Molecular Biology and Water Potential Exam Prep

Gel Electrophoresis

  • A key topic frequently tested on the AP exam.
  • Separates DNA fragments based on length using an electric field.
  • DNA is negatively charged due to its phosphate groups. This allows it to be pulled through the gel towards a positive charge.
  • Shorter fragments travel faster and farther through the gel than longer fragments.

Interpreting Gel Electrophoresis Results

  • Each band represents a collection of many DNA fragments of the same length, not necessarily the same sequence.
  • Restriction Fragment Length Polymorphisms (RFLPs): Differences in DNA fragment lengths due to variations in restriction sites.

Example

  1. Normal DNA has restriction sites that result in fragments of 201 base pairs (bp) and 175 bp when cut.
  2. Mutated DNA lacks a restriction site, resulting in a single combined fragment of 376 bp (201+175).
  3. On a gel, the normal DNA will show two bands (201 bp and 175 bp), while the mutated DNA will show one band (376 bp).
  • Thicker bands indicate more DNA is present.
  • Homozygous individuals will show one band, while heterozygous individuals may show two bands.
  • Practice interpreting gel results is crucial for the AP exam.

Other Molecular Biology Techniques

  • PCR (Polymerase Chain Reaction): Used to amplify (copy) specific DNA sequences through repeated cycles of heating and cooling.

Genetic Engineering

  • Benefits: Producing human insulin in bacteria is a prime example of genetic engineering's benefits. A human insulin gene is inserted into a bacterial plasmid, allowing bacteria to produce human insulin.
  • Ethics of GMOs: Requires consideration and informed opinions.

Process of Inserting Eukaryotic Gene into Plasmid

  1. A eukaryotic gene (e.g., human insulin gene) is inserted into a bacterial plasmid.
  2. Bacteria contain plasmids separate from their main chromosome, which can be modified.
  3. The modified plasmid is introduced into bacteria.
  4. Bacteria replicate, including the plasmid with the human gene.
  5. The human gene is expressed, producing the desired protein (e.g., insulin), which can then be purified.
  6. Restriction Enzymes: Cut DNA at specific restriction sites, creating sticky ends that facilitate the insertion of the gene into the plasmid.

SNPs (Single Nucleotide Polymorphisms)

  • SNPs: Represent the single nucleotide differences between individuals.

Water Potential

  • Water potential: determines the direction of water movement (high to low potential).
  • Formula: \Psi = \Psip + \Psis where \, \Psi is the water potential, \, \Psip is the pressure potential, \, \Psis is the solute potential.
  • Pressure potential (\Psi_p): increasing pressure potential increases the water potential. In open containers, pressure potential is often considered zero.
  • Solute potential (\Psi_s): adding solutes lowers the solute potential; solute potential is always zero or negative. Pure water has a solute potential of zero.
  • If a cell as a pressure potential of -3 and a solute potential of -10, the water will move outside the cell.

Calculating Solute Potential

  • Formula: \Psi_s = -iCRT
    • i = ionization constant (number of particles the solute dissociates into; for NaCl, i = 2; for glucose, i = 1).
    • C = molar concentration (determined from a graph where the line crosses 0% change in mass).
    • R = pressure constant (0.0831 liter bar / mol K).
    • T = temperature in Kelvin (273 + degrees Celsius).
  • The molar concentration (C) is determined from a graph where the line crosses the 0% change in mass.
  • Graphs:
    • Line graphs: used to show change.
    • Bar graphs: used to compare unlike groups.
  • Graphing Water Potential Data:
    • x-axis: independent variable (solution used).
    • y-axis: dependent variable (percent change).
    • The point where the line crosses 0% change on the y-axis indicates the molar concentration (C) of the solution (solute potential).