The Discovery of DNA and Transformation Experiment

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

Which component of a nucleotide is different in DNA compared to RNA?

Pentose sugar (correct)

Phosphate group

Nitrogenous base

Nucleoside

What type of bond links nucleotides in a polynucleotide chain?

Glycosidic bond

Peptide bond

Hydrogen bond

Phosphodiester bond (correct)

Which nitrogenous bases are classified as purines?

Cytosine and Thymine

Adenine and Uracil

Adenine and Guanine (correct)

Thymine and Cytosine

What is the primary structure of DNA characterized by?

Double helix with complementary base pairing Signup and view all the answers

What are nucleosides composed of?

A nitrogenous base and a pentose sugar Signup and view all the answers

What model did James Watson and Francis Crick propose for DNA?

Double helix model Signup and view all the answers

How many nucleotides are there per turn of the B-DNA helix?

10 base pairs Signup and view all the answers

Which nucleotides pair according to Chargaff's rules?

Adenine with Thymine Signup and view all the answers

What type of DNA is primarily found in prokaryotes?

Circular DNA Signup and view all the answers

Which of the following statements about plasmids is correct?

They replicate independently of nuclear DNA. Signup and view all the answers

What type of DNA do mitochondria possess?

Circular DNA Signup and view all the answers

What is the primary function of chloroplasts in plant cells?

Photosynthesis Signup and view all the answers

What is RNA typically characterized as?

Single-stranded Signup and view all the answers

What did Frederick Griffith discover in 1928?

The phenomenon of transformations in bacteria Signup and view all the answers

Which strain of Streptococcus pneumoniae was virulent?

S-Strain Signup and view all the answers

What was the outcome when heat-killed S-strain and live R-strain bacteria were injected into mice?

Mice died, and live S-strain bacteria were recovered Signup and view all the answers

Who identified DNA as the transforming principle in 1944?

Oswald Avery, Colin MacLeod, and Maclyn McCarty Signup and view all the answers

What type of virus did Hershey and Chase use in their experiment?

T2 phage Signup and view all the answers

What did the Hershey-Chase experiment use to label the DNA and protein of the phage?

Radioisotopes of phosphorus and sulfur Signup and view all the answers

Which of the following statements about transformation is true?

Transformation can occur between different species Signup and view all the answers

What overall conclusion was drawn from the research on DNA as genetic material?

DNA is the essential carrier of genetic information Signup and view all the answers

What is the primary function of ribosomal RNA (rRNA)?

Makes up part of ribosomes Signup and view all the answers

Which enzyme is responsible for catalyzing the transcription process?

RNA polymerase Signup and view all the answers

What occurs during the elongation phase of transcription?

RNA polymerase copies the template DNA strand Signup and view all the answers

Which of the following is NOT a stage in the post-transcriptional processing of mRNA?

Replication Signup and view all the answers

Which type of RNA carries amino acids to the ribosomes?

Transfer RNA (tRNA) Signup and view all the answers

During which phase of DNA replication does the double helix unwind?

Initiation Signup and view all the answers

What does the poly(A) tail do during mRNA processing?

Protects the mRNA from degradation Signup and view all the answers

What is the result of semiconservative replication?

One new DNA strand and one old strand in each duplex Signup and view all the answers

Study Notes

The Discovery of DNA as Genetic Material

In 1953 James Watson and Francis Crick discovered the structure of DNA.
Deoxyribose Nucleic Acid was considered the basic building block of DNA.
The discovery involved years of observation and experimentation.

Griffith's Transformation Experiment

Frederick Griffith discovered the phenomenon of bacterial transformation in 1928.
Transformation is the modification of a genome by receiving external DNA from a cell of a different genotype.
It can also be said that transformation is the conversion of a normal higher eukaryotic cell in tissue culture to a cancerous state of uncontrolled division.
Griffith experimented with the bacterium Streptococcus pneumoniae.
- He used two strains of the bacterium:
  - S-Strain: Virulent (causes disease).
    - A polysaccharide capsule surrounds the cells.
    - Colonies appeared smooth.
  - R-Strain: Non-virulent (does not cause disease).
    - No polysaccharide capsule.
    - Colonies appeared rough.
Griffith's experiment involved injecting mice with various combinations of S and R-strain bacteria.
- Mice injected with heat-killed S-strain bacteria lived.
- Mice injected with live R-strain bacteria lived.
- Mice injected with a mixture of heat-killed S-strain and live R-strain bacteria died.
  - Live S-strain bacteria were recovered from the dead mice.
  - The R-strain bacteria were transformed by the heat-killed S-strain bacteria.
This process was called transformation.

Avery, MacLeod, and McCarty's Transforming Principle Experiment

In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty identified DNA as the transforming principle.
They separated different classes of molecules in a cell and tested each for transforming capacity, finding that only DNA was effective.
They concluded that DNA was the transforming principle.

Hershey-Chase Experiment on Hereditary Material

Alfred Hershey and Martha Chase solidified the idea of DNA as genetic material in 1952.
Their experiment used T2 phage, a bacterial virus that infects Escherichia coli.
T2 phage is composed of protein and DNA.
They used radioisotopes to label the phage: - 32P to label DNA - 35S to label protein
Phage infected the E.coli bacteria.
The phage ghosts (empty outer shells) were separated from the bacteria by centrifugation.
They found that the phage DNA entered the bacteria and was incorporated into the new phages.
However, the protein remained outside the bacteria.
Their results confirmed Avery's results and established that DNA is the hereditary material.

Chemical Composition and Structure of Nucleic Acids

Nucleic acids are macromolecules present in the nucleus.
They are polymers of nucleotides.
A nucleotide is composed of:
- A nitrogenous base
- A pentose sugar
- A phosphoric acid
There are two types of nucleic acids: DNA and RNA.
- They differ in sugar and one of the bases:
  - DNA: Deoxyribose, thymine
  - RNA: Ribose, uracil

Nitrogenous Bases

Purines: Adenine (A) and Guanine (G)
Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U)

Nucleosides

A nucleoside is a nitrogenous base linked to a pentose sugar.
- They are named according to the sugar:
  - Ribonucleosides - if the pentose is ribose
  - Deoxyribosides - if the pentose is deoxyribose

Nucleotides

A nucleotide is a nucleoside linked to a phosphate group.
- They are named according to the base:
  - Adenylic acid or adenosine monophosphate (AMP)
  - Guanylic acid or guanosine monophosphate (GMP)
  - Cytidylic acid or cytidine monophosphate (CMP)
  - Uridylic acid or uridine monophosphate (UMP)

Polynucleotides and 3'-5' Phosphodiester Linkage

A polynucleotide is a large polymer of nucleotides joined together.
The nucleotides are linked by phosphodiester bonds.
- The phosphate of one nucleotide links to the 3' carbon of the sugar of the next nucleotide.
- The 5' carbon of the sugar is linked to the phosphate group.
The backbone of a polynucleotide consists of alternating phosphates and pentoses.
- This gives the nucleic acid an acidic character.
The nitrogenous bases project inwards toward the centre of the helix.

Watson-Crick Model of DNA

James Watson and Francis Crick proposed the double helix model for DNA.
Their model was based on:
- X-ray diffraction data: Provided by Rosalind Franklin and Maurice Wilkins
- Chemical data: Regarding base composition, provided by Erwin Chargaff
The two strands of DNA are antiparallel.
There are ten nucleotides per turn of the helix.
The bases are paired according to Chargaff's rules:
- Adenine (A) pairs with Thymine (T)
- Guanine (G) pairs with Cytosine (C)
The two strands are linked by hydrogen bonds between the bases.
- A-T pairs have 2 hydrogen bonds.
- G-C pairs have 3 hydrogen bonds.
The double helix structure has two grooves:
- Major groove: Wider and deeper.
- Minor groove: Narrower and shallower.

Types of DNA

A-DNA: 11 base pairs per turn, right-handed helix
- Less common.
B-DNA: 10 base pairs per turn, right-handed helix
Most common in cells.
Z-DNA: Left-handed helix, zig-zag sugar-phosphate backbone
- Found in certain regions of DNA, and is rich in guanine and cytosine.
Circular DNA: Found in bacteria and some viruses.

DNA in Prokaryotes

Prokaryotes are single-celled organisms without a nucleus.
- They have a single circular DNA molecule in the nucleoid region.
Prokaryotes also have small circular DNA molecules called plasmids.
Plasmids: Autonomous genetic elements that replicate and are inherited independently of the chromosome.
- They can be transferred between bacteria.
- They often carry genes for antibiotic resistance and other advantageous traits.

Organelle DNA (Extranuclear DNA)

Some DNA is found in organelles like mitochondria and chloroplasts.
This DNA is circular and replicates independently of the nuclear DNA.

Mitochondria

Mitochondria are organelles found in all eukaryotic cells that perform oxidative phosphorylation.
They have their own DNA, ribosomes, and protein synthesis machinery.
Mitochondrial DNA is circular and encodes some of the mitochondrial proteins.

Chloroplasts

Chloroplasts are organelles found in plant cells that perform photosynthesis.
They have their own DNA, ribosomes, and protein synthesis machinery.
Chloroplast DNA is circular and encodes some of the chloroplast proteins.

RNA

Ribonucleic acid (RNA) is a polymer of nucleotides that is usually single-stranded.
Three main types of RNA are involved in protein synthesis:
- Ribosomal RNA (rRNA): Makes up part of ribosomes, which are the sites of protein synthesis.
- Messenger RNA (mRNA): Copies the genetic code from DNA, carries it to ribosomes, and directs protein synthesis.
- Transfer RNA (tRNA): Carries specific amino acids to ribosomes, matching them to codons in mRNA.

Transcription

Transcription is the process of copying a DNA sequence into an RNA molecule.
The RNA polymerase enzyme catalyzes this reaction.
In eukaryotes, there are three main types of RNA polymerase:
- RNA polymerase I: Transcribes rRNA genes.
- RNA polymerase II: Transcribes mRNA and some snRNA genes.
- RNA polymerase III: Transcribes tRNA, 5S rRNA, and other small RNA genes.
Steps of eukaryotic Transcription:
- Initiation:
  - The RNA polymerase II complex binds to the promoter region of the gene.
  - Several transcription factors are involved in this process. - Elongation:
  - RNA polymerase II moves along the DNA template, unwinding and copying the template strand. - Termination:
  - The RNA polymerase II complex detaches from the DNA template, stopping transcription.

Post-Transcriptional Processing of mRNA

In eukaryotes, newly synthesized mRNA undergoes three main stages of processing:
- Capping: A methylated guanine nucleotide is added to the 5' end of the mRNA.
- Splicing: Introns, which are non-coding regions of the mRNA, are removed.
- Polyadenylation: A poly(A) tail is added to the 3' end of the mRNA.

Replication

Replication is the process of copying the DNA molecule.
It is essential for cell division is based on the complementary nature of two DNA strands.
Steps of DNA Replication:
- Initiation: The process begins at specific origins of replication, where the DNA double helix is unwound.
- Elongation: DNA polymerase enzymes add nucleotides to a new strand, using the old strand as a template according to the base-pairing rules (A with T, C with G).
- Termination: The replication process stops when the entire chromosome has been copied.
Semiconservative Replication:
- Each new DNA molecule is composed of one old strand and one new strand.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the groundbreaking discoveries by Watson and Crick on the structure of DNA and Griffith's transformation experiment with Streptococcus pneumoniae. Learn how these pivotal studies shaped our understanding of genetics and the mechanisms of bacterial transformation. Test your knowledge on these key historical moments in molecular biology.