Genetics and Evolution: Molecular Basis of Inheritance - Griffith Experiment

Slide 1

  • Topic: Molecular Basis of Inheritance - Griffith Experiment
  • Aim: To understand the significance of the Griffith Experiment
  • Introduction to the Griffith Experiment

Slide 2

  • In 1928, Frederick Griffith conducted an experiment with Streptococcus pneumoniae
  • He observed that bacteria could transfer genetic information
  • Griffith’s experiment laid the foundation for future research in genetics

Slide 3

  • Aim of Griffith’s Experiment: To determine how bacteria acquire new traits
  • Used two strains of Streptococcus pneumoniae:
    • S strain (smooth colonies) - Virulent strain
    • R strain (rough colonies) - Non-virulent strain

Slide 4

  • Griffith’s experimental procedure:
    1. Injected mice with the S strain; mice died due to pneumonia
    2. Injected mice with the R strain; mice survived, but did not develop pneumonia
    3. Heat-killed S strain injected into mice with the R strain; mice died due to pneumonia

Slide 5

  • Conclusion from Griffith’s experiment:
    • Live R strain bacteria transformed into S strain when exposed to heat-killed S strain
    • Transformation occurred, resulting in virulence in the R strain bacteria
    • Genetic material transferred from heat-killed S strain to the live R strain

Slide 6

  • This experiment suggested that there exists a transforming principle
  • This transforming principle is responsible for the transfer of genetic information
  • Later experiments proved that DNA is the genetic material and the transforming principle

Slide 7

  • Explanation of Griffith’s Experiment:
    • The DNA from the heat-killed S strain bacteria was taken up by the live R strain bacteria
    • This introduced the genetic information for virulence into the R strain bacteria
    • The R strain then began producing a capsule and exhibited virulence

Slide 8

  • Griffith’s experiment laid the foundation for further research on genetics and inheritance
  • It provided evidence that DNA carries genetic information and can be transferred between bacteria
  • This experiment led to the discovery of DNA as the molecule responsible for inheritance

Slide 9

  • Significance of the Griffith Experiment:
    • Paved the way for the discovery of the structure and function of DNA
    • Established the importance of DNA in the molecular basis of inheritance
    • Demonstrated the transferability of genetic information

Slide 10

  • Key points to remember from the Griffith Experiment:
    • Two strains of Streptococcus pneumoniae were used: S and R
    • Heat-killed S strain transformed the R strain into the S strain
    • DNA is the genetic material responsible for transformation
    • This experiment laid the foundation for future genetic research
  1. Still there remains something mysterious about genes:
  • Genes remained mysterious until the discovery of DNA as the genetic material
  • Researchers were perplexed about the nature and composition of genes
  1. Discovery of DNA as the genetic material:
  • In 1944, Avery, MacLeod, and McCarty experimentally proved that DNA is the genetic material
  • Their experiments showed that DNA is responsible for the transformations observed in Griffith’s experiment
  1. Structure of DNA:
  • DNA is a double-stranded helical molecule
  • It consists of nucleotides, which are made up of a sugar (deoxyribose), phosphate, and a nitrogenous base (adenine, thymine, cytosine, or guanine)
  • The structure of DNA was determined by James Watson and Francis Crick in 1953
  1. Complementary base pairing in DNA:
  • 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 strands to be replicated accurately
  1. DNA replication:
  • DNA replicates during cell division to ensure that each daughter cell receives an identical copy of the genetic material
  • The process involves unwinding of the DNA helix, separation of the two strands, and synthesis of complementary strands by DNA polymerase
  1. Importance of DNA replication:
  • Accurate DNA replication ensures the continuity of genetic information from one generation to the next
  • Errors in DNA replication can lead to mutations, which may have harmful effects or provide opportunities for evolutionary changes
  1. Genetic code:
  • The genetic code is a set of rules that determines how the information in DNA is translated into proteins
  • The code is based on the sequence of nucleotides in DNA, with each three-nucleotide sequence (codon) corresponding to a specific amino acid
  1. Transcription:
  • Transcription is the process by which DNA is used as a template to synthesize RNA
  • It involves the synthesis of an RNA molecule complementary to a specific DNA sequence
  1. Types of RNA involved in transcription:
  • Messenger RNA (mRNA): Carries the genetic information from DNA to the ribosomes for protein synthesis
  • Transfer RNA (tRNA): Transfers amino acids to the ribosomes during protein synthesis
  • Ribosomal RNA (rRNA): Forms the structural and functional components of ribosomes
  1. Translation:
  • Translation is the process by which the genetic information in mRNA is used to synthesize proteins
  • It takes place in the ribosomes, where tRNA brings amino acids and binds with the mRNA codons to form a polypeptide chain
  1. Mutation:
  • Mutation is a change in the DNA sequence of an organism
  • It can result from errors during DNA replication or exposure to mutagens (e.g., radiation, chemicals)
  • Mutations can be harmful, beneficial, or have no effect on an organism
  1. Types of mutations:
  • Point mutation: A change in a single nucleotide, which can include substitutions, insertions, or deletions
  • Frameshift mutation: Caused by the insertion or deletion of nucleotides, which shifts the reading frame during translation
  • Chromosomal mutation: Involves changes in the structure or number of chromosomes, such as deletions, duplications, inversions, or translocations
  1. Examples of mutations:
  • Sickle cell anemia: A point mutation results in the substitution of one amino acid in the hemoglobin protein, leading to the abnormal shape of red blood cells
  • Down syndrome: Caused by an extra copy of chromosome 21, resulting from a chromosomal mutation
  1. Genetic variation:
  • Mutations contribute to genetic variation, which is essential for the survival and adaptation of species
  • Genetic variation enables individuals within populations to respond differently to environmental changes
  • It increases the chances of species survival and evolution
  1. Evolution:
  • Evolution is the process of gradual change in a population over time
  • It occurs as a result of mechanisms such as natural selection, genetic drift, migration, and mutation
  • Evolution leads to the development of new species and the diversity of life forms on Earth
  1. Natural selection:
  • Natural selection is the mechanism of evolution proposed by Charles Darwin
  • It involves the differential survival and reproduction of individuals with favorable traits that are better adapted to their environment
  • Over time, these traits become more common in the population
  1. Example of natural selection:
  • Peppered moth in England: During the Industrial Revolution, the population shifted from predominantly light moths to dark moths, as the dark color provided better camouflage on polluted tree trunks
  • Antibiotic resistance: Bacteria evolve resistance to antibiotics through natural selection, leading to the emergence of drug-resistant strains
  1. Speciation:
  • Speciation is the process by which one species splits into two or more distinct species
  • It usually occurs when populations become reproductively isolated and no longer interbreed
  • Reproductive isolation can be due to geographical, ecological, or behavioral factors
  1. Steps in speciation:
  • Geographic isolation: Populations become separated by a physical barrier, such as a mountain range or body of water
  • Genetic divergence: Different mutations, natural selection, and genetic drift act on the separated populations, leading to genetic differences
  • Reproductive isolation: The populations can no longer interbreed due to genetic, behavioral, or temporal differences
  • Formation of new species: When reproductive isolation is complete, new species are formed
  1. Conclusion:
  • The Griffith Experiment provided valuable insights into the molecular basis of inheritance and the role of DNA
  • Understanding genetics and evolution is essential for comprehending the diversity of life forms and how they adapt to their environment
  • Genetic variation, mutations, natural selection, and speciation are key processes driving evolutionary change