signup and view all the answers

What is the main conclusion of the complementation tests performed by Beadle and Tatum?

They showed that multiple genes can control different steps in biosynthesis. Signup and view all the answers

What is a characteristic of the heterokaryons formed by mating two different auxotrophs?

They exhibit the ability to synthesize necessary products that were previously blocked. Signup and view all the answers

What was the conclusion of Beadle and Tatum’s experiments on arginine biosynthesis?

Each gene controls a specific step in a metabolic pathway. Signup and view all the answers

What did the Griffith experiments of 1928 demonstrate?

Dead S cells can transform living R cells into pathogenic S cells. Signup and view all the answers

What was the key finding of the Avery–MacLeod–McCarty experiment (1944)?

DNA is the genetic material responsible for transformation. Signup and view all the answers

What enzyme treatment in the Avery–MacLeod–McCarty experiment prevented transformation?

DNase Signup and view all the answers

What did Hershey and Chase demonstrate using bacteriophage T2?

DNA, not protein, is the hereditary material. Signup and view all the answers

What isotopes were used by Hershey and Chase to label phage components?

Sulfur-35 and Phosphorus-32 Signup and view all the answers

What was the purpose of using Sulfur-35 in the Hershey and Chase experiment?

To label the phage protein coat. Signup and view all the answers

Which of the following steps occurs in the complementing heterokaryons during arginine biosynthesis?

Both defects are masked by functional steps from each nucleus. Signup and view all the answers

What is the significance of complementing mutations in heterokaryons?

They demonstrate that different genes control distinct steps of a pathway. Signup and view all the answers

What did Friedrich Miescher discover in 1869?

Nuclein, a phosphorus-rich substance in leukocyte nuclei. Signup and view all the answers

What did the Hershey–Chase experiment reveal about the role of DNA in phages?

DNA enters the host cell and directs replication. Signup and view all the answers

What do rough (R) colonies of Streptococcus pneumoniae lack compared to smooth (S) colonies?

A polysaccharide capsule. Signup and view all the answers

In the Beadle and Tatum experiment, what did they use minimal medium supplemented with arginine to identify?

Auxotrophs requiring arginine for growth. Signup and view all the answers

What does a prototroph refer to in genetic studies?

A strain that grows on minimal media without supplements. Signup and view all the answers

What was demonstrated by complementing mutations in Beadle and Tatum’s experiments?

Different genes control distinct biochemical steps. Signup and view all the answers

False Signup and view all the answers

Which step of the Avery–MacLeod–McCarty experiment confirmed DNA as the genetic material?

Destruction of DNA prevented transformation. Signup and view all the answers

How did Hershey and Chase distinguish between DNA and protein in their experiments?

By using isotopes Sulfur-35 for protein and Phosphorus-32 for DNA. Signup and view all the answers

Which phage component was labeled with Sulfur-35 in the Hershey–Chase experiment?

Protein coat Signup and view all the answers

Why is DNA replication critical in the Central Dogma of Molecular Biology?

It ensures genetic information is passed during cell division. Signup and view all the answers

What is the significance of the phrase "one gene, one enzyme" in Beadle and Tatum's work?

Each gene encodes a specific enzyme for a step in a pathway. Signup and view all the answers

Study Notes

Lecture 6 - DNA is the Genetic Material: Part 1 - What Do Genes Do?

Sir Archibald Garrod proposed the concept of inborn errors of metabolism.
Examples include Albinism (lack of pigment) and alkaptonuria (individuals secrete homogentisic acid into urine, turning it black).
Inherited factors play a critical role in metabolic steps.
Beadle and Tatum used Neurospora crassa to study gene function.
Neurospora crassa can grow on a simple medium containing salts, carbon, nitrogen, and biotin (vitamin H).
The life cycle of Neurospora crassa involves haploid ascospores, germination, vegetative mycelium, germination of conidia, fruiting bodies, and meiosis followed by mitosis.
Haploid nuclei fuse creating diploid nuclei.
Through meiosis, this then create ascospores.
Mutants with altered metabolic steps were generated to study gene roles.
A key question arises: does one gene control the entire conversion or is there one gene for each step?

Beadle and Tatum's Experiment: Step One, Genetic Explanation

In each haploid nucleus, there is only one gene copy.
If a gene is mutated, there's no other copy to mask the mutation.
Beadle and Tatum's experiment demonstrated how to find the gene responsible for a specific metabolic function.
The experiment involved multinucleate conidia, X-rays, and genetic analysis of survivors in different growth media.
Independent segregation of alleles occurred during the process.

Beadle and Tatum's Experiment: Step Two - Growing Survivors

Survivors were grown in various media in order to identify nutritional requirements.
Growth on minimal medium, supplemented with specific amino acids or vitamins, identified missing nutritional requirements.
For example: growth on minimal medium with added amino acids indicated a requirement for a specific amino acid.
Arginine auxotrophs were generated by X-ray treatment, and their characteristics were explored

Identifying Nutritional Requirements (Step 4)

Testing growth on media with different components (amino acids, vitamins) pinpoints the exact nutritional requirements of each mutant.

Identifying Arginine Auxotrophs (Step 5)

Identifying mutants that need arginine to grow.

Beadle and Tatum, 1941: Making Arginine Auxotrophs – Recap

The experiment focused on arginine auxotrophs of Neurospora crassa.
Growth on minimal medium supplemented with arginine indicated a nutritional requirement for arginine.

Testing Arginine Auxotrophs

If auxotrophs come from one ascus/nucleus, they share the same mutation.
If they come from different asci/nuclei, they have different mutations.

How Complementation Works

A biosynthetic pathway converts precursor 1 to precursor 2 to product.
If one nucleus has a mutation preventing conversion of precursor 1 to precursor 2, it blocks the pathway.
If the second nucleus has a mutation preventing conversion of precursor 2 to product, it also stops the pathway.
However, if the two nuclei together have both mutations affecting different parts of the pathway, the cells can produce progeny.

Beadle and Tatum, 1941: Their Results

They isolated three classes of arginine auxotrophs.
Each class had a specific defect in the arginine synthesis pathway.
This discovery led to the "one gene, one enzyme" hypothesis (later revised to "one gene, one protein").

A Modern View

Genes code for proteins, but not all proteins are enzymes.
The hypothesis of "one gene, one enzyme" evolved into "one gene, one protein."

1944: The Discovery of DNA as the Genetic Material

The Avery-MacLeod-McCarty experiment showed that DNA, and not protein, transformed nonpathogenic bacteriainto pathogenic bacteria.

1952: Protein Is Not the Hereditary Material

Hershey and Chase used bacteriophage T2 to further examine the role of protein and DNA.
They determined experimentally that the DNA of the bacteriophage, not protein, entered the bacterial cells during infection. This provided powerful confirmation of DNA as the genetic material.

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Description

Explore Lecture 6 on how DNA functions as the genetic material, focusing on the contributions of Sir Archibald Garrod and the experiments by Beadle and Tatum with Neurospora crassa. This session addresses inborn errors of metabolism, gene functions, and the life cycle of the organism. Join us to unravel how genes influence metabolic processes!

Lecture 6 - DNA as Genetic Material - Part 1

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