Genetics and Evolution: Molecular Basis of Inheritance
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.
- The mixture of bacteriophages and bacterial cells was placed in a blender and gently blended.
- 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.
- 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.
- Initiation: DNA helicase unwinds the double helix and separates the DNA strands.
- Elongation: DNA polymerase synthesizes new strands by adding complementary nucleotides to the template strands.
- 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