KC

Chp 8

Chapter 8: Evolution and Genetic Mechanisms

I. Evolution

  • Definition: Evolution is defined as the process through which populations of organisms undergo changes over time.

    • The smallest unit that can evolve is a population, not an individual.

    • While individuals do not evolve, the survival and reproduction of individuals can impact the evolution of the population.

II. Genetic Diversity

  • Genetic Diversity: It is the raw material for evolution.

    • Genetic diversity is essential for populations to change.

    • The ultimate cause of all genetic diversity is mutation.

A. Effects of Mutation

  • Types of Mutations:

    1. Most mutations are neutral (neither beneficial nor harmful).

    2. Some mutations are deleterious (harmful to the organism).

    3. A few mutations are beneficial (provide an advantage to the organism).

III. Fitness

  • Fitness: Refers to reproductive potential in evolutionary terms.

    1. It is the ability to survive under current environmental conditions, which allows a greater number of offspring to be produced before the death of an organism.

IV. Natural Selection

  • Natural Selection: A process where individuals with higher fitness levels are more likely to survive longer than those with lower fitness.

    1. Longer survival leads to those with high fitness producing more offspring compared to those with lower fitness.

    2. Over time, genes associated with high fitness will pass on to the next generation with greater frequency than genes for low fitness.

    3. Expectations: It is expected that genes that confer high fitness will increase in frequency within the population over time.

A. Environmental Context

  • Environmental Relevance: Fitness levels are applicable only under current environmental conditions. Environmental changes can shift which traits are advantageous. For instance, under global warming, certain organisms that were previously successful are now endangered.

B. Example of Natural Selection

  1. A rare beneficial mutation provides a bacterial cell with resistance to an antibiotic.

    • The mutation is passed to the cell's offspring.

    • Cells lacking resistance may have a fitness advantage when antibiotics are absent, as they conserve energy.

    • With the introduction of widespread antibiotic use during World War II, susceptible cells die, allowing resistant cells to thrive due to reduced competition.

    • There has been a marked increase in antibiotic-resistant bacteria since the 1940s.

    • It is crucial to note that mutations occurred randomly and that antibiotics do not induce resistance but eliminate non-resistant cells.

V. Horizontal Gene Transfer

  • Horizontal Transfer: Refers to the transfer of DNA between two cells of the same generation.

    • This is different from vertical transfer, which is the passing of DNA from parent to offspring.

A. Types of Horizontal Transfer

  • Three Types of Horizontal Transfer (Refer to Figure 8.18 on page 216):

    1. Transformation: Uptake of naked DNA from the environment.

    2. Conjugation: Direct transfer of DNA between bacteria through cell-to-cell contact.

    3. Transduction: Transfer of bacterial DNA via bacteriophages (viruses that infect bacteria).

VI. Transformation

  • Definition: The uptake of naked DNA from the environment into a bacterial cell.

A. Naked DNA

  • Naked DNA: Refers to DNA that is not surrounded by any cellular membrane.

    • Upon bacterial cell lysis, DNA is released into the environment in damaged chromosomal fragments.

B. Competence

  • Competence: The ability to take up naked DNA.

    1. Only specific bacterial genera possess natural competence, including Streptococcus, Haemophilus, and Neisseria.

    2. Bacteria are not always competent; they become so under certain conditions, such as starvation.

C. Steps of Transformation**

  1. DNA binds to a surface protein on a competent cell.

  2. DNA uptake: DNA enters the cell.

  3. Bacterial enzymes fragment the DNA, allowing for integration into the bacterial chromosome.

VII. Griffith Experiment

  • Streptococcus pneumoniae Study:

    1. Smooth (S) strain: Encapsulated, virulent, and lethal to mice when injected.

    2. Rough (R) strain: Non-encapsulated, non-virulent, and harmless when injected.

  • Transformation Explanation: Heat-killed S strain bacteria donate DNA to viable R strain bacteria. R strain cells gain the capability to form a capsule and convert to S strain, demonstrating genetic transformation.

VIII. Conjugation

  • Definition: Conjugation involves the direct connection of two bacteria through a pilus for DNA transfer.

A. Plasmids

  1. Conjugative plasmids are small, circular pieces of DNA that replicate independently from chromosomal DNA.

  2. Plasmids typically carry genes, including those for antibiotic resistance and virulence factors, which contribute to the ability to cause disease. (Refer to Figure 8.26 on page 225)

B. Conjugation Process

  1. F factor: A specific conjugative plasmid utilized in E. coli

  2. F+ Cell: The donor cell with F factor.

  3. F− Cell: The recipient cell lacking F factor.

C. Steps of F Factor Conjugation** (Refer to Figure 8.22 on page 221):

  1. F+ cell synthesizes a pilus, which attaches to F− cell.

  2. Pilus contracts, bringing cells into contact.

  3. F plasmid is replicated and transferred to the F− cell.

  4. F− cell transforms to become an F+ cell (a new DNA donor).

  5. This mechanism of conjugation can spread genes for antibiotic resistance, complicating public health.

IX. Transduction

  • Bacteriophage: A virus specific to bacteria.

  • Transduction: This process involves the transfer of bacterial DNA from one cell to another via a bacteriophage.

  • More detailed explanations of transduction will follow in later chapters.

X. Genetic Engineering

  • Application in Medicine: Genetic engineering has significantly enhanced the production of insulin for diabetes treatment.

  • Process Overview:

    1. The human gene for insulin is isolated and inserted into bacterial cells.

    2. Bacteria express and produce large quantities of human insulin protein due to their ability to multiply rapidly.

  • Steps of Genetic Engineering:

    1. Remove the DNA of interest (insulin gene).

    2. Cut both the DNA of interest and a plasmid using restriction enzymes.

    3. Use ligase to connect the DNA fragments, forming a recombinant plasmid.

    4. Introduce the plasmid into bacteria through transformation.

    5. Allow bacterial cells to reproduce, subsequently expressing insulin.

    6. Purify the produced protein for medical use.

A. Implications of DNA Technology

  • Genetic engineering has led to advancements in health, agriculture, and other industries.

  • Encouragement to take further biotechnology courses for deeper knowledge on practical applications of genetic science.