Hershey-Chase Experiment

• In the 1950s, Alfred Hershey and Martha Chase conducted an experiment to determine if DNA or proteins were the genetic material.
• They used bacteriophages, which are viruses that infect bacterial cells.
• They labeled the DNA and proteins of the bacteriophages with different radioactive isotopes.
• The goal was to see which radioactive material would be passed on to the next generation of bacteriophages.

Experimental Setup

• Bacteriophages were grown in a culture of E. coli bacteria.
• The two types of radioactive isotopes used were:
  • 32P, which labels DNA (phosphorus is found in DNA)
  • 35S, which labels proteins (sulfur is found in proteins)
• The bacteriophages were allowed to infect the bacterial culture, and then the mixture was blended to separate the bacteriophages from the bacterial cells.

Experimental Procedure

1. The mixture of bacteriophages and bacterial cells was placed in a blender and gently blended.

2. The blend was then centrifuged, causing the bacterial cells to form a pellet at the bottom of the tube while the bacteriophages remained in the supernatant.

3. The supernatant containing the bacteriophages was collected and subjected to another round of centrifugation to obtain a purer suspension of bacteriophages.

Experimental Results

• Two sets of experiments were performed:
  • In one set, the bacteriophages were labeled with 32P.
  • In the other set, the bacteriophages were labeled with 35S.
• After each experiment, the infected bacterial cells were collected and analyzed for radioactivity.

Results: 32P-Labeled Bacteriophages

• Analysis of the infected bacterial cells showed the presence of radioactivity.
• This indicated that the radioactive material (32P-labeled DNA) was passed on to the next generation of bacteriophages.
• The radioactivity was found in the cell pellet, confirming that the DNA was the genetic material being inherited.

Results: 35S-Labeled Bacteriophages

• Analysis of the infected bacterial cells showed no presence of radioactivity.
• This indicated that the radioactive material (35S-labeled proteins) was not passed on to the next generation of bacteriophages.
• Since the radioactivity was not found in the cell pellet, it was concluded that proteins did not serve as the genetic material.

Conclusion

• The Hershey-Chase experiment provided strong evidence that DNA is the genetic material.
• The presence of radioactivity in the infected bacterial cells only when the bacteriophages were labeled with 32P-labeled DNA confirmed this.
• This experiment laid the foundation for understanding the molecular basis of inheritance and the role of DNA in passing on genetic information.

Significance

• The Hershey-Chase experiment helped establish the field of molecular biology and revolutionized our understanding of genetics.
• It paved the way for subsequent discoveries, such as the structure of DNA by Watson and Crick.
• This experiment provided evidence for the DNA-centric nature of the genetic code and its role in heredity.

Key Points

• The Hershey-Chase experiment aimed to determine if DNA or proteins were the genetic material.
• Bacteriophages labeled with radioactive isotopes were used to infect bacterial cells.
• Radioactive DNA (32P-labeled) was found in the infected bacterial cells, confirming that DNA is the genetic material.
• Radioactive proteins (35S-labeled) were not found in the infected bacterial cells, indicating that proteins do not serve as the genetic material.

Summary

• The Hershey-Chase experiment provided solid evidence that DNA is the genetic material.
• This experiment has had a profound impact on our understanding of genetics and paved the way for subsequent discoveries.
• DNA’s role as the informational molecule and its ability to pass on genetic traits has been firmly established.

DNA Structure: Double Helix

• DNA (deoxyribonucleic acid) is a double-stranded molecule.
• It consists of two strands of nucleotides which are held together by hydrogen bonds.
• The double helix structure was proposed by James Watson and Francis Crick.
• The strands are anti-parallel, meaning they run in opposite directions.

Nucleotide Structure

• Nucleotides are the building blocks of DNA.
• Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.
• The four nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).
• The nucleotide sequence determines the genetic code.

Base Pairing

• Adenine (A) always pairs with thymine (T) through two hydrogen bonds.
• Cytosine (C) always pairs with guanine (G) through three hydrogen bonds.
• This complementary base pairing allows DNA replication and accurate transmission of genetic information.

DNA Replication

• DNA replication is the process by which DNA is copied to produce identical copies.
• It occurs during the S-phase of the cell cycle.
• The enzyme responsible for DNA replication is DNA polymerase.
• Each strand of the DNA double helix serves as a template for the synthesis of a new complementary strand.

Steps in DNA Replication

1. Initiation: DNA helicase unwinds the double helix and separates the DNA strands.

2. Elongation: DNA polymerase synthesizes new strands by adding complementary nucleotides to the template strands.

3. Termination: DNA replication is completed when the entire DNA molecule has been replicated.

DNA Repair and Proofreading

• DNA replication is highly accurate, but errors can occur.
• DNA polymerase has proofreading capabilities to detect and correct errors.
• Repair mechanisms are also present to fix damaged DNA, preventing mutations.

Mutations

• Mutations are changes in the DNA sequence.
• They can occur spontaneously or be induced by external factors such as radiation or chemicals.
• Mutations can have different effects, ranging from no impact to causing genetic disorders or cancer.
• Mutations are the basis of genetic diversity and drive evolution.

Gene Expression

• Gene expression is the process by which information from a gene is used to make a functional product, such as a protein.
• It consists of two main steps: transcription and translation.
• Transcription converts DNA into RNA, while translation uses RNA to synthesize proteins.

Transcription

• Transcription occurs in the nucleus.
• RNA polymerase binds to the DNA template strand and synthesizes a complementary RNA molecule.
• The resulting RNA molecule, called messenger RNA (mRNA), carries the genetic information to the ribosomes.

Translation

• Translation occurs in the cytoplasm.
• mRNA binds to the ribosome, and transfer RNA (tRNA) carries specific amino acids to the ribosome.
• The ribosome reads the mRNA codons and assembles the amino acids into a polypeptide chain.
• This process continues until a stop codon is encountered, resulting in the formation of a functional protein.

DNA Replication

• Semi-conservative replication
• Leading and lagging strands
• Okazaki fragments
• DNA ligase
• Replication fork

Central Dogma

• DNA → RNA → Protein
• Transcription
• RNA processing
• Translation

Transcription

• Initiation
• Elongation
• Termination
• Promoter
• Transcription factors

RNA Splicing

• Introns and exons
• Spliceosome
• Alternative splicing
• mRNA processing

Genetic Code

• Codons
• Start codon (AUG)
• Stop codons (UAA, UAG, UGA)
• Degeneracy
• Reading frame

Translation

• Ribosomes
• tRNA structure
• Anticodons
• Peptide bond formation
• Polyribosomes

Regulation of Gene Expression

• Transcription factors
• Enhancers and silencers
• Operons (lac and trp)
• Post-transcriptional modification
• Epigenetics

Mutations

• Point mutations (missense, nonsense, silent)
• Frameshift mutations
• Insertions and deletions
• Mutagens
• Genetic disorders (sickle cell anemia, cystic fibrosis)

Recombinant DNA Technology

• Restriction enzymes
• DNA cloning
• Polymerase chain reaction (PCR)
• Gel electrophoresis
• DNA sequencing

Applications of Biotechnology

• Genetically modified organisms (GMOs)
• Gene therapy
• DNA fingerprinting
• Forensic science
• Pharmaceutical production