Experiments on DNA as Genetic Material

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Questions and Answers

Which of the following is NOT a property of DNA?

  • It is susceptible to mutations.
  • It is self-replicating.
  • It is essential for the formation of lipids within the cell. (correct)
  • It stores genetic information in its base pairs.

Which model of DNA replication proposes that the daughter strands contain both parent and newly synthesized DNA?

  • Semiconservative
  • Conservative
  • Dispersive (correct)
  • None of the above

What does the presence of major and minor grooves in DNA allow for?

  • The formation of hydrogen bonds between nitrogenous bases.
  • Replication of the DNA strand
  • The correct pairing of nucleotides during transcription
  • The attachment of regulatory proteins to the DNA molecule. (correct)

What is the difference between a conservative and a semiconservative model of DNA replication?

<p>The conservative model suggests that the parent strands are templates but not included in the daughter strand. (D)</p> Signup and view all the answers

What is the role of nitrogen isotopes in the Meselson-Stahl experiment?

<p>To distinguish between the parent and daughter strands of DNA by density. (C)</p> Signup and view all the answers

Which of the following is NOT a component of Griffith's experiment?

<p>The identification of a specific biomolecule that causes bacterial transformation (C)</p> Signup and view all the answers

Which of the following was a key finding of Avery's experiment?

<p>DNA is the primary component of the transforming substance (C)</p> Signup and view all the answers

What was the hypothesis tested by Hershey and Chase in their experiment?

<p>DNA is the hereditary material responsible for directing the formation of new viruses (D)</p> Signup and view all the answers

In Griffith's experiment, what was the result of injecting mice with a mixture of heat-killed S-type cells and living R-type cells?

<p>The mice died (A)</p> Signup and view all the answers

Which of the following is NOT a defining feature of the DNA molecule?

<p>The strands are parallel, meaning they run in the same direction (A)</p> Signup and view all the answers

How was the Hershey and Chase experiment designed to determine which molecule carried genetic information?

<p>They used radioactive isotopes to label both DNA and protein, and then tracked the labeled molecules (C)</p> Signup and view all the answers

Which of the following is a reason why DNA's location in the nucleus and chromosomes provided circumstantial evidence for its role as the genetic material?

<p>The nucleus is the control center of the cell and chromosomes are essential for cell division (A)</p> Signup and view all the answers

In Hershey and Chase's experiment, what was the purpose of using radioactive isotopes to label the viral DNA and proteins?

<p>To determine which component, DNA or protein, was responsible for entering the bacterial cell (C)</p> Signup and view all the answers

What is the significance of the finding that ³²P was associated with the bacterial pellet in Hershey and Chase's experiment?

<p>It proved that DNA, not protein, is the genetic material that enters the bacterial cell and directs viral replication. (A)</p> Signup and view all the answers

Why did Avery treat dead S-type cells with different enzymes in his experiment?

<p>To eliminate different components of the cells, isolating the transforming substance (B)</p> Signup and view all the answers

What was Griffith's key conclusion from his experiment?

<p>A chemical substance from one cell can genetically transform another cell (D)</p> Signup and view all the answers

Which of the following scientists contributed significantly to understanding the structure of DNA?

<p>All of the above (D)</p> Signup and view all the answers

According to Chargaff's rules, if a DNA sample contains 20% Adenine (A), what percentage of Guanine (G) would it contain?

<p>30% (C)</p> Signup and view all the answers

Why is it important that phosphate groups are located on the outside of the DNA helix?

<p>They repel each other, helping to maintain the helical shape. (B)</p> Signup and view all the answers

Which of the following is NOT a characteristic of the nitrogenous bases in DNA?

<p>They are composed of a sugar and a phosphate group. (B)</p> Signup and view all the answers

What is the primary function of the sugar-phosphate backbone in DNA?

<p>It provides the structural support for the helix. (D)</p> Signup and view all the answers

What characteristic of DNA replication is demonstrated by the presence of intermediate DNA after one generation?

<p>Semiconservative replication (D)</p> Signup and view all the answers

Which enzyme is responsible for unwinding the DNA double helix during replication?

<p>DNA helicase (B)</p> Signup and view all the answers

What is the function of RNA primers in DNA replication?

<p>To initiate the synthesis of the new strand (C)</p> Signup and view all the answers

What best describes the growth of the leading and lagging strands during DNA replication?

<p>The leading strand grows continuously in one direction, while the lagging strand grows discontinuously. (C)</p> Signup and view all the answers

Why do chromosomes lose part of their telomeres with each cell division?

<p>The lagging strand does not have a 3' end to extend after primer removal. (B)</p> Signup and view all the answers

What role does telomerase play in cellular functioning?

<p>It adds missing telomeric sequences to prevent loss. (C)</p> Signup and view all the answers

How do DNA polymerases contribute to genomic integrity?

<p>They correct errors through proofreading. (D)</p> Signup and view all the answers

What are Okazaki fragments?

<p>They are the portions of lagging strand synthesized discontinuously. (B)</p> Signup and view all the answers

What is the result of DNA damage caused by external mutagen agents?

<p>It can create mismatches or mutations in the DNA sequence. (C)</p> Signup and view all the answers

Which of the following proteins is part of the pre-replication complex?

<p>DNA helicase (B)</p> Signup and view all the answers

What amino acid is coded by the codons derived from the artificial mRNA poly U?

<p>Phenylalanine (B)</p> Signup and view all the answers

How many sense codons are involved in coding for amino acids?

<p>61 (C)</p> Signup and view all the answers

What role does ribosomal RNA (rRNA) play in protein synthesis?

<p>Forms part of the ribosome (B)</p> Signup and view all the answers

In translation, what does the A site of the ribosome do?

<p>Binds tRNA carrying an amino acid (B)</p> Signup and view all the answers

What is the significance of wobble base pairing in codon-anticodon interaction?

<p>Enables fewer tRNAs to code for more amino acids (D)</p> Signup and view all the answers

What type of RNA serves as the template that determines the sequence of amino acids in a polypeptide?

<p>Messenger RNA (mRNA) (D)</p> Signup and view all the answers

What facilitates the termination of translation?

<p>Nonsense codons (D)</p> Signup and view all the answers

Which of the following statements is true about the genetic code?

<p>The genetic code is nearly universal across organisms. (A)</p> Signup and view all the answers

What is the role of DNA polymerase III during the proofreading mechanism?

<p>It excises the incorrect nucleotide and adds the correct base pair. (B)</p> Signup and view all the answers

Which mechanism is specifically used to repair DNA mismatches after replication?

<p>Mismatch repair mechanism (D)</p> Signup and view all the answers

What did Archibald Garrod conclude from his observations of alkaptonuria?

<p>One gene codes for one enzyme. (A)</p> Signup and view all the answers

What was the purpose of Beadle and Tatum's experiment with Neurospora fungus?

<p>To establish a link between genes and their respective enzymes. (C)</p> Signup and view all the answers

What are introns in eukaryotic cells?

<p>Non-coding sequences that are transcribed but spliced out. (D)</p> Signup and view all the answers

How does RNA polymerase differ from DNA polymerase in terms of primer requirement?

<p>RNA polymerase does not require a primer to initiate synthesis. (D)</p> Signup and view all the answers

What does the central dogma of molecular biology describe?

<p>Information flow from genes to proteins. (D)</p> Signup and view all the answers

In the context of eukaryotic mRNA processing, what is added to the 5' end of the mRNA?

<p>A 5' cap. (D)</p> Signup and view all the answers

What is the triplet code in the genetic code used for?

<p>To encode amino acids in proteins. (C)</p> Signup and view all the answers

How does RNA transcription differ from DNA replication in terms of direction?

<p>Both are synthesized in the 5' to 3' direction. (B)</p> Signup and view all the answers

Which process involves the addition of a poly-A tail to mRNA?

<p>mRNA processing. (B)</p> Signup and view all the answers

What is the purpose of the termination step in transcription?

<p>To release the finished RNA molecule and DNA template. (B)</p> Signup and view all the answers

What is the main source of DNA damage that can lead to mutations?

<p>Chemical exposure and radiation. (B)</p> Signup and view all the answers

In the context of genetics, what does it mean to say 'one gene corresponds to one polypeptide'?

<p>One gene only encodes for a single polypeptide or protein. (B)</p> Signup and view all the answers

What role do transcription factors play in eukaryotic transcription?

<p>They direct RNA polymerase to the promoter region. (C)</p> Signup and view all the answers

Flashcards

Circumstantial evidence

Evidence indicating that DNA is genetic material including its location and amount in cells.

Griffith's Experiment

Demonstrated bacterial transformation using S and R strains of pneumococcus in mice.

Avery's Experiment

Showed that DNA is the transforming substance by using enzymes to destroy RNA and proteins.

Hershey and Chase's Experiment

Investigated whether DNA or protein is the genetic material by studying bacteriophage T2.

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S strain vs. R strain

S strain is virulent and deadly, while R strain is non-virulent and harmless.

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Bacterial transformation

Process where genetic material is transferred from one bacterial cell to another.

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Role of DNase in Avery's experiment

DNase specifically destroys DNA, confirming its role in transformation.

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Conclusion of Hershey and Chase's experiment

Concluded that DNA, not protein, is the hereditary material in viruses.

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DNA Structure

DNA is a polymer of nucleotides with a phosphate group, deoxyribose sugar, and nitrogenous base.

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Double Helix

DNA's shape; two strands twisted around each other.

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Chargaff's Rule

A + G = T + C; amounts of A equal T and G equal C in DNA.

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X-ray Diffraction

Technique used by Franklin to discover DNA's double helix structure.

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Antiparallel Strands

DNA strands run in opposite directions, one 5' to 3' and the other 3' to 5'.

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Right-Handed Helix

The orientation of the DNA helix is right-handed, spiraling clockwise.

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Complementary Base Pairing

Specific pairing: A with T and C with G, stabilizing DNA structure.

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Supercoiling

DNA is tightly coiled to fit in the cell's nucleus.

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DNA grooves

Major and minor grooves caused by the uneven sugar-phosphate backbone.

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Function of DNA

DNA stores genetic information, is prone to mutations, replicates using complementary base pairing, and codes for proteins.

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Semiconservative replication

Each parent strand serves as a template for a new daughter strand.

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Meselson and Stahl experiment

Experiment that demonstrated DNA replicates semiconservatively using isotopes of nitrogen.

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Mutations

Permanent changes in nucleotide sequences affecting genetic information.

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DNA Replication Steps

Three main processes: initiation, elongation, termination.

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Initiation

First step in DNA replication where helicase unwinds the double helix.

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Elongation

Synthesis of new DNA strands by adding nucleotides.

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Termination

End of DNA replication; replaces RNA primers with DNA.

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Lagging Strand

DNA strand synthesized discontinuously in short fragments.

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Okazaki Fragments

Short strands of DNA formed on the lagging strand.

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Telomeres

Repeated DNA sequences at chromosome ends preventing loss of important genes.

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Telomerase

An enzyme that adds missing telomere sequences to protect chromosome ends.

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DNA Polymerase Errors

Occasional incorrect base insertions by DNA polymerase during replication.

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Codons

Three-nucleotide sequences in mRNA that specify amino acids.

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Poly U

Synthetic RNA made only of uracil that produces phenylalanine.

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Redundant Genetic Code

Multiple codons can code for the same amino acid.

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tRNA

Transfer RNA that carries amino acids to the ribosome.

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Wobble Base Pairing

Variation in pairing for third base of a codon allowing multiple codons for an amino acid.

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Ribosome Structure

Complex of rRNA and proteins, has large and small subunits.

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Translation Steps

Includes initiation, elongation, and termination processes.

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Anticodon

A sequence of three bases on tRNA that pairs with a codon on mRNA.

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DNA Proofreading Mechanism

A process during replication where incorrect nucleotides are removed and replaced.

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Mismatch Repair Mechanism

A system that corrects errors after DNA replication by excising incorrect nucleotides.

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Excision Repair Mechanism

Repairs DNA damage by removing incorrect nucleotides during the cell's life cycle.

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Alkaptonuria

A genetic disorder causing dark urine due to a recessive mutation in enzyme production.

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One Gene One Enzyme Hypothesis

Proposed that each gene encodes a specific enzyme related to a metabolic pathway.

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Neurospora Experiment

Beadle and Tatum's experiment illustrating that genes code for enzymes using fungi.

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Central Dogma of Molecular Biology

Principle that explains the flow of genetic information from DNA to RNA to proteins.

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Transcription Process

The synthesis of RNA from a DNA template involving initiation, elongation, and termination.

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mRNA Processing

Modification of pre-mRNA in eukaryotes, including capping and splicing.

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Introns

Non-coding regions of RNA that are removed during mRNA splicing.

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5' Capping

Addition of a modified guanine cap to the 5' end of mRNA for stability.

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Poly-A Tail

A string of adenine nucleotides added to the 3' end of mRNA for stability.

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Genetic Code

The set of rules by which information encoded in mRNA is translated into amino acids.

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Triplet Code

Group of three nucleotides (codon) in mRNA that encodes a specific amino acid.

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RNA Transcription vs. DNA Replication

RNA transcription does not require a primer and has no proofreading capability unlike DNA replication.

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Study Notes

Experiments Demonstrating DNA as Genetic Material

  • Circumstantial Evidence:

    • DNA is found in the nucleus and chromosomes.
    • Somatic cells have twice the DNA of reproductive cells.
    • DNA amounts vary between species.

  • Griffith's Experiment:

    • Hypothesis: Material in dead bacteria can genetically transform living bacteria.
    • Used two pneumococcus strains: S strain (virulent, kills mice) and R strain (non-virulent, doesn't kill mice).
    • Injecting heat-killed S strain into mice did not kill them.
    • Injecting a mixture of heat-killed S strain and living R strain killed mice.
    • Conclusion: A chemical substance from dead S cells transformed living R cells.

  • Avery's Experiment:

    • Hypothesis: The chemical nature of the transforming substance is DNA.
    • Treated dead S cells with different enzymes (RNase, protease, and DNase).
    • Only the DNase treatment prevented transformation of R cells to S cells.
    • Conclusion: The transforming substance is DNA.

  • Hershey-Chase Experiment:

    • Hypothesis: Either DNA or protein from a bacteriophage enters the bacterial cell and directs the assembly of new viruses.
    • Labeled viral DNA with 32P and viral protein with 35S.
    • Phage DNA entered bacterial cells, not protein.
    • Conclusion: DNA, not protein, is the genetic material of the phage.

DNA Structure

  • DNA is a polymer of nucleotides.
  • Nucleotides consist of: a phosphate group, a deoxyribose sugar, and a nitrogenous base.
  • Nitrogenous bases: purines (adenine, guanine) and pyrimidines (cytosine, thymine).
  • DNA is a double-stranded helix.
  • Phosphate groups face outward.
  • Complementary base pairing: A with T and C with G.
  • Strands run antiparallel (5' to 3' and 3' to 5').
  • A phosphate group is attached to the 5' carbon of the deoxyribose sugar.
  • A hydroxyl group is attached to the 3' carbon of the deoxyribose sugar.
  • DNA has major and minor grooves.

DNA Replication

  • There are three potential DNA replication models:

    • Semiconservative: Each new DNA molecule contains one original strand and one new strand.
    • Conservative: Each new DNA molecule contains two original strands and two new strands.
    • Dispersive: Each new DNA molecule contains scattered fragments of original and new strands.

  • Meselson-Stahl Experiment:

    • Used different isotopes of nitrogen to track DNA replication.
    • Supported the semiconservative model.

  • Steps of DNA Replication:

    • Initiation: DNA helicase unwinds DNA, single-stranded binding proteins stabilize separated strands, and RNA primers are created.
    • Elongation: DNA polymerase adds nucleotides, complementing the template strand. Leading strand is continuous, lagging strand is made in Okazaki fragments.
    • Termination: RNA primers are replaced by DNA, and Okazaki fragments are joined by DNA ligase.

DNA Repair

  • Errors in DNA sequences can occur from spontaneous changes or external factors.
  • Proofreading mechanism: During replication, the complex proteins excise incorrect nucleotide and DNA polymerase adds the correct base pair.
  • Mismatch repair: After replication, proteins excise incorrect and some neighboring nucleotides; DNA polymerase adds new nucleotides, and a ligase repairs the nick.
  • Excision repair: During the life of the cell, proteins excise incorrect nucleotides; DNA polymerase adds new nucleotides, and a ligase repairs the nick.

Experiments Showing Genes Code for Proteins

  • Archibald Garrod:
    • Observed alkaptonuria (black urine).
    • Proposed a relationship between genes (alleles) and enzymes.
  • Beadle and Tatum:
    • Used Neurospora fungus to study the effect of mutations on enzyme production.
    • Concluded that one gene codes for one enzyme.
  • Srb and Horowitz:
    • Studied arginine biosynthesis in Neurospora.
    • Confirmed the one-gene/one-enzyme hypothesis.

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