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No PLTL Sessions Next Week

  • Dates: October 13-October 17

Molecular Basis of Inheritance

  • Date: 10/8/25

Big Ideas to Understand

  • Evidence that DNA is the genetic material

  • DNA replication

  • How DNA is packaged in chromosomes

Transformation of Bacteria

  • Two types of bacterial strains:

    • Pathogenic (disease-causing)

    • Nonpathogenic

  • Experiment:

    • Pathogenic strain killed and mixed with nonpathogenic strain.

    • Result: Living cells became pathogenic and this trait was inherited by all descendants.

    • Process termed as Transformation.

Viral DNA

  • Viruses are comprised of DNA and protein.

  • Method of analysis: Tagged both components to determine which entered bacteria during infection.

  • Conclusion: Only DNA entered the host cell during infection.

DNA

  • Definition: Polymer of nucleotides.

  • Components of a nucleotide:

    • Phosphate group

    • Sugar (deoxyribose in DNA)

    • Nitrogenous base (Adenine, Guanine, Cytosine, Thymine)

Additional Evidence: DNA Base Analysis (Chargaff’s Rules)

  • Key observations:

    • Base composition of DNA varies across species.

    • Example:

    • Sea urchin: 32% Adenine (A)

    • E. Coli: 24% Adenine (A)

    • Base pair rules: The number of A equals T (A=T) and the number of Guanine (G) equals Cytosine (C) (G=C).

DNA Structure: Role in Inheritance

  • Backbone: Phosphate-sugar backbone connected by covalent bonds.

  • Directionality:

    • 5’ end has a free phosphate group.

    • 3’ end has a free hydroxyl (OH) group.

  • Configuration:

    • Structure: Double helix made of two strands.

    • Orientation: Antiparallel (strands run in opposite directions).

DNA Base Pairing

  • Matching rules:

    • Adenine (A) pairs with Thymine (T).

    • Cytosine (C) pairs with Guanine (G).

  • Question: Why doesn't A pair with A or G, and why doesn’t C pair with C or T?

    • Answer: Specific hydrogen bond formations dictate complementary pairing.

Hydrogen Bonding in Base Pairing

  • Hydrogen bond structure:

    • The nitrogenous base pairs are held together by hydrogen bonds:

    • Example pairs:

      • Adenine (A) - Thymine (T)

      • Guanine (G) - Cytosine (C)

Complementary Strands

  • Task: Identify the complementary strand to 5'-TTGGACTGC-3'.

  • Correct answer: 3'-AACCTGACG-5'.

DNA Percentage Analysis

  • Given: A double-stranded DNA molecule with 22% guanine bases.

  • Question: What is the expected percentage of adenine (A) bases?

  • Conclusion: Not enough information to determine percentage based on given data due to pairing rules.

DNA Replication

  • Process:

    • Two complementary strands separate.

    • Each strand serves as a template for a new complementary strand (daughter strand).

    • Model of replication: Semiconservative model.

Eukaryotic Cell Replication

  • Initiation:

    • Origin of replication is established in a eukaryotic cell.

    • Formation of a replication bubble:

    • On either side of the bubble, replication forks form where DNA unwinds.

    • Process continues in both directions until entire molecule is copied.

Role of Enzymes in DNA Replication

  • Key Enzymes:

    • Helicase: Unzips the DNA double helix at replication forks.

    • Single-strand binding proteins: Stabilize and keep strands separated.

    • Topoisomerase: Prevents supercoiling by breaking, swiveling, and rejoining DNA strands.

    • Primase: Synthesizes an RNA primer at the 5' end of leading strand and at the 5' end of each Okazaki fragment of the lagging strand.

    • DNA polymerases: Utilize parental DNA as a template to synthesize new DNA strands by adding nucleotides at the 3' end of the existing DNA strand.

    • DNA ligase: Joins Okazaki fragments on the lagging strand and connects the replaced RNA primer nucleotides with DNA nucleotides.

Elongation of DNA Strand

  • DNA polymerases build a new strand but can only add to the end of an existing chain.

  • RNA primers produced by primase allow nucleotides to be added to the 3’ end of the existing strand.

Antiparallel Elongation of DNA

  • Leading Strand:

    • Synthesized continuously moving toward the replication fork.

  • Lagging Strand:

    • Synthesized discontinuously moving away from the replication fork.

    • Comprised of segments called Okazaki fragments requiring an RNA primer for each fragment.

    • DNA ligase is responsible for joining these fragments once RNA primers are replaced with DNA nucleotides.

Eukaryotic Chromosomes

  • Composition: Consists of a long strand of DNA packed with proteins, forming chromatin.

  • Histones: Proteins that DNA wraps around, helping package and organize DNA into a compact area.

  • Nucleosomes: Units formed by DNA wrapped around histones, coiling and folding into compact chromatin fibers that condense further to form chromosomes.