Study Notes for BCHEM 264 - Biophysics Lecture Three

BCHEM 264 - Biophysics Lecture Three

Instructor Information

Dr. Alexander Kwarteng
Department of Biochemistry & Biotechnology, College of Science, KNUST

Overview of Lecture Content

This lecture focuses on the concepts of DNA melting and renaturation, particularly in relation to DNA complexity illustrated through Cot curves.

DNA Melting and Renaturation
  • DNA Reassociation Kinetics: The groundwork for understanding DNA complexity was laid by examining DNA reassociation kinetics.
  • Definition of Cot:
    • Cot represents the product of the initial DNA concentration (Co) in moles of nucleotides per liter and the time (t) in seconds:
      extCot=C0imestext{Cot} = C_0 imes t
    • The extent of renaturation of complementary strands is dependent solely on the Cot value if conditions such as concentration, pH, and ionic concentration are equal.
    • Cot½: A notable Cot value where half of the DNA has annealed.
Complexity of DNA - Cot Curves
  • There is a linear relationship between DNA structural complexity and Cot½.
  • Example: The calf genome contains approximately 2imes1092 imes 10^9 base pairs (bp), while the T4 bacteriophage genome has approximately 2imes1052 imes 10^5 bp.
    • This means calf DNA is 10,000 times larger, taking 10,000 times longer to anneal, hence a Cot value 10,000 times larger.
Construction of Cot Curves
  1. Heat the DNA sample to denature it.
  2. Cool the sample slowly to allow reassociation.
  3. Dilute the sample to slow down reassociation further.
  4. Measure the fraction of DNA that has reassociated, then plot against log Cot.
  5. Use hydroxyapatite columns for separation, where:
    • Ca²⁺ ions bind to negatively charged phosphate groups in DNA.
    • Low phosphate buffer elutes single-stranded DNA, while high phosphate concentration elutes double-stranded DNA.
Key Calculations and Graphs
  • The percentage of reassociated DNA is calculated as:
    ext{% Reassociated DNA} = rac{ ext{Amount Reassociated}}{ ext{Total Amount}} imes 100
  • Data can be collected by fixing the annealing temperature at 25extoC25^ ext{o}C below Tm and varying annealing time.
    • Y-axis: % Annealed DNA (fraction reassociated).
    • X-axis: Cot on a logarithmic scale.
    • Alternatively, the Y-axis could represent the percentage of the remaining single-stranded DNA.
Characteristics of Cot Curves
  • Features:
    • The shape of the Cot curve is generally sigmoidal.
    • Repetitive sequences reassociate more quickly and dominate the genome.
    • Nonrepetitive sequences are fewer and reassociate at a slower rate.
    • Smooth curves indicate gradual annealing over time.
  • Different regions on the Cot curve represent different types of DNA complexity, with repetitive sequences showing fast reassociation and single copy sequences showing slow reassociation.
Applications of Cot Curves
  • The shape of a Cot curve provides insights into the following:
    • The size and complexity of the genome.
    • The amount and characteristics of repetitive DNA.
    • The number of genes present.
    • Guidance for primer sequences in noncoding regions (e.g., SSRs, VNTRs).
    • Facilitation of genome sequencing efforts.
Conclusions from Cot Curves
  • Increased genome size correlates with a necessity for higher concentration for annealing.
  • Longer annealing times are required as genome size and nonrepetitive sequence content increases.
  • Cot½ is proportional to DNA complexity and can be standardized against E. coli DNA complexity of 4.2imes1064.2 imes 10^6 bp.
Comparison of DNA Complexity - Eukaryotic vs. Prokaryotic
  • Eukaryotic genomes exhibit Cot values spanning up to 8 orders of magnitude, reflecting a more complex structure than prokaryotic genomes, which have a singular kinetic component.
  • Eukaryotic genomes demonstrate various kinetic components, evidencing higher complexity.
  • The reassociation kinetics of eukaryotic DNA can be categorized into three types of components: fast, intermediate, and slow, reflective of differing repetition frequencies and sequence lengths.
Summary of Complexity in Eukaryotic DNA
  • Fast Component: Highly repetitive sequences.
  • Intermediate Component: Moderately repetitive sequences which are generally shorter and often include ribosomal genes.
  • Slow Component: Represents single copy DNA, which includes longer, non-repetitive coding sequences.
Single Copy DNA
  • Constitutes the slow reannealing component, forming the coding sequences of genes.
  • In maize, approximately 2.5 billion bp corresponds to around 32,000 protein-coding genes.
  • The human genome contains approximately 3.2 billion bp with 20,000-25,000 distinct protein-coding genes.